Scott Husband, Toru Shimizu
Evolution of the Avian Visual System. Chapter II. Evolution of the Amniote Brain.
Comparative Cognition Press
By the end of the Carboniferous, amniotes could be categorized into three groups on the basis of skull anatomy.  There are openings in the dermal skull roof behind the orbits, whose pattern is used for classifying the ancestral amniotes.  The earliest amniotes, including the Captorhinida, had no such opening, and they are called Anapsida.  Turtles are often included in this group since their skull is entirely covered with bone.  The first group which diverged from the early anapsids are the Synapsida (one temporal opening), which ultimately evolved into mammals.  The second group which separated from the basal anapsids were the Diapsida (two openings), which evolved into reptiles (including dinosaurs) and birds. [Full Text]

ten Donkelaar HJ.
Some introductory notes on the organization of the forebrain in tetrapods.
Eur J Morphol. 1999 Apr;37(2-3):73-80.
As an introduction to the main theme of this conference an overview of the organization of the tetrapod forebrain is presented with emphasis on the telencephalic representation of sensory and motor functions. In all classes of tetrapods, olfactory, visual, octavolateral, somatosensory and gustatory information reaches the telencephalon. Major differences exist in the telencephalic targets of sensory information between amphibians and amniotes. In amphibians, three targets are found: the lateral pallium for olfactory input, the medial pallium for visual and multisensory input, and the lateral subpallium for visual, octavolateral and somatosensory information. The forebrains of reptiles and mammals are similar in that the dorsal surface of their cerebral hemisphere is formed by a pallium with three major segments: (a) an olfactory, lateral cortex; (b) a 'limbic' cortex that forms the dorsomedial wall of the hemisphere, and (c) an intermediate cortex that is composed entirely of isocortex in mammals, but in reptiles (and birds) consists of at least part of the dorsal cortex (in birds the Wulst) and a large intraventricular protrusion, i.e. the dorsal ventricular ridge. In birds, the entire lateral wall of the hemisphere is involved in this expansion. The intermediate pallial segment receives sensory projections from the thalamus and contains modality-specific sensory areas in reptiles, birds and mammals. The most important differences between the intermediate pallial segment of amniotes concern motor systems. [Abstract]

Smeets WJ, Marin O, Gonzalez A.
Evolution of the basal ganglia: new perspectives through a comparative approach.
J Anat. 2000 May;196 ( Pt 4):501-17.
The striatum is the major receptive structure of the BG in all tetrapods and receives its main inputs from the cortex (or pallium), the thalamus and the dopaminergic neurons of the VTA-SN complex. Afferents from the cortex (or pallium) and the thalamus provide the striatum with a direct access to diverse and multimodal information, although substantial differences exist in the extent and degree of organisation of these projections among tetrapods. In mammals, virtually all cortical areas contribute to the innervation of the striatal territories, giving rise to a complex representation of the functional cortical map at the striatal level (Parent & Hazrati, 1995a). In reptiles and birds, the major source of cortical/pallial inputs to the striatum is the dorsal ventricular ridge (DVR), a telencephalic structure that is embryologically derived from the pallium. Although some authors consider this structure comparable, to some extent, to the isocortex of mammals (Butler, 1994; Reiner et al. 1998), its true nature is still a matter of debate (for reviews, see Lohman & Smeets, 1991; Striedter, 1997). In addition, the striatum of reptiles and birds receives projections from other pallial regions, such as the dorsal cortex and the Wulst, respectively. Recently, it has been shown that palliostriatal connections are also present in amphibians (Marin et al. 1997a), thus underscoring the notion that the existence of striatal afferents from the telencephalic mantle is a feature shared by all tetrapods (Fig. 2). Nevertheless, it is obvious that a
dramatic increase in the number and complexity of the cortico/palliostriatal projections characterises the anamniote-amniote as well as the nonmammalian-mammalian transitions.

The evolution of thalamic afferents to the striatum has been related to the expansion of the cortex and, consequently, to the elaboration of the corticostriatal system. In mammals, direct thalamic afferents to the striatum originate primarily in the midline and intralaminar nuclear complex that relays diverse multimodal ('nonspecific') information to specific parts of the striatum and the cortex (Groenewegen & Berendse, 1994). Specific sensory information, on the contrary, reaches the BG primarily via thalamocorticostriatal connections, although striatal afferents from certain specific relay nuclei in the thalamus do exist (Heimer et al. 1995). In sharp contrast, projections from specific sensory thalamic nuclei are the main afferents of the striatum in living amphibians (Marin et al. 1997a) and, therefore, sensory information of different modalities is essentially relayed
to the amphibian BG without involvement of the telencephalic pallium. Multimodal sensory and limbic information is relayed to the amphibian striatum and pallium by the anterior thalamic region. An intermediate condition is found in reptiles and birds, where specific sensory thalamic nuclei project to the dorsal cortex and the DVR, but also to the striatal components of the BG (Gonzalez et al. 1990; Veenman et al. 1997). Furthermore, a region of the dorsal thalamus of birds and reptiles appears to be largely comparable to the entire intralaminar, mediodorsal and midline nuclear complex of mammals, providing widespread projections to the striatum and the pallium (Veenman et al. 1997). In conclusion, the existence of direct sensory inputs from the thalamus to the striatum seems to be a primitive feature of the BG in tetrapods (Fig. 2). A major evolutionary trend is, however, the progressive involvement of the cortex or cortical-like structures in the processing of the thalamic sensory information relayed to the BG of tetrapods. It also seems likely that the striatum of the common ancestors of tetrapods has been the target of the projections of a dorsal thalamic nuclear complex that receives inputs of diverse and multimodal nature (Fig. 2). In contrast to the specific sensory thalamic nuclei, the midline intralaminar nuclei are involved in nondiscriminative or affective aspects of the information, which might be required to prevent the organism from potentially dangerous situations and, therefore, possess an obvious adaptive value (Groenewegen & Berendse, 1994). [Full Text]

