Amount of surround and test-inducing separation in simultaneous brightness contrast

Simultaneous brightness contrast is investigated in the fovea as a function of (1) amount of surround of the inducing field (Experiment 1) and (2) separation between the test and inducing fields (Experiment 2). Circular test and match fields subtending 14 min (radius) are used throughout. The inducing field, held constant in area, is a circular annulus (615 sq min) varying from a quadrant on one side of the test circle to an annulus completely surrounding the test circle. Test-field apparent brightness is not significantly affected by amount of inducing-field surround when the separation between centers of the test and inducing fields is held constant (Experiment 1). Experiment 2, though, shows that apparent brightness increases significantly as the separation between the centers of the test and inducing fields is increased.