changes delay the perceptual availability of form information

In natural visual situations, unlike most psychophysical experiments, when a new stimulus appears in a portion of the visual field, the surrounding background changes simultaneously. In recordings from macaque V1 we found that a visual stimulus presented simultaneously with a background change evokes a response that is qualitatively different from the response to the same stimulus flashed on a static background. With the changing background, information about stimulus orientation and contrast is significantly delayed compared to the static background situation. Our physiological results make several predictions that we test in the present paper with human psychophysical experiments. In a backward masking paradigm, a bar stimulus was either flashed onto a static background or presented simultaneously with a change in background luminance or pattern. Subjects discriminated bar orientation or detected that the scene changed before the mask. To achieve an equivalent contrast threshold for orientation discrimination, a longer SOA was needed in the changing than in the static background condition; to match the orientation discrimination performance in the static and changing background conditions at a fixed SOA, a higher bar contrast was needed when the background changed. Moreover, in the changing background condition, a longer SOA was needed to discriminate bar orientation than to detect the scene change. These results suggest that orientation information is available more slowly when the background changes; orientation information is available earlier as stimulus contrast