Morphological characteristics of the neuronal population in the feline auditory cortex projecting into the medial geniculate body

The location and morphological profile of auditory cortex neurons projecting to the medial geniculate body were investigated in adult cats using horseradish peroxidase retrograde axonal transport techniques. Sources of descending projections to the medial geniculate body from auditory cortex areas I and II were found to be neurons belonging to deep-lying layers (layer VI and layer V to a lesser extent). By far the majority of corticogeniculate neurons in the auditory cortex were pyramidal cells. In layer VI of the primary auditory area (A1), the number of corticogeniculate neurons reaches 60% of all cells belonging to that layer. The average area (M±m) of the profile of perikarya of corticogeniculate neurons in layer VI, area Al equaled 139.3±2.5 µm² and 219.5±7.0 µm² in layer V neurons; average size of long diameter: 15.0±0.19 and 18.3±0.4 µm respectively. The lower regions of layers III and IV in area Al were found to be the termination point of the greater mass of anterogradely-labeled geniculocortical fibers (terminals of relay neuron axons belonging to the medial geniculate body).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 4, July–August, pp. 513–521, 1989.     

Ultrastructural characteristics of callosal neurons in deep layers of cat primary auditory cortex (AI)

We have carried out an electron microscopic investigation of retrogradely HRP-labeled nonpyramidal neurons in layers V and VI of the primary auditory cortex (AI), which are sources of transcallosal projections. We have established that on average 15.8±1.7% of the perikaryon surface of these cells is occupied by axo-somatic synapses. We detected in ultrathin sections from two to nine synapses on the profiles of the perikaryon of callosal neurons. All of these axo-somatic synapses are formed by axon terminals containing small flat synaptic vesicles and are characterized by symmetrical contacts. The length of the cross section of the contacts was on average 1.6±0.1 µm. The axon terminals of callosal fibers, antegradely labeled by the enzyme, form in the deep layers of the cortex asymmetrical synapses on the spines and stems of the dendrites. A possible functional significance of the axo-somatic synapses in the production of the impulse activity of callosal neurons in the deep layers of the AI region, is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 549–556, May, 1991.     

Neuronal composition and interneuronal connection of area 5 in the cat parietal association cortex

Area 5 of the cat cortex was studied by Nissl's method and by Golgi's chromate-silver impregnation method. Its typical six-layered structure with well-developed layers of pyramidal cells was revealed. The characteristic features of area 5 are: predominance of pyramidal cells in layers II–III and the presence of large forms (40×26 µ) among them (in layer III); giant pyramidal neurons (70×23 µ) arranged singly or nidally in layer V; large (diameter 25–30 µ) and giant (diameter 40–45 µ) stellate cells with radial dendrites, arranged singly or in groups in layers V–VI; infrequent efferent fusiform neurons (40×20 µ) in layers V–VI. Stellate cells connecting pyramidal neurons in the same or in different layers were found in layers II–VI. Some stellate cells in layers II–III form long horizontal connections within area 5. Interneuronal connections are effected by axosomatic and axodendritic terminals, the latter being more numerous; Dendrodendritic and axoaxonal synapses are less common.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 1, pp. 35–42, January–February, 1979.     

Ontogeny of corticocortical projections of the rat somatosensory cortex.

Rhodamine-coated microspheres (RCMs) were injected into the primary somatosensory cortex (SI) of rats ranging in age from postnatal (PN) day 1 to adulthood. Ipsilateral corticocortical and callosal projections within the SI were identified as early as PN day 1. At the end of the first PN week, ipsilaterally projecting neurons located in sublayer VIb were the first to assume an adult-like pattern of connectivity. Injections at subsequent postnatal ages revealed that an adult pattern of lamination of ipsilateral corticocortical projections within the SI is established between PN weeks 2 and 3, comprising projection neurons from layers II/III, layer V, and sublayer VIb. Therefore, local interactions in the rat SI are mediated not only by pyramidal neurons of layers III and V, derived from the cortical plate, but also by a subpopulation of ontogenetically older neurons located in the sublayer VIb, which may correspond to the subplate neurons of other species. Overall, these results suggest the existence of three independent short-range corticocortical systems of projections within the rat SI, which differ in terms of the laminar distribution and ontogenetic origin of their cells.     

