A type of basket cell in superficial layers of the cat visual cortex. A Golgi-electron microscope study

Neocortical layers I and II of the hedgehog (Erinaceus europaeus). I. Intrinsic organization

The intrinsic organization and interlaminar connections in neocortical layers I and II have been studied in adult hedgehogs (Erinaceus europaeus) using the Golgi method. Layer I contains a dense plexus of horizontal fibers, the terminal dendritic bouquets of pyramidal cells of layer II and of underlying layers, and varieties of intrinsic neurons. Four main types of cells were found in layer I. Small horizontal cells represent most probably persisting foetal horizontal cells described for other mammals. Large horizontal cells, tufted cells, and spinous horizontal cells were also found in this layer. Layer II contains primitive pyramidal cells representing the most outstanding feature of the neocortex of the hedgehog. Most pyramidal cells in layer II have two, three or more apical dendrites, richly covered by spines predominating over the basal dendrites. These cells resemble pyramidal cells found in the piriform cortex, hippocampus and other olfactory areas. It is suggested that the presence of these neurons reflects the retention of a primitive character in neocortical evolution. Cells with intrinsic axons were found among pyramidal cells in layer II. These have smooth dendrites penetrating layer I and local axons forming extremely complex terminal arborizations around the bodies and proximal dendritic portions of pyramidal cells. They most probably effect numerous axo-somatic contacts resembling basket cells. The similarity of some axonal terminals with the chandelier type of axonal arborization is discussed. Other varieties of cells located in deep cortical layers and having ascending axons for layers I and II were also studied. It is concluded that the two first neocortical layers represent a level of important integration in this primitive mammal.

Synaptic connections of intracellularly filled clutch cells: a type of small basket cell in the visual cortex of the cat

Light and electron microscopic quantitative analysis was carried out on a type of neuron intracellularly filled with horseradish peroxidase. Two cells were studied in area 17, one of which was injected intra-axonally, and its soma was not recovered. One cell was studied in area 18. The two somata were on the border of layers IVa/b; they were radially elongated and received synapses from numerous large boutons with round synaptic vesicles. The dendrites were smooth and remained largely in layer IV. The cells can be recognised on the basis of their axonal arbor, which was restricted to layer IV (90-95% of boutons) with minor projections to layers III, V, and VI. Many of the large, bulbous boutons contacted neuronal somata, short collaterals often forming "claw"-like configurations around cells. The name "clutch cell" is suggested to delineate this type of neuron from other aspiny multipolar cells. Computer-assisted reconstruction of the axon showed that in layer IV the axons occupied a rectangular area about 300 X 500 microns, elongated anteroposteriorly in area 17 and mediolaterally in area 18. The distributions of synaptic boutons and postsynaptic cells were patchy within this area. A total of 321 boutons were serially sectioned in area 17. The boutons formed type II synaptic contacts. The postsynaptic targets were somata (20-30%), dendritic shafts (35-50%), spines (30%), and rarely axon initial segments. Most of the postsynaptic somata tested were not immunoreactive for GABA and their fine structural features suggest that they are spiny stellate, star pyramidal, and pyramidal neurons. The characteristics of most of the postsynaptic dendrites and spines also suggest that they belong to these spiny neurons. A few of the postsynaptic dendrites and somata exhibited characteristics of cells with smooth dendrites and these somata were immunoreactive for GABA. It is suggested that clutch cells are inhibitory interneurons exerting their effect mainly on layer IV spiny neurons in an area localised perhaps to a single ocular dominance column. The specific laminar location of the axons of clutch cell also suggests that they may be associated with the afferent terminals of lateral geniculate nucleus cells, and could thus be responsible for generating some of the selective properties of neurons of the first stage of cortical processing.

