Following a contrast decrement, scene fading increased dramatically, mean slopes: 1.83%/s (before) and 8.32%/s (after),
t(12) = 6.22,
p = .00002, with the mean maximum fading (
M = 68.71%) exceeding that for an unchanging low-contrast scene (
M = 29.65%),
t(12) = 6.68,
p < .00001 (
Figure 7). In some cases, observers reported that the change induced the complete disappearance of all scene content, resulting in a display of uniform luminance and hue. Observers experienced greater than 90% fading on 36.92% of trials in the contrast decrement condition. None of the other conditions had more than 5% of trials with 90% or more fading. This effect must be attributed to the change from high to low contrast rather than to generally better fading of low-contrast scenes; prior to 10 s, the extent of fading did not differ significantly for the high-contrast (
M = 1.98%/s) and low-contrast (
M = 1.70%/s) trials,
t(12) = 1.32,
p = .205. Whereas contrast decrements induced fading, contrast increments slightly reduced fading, mean slope: 2.05%/s (before) and 2.01%/s (after), making the original scene slightly more visible,
t(12) = .112,
p = .912. Although the results of
Experiment 4 could be explained by contrast adaptation mechanisms (Blakemore & Campbell,
1969; Blakemore, Muncey, & Ridley,
1973), they are also consistent with the idea that a contrast decrement alone can induce fading.