RAMP EDGES, MACH BANDS, AND THE FUNCTIONAL-SIGNIFICANCE OF THE SIMPLECELL ASSEMBLY

The responses of ''complex'' simple cells to sharp and blurred ramp ed ges were studied. These responses are quite similar to those in the ca se of lines, which implies that phase information cannot be used to di scriminate between ramp edges and lines. Furthermore, if the maximum o f the modulus is used as a position estimate, a systematic bias toward the ramp side results, and this bias increases with edge blur. In con trast, a local extremum in the real part of the cell responses provide s a precise position estimate, even for strongly blurred edges. Possib le multiscale detection strategies are discussed in the context of a s yntactical visual reconstruction. This is illustrated by an explanatio n of Mach bands as perceived at trapezoidal edges, including Ratliff's Mach-band cancellation stimulus, and criteria for local probability s ummation in the prediction of Mach-band detection thresholds are prese nted.