The Neural Control of Vision
O. Illusions

Specification of the S1 simple-cell theory:

A. The receptive field locations of 4 OFF S1 cells selective for vertical line segments. Cross-hatched area shows receptive field center location. The elongated regions show area over which response summates. Cells 1 and 2 produce small discharge to the appearance of the dark squares thereby eliciting the perception of darkness at the location of their receptive field centers which are in the white regions of the figure. Cells 3 and 4 are unresponsive, thus not eliciting any sensation of darkness at those receptive field center sites. The same arrangement holds for OFF S1 cells selective to horizontal line segments.

B. The more the orientation selectivity of cells deviates from horizontal and vertical, the less the cells respond as depicted on the right for 24 cells whose orientation specificities are indicated by straight lines and their response magnitudes to the horizontal and vertical line segments by the contrast of the lines. As a consequence of this arrangment the aggregate response of S1 cells whose receptive field centers fall into the white areas is greater at the intersections than along the space between the dark squares. When contrast conditions are reversed, the same kind of activation takes place in ON S1 cells yielding smudges that appear lighter at the intersections as had been shown in Figure 2, slide A15-6.