Receptive field and orientation scatter studied by tetrode recordings in cat area 17

The receptive-field positions and orientation preferences of neurons occupy ing the same tangential location in visual cortex are thought to be similar but to have an associated random scatter. However, previous estimates of t his scatter may have been inflated by the use of subjective plotting method s, sequential recording of single units, and residual eye movements. Here w e report measurements of receptive-field position and orientation scatter i n cat area 17 made with tetrodes, which were able to simultaneously isolate and record up to 11 nearby neurons (ensembles). We studied 355 units at 72 sites with moving light and dark bars. Receptive-field sizes and positions were estimated by least-squares fitting of Gaussians to response profiles. We found that receptive-field position scatter was about half of the ensem ble average receptive-field size. We confirmed previous estimates of orient ation scatter, but calculations suggested that much of it may be accounted for by anatomical scatter in the positions of recorded neurons relative to the tetrode in a smooth map. Orientation tuning width was positively correl ated with the degree of orientation scatter. Scatter was not independent in the two eyes: deviations from the local mean for both preferred orientatio n and receptive-field position were correlated although a significant amoun t of residual inter-ocular orientation and receptive-field position scatter was present. We conclude that cortical maps of orientation and receptive-f ield position are more ordered than was previously thought, and that random scatter in receptive-field positions makes a relatively small contribution to cortical point image size.