Ultrastructural characteristics of layer III pyramidal neurons in the cat primary auditory cortex (Al)

Electron microscope studies were made of retrogradely horseradish peroxidase-labeled pyramidal neurons forming transcallosal projections in layer III of the cat primary auditory cortex (Al). These showed a significant proportion of the somatic membrane to be covered with processes of astroglia, while synapses occupy 20% of the synaptic surface on average. Between 4 and 10 axosomatic synapses were identified on the profiles of callosal cell somata. All these were formed by axonal terminals containing small, flattened synaptic vesicles and had symmetrical contacts. Average length of these synaptic contacts equaled 1.6 µm. Numerous anterogradely horseradish peroxidase-labeled axonal terminals of callosal fibers were found in cortical area Al in amongst retrogradely HP-labeled neurons. The ultrastructural pattern of these is described.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 520–526, July–August, 1990.     

Morphological properties of tectal neurons that project to the nucleus geniculatus lateralis,pars ventralis (GLv) and the surrounding ventral thalamus in chicks

Layer 10 neurons of the chick tectum were morphologically investigated. The layer 10 neurons displayed heterogeneous immunoreactivities to calcium-binding proteins (CaBPs). Calbindin (CB)-immunoreactive (ir) neurons had pyramidal or round somata, primarily found in layers 5, 9, and 13. Parvalbumin (PV)-ir neurons were of various shapes with small to large somata (109.7 ± 48.6 μm²) that were located mainly in layers 4 and 10. Calretinin (CR)-ir neurons had small to middle-sized somata (79.3 ± 9.7 μm²) located primarily in layers 10 and 13, and most of them were similar to typical radial cells in size and shape. Two distinct types of neurons that projected to the nucleus geniculatus lateralis, pars ventralis (GLv) and ventral thalamus were demonstrated in layer 10. Type 1 cells had small to middle-sized somata (74.3 ± 33 μm²), and each cell had a single apical dendrite that ramified into bush-like branches in layer 7. These cells corresponded to CR-ir neurons and radial cells in size and shape. Type 2 cells had larger somata (124.7 ± 52.6 μm²), and their shapes were pyramidal, polygonal, or oval. They had multiple obliquely ascending dendrites that ramified into bush-like branches in layer 7. These cells often appeared similar to PV-ir neurons.     

Cortico-cortical connections between the auditory fields and the sensomotor area of the cortex]

In 18 cats by means of two methods--anterograde degeneration and retrograde transport of exogenic horseradish peroxidase--cortico-cortical connections of the auditory fields to the cortical sensomotor area have been studied. These connections have been stated to terminate in layers V-III of certain parts of the sensomotor area corresponding to the projections of the foreleg and the head. Initial neurons of the connections studied are pyramidal cells in layers III and II. They are situated in rostral and caudal parts of the fields AI and AII, but within these levels they occur in different areas of the auditory fields.     

Modular organization of callosal neurons in the rabbit sensomotor cortex

The density of distribution of callosal neurons in the rabbit sensomotor cortex was studied by injecting horseradish peroxidase into the symmetrical region of the cortex. The degree of inequality of distribution of labeled neurons was determined visually and by statistical analysis. Stained callosal neurons were mainly small and medium-sized pyramidal cells, located chiefly in layer III–IV, and substantially less frequently in layers V and VI. Different forms of grouping of labeled neurons were observed in layer III–IV: two cells at a time, five to eight cells arranged vertically, or in concentrations, whose width was usually 120–200µ, and separated by areas with reduced density. The results are regarded as confirmation of those drawn previously from results of electrophysiological investigations on the modular organization of callosal connections in the rabbit sensomotor cortex.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Brain Institute, Academy of Medical Sciences of the USSR, Moscow. I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 451–457, July–August, 1984.     

