Results showing how to assess an interaction using an event-related design. Subjects viewed stationary monochromatic stimuli that occasionally changed color and moved at the same time. These compound events were presented under two levels of attentional set (attention to color and attention to motion), The event-related responses are modeled, in an attention-specific fashion by the first four regressors (delta functions convolved with a hemodynamic response function and its derivative) in the design matrix on the right. The simple main effects of attention are modeled as similarly convolved boxcars. The interaction between attentional set and visually evoked responses is simply the difference in evoked responses under both levels of attention and is tested for with the appropriate contrast weights (upper right). Only the first 256 rows of the design matrix are shown. The most significant modulation of evoked responses, under attention to motion, was seen in left V5 (insert). The fitted responses and their standard errors are shown on the left as functions of peristimulus time.