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How do attention and adaptation affect contrast sensitivity?

Attention and adaptation are both mechanisms that optimize visual performance. Attention optimizes performance by increasing contrast sensitivity for and neural response to attended stimuli while decreasing them for unattended stimuli; adaptation optimizes performance by increasing contrast sensitivity for and neural response to changing stimuli while decreasing them for unchanging stimuli. We investigated whether and how the adaptation state and the attentional effect on contrast sensitivity interact. We measured contrast sensitivity with an orientation-discrimination task, in two adaptation conditions--adapt to 0% or 100% contrast--in focused, distributed, and withdrawn attentional conditions. We used threshold and asymptotic performance to index the magnitude of the attentional effect--enhancement or impairment in contrast sensitivity--before and after adapting to high-contrast stimuli. The results show that attention and adaptation affect the contrast psychometric function in a similar but opposite way: Attention increases stimulus salience, whereas adaptation reduces stimulus salience. An interesting finding is that the adaptation state does not modulate the magnitude of the attentional effect. This suggests that attention affects the normalized signal once the effect of contrast adaptation has taken place and that these two mechanisms act separately to change contrast sensitivity. Attention can overcome adaptation to restore contrast sensitivity.