Structure of layer II in cat primary auditory cortex (AI)

The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150-200 micron to about 400 micron beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendritic arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included small and medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; bipolar and bitufted cells with polarized, sparse dendritic arbors; small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; large sparsely spinous multipolar cells with smooth, robust apical dendrites; tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and extraverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of choline acetyltransferase in rat cerebral cortex: a study of cholinergic neurons and synapses

Choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme and a definitive marker for cholinergic neurons, was localized immunocytochemically in the motor and somatic sensory regions of rat cerebral cortex with monoclonal antibodies. ChAT-positive (ChAT+) varicose fibers and terminal-like structures were distributed in a loose network throughout the cortex. Some immunoreactive cortical fibers were continuous with those in the white matter underlying the cortex, and many of these fibers presumably originated from subcortical cholinergic neurons. ChAT+ fibers appeared to be rather evenly distributed throughout all layers of the motor cortex, but a subtle laminar pattern was evident in the somatic sensory cortex, where lower concentrations of fibers in layer IV contrasted with higher concentrations in layer V. Electron microscopy demonstrated that immunoreaction product was concentrated in synaptic vesicle-filled profiles and that many of these structures formed synaptic contacts. ChAT+ synapses were present in all cortical layers, and the majority were of the symmetric type, although a few asymmetric ones were also observed. The most common postsynaptic elements were small to medium-sized dendritic shafts of unidentified origin. In addition, ChAT+ terminals formed synaptic contacts with apical and, probably, basilar dendrites of pyramidal neurons, as well as with the somata of ChAT-negative nonpyramidal neurons. ChAT+ cell bodies were present throughout cortical layers II-VI, but were most concentrated in layers II-III. The somata were small in size, and the majority of ChAT+ neurons were bipolar in form, displaying vertically oriented dendrites that often extended across several cortical layers. Electron microscopy confirmed the presence of immunoreaction product within the cytoplasm of small neurons and revealed that they received both symmetric and asymmetric synapses on their somata and proximal dendrites. These observations support an identification of ChAT+ cells as nonpyramidal intrinsic neurons and thus indicate that there is an intrinsic source of cholinergic innervation of the rat cerebral cortex, as well as the previously described extrinsic sources.

The morphology and synaptic connections of spiny stellate neurons in monkey visual cortex (area 17): a Golgi-electron microscopic study

Based on a gold-toning, Golgi-electron microscope examination of 12 small and medium-sized spiny stellate neurons in laminae 4A, 4B, and 4C of the monkey visual cortex (area 17), the ultrastructure of the cell somata, dendrites, and axons of these neurons is described. Particular attention is paid to the synapses involving the surface of different parts of these neurons. Only symmetric synapses occur on the somata of spiny stellate neurons, and these occur with a frequency of 11.0-15.9 synapses/100 microns2 perikaryal surface. Symmetric synapses also occur on dendritic shafts and, occasionally, on dendritic spines. Asymmetric synapses are occasionally present along the dendritic shafts of spiny stellate neurons, but the majority of asymmetric synapses (75-95%) occur on their dendritic spines. The initial axon segments of the smallest spiny stellate neurons possess no axo-axonal synapses, but several symmetric synapses are present along the initial segment of a medium-sized, spiny stellate neuron in layer 4B. Fifty-three synapses made by boutons of the axons of these spiny stellate neurons have been identified, and all are asymmetric. Sixty per cent of the synapses are formed by boutons en passant and the remainder by the terminal swellings of spine-like axonal appendages, boutons terminaux. Of the synapses formed by the axons of spiny stellate cells, axo-spinous synapses outnumber axo-dendritic synapses two to one, and axo-dendritic synapses involve both spinous and aspinous dendrites. Evidence is presented which suggests that many of the axon terminals forming asymmetric synapses with the dendritic shafts and spines of spiny stellate neurons are derived from other spiny stellate neurons.

