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Oliver Braddick, Thomas Bloomfield, Oliver Wright, Janette Atkinson, John Wattam-Bell, Jin Lee; Temporal integration and interaction in the mechanisms underlying orientation- and direction-specific VEP. Journal of Vision 2011;11(11):763. doi: https://doi.org/10.1167/11.11.763.
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Cortical responses arising from orientation- and direction-selective neurons can be isolated using stimuli in which orientation reversals (OR) or direction reversals (DR) are embedded in a sequence of ‘jitter’ events (Braddick et al., Nature, 1986; Wattam-Bell, Vision Res., 1991), providing a tool to study cortical function in normal and atypical development.
We examined how the OR and DR responses are affected by the frequency of these jitter events. OR or DR events were presented at 3.125 or 6.25 reversals/sec, with the interleaved jitter events at 12.5, 25, 50 or 100 jitter/sec (100 Hz frame rate). For OR, the orientation-specific signal/noise ratio (snr) declined steadily with jitter frequency. snr was a function of absolute jitter frequency, rather than the ratio of jitter to reversal rate. DR showed a quite different pattern of results: for 3 direction reversal/sec, snr was largely independent of jitter rate, while for 6 reversal/sec, snr increased with increasing jitter frequency.
We conclude that the two forms of cortical selectivity show quite different temporal properties: (1) OR responses depend on integrating the contrast pattern providing orientation information over a longer period than the equivalent for DR; (2) OR shows no evidence of interaction between the jitter events and orientation reversals; (3) in contrast, DR shows a strong interaction between the timing of reversal and jitter events; but (4) DR requires only the minimum two frames required to define directional motion and is not enhanced by integrating information from more than 2 frames. As well as characterising cortical dynamics, these results help to select the optimal timing parameters for OR- and DR-VEP when investigating cortical selectivity in infants and adults, including the effects of prematurity and early brain damage.
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