AN EVENT RELATED OPTICAL SIGNAL (EROS) STUDY OF LEFT FRONTAL CORTEX ACTIVITY ELICTED BY TASK IRRELEVANT STIMULUS REPETITION

The event related optical signal (EROS) is a measure of the phase changes that occur in frequency modulated near-infrared light as it passes through activated brain tissue. It has a temporal resolution of tens of milliseconds and a spatial resolution of a few millimetres. We used EROS to examine changes in left frontal cortex activity elicited by task-irrelevant stimulus repetition. Participants saw pictures of everyday objects (e.g., a hat), presented one at a time, and indicated whether the real-world equivalent was longer than one meter. Half of the stimuli were presented once and half were repeated. For the repeated stimuli, the repetition was immediate for half the pictures and followed a lag of 3 – 6 pictures for the other half. Compared to first presentations, stimulus repetitions elicited reduced left frontal activity from 350 to 400 ms after stimulus onset. These results are consistent with earlier ERP and fMRI studies of repetition suppression and provide the first EROS imaging data demonstrating the repetition suppression phenomenon.