Abstract
The visibility of a target stimulus canbe affected by the presence of other stimuli on different parts of the visual fields. This lateral effect depends on many spatiotemporal properties, such as orientation or spatial frequency but is less affected by phase. The lack of phase effect suggests a possible involvement of phase-independent second order mechanisms. We investigated the role of second order information in lateral effects by studying lateral masking between patterns modulated in luminance or contrast. The luminance modulation stimuli (LM) were vertical 2 c/deg Gabor patches. The contrast modulation stimuli (CM) were vertical or horizontal 16 c/deg carrier waves whose contrast was modulated by a vertical 2 c/deg Gabor function. We measured contrast detection threshold for LM or contrast-contrast threshold for CM target with or without the presence of another stimulus (mask). The distance between the target and the mask varied from 0 to 4 degrees. We used a 2AFC paradigm and the PSI algorithm to measure threshold. When a target was superimposed on a mask, the detection threshold was elevated regardless the type of the target and the mask. The threshold elevation decreased as the mask moved away from the target. The lateral LM mask facilitated LM target detection with maximum facilitation occurred at a distance of three wavelength units but had little, if any, effect on CM target detection at any distance. The lateral CM mask facilitated CM target regardless the orientation of the carrier wave but had no effect on LM target detection. The lateral masking is specific to the type of stimuli. Our result suggests that the lateral effect for LM patterns is unlikely a result of feedback from the second order or complex mechanisms. The lack of phase effect may be due to rectification on the first order responses.
Supported by NSC 95-2413-H-002-001