Marin O, Gonzalez A, Smeets WJ.
Basal ganglia organization in amphibians: afferent connections to the striatum and the nucleus accumbens.
J Comp Neurol. 1997 Feb 3;378(1):16-49.
As part of a research program to determine if the organization of basal ganglia (BG) of amphibians is homologous to that of amniotes, the afferent connections of the BG in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl were investigated with sensitive tract-tracing techniques. Hodological evidence is presented that supports a division of the amphibian BG into a nucleus accumbens and a striatum. Both structures have inputs in common from the olfactory bulb, medial pallium, striatopallial transition area, preoptic area, ventral thalamus, ventral hypothalamic nucleus, posterior tubercle, several mesencephalic and rhombencephalic reticular nuclei, locus coeruleus, raphe, and the nucleus of the solitary tract. Several nuclei that project to both subdivisions of the BG, however, show a clear preference for either the striatum (lateral amygdala, parabrachial nucleus) or the nucleus accumbens (medial amygdala, ventral midbrain tegmentum). In addition, the anterior entopeduncular nucleus, central thalamic nucleus, anterior and posteroventral divisions of the lateral thalamic nucleus, and torus semicircularis project exclusively to the striatum, whereas the anterior thalamic nucleus, anteroventral, and anterodorsal tegmental nuclei provide inputs solely to the nucleus accumbens. Apart from this subdivision of the basal forebrain, the results of the present study have revealed more elaborate patterns of afferent projections to the BG of amphibians than previously thought. Moreover, regional differences within the striatum and the nucleus accumbens were demonstrated, suggesting the existence of functional subdivisions. The present study has revealed that the organization of the afferent connections to the BG in amphibians is basically similar to that of amniotes. According to their afferent connections, the striatum and the nucleus accumbens of amphibians may play a key role in processing olfactory, visual, auditory, lateral line, and visceral information. However, contrary to the situation in amniotes, only a minor involvement of pallial structures on the BG functions is present in amphibians. [Abstract]

Pritz MB.
The thalamus of reptiles and mammals: similarities and differences.
Brain Behav Evol. 1995;46(4-5):197-208.
Certain aspects of thalamic organization in reptiles and mammals are reviewed. Features shared by the dorsal thalamus of reptiles and that of mammals include projection to the telencephalon, specific and non-specific non-telencephalic afferents, and input from the thalamic reticular nucleus. Differences between the dorsal thalamus of reptiles and that of mammals are the absence of reciprocal telencephalic efferents to the dorsal thalamus and lack of local circuit neurons in reptiles (with the exception of the dorsal geniculate complex in turtles) and their presence in mammals. A thalamic reticular nucleus is present in both reptiles and mammals. In both of these classes of vertebrates, this neuronal aggregate surrounds the dorsal thalamus along its lateral surface, projects to the dorsal thalamus, and is organized into sectors. In one group of reptiles, Caiman crocodilus, the sole reptilian group in which immunocytochemical features have been investigated in detail, the reticular nucleus contains at least three neuronal subpopulations: neurons immunoreactive for glutamic acid decarboxylase (GAD); neurons immunoreactive for parvalbumin; and cells that are not immunoreactive for parvalbumin or, probably, GAD. On the other hand, the reticular nucleus of mammals contains a single population of neurons immunoreactive for GAD, gamma amino butyric acid, and parvalbumin. [Abstract]

Pritz MB, Stritzel ME.
Anatomical identification of a telencephalic somatosensory area in a reptile, Caiman crocodilus.
Brain Behav Evol. 1994;43(2):107-27.
Telencephalic projections of a thalamic somatosensory area were investigated in a reptilian species, Caiman crocodilus, by means of horseradish peroxidase (HRP) neurohistochemistry. Injections of HRP into the medialis complex labeled axons that leave the ventral aspect of this nucleus to enter the dorsal peduncle of the lateral forebrain bundle where they ascend. These fibers course rostrally to enter the central portion of the lateral forebrain bundle. Here, axons turn dorsally, pass through, and probably synapse on, interposed neurons of the ventrolateral area, to end in a central portion of the dorsal ventricular ridge (DVR). These findings were confirmed by HRP injections into this somatosensory area of the DVR that retrogradely labeled neurons in the medialis complex. This telencephalic somatosensory area identified in transverse sections was then reconstructed onto a surface view of the DVR. The locus of this forebrain somesthetic region was then compared with the location of the DVR projection areas of auditory (nucleus reuniens) and visual (nucleus rotundus) thalamic nuclei. This analysis suggested two features. First, telencephalic terminal zones of dorsal thalamic nuclei were largely separate and non-overlapping. Second, the locus of termination in the DVR of each thalamic nucleus reflected each nucleus's topography in the dorsal thalamus. Additional parallels in the neural circuitry of auditory, visual, and somatosensory systems that synapse in the midbrain and project to the DVR are documented. [Abstract]

Smeets WJ, Gonzalez A.
Sensorimotor integration in the brain of reptiles.
Eur J Morphol. 1994 Aug;32(2-4):299-302.
Visual, auditory, and somatosensory information is, via several thalamic nuclei, relayed to the reptilian forebrain. These thalamotelencephalic projections terminate primarily in the dorsal ventricular ridge in a non-overlapping way. Subsequent parallel processing of the three sensory modalities throughout the DVR, striatum and globus pallidus resembles the fundamentally parallel form of organization in the mammalian basal ganglia. The lack of direct projections to the thalamus and the absence of extensive, intrinsic connections make a comparison of the dorsal ventricular ridge with the mammalian neocortex highly questionable. [Abstract]

Pritz MB, Stritzel ME.
Anatomical identification of a telencephalic somatosensory area in a reptile, Caiman crocodilus.
Brain Behav Evol. 1994;43(2):107-27.
Telencephalic projections of a thalamic somatosensory area were investigated in a reptilian species, Caiman crocodilus, by means of horseradish peroxidase (HRP) neurohistochemistry. Injections of HRP into the medialis complex labeled axons that leave the ventral aspect of this nucleus to enter the dorsal peduncle of the lateral forebrain bundle where they ascend. These fibers course rostrally to enter the central portion of the lateral forebrain bundle. Here, axons turn dorsally, pass through, and probably synapse on, interposed neurons of the ventrolateral area, to end in a central portion of the dorsal ventricular ridge (DVR). These findings were confirmed by HRP injections into this somatosensory area of the DVR that retrogradely labeled neurons in the medialis complex. This telencephalic somatosensory area identified in transverse sections was then reconstructed onto a surface view of the DVR. The locus of this forebrain somesthetic region was then compared with the location of the DVR projection areas of auditory (nucleus reuniens) and visual (nucleus rotundus) thalamic nuclei. This analysis suggested two features. First, telencephalic terminal zones of dorsal thalamic nuclei were largely separate and non-overlapping. Second, the locus of termination in the DVR of each thalamic nucleus reflected each nucleus's topography in the dorsal thalamus. Additional parallels in the neural circuitry of auditory, visual, and somatosensory systems that synapse in the midbrain and project to the DVR are documented. [Abstract]

