Spatial properties and direction selectivity of single neurons in area 21bof the cat

The receptive field properties of single units were assessed in area 21b of the cat visual cortex. Visual cells in this area were binocular and showed relatively large receptive fields. Most cells were strongly sensitive to t he direction of drifting gratings. The mean value of the half-widths of the direction tuning curves (32 degrees) suggests broader direction tunings th an are typically found in other visual areas. The spatial frequency tuning functions were either band-pass or low-pass. Cells responded optimally to l ow spatial frequencies (mean = 0.08 c/deg) and also showed low spatial reso lution (mean = 0.29 c/deg.). The estimated values of spatial bandwidths (me an = 2.2 octaves) suggest that area 21b cells act as relatively good spatia l filters. Although some cells exhibited a low contrast threshold, most cel ls began to respond at intermediate or high contrast values (mean threshold = 15.5%). Temporal frequency tuning functions were mostly band-pass and us ually broad (mean temporal bandwidth = 3.3 octaves). Cells were found that responded optimally to various temporal frequencies (mean optimal temporal frequency = 3.2 Hz), although the majority preferred a temporal frequency b elow 4 Hz. These results suggest that visual properties (receptive fields sizes, spati al resolution and orientation/direction selectivity) of cells in area 21b d iffer from those of cells previously observed in the adjoining area 21a. Th ese differences provide evidence in support of functional distinction betwe en these two visual areas. (C) 2000 IBRO. Published by Elsevier Science Ltd .