Synaptic action of specific visual inpulses upon cat's parastriate cortex

A model for self-organization of receptive fields and orientation-selective columns in the striate cortex

In area 17 of the cat visual cortex, simple cells form a hypercolumn in which the optimum orientation from one column to the next gradually changes, composing a complete set of orientation-selective columns (orientation column). This article proposes a model for the development of the bar-shape receptive field of a simple cell and the self-organization of orientation columns. The receptive field of an immature cell in area 17 is assumed to be composed of a circular center and surrounding regions whose synaptic modification rules are different. The synaptic modifications also differ depending on whether the response of a cell is locally maximal or not. The modification of the efficacy of both excitatory and inhibitory synapses is determined according to the combination of activities of the visual cortical cell and the lateral geniculate neuron. The simulation of this model shows the development of the bar-shape receptive field and the self-organization of orientation columns of more than one cycle from 0 degrees to 180 degrees. The abnormal presentations of visual stimuli to this model result in the abnormal development of the orientation column. These simulation results are in good agreement with reported experimental results. Possible neural circuits to achieve this model are proposed. The neural circuits for the synaptic modification are built on the assumption that cortical cells release molecules to modify synaptic efficacies. The neural circuits for the detection of the maximally responding cell are composed of two kinds of inhibitory interneurons. The bar-shape receptive field is assumed to be a consequence of the topographic projection of visual afferents, radial branching of dendrites of a simple cell, and the existence of an inhibitory interneuron.

Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections

Fifteen dark-reared, 4- to 5-week-old kittens were stimulated monocularly with patterned light while they were anesthetized and paralyzed. Six of these kittens were exposed to the light stimuli only, in four kittens the light stimuli were paired with electric stimulation of the mesencephalic reticular formation and in five kittens with electric activation of the medial thalamic nuclei. Throughout the conditioning period, the ocular dominance of neurons in the visual cortex was determined from evoked potentials that were elicited either with electric stimulation of the optic nerves or with phase reversing gratings of variable spatial frequencies. In two kittens, ocular dominance changes were assessed after the end of the conditioning period by analyzing single unit receptive fields. Monocular stimulation with patterned light induced a marked shift of ocular dominance toward the stimulated eye, when the light stimulus was paired with electric activation of either the mesencephalic reticular formation or of the medial thalamus. Moreover, a substantial fraction of cells acquired mature receptive fields. No such changes occurred with light or electric stimulation alone. It is concluded that central core projections which modulate cortical excitability gate experience-dependent modifications of connections in the kitten visual cortex.

On the function of cell systems in area 18. Part II

In addition to the asymmetry of the spatial coupling and of the specific temporal combination of excitation and inhibition, the non-linearity is very pronounced in area 18. Taking the sequence of a linear operation and a stationary nonlinear characteristic as a model, the experimental findings can be systematized and cell classification specified which departs from the customary ones. The hypercomplex cell system probably originates in recurrent inhibition and leads to differentiation of the patterns along their contour line. Problems of cell classification and of the type of parallelism in the visual cortex are discussed.

The afferent connexions and laminar distribution of cells in area 18 of the cat

1. The receptive field properties, laminar distribution and afferent connectivity of cells in area 18 of the cat are described. 2. Testing with both moving and stationary stimuli revealed three main receptive field types which have been termed S, C and B, respectively (cf. Henry, 1977; Henry, Lund & Harvey, 1978). All three classes may show end-zone inhibition and units exhibiting this property have been designated SH, CH and BH. 3. S cells can be divided into spatially separate lights and/or dark edge response regions when tested with moving edges and usually have separate ON and/or OFF areas when tested with stationary flashing stimuli. They are the most commonly encountered cell type in area 18 and occur most frequently in laminae IIIb, IVa and VI. 4. Both C and B cells have spatially coincident light and dark edge response regions and give mixed ON and OFF discharges when tested with stationary flashing stimuli. Compared to B cells however, C cells have large receptive fields, they are broadly tuned for stimulus orientation and generally have a relatively high rate of spontaneous activity. C cells are more common than B cells and are encountered most often in laminae IVb and V. 5. Electrical stimulation of the optic chiasm (OX) and optic radiation (OR) was used to examine the afferent connectivity of parastriate neurons. Cells driven from both OX and OR have been divided into two main groups and it is argued that group 1 cells are directly, and group 2 cells are indirectly, excited by rapidly conducting afferent fibres. Group 1 cells are found most often in laminae IIIb, IVa, IVb and VI, and their distribution closely follows the anatomically defined laminar disposition of geniculocortical afferent terminals. Group 2 neurones predominate in laminae II-IIIa, IIIA and V. 6. The majority of S and SH cells are directly driven, whereas most C and CH cells have OX and OR latencies suggestive of indirect activation by thalamic afferents. 7. The intrinsic organization and possible functional role of area 18 is discussed in the light of these results.

