Abstract
Purpose: :
High contrast Gabor patches (GPs) improve the detection of low-contrast GPs with collinear orientation (perceptual facilitation). Our recent findings provided neurophysiological evidence for these interactions between neurons in the visual cortex (Sterkin et al., 2008, Vision Research 48:1641-7). We also reported evidence for the abolishment of these interactions by temporal masking (backward masking, BM; Sterkin et al., 2009, Vision Research 49:1784-94). We have earlier suggested that the suppression induced by BM eliminates the facilitation (Polat & Sagi 2006, Vision Research 46:953-60). Moreover, there is evidence for accelerated processing speed and a subsequent improvement in visual functions following the training in subjects with presbyopia (Polat 2009, Vision Research 49:2566-73).
Methods: :
We applied perceptual learning that accelerates the processing speed of neurons in order to study whether reinforced facilitation can overcome the suppression induced by BM, in young subjects (ages 18-35). Event-Related Potentials (ERPs) were recorded before and after the training in 10 overnight sessions. Low-contrast, foveal target GP was simultaneously flanked by two collinear high-contrast GPs. In the BM, another identical mask was presented at different time-intervals (ISIs).
Results: :
Before training, BM induced suppression of target detection, at the ISI of 50 ms, in agreement with earlier behavioral findings. This ISI coincides with the active time-window of facilitation. After training, our results show a remarkable improvement in all behavioral measurements, including percent correct, sensitivity (d'), reaction time and the decision criterion for this ISI. The ERP results show that before training BM canceled the physiological markers of facilitation at the same ISI of 50 ms, measured as the amplitude of the negative N1 ERP peak (latency of 260 ms). After the training, the sensory representation, reflected by P1 peak, has not changed, consistent with the unchanged physical parameters of the stimulus. Instead, the shorter latency (by 20 ms, 240 ms) and the increased amplitude of N1 represent the development of facilitation between the target and the collinear flankers.
Conclusions: :
Thus, previously suppressive temporal masking became ineffective in disrupting the facilitation. We suggest that perceptual training of spatial facilitation in a temporally demanding paradigm results in a faster processing speed.
Keywords: perception • plasticity • visual cortex