Table 2.

Mean values (±SEM in gray scale units) obtained for the indicated tasks by adjusting the diamond on dark surround to match the diamond on the light surround (left column), compared to adjusting the diamond on the light surround to match the diamond on the dark surround (right column)

IMAGE	Mean adjustment of diamond on dark surround	Mean adjustment of diamond on light surround
	61.3  ± 5.9	54.9  ± 4.0
	43.4  ± 5.9	42.8  ± 4.7
	52.6  ± 6.9	49.4  ± 5.2
	12.6  ± 4.5	8.3  ± 4.1
	19.2  ± 3.3	15.4  ± 3.2
	12.0  ± 2.6	8.8  ± 2.5
	65.5  ± 3.0	59.8  ± 4.9
	75.9  ± 1.4	73.8  ± 4.2
	82.1  ± 1.9	81.2  ± 3.7
	60.8  ± 3.2	66.4  ± 7.2
	27.1  ± 4.2	26.3  ± 5.8
	52.7  ± 6.2	47.3  ± 5.5
	32.0  ± 6.1	36.9  ± 4.7

The results of these two procedures were in all cases similar. In control trials between these tasks, subjects were asked to adjust the diamonds in each of the scenes in the absence of any context. The result (not shown) indicated that subjects were able to do this with great accuracy (±3 gray scale units) and that their performance was consistent over the testing period.