Functional aspects of plasticity in the visual system of adult cats after early monocular deprivation.
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Action Potentials↗
Biomedical subjects
M Cynader
Publications and source records attributed to M Cynader.
Abolition of direction selectivity in the visual cortex of the cat.
Cats were reared in a stroboscopically illuminated environment, which deprived them of expereience with visual movement but allowed them form vision. In these cats, neurons of the visual cortex displayed normal orientation selectivity, but direction selectivity was virtually abolished. The effect remained unaltered by long periods of normal visual exposure.
Animals↗
Early versus late visual cortex lesions: effects on receptive fields in cat superior colliculus.
Cats that sustain lesions of the visual cortex early in life appear to perform certain visual discrimination tasks better than those operated as adults. This study sought to determine whether this recovery is visual capacities was accompanied by reorganization of single cell responses at the level of the superior colliculus. Areas 17 and 18 were ablated in adult cats and in kittens at various times during the neonatal period. Responses of units in superior colliculus ipsilateral to the lesion were recorded following a prolonged recovery period. Following cortical lesions, collicular units rarely exhibited direction selectivity, binocularity was reduced in the majority of animals, and the ocular dominance distribution was biased toward the contralateral eye. The reduction of direction selectivity and binocularity were unrelated to the animal's age at operation.
Age Factors↗
Recovery of function in cat visual cortex following prolonged deprivation.
Evidence that there is a critical period during which response characteristics of neurons in visual cortex of the cat may be influenced has been provided in several studied, which suggest that the period of influence is restricted to the first few months of life. Using a somewhat different experimental procedure, we have obtained evidence that cortical units retain plasticity long after the end of this period has passed. In our procedure prolonged visual deprivation was followed by exposure in a normal visual environment. The animals were maintained throughout the first year of life either in total darkness or in an enclosure illuminated intermittently by a strobe light. Following the period of deprivation, electrophysiologic recordings were taken from some of these animals. The remaining cats were permitted 6-12 months in a normally-illuminated environment prior to recording. Cats of the same age reared from birth in a normally lit environment were also recorded. Cortical neurons in cats deprived of any normal visual experience rarely show orientation selective responses. In animals allowed subsequent normal visual experience about one-half of the units studied exhibited this property. This level of response specificity is intermediate between that of normally-reared and recently-deprived animals. While most cortical units in normally-reared cats exhibited direction selectivity, this property is rarely observed in the "recovery" cats. A number of unit types which are rarely observed in either normal or totally deprived animals were encountered in cats that had normal exposure following prolonged deprivation. A convergent strabismus was observed, in contrast with the divergent strabismus often shown by cats immediately following prolonged visual deprivation. This shows that ocular alignment as well as cortical unit properties can remain plastic in the adult.
Blindness↗
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.
Animals↗
Cats raised in a one-directional world: effects on receptive fields in visual cortex and superior colliculus.
Cats were reared in a visual environment in which irregularly-shaped patches of luminescent paint moved constantly leftward. The distribution of preferred directions and orientations of cortical neurons in these cats was examined. Most cortical neurons encountered had leftward components in their preferred directions, and although no anisotropy of orientation was present in the rearing environment, most cortical neurons responded optimally to stimuli oriented at or near vertical. Variations in the strength of the induced bias of direction and orientation were noted among the different subclasses of cortical neurons. Preferred velocities of cortical neurons did not appear matched to the velocity of stimuli in the rearing environment. The ocular dominance distribution among cortical neurons in the unidirectional cats was skewed toward the contralateral eye relative to normal cats. The distribution of preferred directions in collicular neurons was largely unaltered by the rearing procedures employed. As in normal cats, units in the left colliculus more frequently responded best to rightward stimulus movement while those in the right colliculus preferred leftward movement. The ocular dominance distribution among collicular units was somewhat skewed toward the contralateral eye.
Animals↗
Modification of direction selectivity of neurons in the visual cortex of kittens.
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Animals↗
Receptive fields in cat superior colliculus after visual cortex lesions.
