Neural responses in the primary visual cortex of the monkey during perceptual filling-in at the blind spot

The phenomenon of perceptual filling-in demonstrates that physical stimuli presented on the retina do not necessarily correspond to surface perception, and that our visual system has mechanisms with which to interpolate missing information in order to construct continuous surfaces. Among its various forms, filling-in at the blind spot is one of the most remarkable. To study the neural mechanisms involved in filling-in at the blind spot, we recently conducted a recording experiment aimed at determining whether the neurons in the primary visual cortex (V1) that represent the visual field corresponding to the blind spot are activated when filling-in occurs. We found that neurons located in deep layers of the V1, particularly layer 6, respond to large stimuli that cover the blind spot and induce perceptual filling-in. These neurons tended to have very large receptive fields, which extended out of the blind spot, and preferred relatively large stimuli. We believe that neurons in the V1 region representing the blind spot encode information essential for perceptual filling-in at the blind spot.     

Intracerebral influences on the microstructure of receptive fields of cat visual cortex

Summary Effects of electrical stimulation of the basal ganglia (caudate nucleus and putamen) and cortex (gyrus proreus and compositus) on the receptive fields and response properties of units in the visual cortex of cats were assessed using single lines, double lines and multiple lines (gratings). In the single line experiment caudate stimulation significantly increased the spontaneous activity, optimal firing rate and receptive field size of visual cortex neurons whereas putamen stimulation decreased these parameters. Stimulation of gyrus proreus enhanced, while that of gyrus compositus diminished optimal firing rate without affecting spontaneous activity; in addition, stimulation of ipsilateral proreus and compositus increased the receptive field size whereas their contralateral homologues decreased it. In the double line experiment, proreus and caudate stimulation increased the magnitude of the facilitatory effect of progressive separation of the lines over certain ranges whereas compositus and putamen stimulation increased the inhibitory influences. Orientation selectivity and spatial frequency tuning characteristics were unaffected by the electrical stimulations of any of the four sites. Thus three categories of network properties were delineated: those characterized by remaining invariant to any cerebral stimulation; those characterized by overall activation as by basal ganglia stimulation; and those characterized as interactive which were responsive especially to cortical stimulation.This research was supported by a postdoctoral fellowship from the Medical Research Council of Canada to Maurice Ptito, a predoctoral fellowship from the National Research Council of Canada to Maryse C. Lassonde and NIMH Grant MH12970 and NIMH Career Research Award MH15214 to Karl H. Pribram     

Organization of suppression in receptive fields of neurons in cat visual cortex.

1. The response to an optimally oriented stimulus of both simple and complex cells in the cat's striate visual cortex (area 17) can be suppressed by the superposition of an orthogonally oriented drifting grating. This effect is referred to as cross-orientation suppression. We have examined the spatial organization and tuning characteristics of this suppressive effect with the use of extracellular recording techniques. 2. For a total of 75 neurons, we have measured the size of each cell's excitatory receptive field by use of rectangular patches of drifting sinusoidal gratings presented at the optimal orientation and spatial frequency. The length and width of these grating patches are varied independently. Receptive-field length and width are determined from the dimensions of the smallest grating patch required to elicit a maximal response. 3. The extent of the area from which cross-orientation suppression originates has been measured in an analogous manner. Each neuron is excited by a patch of drifting grating the same size as the receptive field. The response to this stimulus is modulated by a superimposed patch of grating having an orthogonal orientation. After selecting the spatial frequency that produces maximal suppression, the response of each cell is examined as a function of the length and width of the orthogonal (suppressive) grating patch. Results from 29 cells show that the dimensions of the orthogonal grating patch required to elicit maximal suppression are similar to, or smaller than, the dimensions of the excitatory receptive field. Thus cross-orientation suppression originates from within the receptive field. 4. For some cells the spatial frequency tuning of the suppressive effect is much broader than the spatial frequency tuning for excitation. In these cases it is possible to find a spatial frequency that produces suppression but not excitation. With the use of a suppressive stimulus having this spatial frequency, we examined the strength of suppression as a function of orientation for 11 cells. These tests show that suppression occurs at all orientations, including the preferred orientation for excitation. In some cases, suppression is somewhat stronger at the preferred orientation for excitation than at any other orientation. 5. For 12 cells we varied the relative spatial phase between the optimally oriented and orthogonal gratings. In all cases the magnitude of suppression is largely independent of the relative spatial phase. 6. For three binocular cells we examined whether the suppressive effect of a grating oriented orthogonal to the optimum could be mediated dichoptically.(ABSTRACT TRUNCATED AT 400 WORDS)     

Spatio-temporal plasticity of cortical receptive fields in response to repetitive visual stimulation in the adult cat