Variability in the terminations of GABAergic chandelier cell axons on initial segments of pyramidal cell axons in the monkey sensory-motor cortex

Chandelier cell axons were studied in the sensory-motor cortex of adult monkeys. The axonal fields of Golgi-impregnated chandelier cells in layer II in motor cortex are flattened sagittally. The vertical terminal portions of the axons varied both in length and in the numbers converging to form terminations of greater or lesser complexity. Golgi-impregnated plexuses were embedded in plastic and resectioned serially at 2.5-3.0 micrograms. A single axonal field could have as many as 400 terminal rows. All lie 3-13 micrograms beneath pyramidal cell somata. These terminations are not randomly distributed but instead, form clusters. Further resectioning the plastic sections for electron microscopy revealed that all the terminations are on the initial axon segments of pyramidal cells and all form symmetric synaptic contacts. In immunocytochemical material stained for glutamic acid decarboxylase (GAD), the enzyme involved in the synthesis of GABA, GAD-positive boutons were found to form symmetric synaptic contacts with a variety of postsynaptic elements including the axon hillocks and axon initial segments of pyramidal cells. Serial reconstructions from electron micrographs revealed GAD-positive terminals synapsing with the axon initial segment of pyramidal cells joined by cytoplasmic bridges and forming vertically oriented rows identical to those of chandelier cell terminals identified positively in the resectioned Golgi material. The GAD-positive terminals forming initial segment synapses were never continuous with GAD-positive terminals forming axo hillock synapses. The latter probably arise from basket cell axons. Initial segments of pyramidal cell axons in layers II and III were contacted by more GAD-positive terminals than the initial segments of pyramidal cell axons in layer V. The largest pyramidal cells in layer III received the most synapses. Many larger pyramidal cells, identified as callosally projecting cells by the retrograde transport of horseradish peroxidase (HRP), were shown in serial electron micrographs to possess large numbers of initial segment synapses, comparable to those seen in the immunocytochemical material. Serial reconstructions of pyramidal cell axons from axon hillock to the first myelin internode in resectioned Golgi, immunocytochemical and HRP material showed that the number of synapses varied from 2 to 52 for layers II and III and from 2 to 26 for layer V. The number of synapses on the axon hillocks varied from zero to 12. The variability in these terminations may be an important factor in the shaping of the functional properties of the pyramidal cells.

Development of synapses on pyramidal and multipolar non-pyramidal neurons in the visual cortex of rabbits. A combined Golgi-electron microscope study

A combined Golgi-electron microscope method was used to study the ultrastructural maturation of synapses on identified pyramidal and multipolar non-pyramidal neurons in the visual cortex of young and adult rabbits. In samples of 10 (time of eye opening), 14, 20 day old and 7 month old animals, fully impregnated pyramidal neurons within the layers II-V and multipolar non-pyramidal neurons mainly located in lower layer III and layer IV was studied. We found that synapses in 10 and 14 day old animals were occasionally immature in appearance. They were characterized by either a poorly defined postsynaptic band or equal rims of pre- and postsynaptic electron-dense material and could therefore not be classified as Gray type I or II. The distinction between both types of synapses was easier at day 20 and in the adults when the postsynaptic band of the asymmetrical (type I) synapses had become remarkably thicker. In pyramidal neurons the cytoplasmic organelles increased in number during development. Although a few symmetrical synapses were present on dendritic spines of pyramidal neurons in 14 and 20 day old animals, all pyramidal neurons exhibited the same types of synapses on specific sites of their neuronal surface. They received exclusively type II synapses on their somata, type I synapses on their dendritic spines and both types of synapses on their dendritic shafts. However, in the adult animals the frequency of occurrence of type II synapses, especially on basal dendritic shafts, had increased. In some cases only type II and no type I synapses were present. A striking finding in all young and adult animals was that synapses at the borderline between somata and apical dendritic shafts as well as on dendritic spines were frequently complex or interrupted. The characteristic ultrastructural features of adult spine-free and sparsely spiny multipolar non-pyramidal neurons e.g. the many cytoplasmic organelles and type I and II synapses on somata and on dendrites were already present at day 10. After day 10 the number of organelles and synapses increased prominently and in adult animals the different types of synapses on dendrites were located at relatively short intervals of about 4 microns. In contrast with the dendritic shafts of pyramidal neurons many asymmetrical synapses were observed on dendritic shafts of the non-pyramidal neurons analysed in the adult animals. Furthermore, it appeared that the number of synapses on these non-pyramidal neurons is about twice that on pyramidal neurons in day 20 old animals and about four times in adult animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinotopy and orientation columns in the monkey: a new model

