Abstract
Practice improves contrast discrimination by a factor of two if Gabor stimuli of various contrasts are presented in blocked trials (Yu, Klein & Levi, VSS03, JOV04). However, no significant learning is evident when stimuli are temporally randomly interleaved (Sagi et al, VSS03; Yu et al, VSS03, JOV04). Recently we studied the factors limiting the effective learning of contrast discrimination and found that, among many stimulus properties (stimulus duration, simultaneous vs. successive presentation, same vs. different retinal locations for different contrasts, etc.), the temporal pattern of stimulus contrasts is the key for contrast discrimination learning. When stimulus contrasts are interleaved temporally in an ascending ramp pattern, contrast discrimination can be improved by practice nearly as much as in the blocked trials condition. To determine whether contrast discrimination learning is due to the temporal stimulus pattern itself or due to the fact that the subject are aware of the stimulus contrast before each interleaved trial, we had subjects practice the random interleaving condition with additional exogenous cueing (an identical Gabor at the same contrast) or endogenous cueing (letters ABCD representing four contrasts levels) preceding each trial. No significant improvement of contrast discrimination is shown in either cueing condition. Evidently cueing does not produce memory trace useful for establishing a contrast template. Our surprising results eliminate the role of contrast uncertainty in contrast learning and point to a role of temporal patterning — but stay tuned.
A start-up fund from the Chinese Academy of Sciences and NEI grants R01EY01728 and R01EY04776