VERNIER ACUITY WITH PLAID MASKS - THE ROLE OF ORIENTED FILTERS IN VERNIER ACUITY

Superimposition of oriented grating masks on vernier targets results i n bimodal patterns of vernier threshold elevation, with peaks occurrin g on either side of vernier target orientation. These bimodal masking effects suggest a contribution to vernier acuity from spatial filters toned to orientations on either side of the target. We report similar bimodal threshold elevation with plaid masks composed of symmetrically oriented pairs of gratings. Since filters oriented to either side of the vernier stimulus will be affected similarly by plaid masks, it is unlikely that threshold elevation reflects disruption of relative filt er activity that is used to code for change in target orientation. Ins tead, the results support the proposition that misalignments are detec ted on the basis of differential (i.e. absolute rather than relative) activity of spatial filters. Our plaid-mask data also rule out the pos sibility that: (i) ''off-channel'' looking; or (ii) detection of orien tation shifts (e.g. tilt illusions), underlie bimodal masking effects. The finding that weak bimodal threshold elevation occurs with dot tar gets separated by 40 min are further suggests that the mechanisms invo lved in detecting misalignments over large regions [possibly collator/ collector-type mechanisms] also do so via analysis of their differenti al activity. (C) 1997 Elsevier Science Ltd.