Many psychophysical experiments on perceptual learning in humans show increases of performance that are most probably based on functions of early visual cortical areas. Long-term plasticity of the primary visual cortex has so far been shown in vivo with the use of visual stimuli paired with electrical or pharmacological stimulation at the cellular level. Here, we report that plasticity in the adult visual cortex can be achieved by repetitive visual stimulation. First, spatial receptive field profiles of single units (n=38) in area 17 or 18 of the anesthetized cat were determined with optimally oriented flashing light bars. Then a conditioning protocol was applied to induce associative synaptic plasticity. The receptive field center and an unresponsive region just outside the excitatory receptive field were synchronously stimulated ('costimulation', repetition rate 1 Hz; for 10-75 min). After costimulation the receptive field and its adjacent regions were mapped again. We observed specific increases of the receptive field size, changes of the receptive field subfield structure as well as shifts in response latency.In 37% of the cells the receptive field size increased specifically towards the stimulated side but not towards the non-stimulated opposite side of the receptive field. In addition, changes in the relative strength and size of the on and off subfield regions were observed. These specific alterations were dependent on the level of neuronal activity during costimulation. During recovery, the new responses dropped down to 120% of the preconditioning value on average within 103 min; however, the decay times significantly depended on the response magnitude after costimulation. In the temporal domain, the latency of new responses appeared to be strongly influenced by the latency of the response during costimulation.Twenty-nine percent of the units displayed no receptive field enlargement, most likely because the activity during costimulation was significantly lower than in the cases with enlarged receptive fields. An unspecific receptive field enlargement towards both the stimulated and non-stimulated side was observed in 34% of the tested cells. In contrast to the cells with specifically enlarged receptive fields, the unspecific increase of receptive field size was always accompanied by a strong increase of the general activity level.We conclude that the receptive field changes presumably took place by strengthening of synaptic inputs at the recorded cells in a Hebbian way as previously shown in the visual cortex in vitro and in vivo. The observed receptive field changes may be related to preattentive perceptual learning and could represent a basis of the 'filling in' of cortical scotomas obtained with specific training procedures in human patients suffering from visual cortex lesions.     

The corticopontine projection from area 20 and surrounding areas in the cat: terminal fields and distribution of cells of origin as compared to other visual cortical areas

Visual area 20 in the cat projects to the pontine nuclei and thereby gives input to the cerebellum. The termination of the fibres and the distribution of the cells of origin were studied with anterograde and retrograde transport of horseradish peroxidase-wheat germ agglutinin. Fibres from area 20 terminate ipsilaterally in the medial half of the rostral three-quarters of the pontine nuclei. The labelled fibres occur in multiple well-restricted patches usually some distance away from the peduncle. The retrogradely labelled cells in areas 20a, 20b, and the adjacent posterior suprasylvian visual area were quantitatively mapped. Areal borders were placed according to the maps of Tusa and Palmer (J. comp. Neurol. 193, 147-164) and Updyke (J. comp. Neurol. 246, 265-280). In terms of cortical densities (cells per mm2 of cortex) area 20a is among the visual areas with the highest densities of corticopontine neurons (data from other visual areas from the author's previous works). Densities are 30-60% lower in area 20b than in 20a. The posterior suprasylvian visual area was found to be among the visual areas with the lowest densities of corticopontine neurons. Due to the small size of the areas investigated, the total number of corticopontine cells within them is small compared to many other areas. Within areas 20a and 20b, cortical densities are higher in the representation of visual space below than above the horizontal meridian. Furthermore, cortical densities are generally somewhat higher in the region devoted to central vision compared to regions devoted to the visual periphery. Since the part of cortex in areas 20a and 20b devoted to central vision is weakly over-represented also in terms of cortical volume, it follows that the "visual field density" (cells per degree of visual field) of corticopontine cells is highest in the central visual field representation. The finding of an over-representation of central vision compared to peripheral vision in the corticopontine projection from areas 20a and 20b is particularly interesting in conjunction with previous findings in other visual areas. In the cortical representations of central vision, areas with high magnification factors (i.e. areas that greatly emphasize central vision in terms of cortical volume, such as areas 17, 18, and 19) have relatively low cortical densities of pontine projecting cells, whereas areas with low magnification factors (such as areas 20a, 20b, and some of the lateral suprasylvian visual areas) have relatively high cortical densities. The "visual field density" of corticopontine neurons appears therefore to be fairly constant from area to area as concerns central vision.     