Cholinergic structures in the cat auditory cortex (area AI)

Plexuses of cholinergic varicose fibers, differing in density in different layers of the neuropil, were found in area AI of the cat's auditory cortex by the histochemical reaction for acetylcholinesterase: Their density was maximal or average in layer I or deeper layers and minimal in layers II and III. Among cells in area AI those which are cholinergic are a few stellate neurons located in layers II–VI. Axons of some neurons terminate on neighboring cells, those of others (some neurons in layer VI) run into the subcortical layer of arcuate association fibers. Cholinergic terminals are located on the bodies and proximal areas of dendrites of neurons most of which do not contain acetylcholinesterase. Choliniceptive neurons of different sizes and shapes are found in all layers of this region of the auditory cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 75–81, January–February, 1984.     

Structural bases of sensomotor integration in the cat's cerebral cortex

Connections of the somatosensory cortex surrounding the postcruciate fossa with the lateral region of the motor area (in the cruciate sulcus) were established by the Nauta-Gygax and Fink-Heimer methods and also by the retrograde horse-radish peroxidase transport method. A high degree of differentiation was found in the organization of transcortical sensomotor projections. The pyramidal, stellate, and inverted pyramidal neurons in the third layer of the cortex were shown to take part in the formation of these pathways. Results obtained by the experimental degeneration method combined with electron microscopy showed that afferentation from the first somatosensory area of the cortex reaches mainly cells in layers III and V. It is suggested that the influence of the association fibers on projection neurons in the motor area is transmitted either directly or through interneurons.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 460–466, September–October, 1981.     

Electron microscope study of sources in the cat primary auditory cortex (AI) projecting to the medial geniculate body

An electron microscope study of retrogradely labeled pyramidal neurons in layer VI of the primary auditory cortex (AI) after injecting horseradish peroxidase (HP) into the medial geniculate body was carried out in cats. Not less than 57.8±1.9% on average of the perimeter of perikaryon profiles of corticogeniculate neurons labeled with HP were found to be covered with astroglia processes. Between three and eight synapses occupying an average of 10.8±1.0% of the perimeter length were found on the perikaryon profiles of these neurons. Nearly all synapses (a total of 98.7%) at the soma of corticogeniculate neurons had symmetrical active zones, being made up of axonal terminals with flattened synaptic vesicles. Anterogradely HP-labeled axonal terminals of geniculocortical fibers were also found in the neuropil of layer VI in area AI, in addition to retrogradely labeled neurons. They contained large round synaptic vesicles and formed asymmetrical synapses. The potential role of axosomatic synapses in the shaping of corticogeniculate neuronal activity is discussed.A. A. Bogomolets Institute, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 171–178, March–April, 1990.     

Properties of monosynaptic connections between callosal neurons and specific thalamic nuclei

A comparative analysis of monosynaptic afferent and efferent connections of callosal neurons and target neurons of transcallosal fibers with neurons of the specific ipsilateral thalamic nuclei (ventral posterolateral, ventral posteromedial, ventral lateral, and anteroventral) was undertaken on the sensomotor cortex of unanesthetized rabbits, using an electrophysiological method. Differences were demonstrated between callosal neurons and target neurons of transcallosal fibers with respect to monosynaptic inputs from the thalamic nuclei and pathways proceeding toward these structures and (or) entering the pyramidal tract. Among target neurons, compared with callosal neurons, more cells had descending projections (54 and 14%, respectively). Monosynaptic action potentials arose in 22% of target neurons in response to stimulation of specific thalamic nuclei, whereas no such responses occurred in callosal neurons. Projections of target neurons into thalamic nuclei were shown to be formed both by independent fibers and by axon collaterals of the pyramidal tract. It is postulated that the distinctive properties thus discovered indicate significantly greater convergence of influence of thalamic relay neurons on the target neurons; this determines differences known to exist in characteristics of receptive fields and spontaneous and evoked activity of callosal neurons, on the one hand, and of neurons excited synaptically by transcallosal stimulation, on the other hand.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 3, pp. 305–314, May–June, 1985.     

