Individual sequence representations in the medial temporal lobe.

Much of what we need to remember consists of sequences of stimuli, experiences, or events. Repeated presentation of a specific sequence establishes a more stable long-term memory, as shown by increased recall accuracy over successive trials of an STM task. Here we used fMRI to study the neural mechanisms that underlie sequence learning in the auditory-verbal domain. Specifically, we track the emergence of neural representations of sequences over the course of learning using multivariate pattern analysis. For this purpose, we use a serial recall task, in which participants have to recall overlapping sequences of letter names, with some of those sequences being repeated and hence learned over the course of the experiment. We show that voxels in the hippocampus come to encode the identity of specific repeated sequences although the letter names were common to all sequences in the experiment. These changes could have not been caused by changes in overall level of activity or to fMRI signal-to-noise ratios. Hence, the present results go beyond conventional univariate fMRI methods in showing a critical contribution of medial-temporal lobe memory systems to establishing long-term representations of verbal sequences.

Individual differences in premotor and motor recruitment during speech perception.

Although activity in premotor and motor cortices is commonly observed in neuroimaging studies of spoken language processing, the degree to which this activity is an obligatory part of everyday speech comprehension remains unclear. We hypothesised that rather than being a unitary phenomenon, the neural response to speech perception in motor regions would differ across listeners as a function of individual cognitive ability. To examine this possibility, we used functional magnetic resonance imaging (fMRI) to investigate the neural processes supporting speech perception by comparing active listening to pseudowords with matched tasks that involved reading aloud or repetition, all compared to acoustically matched control stimuli and matched baseline tasks. At a whole-brain level there was no evidence for recruitment of regions in premotor or motor cortex during speech perception. A focused region of interest analysis similarly failed to identify significant effects, although a subset of regions approached significance, with notable variability across participants. We then used performance on a battery of behavioural tests that assessed meta-phonological and verbal short-term memory abilities to investigate the reasons for this variability, and found that individual differences in particular in low phonotactic probability pseudoword repetition predicted participants' neural activation within regions in premotor and motor cortices during speech perception. We conclude that normal listeners vary in the degree to which they recruit premotor and motor cortex as a function of short-term memory ability. This is consistent with a resource-allocation approach in which recruitment of the dorsal speech processing pathway depends on both individual abilities and specific task demands.

Neural mechanisms underlying the grouping effect in short-term memory.

Dividing auditory sequence into groups, or imposing rhythmic, tonal, or spatial structure during presentation, improves recall performance. Several competing computational models have been proposed to account for these effects, but little is known about the neural correlates of grouping and hence the representations that encode grouped sequences. The present study used functional magnetic resonance imaging (fMRI) to compare the auditory encoding of grouped and ungrouped lists of sub-span (six letters) and supra-span (nine letters) length in an immediate serial recall (ISR) task. Analysis of activation revealed an extensive premotor and prefrontal network, which was significantly less active when short-term memory (STM) span was exceeded during encoding. Only primary auditory cortex showed an increase in activation when memory span was exceeded. Comparison of activation for grouped and ungrouped lists showed that during the subspan phase bilateral planum temporale showed less activation for grouped stimuli, while during the supra-span phase supramarginal and inferior parietal areas were more active for grouped lists. The magnitude of both temporal and parietal activations predicted enhanced recall of grouped lists. Thus neural signatures of grouping seem to reflect more structured processing in parietal areas instead of reliance on perceptual-auditory processing in temporal regions.