Vernier acuity with plaid masks: the role of oriented filters in vernier acuity

Superimposition of oriented grating masks on vernier targets results in bimodal patterns of vernier threshold elevation, with peaks occurring on either side of vernier target orientation. These bimodal masking effects suggest a contribution to vernier acuity from spatial filters tuned 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 filter activity that is used to code for change in target orientation. Instead, the results support the proposition that misalignments are detected on the basis of differential (i.e. absolute rather than relative) activity of spatial filters. Our plaid-mask data also rule out the possibility that: (i) "off-channel" looking; or (ii) detection of orientation shifts (e.g. tilt illusions), underlie bimodal masking effects. The finding that weak bimodal threshold elevation occurs with dot targets separated by 40 min arc further suggests that the mechanisms involved in detecting misalignments over large regions [possibly collator/collector-type mechanisms] also do so via analysis of their differential activity.