Shortage of binocular cells in area 17 of visual cortex in cats with congenital strabismus

Spatial and temporal matching of receptive field properties of binocular cells in area 19 of the cat

The spatial and temporal properties of single neurons were investigated in area 19 of the cat. We evaluated the matching of binocular receptive field properties with regard to the respective strength of the ipsilateral and contralateral inputs. Results indicate that most cells in area 19 are well tuned to spatial and temporal frequencies and exhibit relatively low contrast threshold (mean=6.8%) when assessed using optimal parameters and tested through the dominant eye. Spatial resolution (mean=0.75 c/degree), optimal spatial frequencies (mean=0.16 c/degree) were relatively low and spatial bandwidths (mean=2.1 octaves) were broader as compared to those of cells in area 17 but comparable to those of cells in other extrastriate areas. On the other hand temporal resolution (mean=10.7 Hz), optimal temporal frequency (mean=4.5 Hz) and temporal bandwidths (mean=2.9 octaves) were higher and broader than in primary visual cortex. A significant relationship exists between most of the cell's properties assessed through either eye. For some parameters, such as spatial and temporal resolution, ocular dominance was shown to be significantly related to the extent of matching between the two eyes. For these parameters, binocular cells that exhibited a balanced ocular dominance were generally well matched with regard to the receptive field properties of each eye whereas the largest mismatches were found in cells that were more strongly dominated by one eye. These results suggest that visual input contributes to the activation of cells in area 19 in a redundant manner, possibly attesting to the multiplicity of parallel pathways to this area in the cat.

Visual responses of neurons from areas V1 and MT in a monkey with late onset strabismus: a case study

One adult monkey (Macaca fascicularis) was investigated psychophysically and electrophysiologically after at least 5 years of late onset esotropic macrostrabismus (squint angle 52 deg). Behavioural tests revealed normal monocular visual and visuomotor functions. No indications of deep amblyopia or oculomotor asymmetry were found. The monkey used the left or right eye alternately at about equal frequencies. Single unit recordings from area VI disclosed a normal ocular dominance distribution. Most VI neurons from both hemispheres received binocular input. Thus, discordant visual information from corresponding retinal locations of the two eyes converged onto the cortical neurons. No evidence for anomalous retinal correspondence was found. Diplopia and confusion must therefore be avoided by suppression of vision through one eye to allow stable, unambiguous perception. Possible suppression was investigated by stimulating a neuron through the same eye when it was actively used for fixation in one set of trials, and when it was not used for fixation in another set of trials. Significant differences in these two stimulus conditions were found in 20/39 neurons from area VI and in 11/34 motion sensitive neurons recorded in the middle superior temporal area (MT). The normalized population activity in VI and MT was higher if cells were stimulated through the fixating eye. The data are discussed with respect to possible suppressive mechanisms helping to prevent double vision in strabismus and in binocular rivalry.

Visual callosal connections and strabismus

Strabismus is a condition that exists when the visual axes of the two eyes fail to intersect at the fixation point under binocular viewing conditions. When it occurs in mammals during the critical period which corresponds to the period of maximal plasticity early in life, strabismus is known to induce both morphological anomalies and abnormal connections from the retina to the cortex; it further leads to binocular neural changes and to spatial vision deficits, especially at the cortical level. After a brief review of the already known data about the consequences of early strabismus in cats, monkeys and humans on the development of the visual system and of visual perception, new data are presented here concerning interhemispheric connections in the cat. In normally-reared kittens, visual callosal transfer is shown to be almost adult-like as soon as 12 days after birth: it is almost limited to the 17/18 border of the visual cortex when using visual stimulations in spite of the presence of still numerous juvenile exuberant callosal projections. In contrast, callosal transfer of visual information is extended to both areas 17 and 18 after strabismus, leading to the conclusion that at least some juvenile exuberant callosal projections are not only anatomically but also functionally stabilized after such an oculomotor disease. The possibility that similar abnormalities might be present in monkeys and humans is discussed.

Depth perception and cortical physiology in normal and innate microstrabismic cats

Evidence is presented that innate microstrabismus and abnormal cortical visual receptive-field properties can occur also in cats without any apparent involvement of the Siamese or albino genetic abnormalities in their visual system. A possible cause for microstrabismus in these cats may be sought in an abnormally large horizontal distance between blind spot and area centralis indicated by a temporal displacement of the most central receptive fields on both retinae. Depth perception was found to be impaired in cats with innate microstrabismus. Behavioral measurements using a Y-maze revealed in four such cats that the performance in recognizing the nearer of two random-dot patterns did not improve when they were allowed to use both eyes instead of only one. The ability of microstrabismic cats to perceive depth under binocular viewing conditions only corresponded to the monocular performance of five normal cats. Electrophysiological recordings were performed in the visual cortex (areas 17 and 18) of four awake cats, two normal, and two innate microstrabismic animals. Ocular dominance and orientation tuning of single neurons in area 17 and 18 were analyzed quantitatively. The percentage of neurons in area 17 and 18 which could be activated through either eye was significantly reduced to 49.7% in the microstrabismic animals when compared to the normal cats (74.8%). "True binocular cells," which can only be activated by simultaneous stimulation of both eyes, were significantly less frequent (1.6%) in microstrabismic cats than in normal animals (10.4%). However, subthreshold binocular interactions were identical in both groups of animals. In the strabismic animals, long-term binocular stimulation of monocular neurons did not give a clear indication of alternating use of one or the other eye. The range of stimulus orientations leading to discharge rates above 50% of the maximal response, i.e. the half-width of the orientation tuning curves, was the same in the two groups of cats. However, orientation sensitivity, i.e. the alternation in discharge rate per degree change in stimulus orientation, was higher in cortical cells of normal cats than in those of microstrabismic cats. In normal and microstrabismic cats, no clear sign of an "oblique effect," i.e. the preference of cortical neurons for vertical and horizontal orientations compared to oblique orientations, could be found neither in the incidence of cells with horizontal or vertical preferred orientation nor in the sharpness of orientation tuning and sensitivity of these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Human amblyopia: structure of the visual field

