The effects of flicker adaptation on spatial contrast thresholds



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The effect of adaptation to structured patterns has been extensively utilized in investigations of spatial vision, revealing mechanisms for the analysis of spatial frequency, orientation and motion. Adaptation to unstructured flickering fields has revealed mechanisms responsible for temporal analysis, but the effect of flicker adaptation on the detection of spatial patterns remains unclear. To investigate this issue psychophysical contrast thresholds were measured after adapting to an unmodulated uniform field or to a uniform field flickering at 8Hz. The stimuli were 8Hz counterphase gratings with spatial frequencies of 0.5, 0.75, 1.0 and 4.0 cycles/degree, and a multiple rating signal detection procedure was used. To provide converging evidence and to reveal possible neuronal mechanisms, electrophysiolgical contrast thresholds were measured, before and after flicker adaptation, using the steady state visual evoked potential with spatial frequencies of 0.5 and 4.0 cycles/degree. The results show, both psychophysically and electrop Iiysiologically, that flicker adaptation does elevate contrast threshold at low spatial frequencies, and conversely lowers thresholds at medium spatial frequencies. The findings lend support to the theory of interchannel interactions of the long range inhibitory type, and suggest that these interactions also occur between mechanisms signalling spatial and temporal properties of a pattern.



Evoked potentials (Electrophysiology), Threshold (Perception)