1. The superior colliculus has been studied in intact cats and in cats with visual cortex lexions by recording the responses of single tectal units to visual stimuli. 2. Three classes of units have been identified in the superficial layers of the colliculus in these visually decorticate cats. 3. One class, comprising 5% of the units studied, has receptive fields organized concentrically in a manner similar to retinal ganglion cells. 4. The second class, comprising 12% of the units studied, responds to stimulus velocities over 300/sec, responds well to both small and large stimuli, and can be driven by strobe flashes at frequencies up to 35--40/sec. These units are termed 'flicker' cells. 5. The third class comprising 83% of the units studied, responds best to stimuli which are not larger than the activating region of the receptive field, moving at relatively low velocities. These units show strong suppressive surrounds which are sensitive to higher velocities of stimulus movement than the central activating region. Responses from the activating region in these units are dramatically inhibited by flickering dhanges in the level of background illumination. 6. In intact cats few units are found which are strongly inhibited by background flicker. 7. It is suggested that a high-velocity sensitive element such as the 'flicker' cell or phasic retinal ganglion cell is responsible for the flicker-induced inhibition of collicular units in the visually decorticate cat.
Action Potentials↗
Binocular interaction in the cat's superior colliculus.
1. Binocularly driven neurones with small receptive fields near the area centralis were recorded in the cat's superior colliculus. 2. Binocular interaction was tested by stimulating both eyes simultaneously with a single moving stimulus at various retinal disparities. 3. Collicular cells in general showed strong summation or even facilitation when the images of the stimulus were in exact correspondence on the receptive fields, sometimes with occlusion when they were out of register. The range of retinal disparity over which there was additive interaction could be as little as 1 or 2 deg, almost as narrow as for the most precisely tuned neurones in the visual cortex. Even cells with large receptive fields sometimes showed a narrow range of binocular interaction. 4. Non-directional cells generally exhibited weaker summation and broader disparity selectivity than did direction-selective cells. 5. Some neurones with virtually no response to a stimulus in one of the eyes can exhibit marked binocular interaction. Other apparently monocular cells show little or no binocular interaction. 6. The disparity of the centres of the receptive fields was measured after correcting for small eye movements, which were assessed by two different techniques. For 132 cells the measured distribution of horizontal disparity (range 4.5 deg; S.D. 0.93 deg) was significantly broader than that of vertical disparity (range 2.2 deg; S.D. 0.52 deg). Sources of error in these measurements are considered. 7. The results are discussed in relation to the known connexions between visual cortex and superior colliculus and the possible role of the latter in the regulation of eye movements.
Action Potentials↗
Organization of cat striate cortex: a correlation of receptive-field properties with afferent and efferent connections.
The purposes of this study were 1) to relate the receptive-field characteristics of area 17 cells to their afferent and efferent connections, and 2) to obtain quantitative data from area 17 neurons for later comparison with area 18 cells. Intra- and extracellular recordings were obtained in paralyzed preparations which were anesthetized with nitrous oxide. The connectivities of the recorded cells were determined from responses to electrical stimulation of afferent and efferent pathways. In parallel to the classification of units as simple and complex cells, the receptive fields were grouped in four classes according to the spatial arrangement of on- and off-areas; class I, fields with exclusive on- or off-areas; class II, fields with spatially separate on- and off-areas; class III, fields with mixed on-off areas; class IV, fields which could not be mapped with stationary stimuli. The results from electrical stimulation suggest two major classes of cells: cells in the first group are driven mainly or exclusively by LGN afferents. They rarely receive additional excitation from intrinsic or callosal afferents and rarely possess corticofugal axons. Cells in the second group receive either converging inputs from LGN afferents and further intrinsic afferents or only from intrinsic afferents. They frequently received additional input from callosum and from recurrent collaterals of corticofugal axons. They project subcortically more often than cells in the first group. Cells in both groups can be driven either by X- or Y-type afferents. Cells in the first group have mainly class I and class II fields or simple fields, whereas the neurons in the second group have mainly class III and class IV fields or complex fields. Thus, simple and complex cells differ in their connectivity patterns, but the discriminative parameter is neither the selective connection to the X- or the Y-system nor, in a strict sense, the synaptic distance from subcortical input. From the combined consideration of receptive-field properties and connectivity patterns it is concluded that class I and class II cells or simple cells are concerned mainly with the primary analysis of subcortical activity, whereas class III and class IV cells or complex cells perform a correlative analysis between highly convergent activity from extrinsic and intrinsic afferents.
