'Coarse-to-fine' cyclopean processing

Previously (Popple et al, 1998 Vision Research 38 319-326) we found, using random-dot stereograms, that initial vergence increases with the size of a cyclopean disc. A corresponding improvement in stereoacuity within the disc was predicted, because disparities in the disc would be brought closer to the plane of current fixation. In the present experiment, we looked at the effect of the spatial extent of a briefly presented (less than or equal to 500 ms) cyclopean depth pedesta l on stereoacuity thresholds. Observers were required to judge the depth of a small, 1.7 deg, central disc relative to a larger surrounding disc in a random-pattern stereogram. The larger disc was set, initially, at a pedesta l disparity of +/-24 min of are against a fixation-plane surround. The size of the larger disc was varied from 2.6 to 8.0 deg. As predicted, stereoacu ity thresholds fell significantly with increasing pedestal disc size. Next, the disparity of the pedestal disc was varied. When pedestal disparity was reduced to +/-2.4 min of are, a disparity too small to demand vergence, th e size effect disappeared except when the pedestal boundary was within 30 m in of are of the test disc boundary. We argue from this result that the eff ect was largely due to vergence and not cyclopean integration alone. Howeve r, the effect of pedestal size was found to persist with stimuli too brief to permit vergence (less than or equal to 100 ms) suggesting that factors o ther than vergence may also play a role.