Gradients as visual primitives

Following J. J. Gibson (1950), it is implicitly assumed in the literature that texture gradients are directly available as perceptual primitives. Yet, the depth response to compression gradients is poor compared with gradients of linear perspective. This may indicate that mechanisms for directly detecting the differential structure that constitutes a compression gradient do not exist. We tested this hypothesis outside the context of depth perception by measuring the speed with which participants could detect a gradient anomaly as a function of the number of elements in the gradient. Only in the case of linear perspective did anomalies "pop out." This was attributable to the emergent feature of alignment of the ends of the elements forming the gradient rather than the direct detection of its differential structure. It is argued that gradients are not perceptual primitives and that the poor depth response to compression in a variety of context (motion parallax, stereo, and perspective) therefore is not surprising.