Animals↗
Cat parastriate cortex: a primary or secondary visual area.
The purpose of this study was to determine to what extent the cat parastriate cortex processes afferent geniculate activity in a way similar to that in area 17. The area explored was located on the lateral gyrus between the Horsley-Clarke coordinates A1 to 4 and L3 to 4. The receptive-field properties of area 18 cells and their responses to electrical stimulation of afferent and efferent pathways were measured with the same methods as described previously in area 17. Mutual correlations among these items were calculated and compared with the respective data from area 17. The results of this correlative analysis revealed numerous similarities between the two areas with regard to their afferent and efferent connections and their intrinsic organization. Consequently, the structure of the receptive fields and their numerical distribution resembled those in area 17. The same was true for the correlations between receptive-field parameters and afferent and efferent connectivity. The main differences were that area 18 cells had larger receptive fields and responded to considerably higher stimulus velocities. It is suggest-d that these differences are caused by the fact that area 18 receives subcortical afferents of the Y-type, whereas the dominant input to area 17 comes from the X-system. It is concluded that the area investigated in this study is organized in parallel to area 17 and deals with other aspects of visual information than area 17.
Animals↗
Response characteristics of single cells in the monkey superior colliculus following ablation or cooling of visual cortex.
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Anesthesia, General↗
Cats reared in stroboscopic illumination: effects on receptive fields in visual cortex.
Cats were reared in a light-tight box in which the only source of illumination was a 9-musec strobe flash every 2 sec. This allowed them to experience visual form but they did not experience visual movement. Receptive-field properties of single units in area 17 of the visual cortex of cats reared in stroboscopic illumination (strobe-reared) were compared with properties of units in area 17 of normally reared cats. In strobe-reared cats both direction selectivity and orientation selectivity were greatly reduced relative to normally reared cats, and some units in the strobe-reared cats responded only to strobe flashes.
Animals↗
Comparison of receptive-field organization of the superior colliculus in Siamese and normal cats.
1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.2. The retinotopic organization of the superior colliculus has been compared in the two breeds. In the normal cat, the contralateral half-field is represented in the central and caudal part of the colliculus, and a vertical strip of the ipsilateral half-field, 15-20 degrees wide, is represented at the anterior tip. The Siamese cat superior colliculus receives an abnormally large projection from the ipsilateral half-field so that units with visual receptive fields which extend as far as 40 degrees into the ipsilateral half-field can be found. The area of the tectal surface devoted to the representation of the ipsilateral half-field is about twice as large in Siamese cats as in normal cats. The enhanced representation of the ipsilateral half-field in Siamese cats is reflected in a displacement of the vertical meridian and the area centralis on the tectal surface.3. The area centralis in the Siamese cat is located at about the same point on the tectal surface as would be occupied by a point in the visual field about 6-7 degrees contralateral to the area centralis in the normal cat. The smallest receptive fields in both breeds are located near the area centralis. The size of the receptive field for a tectal unit seems to be determined by the retinal location of the receptive field and not by the absolute position of the unit on the tectal surface.4. The receptive-field characteristics of tectal units show many similarities in the two breeds. The receptive fields of individual units consist of activating regions flanked by suppressive surrounds. Units respond well to stimuli of different shapes and orientation provided they are moving. The optimum stimulus for a given unit can be much smaller than the size of the activating region. About two thirds of the units studied in both breeds show directional selectivity. Most of the units studied in normal cats can be activated by stimulation of either eye, while in Siamese cats, 80% of the units studied can be driven only by the contralateral eye. A few monocularly driven units with two separated receptive fields have been observed in Siamese cats.5. In the left tectum of both breeds, units respond well to left-to-right stimulus movement. The reverse situation obtains in the right tectum. In Siamese cats, units located at the anterior tip of the tectum with their receptive fields located in the visual half-field ipsilateral to the tectum under study respond better to stimulus movement toward the area centralis than away from it. The preferred direction for a tectal unit seems to be determined by its tectal location rather than by the location of its receptive field in the retina.6. Visual cortex lesions in both breeds increase the responsiveness of tectal units to flashing spots and almost entirely remove the directional selectivity exhibited by tectal units, although units with asymmetric surrounds are still found. In normal cats, the lesions change the ocular dominance distribution, skewing it more strongly toward the contralateral eye. In Siamese cats, the ocular dominance distribution remains unchanged after a visual cortex lesion.7. The squint commonly exhibited by Siamese cats is regarded as a compensation for the anomalous retinotectal topography. It is suggested that, in the absence of an adaptive modification, the anomalous retinotectal projection would lead to mislocalization in Siamese cats just as it does in frogs and hamsters whose retinotectal projection has been experimentally altered. The convergent strabismus which Siamese cats commonly exhibit may be a cure for the abnormal retinal projections rather than a disease.
