Abstract

Attention is thought to alter appearance by intensifying the sensory impression of the attended stimulus. Current debates are about how this increases might operate and what are the underlying neural mechanisms. Three mechanisms have been proposed to account the effect of attention: contrast gain, response gain and baseline shift. However, psychophysical and neuroimaging studies produced results that are not always consistent. We used the functional magnetic resonance imaging (fMRI) to explore how attention alters the appearance of coherently moving dots. The first experiment assessed coherence response functions for different levels of contrast. This experiment showed which kind of pattern should be expected with attention altering appearance by means of contrast gain mechanism. The results showed that contrast produced a leftward shift of the coherence response function. By our knowledge, this is the first fMRI evidence for the assumption that processing of motion coherence is susceptible to contrast gain. The second experiment assessed the effect of attention on coherence response function. The results showed that attention affects the BOLD response with the same strength for all coherence levels. This pattern of response is compatible with the predictions of the baseline shift model. Finally, in order to explain the discrepancy between psychophysical and neuroimaging results, we suggested a signal detection account for the effect of baseline shift on the perceptual level, proposing that baseline shift may produce a leftward shift of the psychometric function although sensitivity remains unchanged.