Effects of covert attention on early vision
What mechanisms are involved in visual attention and where are they localized in the brain? I discuss how relating psychophysics to electrophysiology and neuroimaging has advanced our understanding of visual attention. Covert attention enhances performance on a variety of perceptual tasks carried out by early visual cortex. In this talk, I concentrate on the effect of attention on contrast sensitivity, and discuss evidence from psychophysical, electrophysiological, and neuroimaging studies indicating that attention increases contrast gain.
First, I illustrate how psychophysical studies allow us to probe the human visual system. Specifically, I discuss studies showing that attention enhances contrast sensitivity, and how these studies allow us to characterize the underlying mechanisms, namely external noise reduction and signal enhancement. Then, I relate these findings to single-unit recording studies, which show that attention can reduce external noise by diminishing the influence of unattended stimuli and that it can also boost the signal by increasing the effective stimulus contrast.
Current neuroimaging techniques can be used to link human performance to non-human primate electrophysiology by providing an intermediate level of analysis. Human electrophysiological studies have provided evidence that attention can increase sensory gain, and neuroimaging studies have shown attentional modulation of neural activity in early visual cortex. For instance, we have documented the effect of transient (exogenous) attention on stimulus representations in early visual areas using rapid event-related fMRI. Attention improves performance and the concomitant stimulus-evoked activity in early visual areas. These results provide evidence regarding the retinotopically-specific neural correlate for the effects of attention on early vision. By integrating psychophysical studies with fMRI, we can bridge the gap between single-unit physiology and human psychophysics, and advance our understanding of visual attention.