Cortical representation of space around the blind spot

The neural mechanism that mediates perceptual filling-in of the blind spot is still under discussion. One hypothesis proposes that the cortical representation of the blind spot is activated only under conditions that elicit perceptual filling-in and requires congruent stimulation on both sides of the blind spot. Alternatively, the passive remapping hypothesis proposes that inputs from regions surrounding the blind spot infiltrate the representation of the blind spot in cortex. This theory predicts that independent stimuli presented to the left and right of the blind spot should lead to neighboring/overlapping activations in visual cortex when the blind-spot eye is stimulated but separated activations when the fellow eye is stimulated. Using functional MRI, we directly tested the remapping hypothesis by presenting flickering checkerboard wedges to the left or right of the spatial location of the blind spot, either to the blind-spot eye or to the fellow eye. Irrespective of which eye was stimulated, we found separate activations corresponding to the left and right wedges. We identified the centroid of the activations on a cortical flat map and measured the distance between activations. Distance measures of the cortical gap across the blind spot were accurate and reliable (mean distance: 6-8 mm across subjects, SD approximately 1 mm within subjects). Contrary to the predictions of the remapping hypothesis, cortical distances between activations to the two wedges were equally large for the blind-spot eye and fellow eye in areas V1 and V2/V3. Remapping therefore appears unlikely to account for perceptual filling-in at an early cortical level.     

Relationship between spatial and spatial-frequency characteristics of receptive fields of cat visual cortex

The spatial (magnitude and eccentricity) and spatial-frequency (optimum frequency and width of pass band) characteristics of the receptive fields of the cat visual cortex were investigated. It was shown that in accordance with the predictions of the theory of piecewise Fourier analysis, linear and quasilinear receptive fields of a single size comprise a modulus in each of the fields of which the index of complexity (ratio of size of field to number of periods of its optimum frequency) equals the optimum frequency multiplied by a coefficient that is constant for the given modulus. Five moduli were found with field sizes of 2.6, 3.8, 5.2, 6.2, and 7.0°, shifting with increase in the size of the modulus towards the periphery of the field of view. In accordance with predictions, when the index of complexity is fixed the width of the pass band declines inversely proportionately to the size of the fields. The obtained data directly support the hypothesis according to which the receptive fields effect a piecewise quasi-Fourier expansion of the image.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 69, No. 5, pp. 614–622, May, 1983.     

Weakened feedback abolishes neural oblique effect evoked by pseudo-natural visual stimuli in area 17 of the cat

The psychological oblique effect, a well-known phenomenon that humans and some mammals are more visually sensitive to cardinal (vertical and horizontal) contours than to oblique ones, has commonly been associated with the overrepresentation of cardinal orientations in the visual cortex. In contrast to the oblique effect, however, Essock et al. [E.A. Essock, J.K. DeFord, B.C. Hansen, M.J. Sinai, Oblique stimuli are seen best (not worst!) broad-band stimuli: a horizontal effect, Vision Res. 43 (2003) 1329–1335] reported a psychological ‘horizontal effect’, in which visual stimuli dominated by oblique orientations were best perceived by human subjects when tested with unique natural broad-band stimuli. In this study, using optical imaging and the similar visual stimuli, we found an overrepresentation of cardinal orientations, i.e. the neural oblique effect, but not ‘horizontal effect’, in area 17 of the cat. In addition, the oblique effect was abolished by GABA administration in area 21a due to the preferred orientation shifting (6.0%) and decrease of orientation selectivity strength of neurons (26.9%) in area 17. These results indicate a neuronal basis of the oblique effect when animals watch a more natural scene, whereas no evidence was found for the ‘horizontal effect’.     

Early versus late visual cortex lesions: Effects on receptive fields in cat superior colliculus

Summary 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 of 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.This research was supported by M.R.C. Grant No. MA 5201 and NRC Grant No. A9939(to M.C.) and Grants from NIH (postdoctoral fellowshop to N.B.).     

A blind spot in correct naming latency analyses

Speech errors and naming latencies provide two complementary sets of behavioural data for understanding language production processes. A recent analytical trend—applied to intact and impaired production alike—highlights a link between specific features of correct picture naming latency distributions and the retrieval processes thought to underlie them. Although chronometric approaches to language production typically consider correct response times in isolation, adequately accounting for their distributions in error-prone situations requires also considering the errors that sometimes censor them. In this paper, I illustrate by simulation how excluding incorrect word retrievals predictably alters observed distributions of correct naming latencies. To the extent that naming errors impose a stochastic deadline on successful production, their censoring should tend to reduce the mean, variance, and skew of observed latencies for correct responses, relative to the uncensored underlying distribution.     