Kittens raised with different kinds of abnormal early visual experience (monocular and binocular deprivation, convergent strabismus, eye rotation, asymmetric alternating occlusion, early callosal split) show systematic deficits in the nasal visual field of the affected eye. To test whether abnormal visual experience produces similar deficits in the human visual system, we measured the monocular visual field of humans with subnormal binocular vision (strabismic and anisometropic amblyopes, strabismics with alternating fixation). Eight amblyopes were tested with a computer-assisted static perimetry (Octopus 2000). Twenty other subjects were tested with kinetic perimetry (Goldmann 940), 11 subjects with static perimetry (Goldmann 940). In some of these subjects, we measured the latency of saccades and the accuracy of visually guided pointing toward stimuli presented in the peripheral visual field. Both strabismic and anisometropic amblyopes frequently showed deficits of visual sensitivity in the central part of the visual field, but no systematic deficits in the peripheral field of the amblyopic eyes. Strabismic alternators had practically equal fields in the two eyes. Neither saccadic latency nor pointing accuracy showed a systematic impairment in the nasal visual field. The discrepancy between the field losses in strabismic humans and in cats raised with a surgically induced squint cannot be due to methodological differences, but rather to anatomical differences, or to the different origin of strabismus in the two species.

The pupillary light reflex in normal and innate microstrabismic cats, II: Retinal and cortical input to the nucleus praetectalis olivaris

The anatomical substrate of the pupillary light reflex was investigated in normal and innate microstrabismic cats using anatomical methods as well as electrical stimulation. The bilateral retinal input to the nucleus praetectalis olivaris (NPO), the pretectal relay station in the subcortical pupilloconstrictor pathway, was identified to come from the ventral retina were the upper visual field is represented. Orthodromic electrical stimulation revealed that retinal information is transmitted to on-tonic neurons in the NPO mainly via slowly conducting axons probably originating from W- and X-type retinal ganglion cells. For the first time, a direct cortical input to on-tonic neurons in the NPO could be demonstrated. This cortical input originates from caudolateral parts of the occipital cortex. Putative input structures are those subdivisions of areas 19 and 20a where the upper part of the visual field is represented. A direct, predominantly contralateral projection with a weak ipsilateral component from NPO to the nucleus of Edinger-Westphal, and an interhemispheric connection between the NPOs could be demonstrated. With respect to the anatomical connections as described in this study, no differences between normal and innate microstrabismic cats could be found. The results are discussed with respect to the binocular summation of the pupillary light reflex and its reduction in subjects with impaired binocular vision.

The pupillary light reflex in normal and innate microstrabismic cats, I: Behavior and receptive-field analysis in the nucleus praetectalis olivaris

Neurons in the nucleus praetectalis olivaris (NPO) were antidromically identified by electrical stimulation of the nucleus of Edinger-Westphal (EW), the location of preganglionic pupilloconstrictor motoneurons. Electrical stimulation within the NPO leads to bilateral pupil constriction. Single neurons recorded in the NPO respond tonically to light stimuli, and their discharge frequency increases linearly with logarithmic increase in light intensity. This characteristic identifies NPO neurons as luminance detectors. They have large receptive fields mostly lying in the upper and contralateral quadrant of the visual field. Cats with impaired binocular vision show a significantly reduced binocular summation of the pupillary light reflex (BSP), i.e. the increase of pupil constriction during binocular illumination when compared to monocular illumination is less than in normal animals. The investigation of ocular dominance and subthreshold binocular interactions in the NPO of normal and innate microstrabismic cats revealed two possible mechanisms for BSP and its reduction in strabismic subjects. First, the percentage of neurons increasing their discharge rate by illuminating either eye is significantly reduced in the NPO of innate microstrabismic cats (6.6%) when compared to normal cats (22% of all neurons tested). Second, in most NPO neurons of normal cats the subthreshold influence of the ipsilateral eye leads to an increase in neuronal activity during binocular stimulation when compared to monocular stimulation of the contralateral eye (binocular summation). The subthreshold influence of the ipsilateral eye in most NPO neurons of microstrabismic cats, however, is inhibitory, i.e. the neuronal discharge rate during binocular stimulation is decreased when compared to monocular stimulation of the contralateral eye (binocular inhibition). However, there is no significant correlation between BSP and binocularity in the NPO in individual animals. This suggests that BSP may be additionally influenced by visual structures other than NPO.