Abstract
The magnitude of sensory responses is enhanced when attention is directed to a relevant stimulus (i.e., sensory gain), and this gain modulation adequately accounts for attention-related behavioral improvements in simple two alternative-force-choice (2AFC) tasks (Itthipuripat et al., 2014a). However, other evidence suggests that sensory gain is not always sufficient to account for improved performance, and that attention may also influence post-sensory read-out mechanisms (Pestilli et al., 2011). Based on previous visual search studies (e.g., Palmer and Verghese, 2000), we reasoned that the number of competing items and alternative choices might be the key factor that determines how much sensory gain and efficient read-out contribute to behavioral performance. Fourteen subjects participated in 2AFC and 4AFC contrast discrimination tasks in which they reported the location that contained an incremental contrast within a target stimulus. This contrast increment target was embedded in an array of four competing stimuli flickering at different frequencies and contrasts. Subjects were cued either to the target-location (focused-attention) or all stimulus-locations (distributed-attention). Contrast discrimination thresholds were measured concurrently with electroencephalography (EEG). Steady-state visual evoked potentials (SSVEPs) were used as an index of sensory responses and late event-related potentials in occipital and frontal channels were used as indices of sustained attention and post-sensory decision processes, respectively. To our surprise, the amount of sensory gain measured by SSVEPs is comparable and sufficient to explain attention-related improvement in perceptual sensitivity in both 2 and 4AFC tasks. Similarly, sustained attention-related negative difference over occipital cortex is comparable across the two tasks. Finally, the pattern of late centrofrontal positive potentials suggests that the decision threshold in the 4AFC task is higher than that in the 2AFC task, consistent with the observed main effects of attention and task-type on decision times.
Meeting abstract presented at VSS 2015