The effect of perceptual contour orientation uncertainty on the tilt aftereffect.

The tilt aftereffect (TAE) occurs when, after adapting to an oriented line, a vertical line appears to be tilted in the opposite direction. The magnitude of the TAE has been shown to relate to the salience of the adapting stimulus (e.g., its contrast) as well as to the similarity between the adapting and testing stimuli. However, the relationship between TAE and orientation uncertainty – variability in the perceived orientation of the stimulus – of either the adapting or the testing stimulus and, more importantly, change in orientation uncertainty as a function of adaptation have not previously been explored. We manipulated stimulus salience by using a variety of contour types, including real and illusory contours. Tilt aftereffects were observed even for stimuli that had much weaker or invisible illusory contours. Orientation uncertainty of the adapting stimulus, as measured by the slope of a psychometric function in orientation discrimination, was positively correlated with TAE magnitude for real and illusory contours, but not for stimuli with weak contour percepts. On an individual subject level, orientation uncertainty increased post-adaptation and was correlated with pre-adaptation uncertainty. That is, individuals with more variability in their perception of orientation before adaptation showed increased variability in orientation discrimination following adaptation. This may account for some of the variability in TAE across individuals and stimulus types and is consistent with previous findings on increased orientation discrimination thresholds post-adaptation for nearby orientations.