Shift in excitation-inhibition balance underlies perceptual learning of temporal discrimination.

Temporal perceptual learning (TPL) constitutes a unique and profound demonstration of neural plasticity within the brain. Our understanding for the neurometabolic changes associated with TPL on the other hand has been limited in part by the use of traditional fMRI approaches. Since plasticity in the visual cortex has been shown to underlie perceptual learning of visual information, we tested the hypothesis that TPL of an auditory interval involves a similar change in plasticity of the auditory pathway and if so, whether these changes take place in a lower-order sensory-specific brain area such as the primary auditory cortex (A1), or a higher-order modality-independent brain area such as the inferior parietal cortex (IPC). This distinction will inform us of the mechanisms underlying perceptual learning as well as the locus of change as it relates to TPL. In the present study, we took advantage of a new technique: proton magnetic resonance spectroscopy (MRS) in combination with psychophysical measures to provide the first evidence of changes in neurometabolic processing following 5 days of temporal discrimination training. We measured the (E)xcitation-to-(I)nhibition ratio as an index of learning in the right IPC and left A1 while participants learned an auditory two-tone discrimination task. During the first day of training, we found a significant task-related increase in functional E/I ratio within the IPC. While the A1 exhibited the opposite pattern of neurochemical activity, this relationship did not reach statistical significance. After timing performance has reached a plateau, there were no further changes to functional E/I. These findings support the hypothesis that improvements in temporal discrimination relies on neuroplastic changes in the IPC, but it is possible that both areas work synergistically to acquire a temporal interval.

Effects of stimulus and task structure on temporal perceptual learning.

Our ability to discriminate temporal intervals can be improved with practice. This learning is generally thought to reflect an enhancement in the representation of a trained interval, which leads to interval-specific improvements in temporal discrimination. In the present study, we asked whether temporal learning is further constrained by context-specific factors dictated through the trained stimulus and task structure. Two groups of participants were trained using a single-interval auditory discrimination task over 5 days. Training intervals were either one of eight predetermined values (FI group), or random from trial to trial (RI group). Before and after the training period, we measured discrimination performance using an untrained two-interval temporal comparison task. Our results revealed a selective improvement in the FI group, but not the RI group. However, this learning did not generalize between the trained and untrained tasks. These results highlight the sensitivity of TPL to stimulus and task structure, suggesting that mechanisms of temporal learning rely on processes beyond changes in interval representation.