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Sven P. Heinrich, Kalanit Grill-Spector; Temporal dynamics of object-repetition effects in the human visual cortex. Journal of Vision 2005;5(8):854. doi: https://doi.org/10.1167/5.8.854.
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© ARVO (1962-2015); The Authors (2016-present)
Repetition of the same object typically yields a reduction of the fMR signal (adaptation) over object-selective cortex. EEG studies report repetition effects, but results vary from decreased cortical activity starting at around 200 ms (or later) to increased activity around 170 ms. Since fMRI does not yield information about the time course and the EEG effects are variable it is difficult to judge whether these repetition-effects are driven by bottom-up or top-down processing. Here we measured EEG and fMRI activation in the same subjects and for the same stimuli consisting of repeated and non-repeated animals.
Six subjects participated in 61-channel EEG experiments. From two of these subjects, we obtained fMRI data. Subject viewed repeated images of the same dog (18 repetitions), the same bird (18 repetitions), or non-repeated images of dogs and birds. Images were presented in an event-related design for 150 ms followed by a blank interval of 1850 ms. Conditions appeared in pseudo-randomized order and were intermixed with scrambled images and fixation trials. Subjects participated in 10 blocks of 108 trials and were asked to categorize images to “bird” or “dog” or “other” while fixating. In each block, a different set of images was used.
All subjects showed an occipito-temporal increase for repeated images peaking around 150 ms. This is earlier than previously reported. In later time intervals, there was a high inter-subject variability. Consistent with earlier studies, but in contrast to the EEG, fMRI activations in object-selective cortex were reduced for repeated images. The difference between EEG and fMRI may have at least two sources: The source of the EEG signal is different from the loci of fMRI activation or the early increase seen in the EEG is lost due to temporal integration in fMRI. Based on the EEG time course we conclude that at least some repetition effects occur during bottom-up processing.
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