Ocular convergence-related neuronal responses in the lateral suprasylvian area of alert cats

Temporal interactions in direction-selective complex cells of area 18 and the posteromedial lateral suprasylvian cortex (PMLS) of the cat

Temporal interactions in direction-sensitive complex cells in area 18 and the posteromedial lateral suprasylvian cortex (PMLS) were studied using a reverse correlation method. Reverse correlograms to combinations of two temporally separated motion directions were examined and compared in the two areas. A comparison to the first-order reverse correlograms allowed us to identify nonlinear suppression or facilitation due to pairwise combinations of motion directions. Results for area 18 and PMLS were very different. Area 18 showed a single type of nonlinear behavior: similar directions facilitated and opposite directions suppressed spike probability. This effect was most pronounced for motion steps that followed each other immediately and decreased with increasing delay between steps. In PMLS, the picture was much more diverse. Some cells exhibited nonlinear interactions, that were opposite to those in area 18 (facilitation for opposite directions and suppression for similar ones), while the majority did not show a systematic interaction profile. We conclude that nonlinear second-order reverse correlation characteristics reveal different functional properties, despite similarities in the first-order reverse correlation profiles. Directional interactions in time revealed optimal integration of similar directions in area 18, but motion opponency--at least in some cells--in PMLS.

Spatial disparity sensitivity in area PMLS of the Siamese cat

Previous studies of the visual system of Siamese cats have shown that binocular cells are scarce in areas 17, 18 and 19, yet significantly more abundant in suprasylvian areas such as the postero-medial lateral suprasylvian area (PMLS). The present study aims at evaluating the sensitivity to spatial disparity of PMLS binocular cells in paralyzed and anesthetized Siamese cats. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. Delays were introduced in the arrival of the luminous bars in the receptive fields so as to create the desired spatial disparities. Results indicate that approximately a third of PMLS units are binocular and that these binocular cells can detect spatial disparity cues. Indeed, although the sample was relatively small, cells of the tuned excitatory (14/34), tuned inhibitory (2/34), near (6/34) and far (1/34) types were identified. The spatial selectivity, as measured by the width at half height of the tuning curves of the excitatory and inhibitory cells and the slopes of the near and far cells, was similar to that obtained in PMLS of normal cats but not as precise as that found for primary visual areas in these animals. This suggests that these cells might serve as a substrate for coarse stereopsis.

Effects of localized lesions in the lateral suprasylvian cortex on convergence eye movement in cats

We evaluated the effects of electrolytic lesions in the extrastriate cortical area on the amplitudes and velocities of vergence eye movements in six alert cats that were trained to track a target moving in depth. Bilateral or unilateral lesions in the lateral suprasylvian (LS) cortex reduced the amplitudes and velocities of vergence eye movements, but the positive correlation between them was maintained. Furthermore, unilateral lesions changed the symmetry of eye movements. Movements of the left eye were decreased by lesions in the right LS cortex, resulting in asymmetric vergence eye movement with right eye predominance, and vice versa. These results support the hypothesis that the LS cortex plays an essential role in controlling vergence eye movement.

How is binocularity maintained during convergence and divergence?

The geometrical requirements for binocular fusion are stated, and the main features of horizontal vergence eye movements are described, together with an influential schema of understanding the interaction between vergence and accommodation. The anatomy and physiology of the midbrain region implicated in vergence and accommodation control are discussed. The cortical areas from which suitable sensory signals might be derived are mentioned briefly, and a speculation is made about esotropia.

Roles of the lateral suprasylvian cortex in convergence eye movement in cats

Ocular convergence and lens accomodation were evoked by microstimulation in the lateral suprasylvian area (LS cortex) in the parieto-occipital cortex in the cat. Electrolytic lesions in LS cortex reduced the amplitude and velocity of ocular convergence. Neurons in LS cortex discharged in relation to ocular convergence and/or lens accommodation. These results support the hypothesis that the LS cortex plays an important role in controlling ocular convergence The LS cortex receives visual inputs from cortical visual areas 17, 18 and 19, and in addition from the superior colliculus through the LP nucleus of the thalamus. Electrophysiological recordings have revealed that these visual inputs, which include cues about 3-dimensional target motion, are integrated in the LS cortex. The integrated output from LS cortex may provide the brainstem motor centers with the neural signals that facilitate eye movements, especially when the target is moving at high speeds. Outputs from the LS cortex travel directly to brainstem structures including the superior colliculus and pretectum. Evidence from monkey suggests that information may also travel to the mesencephalic reticular formation, where neurons have been recorded that are related to ocular convergence, lens accomodation or both. Although comparable data is lacking in the cat, it is suggested that the efferent circuit from the LS cortex to the motor nuclei in the brainstem included both the superior colliculus and the mesencephalic reticular formation. It is also suggested that this pathway is rather short, given that the mean latency of the early component of evoked disjunctive eye movements was approximately 60 ms.

Handedness in strabismics. Etiological implications

The incidence of non right handedness was found to be significantly higher in a group of 228 strabismics when compared with a control group of neurologically normal non strabismic subjects. The difference was still present after elimination of strabismics with known neurological or general health problems, or who were born prematurely. The strabismics showed no inter group differences in handedness in relation to the type of retinal correspondence present or the ability to achieve normal or abnormal binocular single vision. Evidence is reviewed for the presence of a developmental defect leading to strabismus, and results of this survey support a previously proposed idea that a developmental defect leads to strabismus, the type of which reflects the severity of the defect (although external factors may also be an influence).

Extrastriate cortical neurons correlated with ocular convergence in the cat

Responses of neurons in the lateral suprasylvian area to visual stimulation in association with ocular convergence were studied in eight alert cats trained to track a visual target moving in depth. Activities of 18 (3%) of 659 cells were related to ocular convergence. These 18 neurons were divided into two groups: activities of seven neurons (40%) correlated with peak velocities of convergence eye movement at both fast and slow target speeds (group I), while those of five neurons (30%) correlated with them only at faster target speed (group II). Activities of six other neurons correlated with peak velocity of ocular convergence at faster target speed but were not tested at slower speed. Activities of four group-I neurons (60%) did not or only weakly correlate with lens accommodation, while those of four group-II neurons (80%) correlated with peak velocity or amplitude of lens accommodation. It can therefore be concluded that the four group-I neurons are primarily related to ocular convergence. Other cells were either convergence-related, lens accommodation-related or both. It is suggested that these different types of neurons contribute in combination to optimal control of convergence eye movement.