J. A. Movshon

Latency of inhibition from inside and outside the classical receptive field in macaque V1 neurons

The response of a neuron to an optimal stimulus in its classical receptive field (CRF) can be modulated by two well-known effects: surround suppression and cross-orientation inhibition. Some researchers have proposed that a common mechanism underlies these suppressive effects (Heeger, 1992) while others have suggested separate mechanisms (Sengpiel et al, 1998). We used a novel stimulus to probe the temporal dynamics of both phenomena and found that on average surround suppression occurs substantially later than the suppression caused by cross-orientation masking.

The role of horizontal intracortical connections in "long range" spatial interactions

Previous studies have used a conservative definition of CRF size, the minimum response field (MRF). But MRF measurement misses parts of the CRF that are too insensitive to generate spikes when stimulated alone. We have measured the size of the CRF in macaque V1 neurons using a grating summation technique. On average the MRF underestimates the area of the CRF by a factor of 4 at high contrast. At low contrast, the suppressive surround is weakened and the area of summation increases by an additional factor of 6. Using published visuotopic maps, we projected our measured CRFs onto the cortical surface, and found that the majority have radii that correspond to horizontal cortical distances of 2-6 mm.