Neurons with choline acetyltransferase immunoreactivity and mRNA are present in the human cerebral cortex

 We examined the cerebral cortex of five autopsied individuals without neurological and psychiatric diseases by immunohistochemistry using an anti-human recombinant choline acetyltransferase (ChAT) polyclonal antibody and in situ hybridization with ³⁵S-labeled human ChAT riboprobes. The immunohistochemistry detected positive neurons which were medium-sized or large pyramidal neurons located predominantly in layers III and V. The density of such neurons was higher in the motor and secondary sensory areas than in other cortical areas; the immunoreactive neurons in layer V were more densely distributed in the motor area and those in layer III were distributed in the secondary sensory areas. Positively stained, non-pyramidal neurons were observed in the superficial layer of the cingulate gyrus and parahippocampus. No immunoreactive neurons were found in the primary sensory areas. The in situ hybridization detected some neurons with signals for ChAT mRNA in the cerebral cortex, most of which were distributed in layer V of the motor area and in layer III of the secondary visual area. These results indicate that the human cerebral cortex contains cholinergic neurons and displays regional and laminal variations in their distribution. Accepted: 17 November 1998     

Comparative study of inter- and intrahemispheric cortico-cortical connections in gerbil auditory cortex

Topographic distributions and laminar pattern of cortico-cortical projections from the primary auditory field (AI), anterior auditory field (AAF), dorsoposterior field (DP), ventroposterior field (VP), dorsal field (D) and ventral field (V) were studied in relation to tonotopic maps in combined anatomical, electrophysiological and 2-deoxyfluoro-D-glucose (2DG) experiments. Distributions of axons were examined by means of retrogradely-transported fluorescent tracer Fast Blue (FB) injected in the primary (AI) and anterior (AAF) auditory field. Injections of fluorescent tracer were placed in electrophysiologically-identified locations of AI and AAF. Neurons in AAF, DP, VP and V project to AI in the ipsilateral hemisphere. This area also receives projections from AI, AAF and D from the contralateral hemisphere. In AI, DP and VP, neurons are connected with AAF in the ipsilateral hemisphere and AI and AAF in the opposite hemisphere. In all cases, patches of labeling are distributed along 2DG bands oriented parallel to the isofrequency line. Substantial numbers of retrogradedly labeled neurons with similar best frequencies (BFs) were observed in the ipsilateral and moderate to scant numbers in the contralateral hemisphere. In general, regions near the injection sites receive more densely-labeled projections than do more distant targets. In both hemispheres, the supragranular layer III contains the greatest concentration of cortico-cortical cells bodies; the granular and infragranular layer V contains a somewhat lower concentration.     

The structure of the first auditory cortex (A I) in the cat. I.--Light microscopic observations on its organization

The cyto- and mieloarchitecture of the first auditory cortex (A I) was studied in the cat. The cortical layers II, III and IV are very densely populated by relatively uniform, round or stellate cells with 20 to 30 micro perikaryal diameter. The separation between these three layers, which is not possible in Nissl stained sections, becomes visible in 1 to 3 micro thick sections of plastic embedded material. nerve cells in layer II are randomly disposed, whilst they form in laver III loose rounded cellular groups, and in layer IV vertical cylinders which have 50 to 60 micro in outside diameter and a cell poor centre. These cylinders are best visible in 100 micro thick Nissl preparations, cut parallel to the pial surface. The cylinders may extend into layer V, which is comparatively cell poor. The VIth layer contains numerous round, stellate or fusiform cells with 20 to 30 micro in diameter. The IIIrd and Vth layers have few pyramidal perikarya which are small. Large or giant pyramidal cells are not found in A I. The overall thickness of the cortex in the convexity of A I is 2,000 micro, measured in sections of plastic blocks. The thickness of the 6 layers is 200 to 250 micro for layer I; 300 micro for layer II; 300 micro for layer III; 300 to 400, for layer IV; 350 micro for layer V; and 400 micro for layer VI. In preparations stained for myelin sheats A I is characterized by the presence of a very dense plexus of fibres running in all directions in the IVth, Vth anti VIth layers. These plexus obscurs the radiations of Meynert, giving a characteristic appearance to A I, since these radiations are prominent in the neighbouring cortical areas. In preliminary studies of Golgi rapid preparations of A I the cell types commonly present in others cortical areas were found. Pyramidal cells have small perikarya, and very long (600 micro) horizontal basal dendrites. Modified pyramidal cells (star pyramids) are the main cellular element in layer II and constitute one of the main sources of efferent fibres of A I. Several types of stellate cells were found, including a particular cell type, found very often in the IVth layer, with a very long horizontal axon. The specific thalamic afferents were identified as fibres with 5 or 8 micro in diameter, which run obliquely and sinuously through the VIth and Vth layers of A I. These fibres give off many branches with 1 to 2 micro in diameter, which pass to the IVth layer where they give off very thin sinuous branches, ending in small terminal knobs. The ramification of one of these fibres may spread horizontally over 800 micros, at the level of the IVth layer.     

