The authors examined whether progressively training humans and rats to perform a difficult auditory identification task led to larger improvements than extensive training with highly similar sounds (the easy-to-hard effect). Practice improved humans' ability to distinguish sounds regardless of the training regimen. However, progressively trained subjects were more accurate and showed more generalization, despite significantly less training with the stimuli that were the most difficult to distinguish. Rats showed less capacity to improve with practice but still benefited from progressive training. These findings indicate that transitioning from an easier to a more difficult task during training can facilitate, and in some cases may be essential for, auditory perceptual learning. The results are not predicted by an explanation that assumes interaction of generalized excitation and inhibition but are consistent with a hierarchical account of perceptual learning in which the representational precision required to distinguish stimuli determines the mechanisms engaged during learning.

Objective Perceptual sensitivities are malleable via learning, even in adults. We trained adults to discriminate complex sounds (periodic, frequency-modulated sweep trains) using two different training procedures, and used psychoacoustic tests and evoked potential measures (the N1-P2 complex) to assess changes in both perceptual and neural sensitivities. Methods Training took place either on a single day, or daily across eight days, and involved discrimination of pairs of stimuli using a single-interval, forced-choice task. In some participants, training started with dissimilar pairs that became progressively more similar across sessions, whereas in others training was constant, involving only one, highly similar, stimulus pair. Results Participants were better able to discriminate the complex sounds after training, particularly after progressive training, and the evoked potentials elicited by some of the sounds increased in amplitude following training. Significant amplitude changes were restricted to the P2 peak. Conclusion Our findings indicate that changes in perceptual sensitivities parallel enhanced neural processing. Significance These results are consistent with the proposal that changes in perceptual abilities arise from the brain’s capacity to adaptively modify cortical representations of sensory stimuli, and that different training regimens can lead to differences in cortical sensitivities, even after relatively short periods of training.

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Training can improve perceptual sensitivities. We examined whether the temporal dynamics and incidental versus intentional nature of training are important. Within the context of a birdsong rate discrimination task, we examined whether the sequencing of pre-testing exposure to the stimuli mattered. Easy-to-hard (progressive) sequencing of stimuli during pre-exposure led to more accurate performance with the critical difficult contrast and greater generalization to new contrasts in the task, compared to equally variable training in either a random or anti-progressive order. This greater accuracy was also evident when participants experienced the progressively-sequenced stimuli in a different incidental learning task that did not involve direct auditory training. The results clearly show the importance of temporal dynamics (sequencing) in learning, and that the progressive training advantages cannot be fully explained by direct associations between stimulus features and the corresponding responses. The current findings are consistent with a hierarchical account of perceptual learning among other possibilities, but not with explanations that focus on stimulus variability.

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