Fig 3. An example of the event-related potential (ERP) subtraction method.

(A) Left column (in purple): schematic of a 61.66% contrast divided nontarget stimulus presented in the left hemifield (termed the stimulus of interest) and paired with target stimuli rendered in all different contrasts (down the rows). Right column (in black): the divided nontarget 0% contrast stimulus in the left hemifield paired with the same set of target stimuli in the right hemifield. (B) In this case, the ERP response evoked by the left divided nontarget stimulus of 0% contrast (A, right; B, top, black dotted traces) was subtracted from the ERP response evoked by the left divided nontarget stimulus of 61.66% contrast (A, left; B, top, dotted purple traces), resulting in the baseline-subtracted ERP response (B, bottom, solid purple traces). A similar subtraction was done to compute the ERPs associated with stimuli of interest rendered at all other contrasts. Note that the stimulus paired with the stimulus of interest (in this case, the right divided target stimulus) was equally likely to have any of the 6 contrast values. Therefore, this method amounts to subtracting out the mean response to all ipsilateral stimuli.