Spatial phase sensitivity of mechanisms mediating discrimination of small orientation differences.

Experiments with spatially superimposed gratings have defined mechanisms that sum signals across spatial frequency bands while mediating discriminations of small differences in orientation. In the present experiments, localized stimuli (Gaussian bars and derivatives of Gaussian bars) occupying different spatial frequency bands were superimposed in different phase and location relationships to assess the sensitivity of the summing mechanisms to these differences. Masking (loss of accuracy when a second component is added as a mask) was unaffected by differences in either phase or location (15 min separation). Summation (increase in accuracy when both components vary together) occurs for all phase relationships but is reduced by spatial separation.