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Stefanie Liebe, Jason M. Gold, Thomas A. Busey, Brian O'Donnell; Electrophysiological correlates of the effects of perceptual learning on signal and noise in the human visual system. Journal of Vision 2004;4(8):297. doi: 10.1167/4.8.297.
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Purpose: Performance in visual tasks often improves with practice. An external noise masking paradigm  has been used previously to measure the relative contributions of changes in signal strength and internal noise to these improvements in performance [2–4]. Here, we estimated the effects of practice on changes in internal noise and signal strength electrophysiologically by measuring contrast ‘thresholds’ derived from Visual Evoked Potential amplitudes under different external noise conditions before and after training. Methods: Steady-state visual evoked potentials (SSVEP's) were recorded from electrode site Oz before and after training while subjects discriminated the orientation of a foveally presented contrast-reversed Gabor pattern embedded in five different levels of external noise. An initial EEG recording was followed by a second recording after 4 subsequent days of behavioral training. Contrast thresholds at each noise level were derived as a function of VEP response at the temporal signal flicker frequency. Behavioral thresholds in the training sessions were measured through an adaptive staircase procedure. Results & Conclusions: Both electrophysiological and behavioral thresholds increased as a function of external noise contrast and decreased as a function of practice. Log behavioral thresholds decreased uniformly as a function of log external noise contrast with practice, indicating learning only served to increase internal signal strength at the level of behavior. However, log electrophysiological thresholds decreased only at low external noise levels, indicating learning served to reduce additive internal noise only. Possible reasons for this apparent contradiction will be discussed.
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