A neuronal subsystem in the cat's area 18 lacks retinotopy.

Single neurones were recorded extracellularly in the visual cortex of anaesthetized and paralyzed cats. In all parts of area 18, neurones with diffuse receptive fields (DRFs) were found. DRFs measured at least 50 degrees in diameter and always included the area centralis and large parts of the ipsi- and contralateral hemifeld; some DRFs even covered the entire visual field. DRF neurones gave phasic responses to diffuse light, and to light spots either presented stationary or moved rapidly across the field; they were not orientation- or direction-sensitive. In contrast to the majority of area 18 neurones, which are part of a retinotopically organized system, DRF neurones constitute a subsystem with only poor retinotopy, or no retinotopy at all. This system might be suitable for detecting sudden changes in global illumination.