Areas PMLS and 21a of cat visual cortex: two functionally distinct areas.

We have compared the receptive field properties of neurons recorded from visuotopically corresponding regions of area 21a and the posteromedial lateral suprasylvian area (PMLS) of cat visual cortex. In both areas, the great majority of neurons were orientation-selective and binocular, and their responses to moving contours were modulated by simultaneous in-phase or anti-phase motion of large textured background stimuli ('visual noise'). However, despite the great hodological similarity between the two areas, PMLS neurons had on average significantly higher peak discharge rates, exhibited substantially greater direction selectivity indices, and preferred substantially higher stimulus velocities than area 21a neurons. Furthermore, the majority of binocular neurons in the PMLS area and in area 21a were dominated respectively by contralateral and ipsilateral eyes. Finally, while 46% of PMLS neurons were excited by movement of visual noise per se, only 25% of area 21a neurons could be excited by such stimuli. We argue that the PMLS area, like its presumed primate homologue the middle-temporal (MT) area, is mainly involved in motion analysis. By contrast, area 21a appears to be involved in pattern analysis rather than motion analysis. It is likely that phylogenetically area 21a derives from the PMLS area.