Transient sensitization by a contrast flash

One line decreases the visibility of a simultaneous identical distant second line

A top line decreased the visibility of a simultaneous, identical, distant bottom line. This context-produced decrease in visibility (DV) occurred when the bottom line was masked by flanking lines and hence was less visible than the top line. It continued when the top line was three times as far from the bottom line. It disappeared without the mask. There was a hint of an opposing context-produced increase in visibility (IV) when the lines were close together. The DV is not accounted for by numerous extant phenomena and theories. It means that the top line decreased the similarity in visibility between it and the bottom line, a contrast effect for visibility, rather than for a typical attribute. Contrast does not occur between two attributes that are perceptually equal. Therefore, the reason why two distant equally visible objects fail to result in a DV may be that their equal visibility precludes the occurrence of contrast. This DV-as-contrast theory is consistent with evidence that two groups (phenomenal wholes) are associated with both contrast and DVs, and thus also with evidence that one group is associated with both assimilation and IVs.

Foveal inhibition and facilitation caused by remote grating jerks: interaction between long-range and short-range effects

Periodic oscillation of a luminance grating imaged upon the peripheral retina reduces the threshold visibility of a foveally presented test spot. This new effect has been named the "jerk effect". The present investigation is concerned with the effect of a single jerk of the remote grating on threshold sensitivity. Foveal sensitivity changes were measured for different delays between grating jerk and test spot presentation. For 0.38 degrees, 100 ms test spot, long-range transient inhibition was found for all delays, with a maximal effect between 0 and 30 ms delay. By combining the jerk effect with the Westheimer paradigm, both facilitatory and inhibitory long-range effects could be demonstrated. For facilitation to occur, it was necessary that the steady background extended into the sensitization zone of the Westheimer area. Inhibition was the only result for smaller backgrounds. Reduced visibility is consistent with the hypothesis that peripheral transient mechanisms inhibit foveal sustained mechanisms. Enhanced visibility indicates that thresholds depend on an interaction between foveal-sustained and foveal-transient units. Transient peripheral stimulation and steady backgrounds of increasing diameter change the balance of inhibitory and facilitatory processes between these units.

Changes of visual sensitivity caused by on- and off-transients

The effects of abrupt increases and decreases of light in one part of the retina on sensitivity in an adjacent part of the retina were studied under low scotopic conditions near absolute threshold. The basic finding is that a brief annular surround decreases sensitivity to test spots occurring near the time of its onset or its offset, but that the surround increases sensitivity to test spots occurring the first few hundred milliseconds following its offset. If the annulus is presented in one eye and the test flash to the other, annulus onset reduces sensitivity, but annulus offset has almost no effect on thresholds in the contralateral eye. In fully dark-adapted observers, absolute sensitivity is somewhat greater when the test flashes are preceded by a flashed surround than when they are presented alone. These results challenge the idea that sensitivity to these stimuli is limited by an irreducible level of noise in the dark and under dim illumination.

Two-pulse discrimination and rapid light adaptation: complex effects on temporal resolution and a new visual temporal illusion

Temporal resolution during rapid light adaptation was studied using a suprathreshold two-pulse discrimination paradigm with stimulus conditions modeled after the classic increment threshold experiments of B. H. Crawford. The two-pulse stimulus (1 degree diam, two 20-ms pulses separated by 40 ms) was presented before, during, and after a 6 degree diam, 500 ms background conditioning field, and discriminability was measured as d' using signal detection methods. Discriminability is depressed at background field onset and offset, but increases during the exposure time of the background. Immediately following background offset, two-pulse discriminability is dramatically enhanced: two pulses are discriminated from one pulse with a d' of 4 or more. When the two-pulse stimulus follows background offset by 80 to 250 ms, a novel temporal illusion occurs: a single pulse consistently appears as a double pulse. Several hypotheses for understanding these results are discussed.