Shape adaptation exaggerates shape differences

Adaptation to different visual properties can produce distinct patterns of perceptual aftereffect. Some, such as those following adaptation to color, seem to arise from recalibrative processes. These are associated with a reappraisal of which physical input constitutes a normative value in the environment- in this case, what appears "colorless," and what "colorful." Recalibrative aftereffects can arise from coding schemes in which inputs are referenced against malleable norm values. Other aftereffects seem to arise from contrastive processes. These exaggerate differences between the adaptor and other inputs without changing the adaptor's appearance. There has been conjecture over which process best describes adaptation-induced distortions of spatial vision, such as of apparent shape or facial identity. In 3 experiments, we determined whether recalibrative or contrastive processes underlie the shape aspect ratio aftereffect. We found that adapting to a moderately elongated shape compressed the appearance of narrower shapes and further elongated the appearance of more-elongated shapes (Experiment 1). Adaptation did not change the perceived aspect ratio of the adaptor itself (Experiment 2), and adapting to a circle induced similar bidirectional aftereffects on shapes narrower or wider than circular (Experiment 3). Results could not be explained by adaptation to retinotopically local edge orientation or single linear dimensions of shapes. We conclude that aspect ratio aftereffects are determined by contrastive processes that can exaggerate differences between successive inputs, inconsistent with a norm-referenced representation of aspect ratio. Adaptation might enhance the salience of novel stimuli rather than recalibrate one's sense of what constitutes a "normal" shape