A model is presented in which orientation columns arise directly out of retinotopy. According to the model, iso-orientation lines are arrayed radially around nodal centers which correspond to cytochrome oxidase patches. The nodal centers form a square matrix superimposed upon the map of ocular dominance stripes. In the supragranular layers horizontal iso-orientation lines run down the centers of ocular dominance stripes, with vertical iso-orientation lines crossing perpendicularly. Diagonal orientations (45 degrees and 135 degrees) are represented as alternating iso-orientation zones at the centers of the interstices in the matrix (internodal centers). Preferred orientations in the infragranular layers are reversed with respect to the supragranular layers. The model is consistent with new data concerning ocularity and preferred orientation in systematic penetrations through striate cortex, and helps to explain some previously puzzling features of the relationship between ocular dominance columns, orientation columns and retinotopy.

Glutamate decarboxylase-immunoreactive terminals of Golgi-impregnated axoaxonic cells and of presumed basket cells in synaptic contact with pyramidal neurons of the cat's visual cortex

Glutamate decarboxylase (GAD)-immunoreactive varicosities were found around cell bodies of nonimmunoreactive and immunoreactive neurons in the cat's visual cortex; they also occurred along apical dendrites and axon initial segments of pyramidal neurons. By examination in the electron microscope of structures first identified in the light microscope, it was established that the GAD-immunoreactive varicosities were boutons in symmetrical synaptic contact with pyramidal cells in layers II-IV. More than 90% of 142 boutons surrounding the cell bodies of 20 pyramidal neurons were immunoreactive for GAD. Since such a high proportion of the axosomatic boutons are GAD-immunoreactive, it is likely that the terminals of basket cells are included in this population and so the basket cell probably uses gamma-aminobutyrate as a transmitter, as suggested by previous authors. Almost all the 68 boutons in symmetrical contact with the axon initial segments of six pyramidal neurons could be shown to be GAD-immunoreactive, which makes it very likely that the boutons of axoaxonic cells contain GAD-immunoreactivity. This was established unequivocally for an individual Golgi-impregnated axoaxonic cell by combining Golgi impregnation and immunocytochemistry in the same sections: A Golgi-impregnated axoaxonic cell whose cell body was in layer II gave rise to numerous terminal segments, some of which were examined in the electron microscope after gold-toning. These boutons were in synaptic contact with axon initial segments and not only contained the Golgi precipitate but were also immunoreactive for GAD. It is concluded that the axoaxonic cell in the visual cortex uses gamma-aminobutyrate as a transmitter. An individual axoaxonic cell in layer II/III was filled with horseradish peroxidase by intracellular iontophoresis. The very extensive local axonal field was composed of 330 terminal bouton rows in layer II/III and a sparse descending collateral projection to infragranular layers. A computer-assisted reconstruction of the axonal field in three dimensions revealed the following: The main output of the cell is to pyramidal neurons that lie deeper than the soma; the axonal arborization occupies an area of 400 micron in the anteroposterior axis and extends 200 micron along the mediolateral axis; the terminal bouton rows in layer II/III form clusters about 50 micron wide running approximately at right angles to the border between areas 17 and 18, with an intercluster interval of about 100 micron. These findings suggest that the terminals of an individual axoaxonic cell could be contained within one ocular dominance column but that there may be inhomogeneities in the weighting of the axoaxonic input to pyramidal cells in the supragranular layers.

