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Abstract:
Visual contrast adaptation decreases contrast sensitivity in visual detection. It has been suggested that decreased sensitivity is due to decreased signal output from visual elements tuned to the adapting stimulus properties, like spatial frequency. Alternatively, adaptation may decrease inhibition of the stimulus-analysing channel on other channels, or both phenomena may take place at the same time. Thus, adaptation may reduce the signal-to-noise ratio (SNR) of the tested visual capacity by affecting stimulus strength, noise, or both. We attempted to examine the origin of the experimentally observed reduction in the SNR of the contrast-detection mechanism. We used an external noise masking paradigm [Pelli, 1990, in Vision: Coding and Efficiency Ed. C Blakemore (Cambridge, Cambridge University Press) pp 3 - 24] to measure the relative contribution of SNR under various experimental conditions. Visual-detection contrast thresholds were measured twice at three noise levels through an adaptive staircase procedure. In one of two experimental blocks the observers were presented with an adapting stimulus for 25 s before the actual test stimulus (or a blank/noise field) appeared. As expected, detection thresholds increased linearly as a function of external noise contrast. Logarithm of thresholds increased uniformly as a function of adaptation irrespective of the logarithm of external noise contrast. The latter observation indicates that adaptation decreases SNR without affecting the noise of the detection mechanism.