Guirado S, Davila JC.
Thalamo-telencephalic connections: new insights on the cortical organization in reptiles.
Brain Res Bull. 2002 Feb-Mar 1;57(3-4):451-4.
Tracer injections into the dorsal tier of the lacertilian dorsal thalamus revealed an extensive innervation of the cerebral cortex. The medial cortex, the dorsomedial cortex, and the medial part of the dorsal cortex received a bilateral projection, whereas the lateral part of dorsal cortex and the dorsal part of the lateral cortex received only an ipsilateral thalamic projection. Thalamocortical fibers were found superficially in all cortical regions, but in the dorsal part of the lateral cortex, varicose axons within the cellular layer were also observed. The bilateral thalamocortical projection originates from a cell population located throughout the dorsolateral anterior nucleus, whereas the ipsilateral input originates mainly from a rostral neuronal subpopulation of the nucleus. This feature suggests that the dorsolateral anterior nucleus consists of various parts with different projections. The dorsal subdivision of the lateral cortex displayed hodological and topological (radial glia processes) features of a dorsal pallium derivative. After tracer injections into the dorsal cortex of lizards, we found long descending projections that reached the striatum, the diencephalic basal plate, and the mesencephalic tegmentum, which suggests that it may represent a sensorimotor cortex. [Abstract]

Adams NC, Lozsadi DA, Guillery RW.
Complexities in the thalamocortical and corticothalamic pathways.
Eur J Neurosci. 1997 Feb;9(2):204-9.
It is now a century since Kölliker (Handbuch der Gewebelehre des Menschen. Nervensystemen des Menschen und der Thiere, Vol. 2, 6th edn. Engelmann, Leipzig, 1896) described the thalamic reticular nucleus as the 'Gitterkern' or lattice nucleus on the basis of the fibrous latticework that is the characteristic feature of this part of the ventral thalamus and adjacent parts of the internal capsule. We suggest that the fibre reorganization produced in this lattice is a fundamental requirement for linking orderly maps in the thalamus to corresponding cortical maps by two-way thalamocortical and corticothalamic connections; these connections involve divergence, convergence and mirror reversals, which all have to occur between the thalamus and the cortex. Apart from the thalamic reticular nucleus, two transient groups of cells, the perireticular nucleus (located in the internal capsule lateral to the reticular nucleus) and the cells of the cortical subplate, are prominent along the course of axons linking the cortex and thalamus early in development. The functions of these two cell groups are not known. However, since early in development complex patterns of reorganization, defasciculation and crossings occur in the regions of these cells, it is likely that they play a role in creating the latticework of the adult. The latticework that characterizes the thalamic reticular nucleus of mammals can also be identified in the ventral thalamus of non-mammalian brains, formed along the course of the fibres that join the dorsal thalamus to the telencephalon. We suggest that the ubiquitous presence of such a zone of fibre reorganization is integral to the functioning of the thalamocortical pathways, and that the complexity of thalamic connections produced in the lattice has been central to the evolutionary success of the thalamotelencephalic system. [Abstract]

Super H, Uylings HB.
The early differentiation of the neocortex: a hypothesis on neocortical evolution.
Cereb Cortex. 2001 Dec;11(12):1101-9.
During development, a cerebral cortex appears in the wall of the telencephalic vesicle in reptiles and mammals. It arises from a cell-dense cortical plate, which develops within a primordial preplate. The neurons of the preplate are essential for cortical development; they regulate the neuronal migration of the cortical plate neurons and form the first axonal connections. In the reptilian cortex and in the hippocampus of the mammalian cerebral cortex, most ingrowing afferent axons run above the cortical plate, in the zone where the receptive tufts of apical dendrites of the cortical pyramidal neurons branch extensively. In the mammalian neocortex, however, axons enter mainly from below the cortical plate where they do not encounter the apical tufts of these pyramidal neurons. In this paper, we discuss the idea that this difference in cortical development has relieved a functional constraint in the expansion of the cortex during evolution. We hypothesize that the entrance of axons below the cell-dense cortical plate, together with the inside-out migration of cortical neurons, ensures that the neocortex remains an "open" system, able to differentiate into new (sub)layers and more cortical areas. [Full Text]

Bruce LL, Butler AB.
Telencephalic connections in lizards. II. Projections to anterior dorsal ventricular ridge.
J Comp Neurol. 1984 Nov 10;229(4):602-15.
Three distinct cytoarchitectonic regions were identified within the anterior dorsal ventricular ridge (ADVR) of two species of lizards, Gekko gecko and Iguana iguana. These regions have been named according to their general topographical positions: medial area, caudolateral area, and rostrolateral area. Injections of horseradish peroxidase throughout the ADVR demonstrated that each of the three areas of the ADVR receives projections from specific thalamic nuclei which are associated with specific sensory modalities. The medial area receives an auditory thalamic projection from nucleus medialis. The caudolateral area receives thalamic projections from nucleus medialis posterior and nucleus posterocentralis. The latter two nuclei were shown to receive projections from the spinal cord and, therefore, are presumed to be associated with body somatosensory information. The rostrolateral area receives a thalamic projection from nucleus rotundus, which receives visual information. In addition, the mesencephalic tegmentum and the thalamic nucleus dorsomedialis project to the entire ADVR. The latter projection is similar to the diffuse cortical projections of the intralaminar thalamic nuclei in mammals. These findings support previous suggestions that the ADVR is comparable to sensory regions of the mammalian neocortex. [Abstract]

Manger PR, Slutsky DA, Molnar Z.
Visual subdivisions of the dorsal ventricular ridge of the iguana (Iguana iguana) as determined by electrophysiologic mapping.
J Comp Neurol. 2002 Nov 18;453(3):226-46.
The dorsal ventricular ridge (DVR) of reptiles is one of two regions of the reptilian telencephalon that receives input from the dorsal thalamus. Although studies demonstrate that two visual thalamic nuclei, the dorsal lateral geniculate and rotundus, send afferents to the dorsal cortex and DVR, respectively, relatively little is known about physiologic representations. The present study determined the organization of the visual recipient region of the iguana DVR. Microelectrode mapping techniques were used to determine the extent, number of subdivisions, and retinotopy within the visually responsive region of the anterior DVR (ADVR). Visually responsive neurons were restricted to the anterior two thirds of the ADVR. Within this region, two topographically organized subdivisions were determined. Each subdivision contained a full representation of the visual field and could be distinguished from the other by differences in receptive field properties and reversals in receptive field progressions across their mutual border. A third subdivision of the ADVR, in which neurons are responsive to visual stimulation is also described; however, a distinct visuotopic representation could not be determined for this region. This third region forms a shell surrounding the lateral, dorsal, and medial aspects of the topographically organized subdivisions. These results demonstrate that there are multiple physiologic subdivisions in the thalamic recipient zone of the ADVR of the iguana. Comparisons to the ADVR of other reptiles are made, homologies to ectostriatial regions of the bird are proposed, and the findings are discussed in relation to telencephalic organization of other vertebrates. [Abstract]