Hierarchical and parallel mechanisms in the organization of visual cortex

We argue that it seems fruitful to regard the retino-geniculate-cortical pathway, and perhaps the visual pathways in general, as comprising distinct neuronal channels which begin with the major groupings of ganglion cells, and subserve distinct functions within the overall operation of the visual system. One problem for future work is to determine the extent and, equally importantly, the limitations of the idea of independently functioning neuronal channels operating within the visual system. Some evidence of those limitations is already available. Kulikowski and Tolhurst have provided evidence suggesting that pattern detection is mediated by the X-like system at high spatial frequencies and by the Y-like system at low frequencies, but that at intermediate frequencies, both systems are likely to contribute to this function. Again, there is already physiological and psychophysical evidence of inhibitory interaction between X- and Y-cell systems, which may contribute to their functioning. That is, although there is little evidence of excitatory interaction between W-, X- and Y-cell systems, at least up to the first cortical synapse, the functioning of, say, the X-cell system may depend on the inhibitory influences impinging on it from Y-cell activity. Further, it may prove to be the case that one cell 'system' may be involved in several distinct functions and considerable work may be required to establish whether or not these functions can be considered constituent parts of an overall function, such as 'ambient' or 'foveal' vision. In the following section we suggest a classification and terminology for visual neurones which may provide a framework for future work on these lines.

Unit responses of the first and second somatosensory areas to stimulation of the ventroposterior thalamic nucleus

Single unit responses of the first (SI) and second (SII) somatosensory areas to stimulation of the ventroposterior thalamic nucleus (VP) were investigated in cats immobilized with D-tubocurarine. In response to VP stimulation 12.0% of reacting SI neurons and 9.5% of SII neurons generated an antidromic spike. In most antidromic responses of both SI and SII neurons the latent period did not exceed 1.0 msec. The minimal latent period of spike potentials during orthodromic excitation was 1.5 msec in SI and 1.7 msec in SII. Neurons with an orthodromic spike latency of not more than 3.0 msec were more numerous in SI than those with a latency of 3.1--4.5 msec. The ratio between the numbers of neurons of these two groups in SII was the opposite. In SII there were many more neurons with a latency of 5.6--8.0 msec than in SI. EPSPs appeared after a latent period of 1.1--9.0 msec in SI and of 1.4--6.6 msec in SII. The latent period of IPSPs was 1.5--6.8 msec in SI and 2.2--9.4 msec in SII. The relative importance of different pathways for excitatory and inhibitory influences of VP on SI and SII neurons is discussed.

The effect of reticular stimulation on spontaneous and evoked activity in the cat visual cortex

The mesencephalic reticular formation (MRF) of cats anesthetized with N2O was stimulated electrically, and the effects of this stimulation on activity in the striate cortex were studied. The variations of intra- and extracellularly recorded unit activity and the changes in the extracellular potassium concentration were investigated. At all levels of analysis the prevailing effect of MRF stimulation was facilitation. Half of the cells reacted with brief bursts of activity to reticular stimuli. A decrease of resting activity was rare. The cells activated by MRF stimulation had in common: (1) to show a high degree of excitatory convergence from extrinsic and intrinsic afferents, (2) to possess often corticofugal axons, and (3) to have preferentially complex receptive fields. In the large majority of cortical cells MRF stimulation facilitated responses evoked by stimulation of the optic radiation or by light stimuli. This facilitation could lead to a loss of orientation and direction selectivity. Reticular activation further led to a large increase of the extracellular potassium concentration, whereas stimulation of specific afferents led to a decrease. It is concluded that these phenomena are not merely a consequence of altered thalamic transmission, but are caused by a projection system which is organized in parallel to the specific projection and exerts a direct control over cortical excitability. The mechanism for this control appears to be a slight and rather unselective depolarization of most neurons. If disinhibitory processes are involved at all, their role is much less prominent than at the thalamic level. The functional implications of such an unselective but powerful modulation of cortical excitability are discussed in respect to corollary reticular activation as it occurs with rapid eye movements.