Intrinsic cholinergic components in cholinergic innervation of the cat auditory cortex (area AI)

Histochemical study of neuronally isolated area AI of the auditory cortex in cats by the reaction for acetylcholinesterase 3 days and 1, 2, and 3 weeks after undercutting showed that the cholinergic neuropil of this area is formed mainly by incoming fibers and to a lesser degree by processes from a few intrinsic cholinergic neurons. The intrinsic cholinergic neurons include, first, cholinergic long-axon association neurons responding to cortical isolation by retrograde changes and by hyperreaction to acetylcholinesterase (Cajal-Retzius cells of layer I and neurons of layer VI, whose axons run into the subcortical layer of association fibers), and, second, cholinergic short-axon association neurons of layers II–VI, preserving their normal cell structure and moderate acetylcholinesterase activity after isolation. Axon collaterals of similar cells terminate on neighboring neurons. Short-axon neurons are more numerous in the lower layers of the cortex, and exceed in number the long-axon association neurons. Choliniceptive neurons (pyramidal and stellate), on whose bodies and proximal dendrites are located terminals formed by axons of cholinergic association neurons, are found in the isolated cortex. Choliniceptive neurons are found more often in the lower layers of the cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 81–87, January–February, 1984.     

Sonic hedgehog expression in corticofugal projection neurons directs cortical microcircuit formation

The precise connectivity of inputs and outputs is critical for cerebral cortex function; however, the cellular mechanisms that establish these connections are poorly understood. Here, we show that the secreted molecule Sonic Hedgehog (Shh) is involved in synapse formation of a specific cortical circuit. Shh is expressed in layer V corticofugal projection neurons and the Shh receptor, Brother of CDO (Boc), is expressed in local and callosal projection neurons of layer II/III that synapse onto the subcortical projection neurons. Layer V neurons of mice lacking functional Shh exhibit decreased synapses. Conversely, the loss of functional Boc leads to a reduction in the strength of synaptic connections onto layer Vb, but not layer II/III, pyramidal neurons. These results demonstrate that Shh is expressed in postsynaptic target cells while Boc is expressed in a complementary population of presynaptic input neurons, and they function to guide the formation of cortical microcircuitry. VIDEO ABSTRACT:     

Expression of nerve growth factor and its receptors in the somatosensory-motor cortex of Macaca nemestrina

The present study was designed to examine the nerve growth factor (NGF) system (ligand and receptor-expressing neurons) in the somatosensory (areas 1, 3a, and 3b) and motor (area 4) cortices of the mature macaque. Light and electron microscope immunohistochemistry was used to assess the distribution and identity of NGF-, p75-, and trk-expressing elements. In each cortical area examined, NGF-positive neuronal somata were distributed through all laminae; most immunolabeled neurons were in layers II, III, and V. Based upon light microscope criteria (e.g., the morphology of proximal dendrites), both pyramidal and stellate neurons expressed NGF. Of the identifiable NGF- immunoreactive cells, 92% were pyramidal neurons and the remainder was stellate neurons. The electron microscope study showed that most (88%) NGF-positive somata formed symmetric synapses, whereas the others formed both symmetric and asymmetric synapses. As the somata of pyramidal neurons form only symmetric synapses and those of inhibitory stellate neurons form both symmetric and asymmetric somatic synapses, the ultrastructural data support the light microscopic analyses. In contrast, neurotrophin receptors, p75 and trk, were expressed chiefly by the cell bodies of layer V pyramidal neurons and the supragranular neuropil. At the ultrastructural level, receptor-positive profiles were post-synaptic elements (e.g., dendritic shafts and spines) and the concentration of immunoreactivity was greatest in the vicinity of post-synaptic densities. Thus, NGF regulatory systems parallel excitatory and inhibitory neurotransmitter systems. Cortex contains the morphological framework by which pyramidal and/or inhibitory stellate neurons can affect the activity of post-synaptic pyramidal neurons via anterograde and autocrine/paracrine NGF systems.     

