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Lynn A. Olzak, Scott H. Gabree; Relative effects of superimposed and lateral masks in discrimination. Journal of Vision 2003;3(9):650. doi: 10.1167/3.9.650.
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We investigated the relative effects of superimposed and surround gratings on the ability to distinguish between two 4-cpd grating patterns that differed slightly in spatial frequency. Superimposed gratings were always orthogonal to the base components, and could act as masks or as second cues to discrimination. When two cues to discrimination were present, they could vary together (i.e., both low frequency in pattern A and high in patterns B), or could vary in opposition (low-vertical + horizontal-high vs. vertical-high + horizontal-low). Surround masks were either simple gratings or plaids, and were either in-phase or 180 deg out of phase with the center when surrounds contained gratings of the same orientation as the test. Test and mask contrasts were held constant at 0.1. Each condition was run in a separate block of 80 trials. Differences to be discriminated were adjusted individually for each observer to yield a d' of approximately 1.5 in no-surround control conditions and fixed for all conditions. A two-alternative signal-detection rating procedure measured how performance changed with condition. Replicating earlier findings (Thomas and Olzak (1999) Vis. Res, 39, 3983–3987), the addition of superimposed masks decreased discrimination performance. In two-cue conditions, performance depended upon the configuration and was superior when cues varied together. The addition of an in-phase sinusoidal surround reduced discrimination performance when the center was a simple sinusoid, but results were more complex when the center was a plaid. The effect of a plaid surround depended upon the center configuration. When the center was a simple sinusoid, the plaid showed less lateral masking than a sin of the same orientation. When the center was also a plaid, lateral masking was maximal. The results are discussed in the context of an extension to the Olzak & Thomas (1999) model.
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