Abstract
Adaptation techniques provide a powerful means of characterizing underlying neural mechanisms of vision. Here we used a “double-pulse” paradigm to examine adaptation of the M170 response, a “face-selective” MEG component. Using this paradigm, Jeffreys (1996) showed reduction in the amplitude of the vertex positive potential (VPP), a face-selective component recorded with ERP, for faces preceded by other faces, but not for faces preceded by nonface stimuli. We replicated and extended this finding by varying the stimulus-onset asynchrony (SOA) between the first (S1) and second (S2) stimulus between 300 and 800 ms. Shortening the SOA dramatically reduced the amplitude of the M170 response to S2. There was also a significant effect of S1 stimulus category: the M170 response to S2 was weaker when S1 was a face than when S1 was a house. Thus, the adaptation of the M170 depends on the category of the S1 stimulus. However, while this differential effect of S1 could be due to stimulus selectivity of the adaptation, it could instead simply reflect the amplitude of the response to the S1 stimulus, with higher S1 amplitude producing greater adaptation of the S2 response. To test these hypotheses, we reduced the amplitude of the S1 face response by adding white noise. If double-pulse adaptation depends on the amplitude of the adapting stimulus, we would expect to see a decrease in adaptation with decreasing amplitude of the S1 response. Also, when the amplitudes of the S1 face and house are equated, the magnitude of S2 response attenuation should be equal. Instead, we found that while, as expected, S1 amplitude decreased with increasing noise, S2 adaptation remained roughly constant across noise levels. Direct comparison of conditions with equal S1 face and house amplitudes also showed greater S2 adaptation when S1 was a face. We conclude that double-pulse adaptation of the M170 response is indeed face-selective and not due to the amplitude of the S1 response.