The test stimulus, which was presented in the top half of the display (
Figure 2), contained a central circular patch of diameter 0.90°. The center patch was surrounded by a dark gap (inner/outer diameter of 0.90°/1.05°) and an annular surrounding field (inner/outer diameter of 1.05°/5.30°). The center and surround were presented to either the same eye (monocular condition) or opposite eyes (dichoptic condition). The fused percept was the same for both conditions (
Figure 2c). The dark circular gap was always present in both eyes. The L-, M-, and S-cone excitation values were calculated using the Smith and Pokorny (
1975) cone fundamentals. The center was modulated sinusoidally in chromaticity over time along the
l = L/(L + M) direction of MacLeod–Boynton chromaticity space (MacLeod & Boynton,
1979) at a Michelson contrast of 0.05 ([
lmax −
lmin]/[
lmax +
lmin]). The surround was also modulated sinusoidally in time at the same temporal frequency as the center and had a Michelson contrast of either 0.025 (peak and trough
l values of 0.684 and 0.650, respectively) or 0.05 (peak and trough
l values of 0.700 and 0.634, respectively). Center and surround frequencies always were identical and set to 3.13, 6.25, or 9.38 Hz. At the two lower frequencies, the relative phases between center and surround were −150°, −120°, −90°, −60°, −30°, 0°, 30°, 60°, 90°, 120°, 150°, and 180°. At 9.38 Hz, the relative phases tested were −135°, −90°, −45°, 0°, 45°, 90°, 135°, and 180°. Positive (negative) phase indicates that the surround's phase physically led (lagged) the center's phase.