Hierarchy within human SI: supporting data from cytochrome oxidase, acetylcholinesterase and NADPH-diaphorase staining patterns

The human primary somatosensory cortex consists of four cytoarchitectonic subdivisions (3a, 3b, 1 and 2) that are likely to contain distinct somatosensory representations. The intraareal organization of these areas as well as that of the primary motor cortex (area 4) has been analyzed using histochemical stains of cytochrome oxidase, acetylcholinesterase and NADPH-diaphorase activity in normal human brains. Cytochrome oxidase activity was revealed in individual cortical neurons and neuropil. Areas 4, 3a and 3b were on average darker than areas 1 and 2. The laminar distribution of cytochrome oxidase activity varied in different areas. A prominent dark band was present in layers IV and lower III in areas 3a and 3b and in layer III in areas 1, 2 and 4. Acetylcholinesterase staining revealed fibers and pyramidal cells in layers III and V; stained layer III pyramids were rare in areas 3a and 3b and numerous in areas 1, 2 and 4. NADPH-diaphorase positive elements included Golgi-like stained non-pyramidal neurons and Nissl-like stained pyramidal neurons; the former were found, in small numbers, in layer II of areas 4, 3a, 3b and 1, and the latter in layers III and V of areas 4 and 3a and in layer V of areas 1 and 2. The dark cytochrome oxidase staining of layer IV and the paucity of acetylcholinesterase positive pyramids in areas 3a and 3b resemble the pattern found in primary visual and auditory areas, whereas the dark cytochrome oxidase staining in layer III and abundance of acetylcholinesterase positive pyramids in areas 1 and 2 that of association areas. These results suggest that the four areas included in human SI constitute hierarchical stages of cortical processing, with 3a and 3b corresponding to primary and 1 and 2 to secondary areas.     

Hierarchy within human SI: supporting data from cytochrome oxidase,acetylcholinesterase and NADPH-diaphorase staining patterns

The human primary somatosensory cortex consists of four cytoarchitectonic subdivisions (3a, 3b, 1 and 2) that are likely to contain distinct somatosensory representations. The intraareal organization of these areas as well as that of the primary motor cortex (area 4) has been analyzed using histochemical stains of cytochrome oxidase, acetylcholinesterase and NADPH-diaphorase activity in normal human brains. Cytochrome oxidase activity was revealed in individual cortical neurons and neuropil. Areas 4, 3a and 3b were on average darker than areas 1 and 2. The laminar distribution of cytochrome oxidase activity varied in different areas. A prominent dark band was present in layers IV and lower III in areas 3a and 3b and in layer III in areas 1, 2 and 4. Acetylcholinesterase staining revealed fibers and pyramidal cells in layers III and V; stained layer III pyramids were rare in areas 3a and 3b and numerous in areas 1, 2 and 4. NADPH-diaphorase positive elements included Golgi-like stained non-pyramidal neurons and Nissl-like stained pyramidal neurons; the former were found, in small numbers, in layer II of areas 4, 3a, 3b and 1, and the latter in layers III and V of areas 4 and 3a and in layer V of areas 1 and 2. The dark cytochrome oxidase staining of layer IV and the paucity of acetylcholinesterase positive pyramids in areas 3a and 3b resemble the pattern found in primary visual and auditory areas, whereas the dark cytochrome oxidase staining in layer III and abundance of acetylcholinesterase positive pyramids in areas 1 and 2 that of association areas. These results suggest that the four areas included in human SI constitute hierarchical stages of cortical processing, with 3a and 3b corresponding to primary and 1 and 2 to secondary areas.