Butler AB.
The evolution of the dorsal pallium in the telencephalon of amniotes: cladistic analysis and a new hypothesis.
Brain Res Brain Res Rev. 1994 Jan;19(1):66-101.
The large body of evidence that supports the hypothesis that the dorsal cortex and dorsal ventricular ridge of non-mammalian (non-synapsid) amniotes form the dorsal pallium and are homologous as a set of specified populations of cells to respective sets of cells in mammalian isocortex is reviewed. Several recently taken positions that oppose this hypothesis are examined and found to lack a solid foundation. A cladistic analysis of multiple features of the dorsal pallium in amniotes was carried out in order to obtain a morphotype for the common ancestral stock of all living amniotes, i.e., a captorhinomorph amniote. A previous cladistic analysis of the dorsal thalamus (Butler, A.B., The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis, Brain Res. Rev., 19 (1994) 29-65; this issue, previous article) found that two fundamental divisions of the dorsal thalamus can be recognized--termed the lemnothalamus in reference to predominant lemniscal sensory input and the collothalamus in reference to predominant input from the midbrain roof. These two divisions are both elaborated in amniotes in that their volume is increased and their nuclei are laterally migrated in comparison with anamniotes. The present cladistic analysis found that two corresponding, fundamental divisions of the dorsal pallium were present in captorhinomorph amniotes and were expanded relative to their condition in anamniotes. Both the lemnothalamic medial pallial division and the collothalamic lateral pallial division were subsequently further markedly expanded in the synapsid line leading to mammals, along with correlated expansions of the lemnothalamus and collothalamus. Only the collothalamic lateral pallial division--along with the collothalamus--was subsequently further markedly expanded in the non-synapsid amniote line that gave rise to diapsid reptiles, birds and turtles. In the synapsid line leading to mammals, an increase in the degree of radial organization of both divisions of the dorsal pallium also occurred, resulting in an 'outside-in' migration pattern during development. The lemnothalamic medial division of the dorsal pallium has two parts. The medial part forms the subicular, cingulate, prefrontal, sensorimotor, and related cortices in mammals and the medial part of the dorsal cortex in non-synapsid amniotes. The lateral part forms striate cortex in mammals and the lateral part of dorsal cortex (or pallial thickening or visual Wulst) in non-synapsid amniotes. Specific fields within the collothalamic lateral division of the dorsal pallium form the extrastriate, auditory, secondary somatosensory, and related cortices in mammals and the visual, auditory, somatosensory, and related areas of the dorsal ventricular ridge in non-synapsid amniotes. [Abstract]

Balaban CD, Ulinski PS.
Organization of thalamic afferents to anterior dorsal ventricular ridge in turtles. I. Projections of thalamic nuclei.
J Comp Neurol. 1981 Jul 20;200(1):95-129.
Dorsal ventricular ridge (DVR) is a thalamorecipient, subcortical telencephalic structure in reptiles and birds. Although there is a fair amount of information about sources of afferents to DVR, little is known about the relationship of projections from individual thalamic nuclei to the organization of the structure. This study examines the relationship between thalamic projections and both areal and zonal divisions of anterior DVR (ADVR; Balaban, '78a) of emydid turtles with orthograde degeneration, autoradiographic and horseradish peroxidase techniques. Individual thalamic nuclei contribute either a diffuse or a restricted projection to ADVR. Diffuse projections arise primarily from the dorsomedial anterior nucleus. These fine-caliber axons distribute bilaterally over a wide region of the telencephalon via both medial and lateral thalamotelencephalic pathways. The terminal regions include septum, striatum and the medial bank of cortex caudal to the lamina terminalis. In ADVR, the fibers are distributed sparsely in zones 2-4 of dorsal, medial and ventral areas. Restricted projections to ADVR originate in nucleus rotundus, nucleus reuniens and nucleus caudalis. They ascend ipsilaterally in the lateral thalamotelencephalic pathway (lateral forebrain bundle), and enter ADVR rostral to the anterior commissure. Nucleus rotundus projects to zone 4 of dorsal area, nucleus caudalis projects to zones 2-4 of dorsal division of medial area, and nucleus reuniens projects to zones 2-4 of both the ventral division of medial area and the ventral area. Comparison of these results with thalamotelencephalic projections in mammals suggests that diffuse and restricted thalamic projection systems are a common feature of both groups. Restricted thalamic projections in reptiles, birds and mammals terminating in anatomically distinct regions, also appear to be associated with different sensory modalities. The significance of diffuse systems is not clear. [Abstract]

Novejarque A, Lanuza E, Martinez-Garcia F.
Amygdalostriatal projections in reptiles: A tract-tracing study in the lizard Podarcis hispanica.
J Comp Neurol. 2004 Nov 15;479(3):287-308.
Whereas the lacertilian anterior dorsal ventricular ridge contains unimodal sensory areas, its posterior part (PDVR) is an associative center that projects to the hypothalamus, thus being comparable to the amygdaloid formation. To further understand the organization of the reptilian cerebral hemispheres, we have used anterograde and retrograde tracing techniques to study the projections from the PDVR and adjoining areas (dorsolateral amygdala, DLA; deep lateral cortex, dLC; nucleus sphericus, NS) to the striatum in the lizard Podarcis hispanica. This information is complemented with a detailed description of the organization of the basal telencephalon of Podarcis. The caudal aspect of the dorsal ventricular ridge projects nontopographically mainly (but not exclusively) to the ventral striatum. The NS projects bilaterally (with strong ipsilateral dominance) to the nucleus accumbens, thus recalling the posteromedial cortical amygdala of mammals. The PDVR (especially its lateral aspect) and the dLC project massively to a continuum of structures connecting the striatoamygdaloid transition area (SAT) and the nucleus accumbens (rostrally), the projection arising from the dLC being probably bilateral. Finally, the DLA projects massively and bilaterally to both the ventral and dorsal striatum, including the SAT. Our findings lend further support to the view that the PDVR and neighboring structures constitute the reptilian basolateral amygdala and indicate that an emotional brain was already present in the ancestral amniote. These results are important to understand the comparative significance of the caudal aspect of the amniote cerebral hemispheres, and specifically challenge current views on the nature of the avian caudal neostriatum. [Abstract]

Jacobs LF.
The evolution of the cognitive map.
Brain Behav Evol. 2003;62(2):128-39.
The hippocampal formation of mammals and birds mediates spatial orientation behaviors consistent with a map-like representation, which allows the navigator to construct a new route across unfamiliar terrain. This cognitive map thus appears to underlie long-distance navigation. Its mediation by the hippocampal formation and its presence in birds and mammals suggests that at least one function of the ancestral medial pallium was spatial navigation. Recent studies of the goldfish and certain reptile species have shown that the medial pallium homologue in these species can also play an important role in spatial orientation. It is not yet clear, however, whether one type of cognitive map is found in these groups or indeed in all vertebrates. To answer this question, we need a more precise definition of the map. The recently proposed parallel map theory of hippocampal function provides a new perspective on this question, by unpacking the mammalian cognitive map into two dissociable mapping processes, mediated by different hippocampal subfields. If the cognitive map of non-mammals is constructed in a similar manner, the parallel map theory may facilitate the analysis of homologies, both in behavior and in the function of medial pallium subareas. [Abstract]

