Functional organization in the cat's pulvinar complex

The responses of 192 single units in the cat's pulvinar-complex, comprising the inferior, medial and lateral pulvinar nuclei, were studied in paralysed cats, lightly anaesthetized with N2O/O2 supplemented with pentobarbitone. About 60% of the cells were visually driven and their receptive fields classified as either diffuse, concentric, movement sensitive, direction sensitive or orientation sensitive. The response fields of such cells were commonly large. Response field maps for the movement and direction sensitive cells formed a heterogenous population with uniform on-off fields to more complex arrangements with on- or off-centres, often with only partial surrounds; other cells responded exclusively to moving stimuli. A dual representation of the visual field was found in the pulvinar-complex corresponding to the striate and tectal recipient zones described anatomically by others. The representation in the striate recipient zone comprised an oblique column running medio-laterally and rostro-caudally through the inferior pulvinar and lateral margin of the medial pulvinar. The peripheral visual field was represented laterally and the vertical meridian medially; the upper visual field was represented dorso-laterally in the medial pulvinar and the lower visual field caudo-ventrally within the inferior pulvinar. That this visuotopic organization corresponded to the striate recipient zone was established by tracing the retrograde transport of HRP. Medial to the striate zone evidence for a second visual field representation was found, apparently more randomly organized than the striate zone, corresponding to the presumed tectal recipient zone. These results support the assertion that cytoarchitectural boundaries do not necessarily delineate functional (visuotopically organized) regions. These observations suggest caution when comparing cytoarchitecturally defined regions between species; rather, 'functionally equivalent' regions should be compared.