Axonal patterns and topography of short-axon neurons in visual areas 17, 18, and 19 of the cat

In a Golgi study of short-axon cells in areas 17, 18, and 19 of the adult cat, 21 main axonal arborization patterns are distinguished and related to laminar position and dendritic morphology. Multipolar neurons exclusive to layer 2/3 exhibit the greatest diversity of axons: These may be arranged in a local tuft, a single descending stem with recurrent collaterals, a columnar descending plexus, horsetaillike bundles, a dense intralaminar plexus, or a varicose local arbor. Multipolar cells of layers 2--5 may have local basket or chandelier axons or a neurogliform axonal plexus, and axons in layer 4 may branch into a columnar basket plexus. Two types of multipolar cells are described in layers 5 and 6. Also, neurons with a bitufted dendritic tree have different axonal patterns: cone-shaped axons, arcade axons, profuse ascending plexuses, and columnar or diffuse ascending and descending arbors. The axons of bitufted cells in layer 6 ascend or form a local tuft. Finally, giant bitufted cells with local axons, bitufted or multipolar cells with horizontal axons, and bipolar cells are described. The neuronal types established on the basis of their axonal arbor are compared to previous classifications of cortical neurons. The location of neurons thus classified has been recorded and evaluated. Most cells show no topographical specificity and occur indistinctly in the three visual areas, while two types are exclusive to area 17. Horsetail neurons accumulate at the 17/18 and 18/19 borders. Multipolar cells of layer 2/3 with a dense intralaminar plexus and chandelier cells are concentrated in the region where the central visual field is represented.

Synaptic connections of morphologically identified and physiologically characterized large basket cells in the striate cortex of cat

Neurons were studied in the striate cortex of the cat following intracellular recording and iontophoresis of horseradish peroxidase. The three selected neurons were identified as large basket cells on the basis that (i) the horizontal extent of their axonal arborization was three times or more than the extent of the dendritic arborization; (ii) some of their varicose terminal segments surrounded the perikarya of other neurons. The large elongated perikarya of the first two basket cells were located around the border of layers III and IV. The radially-elongated dendritic field, composed of beaded dendrites without spines, had a long axis of 300-350 microns, extending into layers III and IV, and a short axis of 200 microns. Only the axon, however, was recovered from the third basket cell. The lateral spread of the axons of the first two basket cells was 900 microns or more in layer III and, for the third cell, was over 1500 microns in the antero-posterior dimension, a value indicating that the latter neuron probably fulfills the first criterion above. The axon collaterals of all three cells often branched at approximately 90 degrees to the parent axon. The first two cells also had axon collaterals which descended to layers IV and V and had less extensive lateral spreads. The axons of all three cells formed clusters of boutons which could extend up a radial column of their target cells. Electron microscopic examination of the second basket cell showed a large lobulated nucleus and a high density of mitochondria in both the perikarya and dendrites. The soma and dendrites were densely covered by synaptic terminals. The axons of the second and third cells were myelinated up to the terminal segments. A total of 177 postsynaptic elements was analysed, involving 66 boutons of the second cell and 89 boutons of the third cell. The terminals contained pleomorphic vesicles and established symmetrical synapses with their postsynaptic targets. The basket cell axons formed synapses principally on pyramidal cell perikarya (approximately 33% of synapses), spines (20% of synapses) and the apical and basal dendrites of pyramidal cells (24% of synapses). Also contacted were the perikarya and dendrites of non-pyramidal cells, an axon, and an axon initial segment. A single pyramidal cell may receive input on its soma, apical and basal dendrites and spines from the same large basket cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Handling Golgi-impregnated tissue for light microscopy

The use of cyanocrylic glue to fix pieces of Golgi-stained nervous tissue on a paraffin blank is proposed for obtaining thick sections of unembedded tissue with a sliding microtome. This procedure makes correct orientation of the tissue easy during sectioning and makes it possible to obtain tissue sections quickly. The sections are flat-mounted using epoxy resin, resulting in permanent preparations with excellent optical properties and enabling further thin-sectioning for light and electron microscopic studies.