Lopez JC, Vargas JP, Gomez Y, Salas C.
Spatial and non-spatial learning in turtles: the role of medial cortex.
Behav Brain Res. 2003 Aug 14;143(2):109-20.
In mammals and birds, hippocampal processing is crucial for allocentric spatial learning. In these vertebrate groups, lesions to the hippocampal formation produce selective impairments in spatial tasks that require the encoding of relationships among environmental features, but not in tasks that require the approach to a single cue or simple non-spatial discriminations. In reptiles, a great deal of anatomical evidence indicates that the medial cortex (MC) could be homologous to the hippocampus of mammals and birds; however, few studies have examined the functional role of this structure in relation to learning and memory processes. The aim of this work was to study how the MC lesions affect spatial strategies. Results of Experiment 1 showed that the MC lesion impaired the performance in animals pre-operatively trained in a place task, and although these animals were able to learn the same task after surgery, probe test revealed that learning strategies used by MC lesioned turtles were different to that observed in sham animals. Experiment 2 showed that the MC lesion did not impair the retention of the pre-operatively learned task when a single intramaze visual cue identified the goal. These results suggest that the reptilian MC and hippocampus of mammals and birds function in quite similar ways, not only in relation to those spatial functions that are impaired, but also in relation to those learning processes that are not affected. [Abstract]

Reiner A.
Neurotransmitter organization and connections of turtle cortex: implications for the evolution of mammalian isocortex.
Comp Biochem Physiol Comp Physiol. 1993 Apr;104(4):735-48.
Telencephalic cortex in turtles is a simple three layered-structure. The dorsal most part of this structure is thought to resemble the reptilian forerunner of at least parts of mammalian isocortex. This dorsal part of turtle cortex contains several functionally distinct regions that show similarity in their connections and function to specific areas in mammalian isocortex. The types of neurons found in turtle dorsal cortex (as defined by their morphology and neurotransmitter content) also show great similarity to those observed in mammals, with the major exception that turtle cortex appears to lack the types of neurons found in granular and supragranular layers of mammalian isocortex. Similar results have also been observed in other living reptiles. Thus, one major step in the evolution of reptilian cortex into mammalian cortex must have been the addition of the types of neurons found in the granular and supragranular layers of mammalian isocortex. These observations for turtles also suggest that turtle cortex in particular and reptilian telencephalic cortex in general must differ functionally from mammalian isocortex with respect to those features associated with the laminar and columnar organization of isocortex. These issues are discussed in more detail below and in Reiner (1991). [Abstract]

Reiner A.
A comparison of neurotransmitter-specific and neuropeptide-specific neuronal cell types present in the dorsal cortex in turtles with those present in the isocortex in mammals: implications for the evolution of isocortex.
Brain Behav Evol. 1991;38(2-3):53-91.
Although it seems highly likely that mammalian isocortex evolved from a structure resembling reptilian telencephalic cortex, it has been uncertain if this occurred by the laminar differentiation of three-layered reptilian cortex into six-layered mammalian isocortex without the addition of new cell types or by laminar differentiation with the addition of new cell types. To distinguish between these two possibilities, immunohistochemical techniques were used to study turtles to see if the same major neuronal cell types, as defined by neurotransmitter or neuropeptide content, present in mammalian isocortex are also present in the specific part of reptilian cortex thought to be the forerunner of at least parts of isocortex, namely the dorsal cortex. Neurons containing the following substances are the major transmitter-specific types of neurons known to be present in mammalian isocortex: cholecystokinin-8 (CCK8), vasoactive intestinal polypeptide (VIP), acetylcholine, substance P (SP), neuropeptide Y (NPY), somatostatin (SS), LANT6, enkephalin, GABA and glutamate (GLUT). In turtles, only those of the above substances that are found in large numbers of neurons in layers V-VI in mammalian isocortex, irrespective of whether they are also present in layers II-IV (i.e. SP, NPY, SS, LANT6, GABA and GLUT), were present in neurons in dorsal cortex. The neurons containing these substances in dorsal cortex in turtles were generally highly similar in morphology to their counterparts in mammalian isocortex. In contrast, neurons labeled for CCK8, VIP or acetylcholine, which are mainly found in neurons of layers II-IV of mammalian isocortex, were absent or extremely rare in dorsal cortex. The absence or paucity of neurons labeled for these latter substances in dorsal cortex in turtles did not reflect an overall staining failure of the antisera used since the same antisera yielded excellent labeling of neurons, fibers and terminals in many other brain regions in turtles. Thus, dorsal cortex in turtles appears to lack several of the major cell types characteristic of layers II-IV of mammalian isocortex, but possesses a number of the major cell types characteristic of layers V-VI of isocortex. The findings support and extend a previous suggestion by Ebner [1976], based on hodological data, that dorsal cortex in turtles may lack the types of neurons found in the more superficial layers of mammalian isocortex. [Abstract]

Nieuwenhuys R.
The neocortex. An overview of its evolutionary development, structural organization and synaptology.
Anat Embryol (Berl). 1994 Oct;190(4):307-37.
By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this "new" structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neocortical population. The evolutionary development of the pyramidal neurons can be traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as "psychic cells". Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1-6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point. [Abstract]

Aboitiz F, Montiel J, Morales D, Concha M.
Evolutionary divergence of the reptilian and the mammalian brains: considerations on connectivity and development.
Brain Res Brain Res Rev. 2002 Sep;39(2-3):141-53.
The isocortex is a distinctive feature of the mammalian brain, with no clear counterpart in other amniotes. There have been long controversies regarding possible homologues of this structure in reptiles and birds. The brains of the latter are characterized by the presence of a structure termed dorsal ventricular ridge (DVR), which receives ascending auditory and visual projections, and has been postulated to be homologous to parts of the mammalian isocortex (i.e., the auditory and the extrastriate visual cortices). Dissenting views, now supported by molecular evidence, claim that the DVR originates from a region termed ventral pallium, while the isocortex may arise mostly from the dorsal pallium (in mammals, the ventral pallium relates to the claustroamygdaloid complex). Although it is possible that in mammals the embryonic ventral pallium contributes cells to the developing isocortex, there is no evidence yet supporting this alternative. The possibility is raised that the expansion of the cerebral cortex in the origin of mammals was product of a generalized dorsalizing influence in pallial development, at the expense of growth in ventral pallial regions. Importantly, the evidence suggests that organization of sensory projections is significantly different between mammals and sauropsids. In reptiles and birds, some sensory pathways project to the ventral pallium and others project to the dorsal pallium, while in mammals sensory projections end mainly in the dorsal pallium. We suggest a scenario for the origin of the mammalian isocortex which relies on the development of associative circuits between the olfactory, the dorsal and the hippocampal cortices in the earliest mammals. [Abstract]

