Summation of concentric orientation structure: seeing the Glass or the window?

Rotational Glass patterns are discriminable from noise at substantially lower signal-to-noise levels than translational patterns, a finding that has been attributed to the operation of concentrically tuned units in cortical area V4 (Wilson, Wilkinson, & Asaad, Vis. Res. 37 (17) (1997) 2325; Wilson & Wilkinson, Vis. Res. 38 (19) (1998) 2933). Under experimental conditions similar to Wilson et al. we found this advantage to be largely contingent on the pattern being viewed through a circular aperture. Because rotation of a random dot set cannot lead to the presence of unmatched dots at the boundary of a circular aperture, the integrity of low spatial frequency information at the boundary reliably indicates the presence of rotational, but not translational, structure. When we removed this cue, either using a square aperture or surrounding a round aperture with noise dots, none of the nine subjects tested showed any statistically significant advantage for rotational Glass patterns (although at least two did take longer to master the task with translational compared to rotational patterns). We go on to show generally similar patterns of global integration for both rotational and translational patterns. We conclude that this paradigm presently offers no concrete psychophysical evidence for specialised concentric orientation detectors.