Spatial properties of the visual detectability of moving spatial white noise

Summary

We determined the spatial parameters that describe the visual detection of spatio-temporal correlation in moving two-dimensional noise patterns. The target field (5.21×5.31 deg of visual angle) was divided into horizontal stripes of equal width D. Adjacent bars alternately contained noise patterns moving with velocity →v₁ and →v₂. We varied D, →v₁ and →v₂.

Roughly three different percepts occurred. If the stripes were very broad the different movements in alternate stripes were perceived together with the division of the field into stripes. If the stripes were very narrow the division into stripes was not seen, but the moving noise patterns with velocities →v₁ and →v₂ were perceived as transparent sheets moving through each other. For intermediate stripe widths the target field looked incoherent and the subject was not clear about the percept. In this region the subject found it difficult and sometimes impossible to discriminate these patterns from a completely uncorrelated spatiotemporal white noise pattern (snow).

To quantify the detectability, the patterns were masked with snow (spatio-temporal white noise). The r.m.s. contrast of the total stimulus was kept at a constant value, whereas the subject set the signal-to-noise ratio (SNR) to a threshold value. At certain barwidths the thresholds reached a maximum value. These critical barwidths depended on the velocities →v₁ and →v₂.

These critical barwidths were interpreted in terms of a simple general model for the detection of spatiotemporal correlation. In these terms the span of the elementary correlators rose monotonically with the velocity to which the correlator is most sensitive.