Five types of basket cell in the hippocampal dentate gyrus: a combined Golgi and electron microscopic study

Five types of basket cell in the hippocampal denate gyrus of rats were analysed with a combined Golgi and electron microscopic method. Light microscopic observations show that the large somata of these different cell types are located either in the granule cell layer or within 30-50 micron of this layer. The somata of basket cells are pyramidal, horizontal, fusiform or multipolar. Dendrites of basket cells are aspinous or sparsely spinous and are found in all layers of the dentate gyrus. Their axons form an extensive plexus in the granule cell and lower molecular layers. Electron microscopic preparations of Golgi-impregnated, gold-toned basket cells revealed gold-labelled neurons with distinct ultrastructural features. All somata of basket cells displayed an extensive perikaryal cytoplasm with large Nissl bodies and nuclei with infoldings, euchromatin, intranuclear rods and sheets, and large nucleoli. The aspinous dendrites as well as the somata had a mixture of asymmetric and symmetric synapses on their surfaces. Basket cell dendrites located in the hilus were contacted by numerous terminals with characteristics of mossy fibres derived from granule cells. Some of these terminals were identified positively in preparations that also contained impregnated granule cells. The axons of basket cells formed exclusively symmetric synapses. The most common postsynaptic structures to these terminals were the somata and dendrites of granule cells. Dendritic spines were rarely contacted by basket cell axons while the axon hillocks and initial segments of granule cells were never contacted. These findings are consistent with previous immunocytochemical, and physiological data that indicate feedback inhibitory mechanisms in the dentate gyrus are mediated via mossy fibre collaterals which synapse with GABAergic basket cells. In addition, the electron microscopic data for basket cells are similar to those for aspinous stellate cells in the neocortex, another type of cortical, GABAergic local circuit neuron. Thus, the basket cells in the dentate gyrus may have a function similar to other inhibitory, cortical local circuit neurons.

The section-Golgi impregnation procedure. 2. Immunocytochemical demonstration of glutamate decarboxylase in Golgi-impregnated neurons and in their afferent synaptic boutons in the visual cortex of the cat

Sections of the cat's visual cortex were stained by an antiserum to glutamate decarboxylase using the peroxidase-antiperoxidase method; they were then impregnated by the section Golgi procedure and finally the Golgi deposit was replaced by gold. Neurons containing glutamate decarboxylase immunoreactivity were found in all layers of the visual cortex, without any obvious pattern of distribution. Fifteen immunoreactive neurons were also Golgi-impregnated and gold-toned, which enabled us to study the morphology and synaptic input of identified GABAergic neurons. These neurons were found to be heterogeneous both with respect to the sizes and shapes of their perikarya and the branching patterns of their dendrites. All the immunoreactive, Golgi-impregnated neurons had smooth dendrites, with only occasional protrusions. The synaptic input of glutamate decarboxylase-immunoreactive neurons was studied in the electron microscope. Immunoreactive neurons received immunoreactive boutons forming symmetrical synapses on their cell bodies. The Golgi-impregnation made it possible to study the input along the dendrites of immunoreactive neurons. One of the large neurons in layer III whose soma was immunoreactive was also Golgi-impregnated: it received numerous non-immunoreactive asymmetrical synaptic contacts along its dendrites and occasional ones on its soma. The same neuron also received a few boutons forming symmetrical synaptic contacts along its Golgi-impregnated dendrites; most of these boutons were immunoreactive for glutamate decarboxylase. Glutamate decarboxylase-immunoreactive boutons were also found in symmetrical synaptic contact with non-immunoreactive neurons that were Golgi-impregnated. A small pyramidal cell in layer III was shown to receive several such boutons along its somatic membrane. It is concluded that the combination of immunoperoxidase staining and Golgi impregnation is technically feasible and that it can provide new information. The present study has shown that there are many morphologically distinct kinds of aspiny GABAergic neurons in the visual cortex; that the predominant type of synaptic input to the dendrites of such neurons is from boutons forming asymmetrical synapses, but that some of the GABAergic neurons also receive a dense symmetrical synaptic input on their cell bodies, and occasional synapses along their dendrites, from the boutons of other GABAergic neurons. These findings provide a morphological basis, firstly, for a presumed powerful excitatory input to GABAergic interneurons and, secondly, for the disinhibition which has been postulated from electrophysiological studies to occur in the cat's visual cortex.