A study of inhibitory antagonism in cat visual cortex

Since there seems to be good evidence that GABA may act as an inhibitory neurotransmitter in the mammalian cortex, we tested the effects of an antagonist of GABA, namely the alkaloid bicuculine, on the response properties of visual cortex neurons, using a computer-controlled stimulus presentation system to assess quantitatively the changes in receptive field organization after the drug. Complex cells were most affected, increasing both evoked and spontaneous activity and losing some of their specificities for stimulus parameters such as orientation and direction. Hyper-complex cells lost their inhibitory flanks, responding equally well to long and short bars after the drug. Simple cells were the least affected, usually becoming somewhat depressed after the drug. Preliminary tests with another inhibitory amino acid antagonist, strychnine, showed that it excited simple cells, indicating that possibly more than one inhibitory transmitter is at work in the cortex. The results are discussed with relation to the synaptic anatomy of the cortex, and it is concluded that a class of stellate cells, using GABA, is a likely candidate for the transmitter of some intracortical inhibition.

Identification of cone mechanisms in graded responses of foveal striate cortex

1. The earliest electrical response detectable in foveal striate cortex of anaesthetized Rhesus monkeys following light stimulation is a graded potential which is positive at the cortical surface and negative in the grey matter and has a peak latency of about 60 msec. The response is similar at both the on- and the off-phase of a light stimulus.2. The relationship of this graded potential to the depth of the recording electrode, to the latency of extracellular impulses and to post-synaptic potentials suggests that it is generated by the depolarization of cortical cells.3. Action spectra obtained in the presence of strong selective chromatic adaptation indicate the participation of all three cone mechanisms in this response.4. Each cone mechanism contributes a similar potential to the response but antagonism between cone mechanisms is apparent. The proportion in which a cone mechanism contributes to the response varies from one area to another implying topographical differences in the representation of cone mechanisms in striate cortex.

Receptive fields of simple cells in the cat striate cortex

1. The excitatory and inhibitory components in the receptive fields of unimodal simple cells in the striate cortex of the cat anaesthetized with nitrous oxide have been described using slits of light and single light-dark edges as stimuli.2. There is a small excitatory region (excitatory complex) centrally located in the receptive field that is made up of various combinations and spatial arrangements of subliminal excitatory and discharge subregions or centres.3. The subliminal excitatory centres were revealed by a binocular facilitation technique. The excitability of the cell was raised by repeated stimulation via one eye while the neurone was tested with single edges via the other eye.4. The subliminal excitatory and discharge centres are each specifically activated by only one type of edge, light-dark or dark-light, and then only in one direction of motion. All the subregions in the excitatory complex have the same optimal stimulus orientation.5. Inhibitory components in the receptive field were identified by stimulating the cell with bars of light and single edges against an artificial background discharge produced by repeated stimulation separately applied either to the same eye (monocular conditioning) or to the other eye (binocular conditioning). There are powerful inhibitory sidebands to either side of the excitatory complex and these inhibitory regions merge to include the excitatory complex when stimulus orientation is angled away from the optimal.6. Excitation is highly stimulus specific whereas inhibition is non-specific.7. The organization of the two receptive fields of a binocularly discharged cell can be closely similar.8. The attempt is made to translate the concept of subliminal excitatory and discharge centres into specific neural mechanisms involving both the geniculo-cortical input and various intracortical circuits.9. These new developments call for only minor modifications to the model we have proposed for the organization of the receptive field.

Interaction effects of visual contours on the discharge frequency of simple striate neurones

1. The discharge frequency of simple neurones in the cat striate cortex responding to the two edges of a slit of light moving over their receptive fields was studied as a function of slit width. While one edge of the slit was discharging the cell, the other edge had a modifying influence on that discharge either by way of facilitation or of inhibition.2. The most common form of the curve relating discharge frequency and slit width had a maximal discharge at narrow slit widths (< 0.5 degrees ) and relative inhibition at medium widths (between 0.5 degrees and 2 degrees ). At greater slit widths there was usually a region of facilitation before the effects of the two edges became independent of one another. Three other response patterns to slits of different width are described.3. The curve relating slit width and response amplitude for a particular cell provides an important clue to the various activity profiles for that cell. An activity profile plots the excitability of a cell along a line through the receptive field in the direction of stimulus movement. Each type of edge, light and dark, has its own set of activity profiles which differ depending upon stimulus parameters such as the direction of the movement of the edge.4. Two other methods were used to provide further data concerning the activity profiles and as a check on the evidence provided by the responses to slits of different width. One of these two methods used the test stimulus against the background of an artificially produced maintained discharge and the other involved the interaction of the two receptive fields of binocularly activated cells.5. A model is put forward to explain the receptive field organization of simple striate neurones which takes into account not only the main features of what is known concerning the synaptology of the visual cortex but also the new data provided by the present paper and the one which precedes it.