Abstract
In 1972, Tolhurst showed a reduced contrast threshold elevation of the 3rd harmonic component of a square-wave grating when observers adapted to the square-wave, as compared to adaptation to the 3rd harmonic by itself. This result can be explained either as an automatic mutual interaction between components in an early stage of visual processing, or as a top-down modulation that suppresses higher order components to maintain an integrated percept of the square wave at the fundamental frequency. The current study tested between these two possibilities, by examining whether extrinsic, top-down, attention is required to facilitate the interaction between components during adaptation leading to a reduced adaptive effect on the 3rd harmonic. We measured the contrast elevation of the 3rd harmonic grating (6 cpd) after adapting to a complex grating consisting of the 1st (2 cpd) + 3rd harmonic, compared to adaptation to the 3rd harmonic alone. Attention was manipulated by two tasks: the “attended” task required the observer to detect the sudden disappearance of the adaptor and the “unattended task” required observers to detect a target letters in a foveal RSVP stream. Our results showed a weaker adapting ability of the complex grating on its 3rd harmonic component in both attended and unattended conditions. Diverting attention attenuated the adaptation effect, and no interaction was found between attention and adaptors: attention affected the adaptation effect of both the complex grating and the single grating equally. These results suggest that the interaction between harmonics that weakens adaptation is an automatic process, likely occurring in early stages of visual processing.