Receptive fields of units in the visual cortex of the cat in the presence and absence of bodily tilt

Summary The receptive fields of units in the visual cortex of anaesthetised cats were studied using spots or slits of light. Some fields were found to be stable when they were repeatedly plotted with the cat maintained in the horizontal position: other fields were not stable and the sharpness of spatial tuning varied though the orientation of the axis did not shift. When the cat was tilted the field axis of the majority of cells followed the tilt. In 14 cells, however, changes occurred in the receptive field which were not observed when the animal remained in the horizontal plane. These changes included drifts of the field axis in a direction which, with one exception, was opposite to the tilt, and alterations in the spatial extent of the field. On returning the animal to horizontal the axis of 4 fields drifted past the original orientation. These effects were not eliminated by either bilateral destruction of the labyrinth or high cervical transection of the spinal cord. The time of onset of the tilt effects varied from cell to cell: some of this variability is probably an effect of anaesthesia.The findings are consistent with the view that the receptive field of certain cells in the visual cortex are capable of being modified, one of the modifying influences being the orientation of the body in space.This work was supported by grants from the Science Research Council to G. Horn and from the U.S. Public Health Service to G. Stechler (Grant MH 16215) and R.M. Hill (Grant NB 05653).     

Aberrant visual projections in the Siamese cat

1. Guillery has recently shown that the Siamese cat has a grossly abnormal lateral geniculate body. His anatomical study suggested that certain fibres originating in the temporal retina of each eye cross in the chiasm instead of remaining uncrossed. They thus reach the wrong hemispheres, but in the geniculate they terminate in the regions that the missing fibres from the ipsilateral eye would normally have occupied. The result is that each hemisphere receives an input from parts of the ipsilateral field of vision, this input being entirely from the opposite eye. The purpose of the present work was to study the physiological consequences of this aberrant projection, in the lateral geniculate body and visual cortex.2. Single-cell recordings from the lateral geniculate body confirmed the presence of projections from the ipsilateral visual field of the contralateral eye. The part of layer A(1) receiving these projections was arranged so that the receptive fields of the cells were situated at about the same horizontal level and at the same distance from the vertical meridian as the fields of cells in the layers above and below (layers A and B), but were in the ipsilateral visual field instead of the contralateral. They thus occupied a region directly across the mid line from their normal position.3. In the cortex of all animals studied, we found a systematic representation of part of the ipsilateral visual field, inserted between the usual contralateral representations in areas 17 and 18. When the visual cortex was crossed from medial to lateral the corresponding region of visual field moved from the contralateral periphery to the mid line, and then into the ipsilateral field for 20 degrees . The movement then reversed, with a return to the mid line and a steady progression out into the contralateral field. The entire double representation was, with some possible exceptions, a continuous one. The point of reversal occurred at or near the 17-18 boundary, as judged histologically, and this boundary was in about the same position as in ordinary cats.4. Cells in the part of the cortex representing the ipsilateral fields had normal receptive fields, simple, complex, or hypercomplex. These fields tended to be larger than those in corresponding parts of the contralateral visual fields. Receptive-field size varied with distance from the area centralis, just as it does in the normal cat, so that cells with the smallest fields, in the area centralis projection, were situated some distance from the 17-18 border.5. Projections originating from the first 20 degrees from the midvertical in both visual half-fields had their origin entirely in the contralateral eye, as would be expected from the abnormal crossing at the chiasm. Beyond this visual-field region, and out as far as the temporal crescents, there were projections from both eyes, but we found no individual cells with input from the two eyes. The cells were aggregated, with some groups of cells driven by one eye and some by the other.6. From previous work it is known that ordinary cats raised with squint show a decline in the proportion of cells that can be driven binocularly, whereas animals raised with both eyes closed show little or no decline. A Siamese cat raised with both eyes closed had binocular cells in the regions of 17 and 18 subserving the peripheral visual fields, suggesting that the absence of binocular cells seen in the other Siamese cats was indeed secondary to the squint.7. In two Siamese cats there were suggestions of an entirely different projection pattern, superimposed upon that described above. In the parts of 17 and 18 otherwise entirely devoted to the contralateral visual field, we observed groups of cells with receptive fields in the ipsilateral field of vision. The electrode would pass from a region where cells were driven from some part of the contralateral visual field, to regions in which they were driven from a part of the ipsilateral field directly opposite, across the vertical mid line. The borders of these groups were not necessarily sharp, for in places there was mixing of the two groups of cells, and a few cells had input from two discrete regions located opposite one another on either side of the vertical mid line. The two receptive-field components of such cells were identical, in terms of orientation, optimum direction of movement, and complexity. Stimulation of the two regions gave a better response than was produced from either one alone, and the relative effectiveness of the two varied from cell to cell. These cells thus behaved in a way strikingly reminiscent of binocular cells in common cats.8. The apparent existence of two competing mechanisms for determining the projection of visual afferents to the cortex suggests that a number of factors may cooperate in guiding development. There seems, furthermore, not to be a detailed cell-to-cell specificity of geniculocortical connexions, but rather a tendency to topographic order and continuity, with one part of a given area such as 17 able to substitute for another. Whether or not these tentative interpretations are ultimately proved correct, it seems clear that this type of genetic anomaly has potential usefulness for understanding mechanisms of development of the nervous system.