NBIDPSTM · Neural basis for individual differences in pSTM in the normal and dyslexic populations

Neural basis for individual differences in pSTM in the normal and dyslexic populations

Project context and objectives The objectives of the current project were twofold. Firstly, to investigate how capacities in phonological short-term memory processes (pSTM) relate to language processing capacities more generally. We specifically looked at individual variability in both phonological processing and pSTM and asked if these can arise from variability in the functional and anatomical underpinnings of speech perception and production processes. Secondly, to address the question of whether the so-called 'phonological deficit' in developmental dyslexia is more directly related to impaired phonological processing per-se or to altered pSTM processes, or, alternatively, that it arises from a disruption that impacts on both functions. Work performed To address the first objective, we recruited and tested healthy adult participants with a broad range of language and verbal memory capacity and subjected them to a battery of behavioural and neuro-imaging experiments. The behavioural tests were designed and aimed at assessing the participants' abilities in language processing and verbal memory as well as their non-verbal IQ. They included a standardised reading test, tests of phonological awareness and four different phonological short-term memory tests. The imaging experiments were designed and aimed at characterising the networks involved in phonological processing and in phonological short-term memory. These were three experiments on language processing and one experiment on pSTM. The language processing experiments were auditory speech perception, single-word speech production, and single-word repetition, (incorporating both perception and production). The pSTM experiment was designed to allow the separate investigation of the three component processes of the phonological short-term memory model (encode, maintain and rehearse). It was implemented by involving listening to, silently rehearsing and recalling sequences of nonsense words. To address the objectives of the second stage, we first ran a large-scale behavioural screening of healthy adults and adults diagnosed with developmental dyslexia. Following the screening, those who met the pre-established criteria for dyslexia and for normal reading and language processing were selected. The behavioural tests were similar to those used in the first stage of the project with some additional tests (e.g. an orthography test). The nature of the imaging experiments was also the same but with a number of small modifications in order to adapt them better to the dyslexic population (e.g. longer pauses between blocks of tests, and shorter sequences to remember). Main results One of the main results obtained in the first stage of the project indicated that capacity in verbal memory (pr pSTM) might play a role in speech perception and/or in the efficient speech processing. Our findings indicated a positive relationship between verbal memory abilities (assessed outside the scanner) and neural activation elicited during speech perception (by listening to speech in the scanner). We therefore concluded that normal listeners vary in the degree to which they recruit their premotor cortex as a function of verbal short-term memory ability. In other words, participants with better verbal memory are likely to recruit their motor regions more (obligatorily involved in speech production) when listening to speech. One plausible hypothesis that one could put forward from these findings is that individuals with impaired short-term memory processes (e.g. dyslexics) may have particular difficulties recruiting their motor regions, even during mere speech perception (auditory motor pathways). In addition, these findings are also important from a theoretical point of view given the current renewed interest in the involvement of the motor cortex during speech perception. Similarly, probably the most interesting result obtained in the experiment on phonological short-term memory indicated that variability in neural activation during the silent maintenance of verbal information (in particular, in the left fronto-parietal network; the left premotor region and the left intraparietal lobule) is associated with better recall. Participants showing stronger neural activation in these two regions during silent maintenance produced a better recall accuracy. These findings, which were obtained in a different experimental set-up, corroborate our previous results that individual differences in phonological short-term memory processes, here during silent rehearsal, are linked to processes in premotor and motor cortices (dorsal stream). Given a number of technical problems during the first stage, the analysis of the experiments of the second stage is currently ongoing, and results are only preliminary. Generally speaking, they suggest potential lateralisation differences between the groups during speech perception: dyslexic participants seem to produce a more bilaterally distributed activation pattern when listening to speech, whereas control participants appear to produce a more left-lateralised response. When producing speech, both groups recruit the same network but to a different degree: dyslexic participants seem to produce reduced neural activation in the phonological output network (underlying speech output) during reading aloud and repetition. This data is currently being investigated in more depth. As for variability in motor region recruitment during speech perception, we observed similar trends to the first dataset, however without reaching significance. The lack of significance may be explained by reduced behavioural variability with groups. In the first stage of the project, we aimed at maximising the behavioural variability among the recruited healthy participants, whereas for the second set, participants were selected according to pre-established criteria of dyslexia and/or normal reading. They were carefully selected in order to maximise the group differences and to avoid any overlap (the dyslexics' performance was by definition more than two standard deviations below the mean of the control group). This resulted in a low variability control group and a variable dyslexic group. The analysis of the pSTM data is currently ongoing. Using the second stage dataset, we hope to shed new light on the nature of language impairment in the dyslexic population. Specifically, we are curious to see whether we can find similar relationships between phonological processing and verbal memory abilities to those we saw in the first stage of the project, and whether we can relate them to any anatomical variability. Verbal memory impairment is well documented in developmental dyslexia but there are few attempts to relate these impairments to speech processing abilities. The current project is therefore an important attempt to relate speech processing abilities to that of verbal memory in both healthy and dyslexic populations, and will hopefully have an influential impact on the direction of dyslexia research.

Data: CORDIS, © European Union