Striedter GF.
The telencephalon of tetrapods in evolution.
Brain Behav Evol. 1997;49(4):179-213.
Numerous scientists have sought a homologue of mammalian isocortex in sauropsids (reptiles and birds) and a homologue of sauropsid dorsal ventricular ridge in mammals. Although some of the proposed theories were enormously influential, alternative theories continued to coexist, primarily because the striking differences in pallial organization between adult mammals, sauropsids, and amphibians enabled different authors to enlist different subsets of similarity data in support of different hypotheses of putative homology. A phylogenetic analysis based on parsimony cannot discriminate between such alternative hypotheses of putative homology, because sauropsids and mammals are sister groups. One solution to this dilemma is to include embryological patterns of telencephalic organization in the comparative analysis. Because early developmental stages in different taxa tend to resemble each other more than the adults do, the embryological data may reveal intermediate patterns of organization that provide unambiguous support for a single hypothesis of putative homology. The validity of this putative homology may then be supported by means of a phylogenetic analysis based on parsimony. A comparative analysis of pallial organization that includes embryological data suggests the following set of homologies. The lateral cortex in reptiles is homologous to the piriform cortex in birds and mammals. The anterior dorsal ventricular ridge in reptiles is probably homologous to the neostriatum and ventral hyperstriatum in birds and to the endopiriform nucleus in mammals. The posterior dorsal ventricular ridge in reptiles is most likely homologous to the archistriatum in birds and to the pallial amygdala in mammals. The pallial thickening in reptiles is probably homologous to the dorsal and intercalated portions of the hyperstriatum in birds and to the claustrum proper in mammals. Finally, the dorsal cortex in reptiles is probably homologous to the accessory hyperstriatum and parahippocampal area in birds and to the isocortex in mammals. These hypotheses of homology imply relatively minor evolutionary changes in development but major changes in neuronal connections. Most significantly, they imply the independent elaboration of thalamic sensory projections to derivatives of the lateral and dorsal pallia in sauropsids and mammals, respectively. They also imply the independent evolution of lamination in the pallium of birds and mammals. [Abstract]

Aboitiz F, Montiel J, Lopez J.
Critical steps in the early evolution of the isocortex: insights from developmental biology.
Braz J Med Biol Res. 2002 Dec;35(12):1455-72.
This article proposes a comprehensive view of the origin of the mammalian brain. We discuss i) from which region in the brain of a reptilian-like ancestor did the isocortex originate, and ii) the origin of the multilayered structure of the isocortex from a simple-layered structure like that observed in the cortex of present-day reptiles. Regarding question i there have been two alternative hypotheses, one suggesting that most or all the isocortex originated from the dorsal pallium, and the other suggesting that part of the isocortex originated from a ventral pallial component. The latter implies that a massive tangential migration of cells from the ventral pallium to the dorsal pallium takes place in isocortical development, something that has not been shown. Question ii refers to the origin of the six-layered isocortex from a primitive three-layered cortex. It is argued that the superficial isocortical layers can be considered to be an evolutionary acquisition of the mammalian brain, since no equivalent structures can be found in the reptilian brain. Furthermore, a characteristic of the isocortex is that it develops according to an inside-out neurogenetic gradient, in which late-produced cells migrate past layers of early-produced cells. It is proposed that the inside-out neurogenetic gradient was partly achieved by the activation of a signaling pathway associated with the Cdk5 kinase and its activator p35, while an extracellular protein called reelin (secreted in the marginal zone during development) may have prevented migrating cells from penetrating into the developing marginal zone (future layer I). [Full Text]

Aboitiz F, Morales D, Montiel J.
The evolutionary origin of the mammalian isocortex: towards an integrated developmental and functional approach.
Behav Brain Sci. 2003 Oct;26(5):535-52; discussion 552-85.
The isocortex is a distinctive feature of mammalian brains, which has no clear counterpart in the cerebral hemispheres of other amniotes. This paper speculates on the evolutionary processes giving rise to the isocortex. As a first step, we intend to identify what structure may be ancestral to the isocortex in the reptilian brain. Then, it is necessary to account for the transformations (developmental, connectional, and functional) of this ancestral structure, which resulted in the origin of the isocortex. One long-held perspective argues that part of the isocortex derives from the ventral pallium of reptiles, whereas another view proposes that the isocortex originated mostly from the dorsal pallium. We consider that, at this point, evidence tends to favor correspondence of the isocortex with the dorsal cortex of reptiles. In any case, the isocortex may have originated partly as a consequence of an overall "dorsalizing" effect (that is, an expansion of the territories expressing dorsal-specific genes) during pallial development. Furthermore, expansion of the dorsal pallium may have been driven by selective pressures favoring the development of associative networks between the dorsal cortex, the olfactory cortex, and the hippocampus, which participated in spatial or episodic memory in the early mammals. In this context, sensory projections that in reptiles end in the ventral pallium, are observed to terminate in the isocortex (dorsal pallium) of mammals, perhaps owing to their participation in these associative networks. [Abstract]

Aboitiz F.
Comparative development of the mammalian isocortex and the reptilian dorsal ventricular ridge. Evolutionary considerations.
Cereb Cortex. 1999 Dec;9(8):783-91.
There has been a long debate about a possible homology between parts of the dorsal ventricular ridge (DVR) of reptiles and birds, and parts of the mammalian isocortex. Correspondence between these structures was originally proposed on the basis of connectional similarities between the DVR of birds and the mammalian auditory and extrastriate visual isocortical areas. Furthermore, the proposal of homology includes the possible embryological similarity of cells that give rise to the DVR and cells that give rise to the isocortex. Against this concept it has been claimed that the DVR and the isocortex originate in topographically different pallial compartments, an interpretation that is supported by recent developmental and molecular data. Other studies indicate that migrating cells can cross the borders between adjacent developmental compartments: cells that originate in subcortical components contribute a number of interneurons to the developing isocortex via tangential migration. This mechanism might reconcile the proposed homology with the developmental evidence, since cells originating in one compartment (the one corresponding to DVR) may become included in structures generated in a different compartment (the one corresponding to isocortex). However, there is no evidence in mammals of a structure homologous to the embryonic DVR that can produce isocortical neurons. In order to fully clarify the problem of isocortical origins, further comparative studies are needed of the embryonic development of the lateral and dorsal aspects of the cerebral hemispheres in amphibians, reptiles and mammals. [Full Text]

