Figure 4.

Topographic plots of the SNRs of the second-harmonic SSVEPs in response to the two surround gratings at high contrast (a, b) and medium contrast (c, d), averaged across observers. a, Frequency-tagged responses to the 16.67 Hz high-contrast surround grating in the left visual field. b, Frequency-tagged responses to 12.50 Hz high-contrast surround grating in the right visual field. c, d, Similar to a and b, but at medium contrast. SNR values smaller than 1 indicate that there was no signal at the driving frequencies. In each figure, the first three columns show the segmented stimulus configurations (0.25° gap, 0.1° gap, and out-of-phase) and the last column shows the unsegmented stimulus configuration (in-phase). The rows show responses when the relevant target was attended (upper), ignored (middle), and the difference between attended and ignored conditions (lower). At both high and medium contrast, the largest responses (SNR) and the largest attention effects occurred at contralateral posterior scalp locations.