It has been debated if attention can penetrate early perceptual representations to alter visual appearance or it simply induces response biases. Here, we tested these alternative accounts by evaluating attentional modulations of EEG responses recorded from human subjects while they compared the perceived contrasts of cued and uncued visual stimuli of varying physical contrasts. We found that attention enhanced the response gain of neural contrast response functions (CRFs) computed based on the amplitude of the P1 component, an early visually evoked potential. Quantitative models suggested that the response gain of the P1-based CRFs could account for attention-induced changes in perceived contrast. Instead, attentional cues induced changes in the baseline offset of the CRFs based on 9-12Hz alpha-band oscillations and these baseline-offset changes better accounted for cue-induced response biases. Together, these results suggest that different neural mechanisms underlie the effects of attention on perceptual experience and on response biases.
Perceptual difficulty is sometimes used to manipulate selective attention. However, these two factors are logically distinct. Selective attention is defined by priority given to specific stimuli based on their behavioral relevance, whereas perceptual difficulty is often determined by perceptual demands required to discriminate relevant stimuli. That said, both perceptual difficulty and selective attention are thought to modulate the gain of neural responses in early sensory areas. Previous studies found that selectively attending to a stimulus or increasing perceptual difficulty enhanced the gain of neurons in visual cortex. However, some other studies suggest that perceptual difficulty can have either a null or even reversed effect on gain modulations in visual cortex. According to Yerkes-Dodson's Law, it is possible that this discrepancy arises due to an interaction between perceptual difficulty and attentional gain modulations yielding a non-linear inverted-U function. Here, we used EEG to measure modulations in the visual cortex of male and female human participants performing an attention-cueing task where we systematically manipulated perceptual difficulty across blocks of trials. The behavioral and neural data implicate a non-linear inverted-U relationship between selective attention and perceptual difficulty: a focused-attention cue led to larger response gain in both neural and behavioral data at intermediate difficulty levels compared to when the task was more or less difficult. Moreover, difficulty-related changes in attentional gain positively correlated with those predicted by quantitative modeling of the behavioral data. These findings suggest that perceptual difficulty mediates attention-related changes in perceptual performance via selective neural modulations in human visual cortex.SIGNIFICANCE STATEMENT:Both perceptual difficulty and selective attention are thought to influence perceptual performance by modulating response gain in early sensory areas. That said, less is known about how selective attention interacts with perceptual difficulty. Here, we measured neural gain modulations in the visual cortex of human participants performing an attention-cueing task where perceptual difficulty was systematically manipulated. Consistent with Yerkes-Dodson's Law, our behavioral and neural data implicate a non-linear inverted-U relationship between selective attention and perceptual difficulty. These results suggest that perceptual difficulty mediates attention-related changes in perceptual performance via selective neural modulations in visual cortex, extending our understanding of the attentional operation under different levels of perceptual demands.
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