Aboitiz F.
Evolutionary origins of the reptilian brain: the question of putative homologues of dorsal ventricular ridge. An overview and proposal.
Biol Res. 1995;28(3):187-96.
The reptilian brain is characterized by a structure that bulges into the lateral ventricle, called dorsal ventricular ridge (DVR). The DVR was originally considered to be a part of the basal ganglia, although more recent studies indicate that it may correspond to the dorsal part of the hemisphere. The anterior portion of the DVR has several connectional and functional similarities with parts of the mammalian neocortex, for which reason it has been claimed that the two structures can be considered as homologues. In this article I review the evidence supporting and refuting homology of the DVR with different telencephalic structures of mammals, concluding that it is still early to unequivocally ascribe structural correspondences between the different components in the two vertebrate classes. However, a way out of the problem is suggested by comparing the embryonic position of DVR with that of lateral cortex in the reptilian hemisphere. The lateral cortex is considered to be quite comparable in reptiles and mammals, and hence may be a good marker for the original position of the DVR. If the DVR originates dorsal to lateral cortex, it may be considered comparable to parts of the mammalian neocortex, while if it develops in its same position or ventral to it, it may not correspond to the neocortex. Early embryological work indicated that the DVR develops in the same position as the lateral cortex, but arises as a late migration wave, after cells destined to lateral cortex are generated. In other words, instead of being interposed between dorsal and lateral cortices, the DVR may originate in a position overlapping with lateral cortex. If this alternative turns out to be the case, it may imply that the DVR arose de novo, through an extension of the ancestral period of neuroblast proliferation. As a consequence, there may be no structures comparable to it in other vertebrate classes. Finally, it is also proposed that, regardless of whether the DVR and the extrastriate neocortex can or cannot be considered phylogenetic homologues, some of the integrative functions performed by them might have a common evolutionary origin, that became localized in the reptilian DVR and in the mammalian extrastriate neocortex. [Abstract]

Reiner AJ.
A hypothesis as to the organization of cerebral cortex in the common amniote ancestor of modern reptiles and mammals.
Novartis Found Symp. 2000;228:83-102; discussion 102-13.
Opinions on the evolutionary origins of mammalian neocortex have divided into two camps: (1) antecedents of the superior neocortex (i.e. occipital, parietal and frontal lobes) and temporal neocortex (i.e. temporal lobe) were present in stem amniotes, and these antecedent regions gave rise to dorsal cortex and dorsal ventricular ridge (DVR), respectively, in living reptiles; (2) the stem amniote antecedent of mammalian superior neocortex gave rise to dorsal cortex in the reptilian lineage, while the stem amniote antecedent of mammal claustrum, endopiriform region and/or basolateral/basomedial amygdala gave rise to DVR in reptiles, with mammalian temporal neocortex being a newly evolved structure with no reptilian homologue. The latter hypothesis has the merit of being more consistent with some current homeobox gene data, but it has the disadvantages of positing that mammalian temporal neocortex arose de novo, and of assuming that the high similarity between DVR and temporal neocortex in the organization of thalamic sensory input and corticostriatal projections and in the topology of sensory areas is coincidental. If one assumes that the antecedent of superior and temporal neocortex in stem amniotes was one continuous field that histologically resembled dorsal cortex in living reptiles, the first hypothesis provides basis for a parsimonious account of the origin of superior and temporal neocortex and their considerable resemblance to dorsal cortex and DVR in reptiles, as well as to Wulst and DVR in birds. [Abstract]

Treves A.
Computational constraints that may have favoured the lamination of sensory cortex.
J Comput Neurosci. 2003 May-Jun;14(3):271-82.
At the transition from early reptilian ancestors to primordial mammals, the areas of sensory cortex that process topographic modalities acquire the laminar structure of isocortex. A prominent step in lamination is granulation, whereby the formerly unique principal layer of pyramidal cells is split by the insertion of a new layer of excitatory, but intrinsic, granule cells, layer IV. I consider the hypothesis that granulation, and the differentiation between supra- and infra-granular pyramidal layers, may be advantageous to support fine topography in their sensory maps. Fine topography implies a generic distinction between "where" information, explicitly mapped on the cortical sheet, and "what" information, represented in a distributed fashion as a distinct firing pattern across neurons. These patterns can be stored on recurrent collaterals in the cortex, and such memory can help substantially in the analysis of current sensory input. The simulation of a simplified network model demonstrates that a non-laminated patch of cortex must compromise between transmitting "where" information or retrieving "what" information. The simulation of a modified model including differentiation of a granular layer shows a modest but significant quantitative advantage, expressed as a less severe trade-off between "what" and "where". The further connectivity differentiation between infra-granular and supra-granular pyramidal layers is shown to match the mix of "what" and "where" information optimal for their respective target structures. [Abstract]

Montagnini A, Treves A.
The evolution of mammalian cortex, from lamination to arealization.
Brain Res Bull. 2003 May 30;60(4):387-93.
We analyse some of the most important anatomical and functional features emerging at different stages of mammalian brain evolution in terms of a possible computational advantage. At the transition from reptiles to mammals, a major anatomical change occurs in the originally sensory dorsal cortex. The principal layer of pyramidal cells is split by the insertion of a new layer of granule cells, giving rise to the laminated isocortex. It has been hypothesized that this qualitative change in the evolution of mammalian brains is necessary to support fine topography in their sensory maps. The simulation of neural network models demonstrates that a nonlaminated patch of cortex must compromise between transmitting "where" information, explicitly mapped, topographically, on the cortical sheet, and retrieving "what" information, represented by the distributed firing pattern across neurons. The differentiation of a granular layer is shown in the model to yield a small quantitative advantage, allowing to transmit a slightly better combination of both information types. Along the same theoretical lines, we are investigating the multiplication of successive sensory areas coding for ever more composite stimuli, such as those in the visual and auditory temporal cortices in primates. In particular we analyse the possible computational advantage for a specific neural population devoted to encode the complex structure of whole stimuli, rather than relying on the coactivation of separate populations encoding their basic elements. [Abstract]

Prechtl JC.
Visual motion induces synchronous oscillations in turtle visual cortex.
Proc Natl Acad Sci U S A. 1994 Dec 20;91(26):12467-71.
In mammalian brains, multielectrode recordings during sensory stimulation have revealed oscillations in different cortical areas that are transiently synchronous. These synchronizations have been hypothesized to support integration of sensory information or represent the operation of attentional mechanisms, but their stimulus requirements and prevalence are still unclear. Here I report an analogous synchronization in a reptilian cortex induced by moving visual stimuli. The synchronization, as measured by the coherence function, applies to spindle-like 20-Hz oscillations recorded with multiple electrodes implanted in the dorsal cortex and the dorsal ventricular ridge of the pond turtle. Additionally, widespread increases in coherence are observed in the 1- to 2-Hz band, and widespread decreases in coherence are seen in the 10- and 30- to 45-Hz bands. The 20-Hz oscillations induced by the moving bar or more natural stimuli are nonstationary and can be sustained for seconds. Early reptile studies may have interpreted similar spindles as electroencephalogram correlates of arousal; however, the absence of these spindles during arousing stimuli in the dark suggests a more specific role in visual processing. Thus, visually induced synchronous oscillations are not unique to the mammalian cortex but also occur in the visual area of the primitive three-layered cortex of reptiles. [Abstract/Full Text]