Animals↗
Receptive-field organization of monkey superior colliculus.
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Animals↗
Disruption of cortical activity prevents ocular dominance changes in monocularly deprived kittens.
Abundant evidence now indicates that atypical visual exposure early in the life of cats and primates can cause profound alterations in cortical organization. In particular, it has been shown that preventing the use of one eye for vision early in life results in a marked shift of ocular preference among neurones of kitten visual cortex in favour of the exposed eye. The cellular mechanisms underlying these alterations remain uncertain, but much recent attention has focused on the possible role of pharmacological agents in modifying cortical plasticity, with particular reference to catecholamines. These experiments, which have shown that agents which modify cortical noradrenaline levels can alter the degree of cortical plasticity, do not specify the mechanism of action, and leave open the possibility that other neurotransmitter systems may also be involved in cortical modifiability. We now report that chronic intracortical administration of L-glutamate during a period of monocular vision imposed on young kittens largely prevents the ocular dominance shift which normally occurs under these circumstances.
Animals↗
Quantitative distribution of GABA-immunopositive and -immunonegative neurons and synapses in the monkey striate cortex (area 17).
The number of GABA-immunoreactive [GABA(+)] neurons and synapses was determined in functionally distinct subregions delineated as rich and poor in cytochrome oxidase (CO) in the visual cortex of adult macaque monkeys. The average numerical density (number per unit volume, Nv) of GABA(+) neurons and synapses was not significantly different between the CO-rich and -poor regions. Twenty percent of the total number of cortical neurons and 17% of the synapses were GABA(+). On average, each visual cortical neuron receives 3900 synapses, 660 of them being GABA(+). The latter were distributed on the target cell in a pattern that predicts the site of GABA influences in cortex. The major targets of GABA(+) synapses were dendritic shafts, comprising nearly two-thirds of the postsynaptic elements. About every fourth and every eighth GABA(+) synapse was devoted to dendritic spines and to neuronal somata, respectively. Axon initial segments, although the exclusive targets of GABA(+) cells, comprise less than 0.1% of structures postsynaptic to GABA(+) boutons. From this distribution, we estimate that in each cubic millimeter of striate cortex there were about 20 million GABA(+) synapses on dendritic spines, 47 million on dendritic trunks, 9 million on somata, and fewer than 0.1 million on axon initial segments. The sites of influences of GABA-immunonegative [GABA(-)] synapses were different in that they target mainly dendritic spines and dendritic trunks. About two-thirds of GABA(-) synapses were on dendritic spines, and the remainder were devoted to dendritic trunks. Only a minute fraction innervate somata. We estimate that in 1 mm3 of striate cortex there were about 235 million GABA(-) synapses on spines, 133 million on dendrites, and about 2 million on somata. The proportions of GABA(+) neurons and synapses and their target distribution did not appreciably differ from those of the visual cortex of the cat even though the numerical density of neurons was 2.5 times higher in the monkey.
Animals↗