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Oliver Braddick, Jin Lee, Katie McKinnon, Isobel Neville, John Wattam-Bell, Janette Atkinson; Relative latency of visual evoked responses to reversals in contrast, orientation, and motion direction. Journal of Vision 2010;10(7):925. doi: 10.1167/10.7.925.
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Conventional VEP recording tests neural responses evoked by reversing pattern contrast. VEPs for orientation-reversal (OR) [Braddick et al (1986), Nature, 320: 617] and direction-reversal (DR) [Wattam-Bell (1991), Vision Res, 31: 287] use stimulus sequences designed to isolate cortical responses to these higher-order changes from responses to contrast change. Since these require more complex processing than contrast changes, we might expect some additional delay of the measured response, reflecting this additional processing.
We have tested this hypothesis by recording pattern-reversal (PR-), OR- and DR responses, at reversal rates up to 4 /sec, from occipital scalp electrodes on adult subjects, and assessing the mean latency of the first positive peak. OR- and DR- sequences isolate the effect of reversals from accompanying contrast changes, by embedding the reversal event within a sequence of equivalent contrast changes (‘jumps’). We use two methods to avoid our latency measures being contaminated by responses to jumps – filtering out harmonics in the signal related to the jump frequency, or subtracting a ‘jump-only’ section of the waveform from the response to reversal + jump.
We find very similar latencies for OR- and PR- responses, suggesting that responses to pattern reversal arise from a level of cortical processing which is already orientation-selective. The DR- response is more complex, but typically contains components with a latency 10-20 ms lower than either PR or OR - evidence against any time penalty associated with motion processing. We will discuss these results in relation to possible differences in the balance of magno- and parvocellular inputs to the three responses, and possible ‘fast’ routes for motion processing bypassing V1. Future work will test the overall temporal properties of the different responses, beyond the initial latency, and also the potential use of this comparison in analysing cortical processing in infancy.
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