UNCERTAINTY EXPERIMENTS SUPPORT THE ROLES OF 2ND-ORDER MECHANISMS IN SPATIAL-FREQUENCY AND ORIENTATION DISCRIMINATIONS

Previous studies of spatial frequency and orientation discrimination [ Vision Res. 32, 1885 (1992)] suggest the existence of two second-order cortical mechanisms: one that mediates spatial frequency discriminati ons and sums signals across orientations and one that mediates orienta tion discriminations and sums signals across spatial frequency bands. The existence of each mechanism is tested in an uncertainty experiment in which the observer does not know which of two hypothetically poole d signals deviates from the standard but must judge whether the deviat ion is an increment or a decrement. No uncertainty effect is expected if the signals are completely pooled. Observed effects are compared wi th this expectation and with both theoretical and empirical estimates of the effects expected if the signals are processed separately. Resul ts support the existence of the first mechanism, but not its exclusive role in mediating spatial frequency judgments, and support the exclus ive role of the second mechanism in mediating orientation judgments. ( C) 1996 Optical Society of America