Prechtl JC, Cohen LB, Pesaran B, Mitra PP, Kleinfeld D.
Visual stimuli induce waves of electrical activity in turtle cortex.
Proc Natl Acad Sci U S A. 1997 Jul 8;94(14):7621-6.
The computations involved in the processing of a visual scene invariably involve the interactions among neurons throughout all of visual cortex. One hypothesis is that the timing of neuronal activity, as well as the amplitude of activity, provides a means to encode features of objects. The experimental data from studies on cat [Gray, C. M., Konig, P., Engel, A. K. & Singer, W. (1989) Nature (London) 338, 334-337] support a view in which only synchronous (no phase lags) activity carries information about the visual scene. In contrast, theoretical studies suggest, on the one hand, the utility of multiple phases within a population of neurons as a means to encode independent visual features and, on the other hand, the likely existence of timing differences solely on the basis of network dynamics. Here we use widefield imaging in conjunction with voltage-sensitive dyes to record electrical activity from the virtually intact, unanesthetized turtle brain. Our data consist of single-trial measurements. We analyze our data in the frequency domain to isolate coherent events that lie in different frequency bands. Low frequency oscillations (<5 Hz) are seen in both ongoing activity and activity induced by visual stimuli. These oscillations propagate parallel to the afferent input. Higher frequency activity, with spectral peaks near 10 and 20 Hz, is seen solely in response to stimulation. This activity consists of plane waves and spiral-like waves, as well as more complex patterns. The plane waves have an average phase gradient of approximately pi/2 radians/mm and propagate orthogonally to the low frequency waves. Our results show that large-scale differences in neuronal timing are present and persistent during visual processing. [Full Text]

Prechtl JC, Bullock TH, Kleinfeld D.
Direct evidence for local oscillatory current sources and intracortical phase gradients in turtle visual cortex.
Proc Natl Acad Sci U S A. 2000 Jan 18;97(2):877-82.
Visual stimuli induce oscillations in the membrane potential of neurons in cortices of several species. In turtle, these oscillations take the form of linear and circular traveling waves. Such waves may be a consequence of a pacemaker that emits periodic pulses of excitation that propagate across a network of excitable neuronal tissue or may result from continuous and possibly reconfigurable phase shifts along a network with multiple weakly coupled neuronal oscillators. As a means to resolve the origin of wave propagation in turtle visual cortex, we performed simultaneous measurements of the local field potential at a series of depths throughout this cortex. Measurements along a single radial penetration revealed the presence of broadband current sources, with a center frequency near 20 Hz (gamma band), that were activated by visual stimulation. The spectral coherence between sources at two well-separated loci along a rostral-caudal axis revealed the presence of systematic timing differences between localized cortical oscillators. These multiple oscillating current sources and their timing differences in a tangential plane are interpreted as the neuronal activity that underlies the wave motion revealed in previous imaging studies. The present data provide direct evidence for the inference from imaging of bidirectional wave motion that the stimulus-induced electrical waves in turtle visual cortex correspond to phase shifts in a network of coupled neuronal oscillators. [Full Text]

Ayala-Guerrero F, Calderon A, Perez MC.
Sleep patterns in a chelonian reptile (Gopherus flavomarginatus).
Physiol Behav. 1988;44(3):333-7.
Individuals of Gopherus flavomarginatus, previously adapted to experimental conditions were chronically implanted for polygraphic recordings. Four different states of vigilance were observed:. Active wakefulness, quiet wakefulness, quiet sleep and active sleep. EEG was polymorphic and irregular showing a decreasing tendency in frequency and amplitude when passing from wakefulness to quiet sleep. Heart rate decreased with sleep but it was slightly higher during active sleep than quiet sleep. Motor automatisms were present during active sleep being sometimes accompanied by ocular movements. This sleep always appeared after long periods of quiet sleep. Its average duration from animal to animal varied between 9.15 and 13.62 sec. Reaction threshold increased during sleep. The conclusion is that Gopherus flavomarginatus shows two phases of sleep similar to slow and paradoxical sleep in mammals. [Abstract]

De Vera L, Gonzalez J, Rial RV.
Reptilian waking EEG: slow waves, spindles and evoked potentials.
Electroencephalogr Clin Neurophysiol. 1994 Apr;90(4):298-303.
Signal spectral analysis procedures were used to compute the power spectrum of Gallotia galloti lizards EEG at different (5-35 degrees C) body temperatures. EEG power spectra were mainly characterized by a low frequency peak between 0.5 and 4 Hz which was present at the different body temperatures. A second spectral peak, corresponding to spindles of similar pattern to the sleep spindles of mammals, also appears in the spectra. The peak frequency of the spindles increased with the body temperature. Flash evoked potentials were characterized by a slow triphasic component upon which a spindle was superimposed, adopting a morphology similar to the K complexes of mammalian sleep. The characteristics of this EEG and evoked potentials support the hypothesis of homology between the waking state of the reptiles and the slow wave sleep of mammals. [Abstract]

Lorenzo D, Velluti JC.
Noradrenaline decreases spike voltage threshold and induces electrographic sharp waves in turtle medial cortex in vitro.
Brain Behav Evol. 2004;64(2):104-14. Epub 2004 Jun 15.
The noradrenergic modulation of neuronal properties has been described at different levels of the mammalian brain. Although the anatomical characteristics of the noradrenergic system are well known in reptiles, functional data are scarce. In our study the noradrenergic modulation of cortical electrogenesis in the turtle medial cortex was studied in vitro using a combination of field and intracellular recordings. Turtle EEG consists of a low voltage background interspersed by spontaneous large sharp waves (LSWs). Noradrenaline (NA, 5-40 microM) induced (or enhanced) the generation of LSWs in a dose-dependent manner. Pharmacological experiments suggest the participation of alpha and beta receptors in this effect. In medial cortex neurons NA induced a hyperpolarization of the resting potential and a decrease of input resistance. Both effects were observed also after TTX treatment. Noradrenaline increased the response of the cells to depolarizing pulses, resulting in an upward shift of the frequency/current relation. In most cells the excitability change was mediated by a decrease of the spike voltage threshold resulting in the reduction of the amount of depolarization needed to fire the cell (voltage threshold minus resting potential). As opposed to the mechanisms reported in mammalian neurons, no changes in the frequency adaptation or the post-train afterhyperpolarization were observed. The NA effects at the cellular level were not reproduced by noradrenergic agonists. Age- and species-dependent properties in the pharmacology of adrenergic receptors could be involved in this result. Cellular effects of NA in turtle cortex are similar to those described in mammals, although the increase in cellular excitability seems to be mediated by a different mechanism. [Abstract]