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L. Olzak, S.A. Saylor, S.H. Gabree, P.I. Laurinen; Lateral Interactions on Discrimination Performance by Simple and Complex Surrounds . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4317.
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Purpose: In previous studies, we have shown that modulated surrounds of similar orientation and frequency disrupt the ability to make fine spatial discriminations. Here, we investigate the effect of complex surrounds on orientation and spatial frequency discriminations. Methods: The patterns to be discriminated were always vertical sinusoids of either 3 or 4 cpd, displayed in a sharp-edged 40-minute window. In control conditions, no surrounds were present. Surrounds were either simple vertical gratings, complex gratings formed by adding a 15 cpd grating to a 3 cpd component, or plaids formed by adding a horizontal component of the same frequency to the surround. Center and surround 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. Results: Performance was reduced in all instances when a mask was present and low-frequency vertical components were in phase, but in some observers, complex gratings showed less masking than simple sinusoids. No masking was found when components were out of phase. Conclusions: We conclude that lateral masking occurs among low-level analyzers tuned to spatial frequency and phase. However, higher level processes may also be involved. One interpretation of the reduced lateral masking effect found with complex gratings is a reduction in surround strength resulting from pooled gain control processes such as those described by Olzak & Thomas (Vis. Res, 39, 3983-3987), which operate over very different frequencies and/or orientations when components are superimposed. A second interpretation is that the differential textures found in center and surround lead to partial segregation by texture, which then weakens lateral influences.
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