Neural reactivation and judgements of vividness reveal separable contributions to mnemonic representation.

Mnemonic representations vary in fidelity, sharpness, and strength-qualities that can be examined using both introspective judgements of mental states and objective measures of brain activity. Subjective and objective measures are both valid ways of "reading out" the content of someone's internal mnemonic states, each with different strengths and weaknesses. St-Laurent and colleagues (2015) compared the neural correlates of memory vividness ratings with patterns of neural reactivation evoked during memory recall and found considerable overlap between the two, suggesting a common neural basis underlying these different markers of representational quality. Here we extended this work with meta-analytic methods by pooling together four neuroimaging datasets in order to contrast the neural substrates of neural reactivation and those of vividness judgements. While reactivation and vividness judgements correlated positively with one another and were associated with common univariate activity in the dorsal attention network and anterior hippocampus, some notable differences were also observed. Vividness judgments were tied to stronger activation in the striatum and dorsal attention network, together with activity suppression in default mode network nodes. We also observed a trend for reactivation to be more closely associated with early visual cortex activity. A mediation analysis found support for the hypothesis that neural reactivation is necessary for memory vividness, with activity in the anterior hippocampus associated with greater reactivation. Our results suggest that neural reactivation and vividness judgements reflect common mnemonic processes but differ in the extent to which they engage effortful, attentional processes. Additionally, the similarity between reactivation and vividness appears to arise, partly, through hippocampal engagement during memory retrieval.

Processing differences between monolingual and bilingual young adults on an emotion n-back task.

Bilingualism is associated with enhancement of executive control (EC) across the lifespan. Working memory and non-verbal emotion regulation both draw upon EC mechanisms so may also be affected by bilingualism, but these relationships are not fully understood. These relationships were explored using an n-back task with distracting emotional stimuli administered to young adults while continuous EEG was recorded. Monolinguals were faster but less accurate on the 2-back than bilinguals, and monolingual accuracy was more impeded by the presence of emotional stimuli than was that of bilinguals. The P300 event-related potential, a neural signature of working memory processing in the n-back, had smaller amplitudes in both groups on the 2-back than the 1-back, but attenuation in response to distracting emotional stimuli was greater for bilinguals than monolinguals. P300 latencies were also differentially affected by emotional stimuli in each group: Bilingual latencies were constant across emotions but monolingual latencies increased from neutral to angry conditions. In general, bilingual performance was less impacted by the emotional distraction than was that of the monolinguals. Additionally, bilinguals adjusted to the changing demands of the 1-back and 2-back conditions by recruiting neural networks to support different behavioral outcomes than monolinguals.

The relation between brain signal complexity and task difficulty on an executive function task.

On a daily basis, we constantly deal with changing environmental cues and perceptual conflicts and as such, our brains must flexibly adapt to current demands in order to act appropriately. Brains become more efficient and are able to switch states more readily by increasing the complexity of their neural networks. However, it is unclear how brain signal complexity relates to behavior in young adults performing cognitively demanding executive function tasks. Here we used multiscale entropy analysis and multivariate statistics on EEG data while participants performed a bivalency effect task-switching paradigm to show that brain signal complexity in young adults increases as task demands increase, that increases in brain signal complexity are associated with both speed gains and losses depending on scalp location, and that more difficult tasks are associated with more circumscribed complexity across the scalp. Overall, these findings highlight a critical role for brain signal complexity in predicting behavior on an executive function task among young adults.

Effects of bilingualism on white matter integrity in older adults.

Bilingualism can delay the onset of dementia symptoms and has thus been characterized as a mechanism for cognitive or brain reserve, although the origin of this reserve is unknown. Studies with young adults generally show that bilingualism is associated with a strengthening of white matter, but there is conflicting evidence for how bilingualism affects white matter in older age. Given that bilingualism has been shown to help stave off the symptoms of dementia by up to four years, it is crucial that we clarify the mechanism underlying this reserve. The current study uses diffusion tensor imaging (DTI) to compare monolinguals and bilinguals while carefully controlling for potential confounds (e.g., I.Q., MMSE, and demographic variables). We show that group differences in Fractional Anisotropy (FA) and Radial Diffusivity (RD) arise from multivariable interactions not adequately controlled for by sequential bivariate testing. After matching and statistically controlling for confounds, bilinguals still had greater axial diffusivity (AD) in the left superior longitudinal fasciculus than monolingual peers, supporting a neural reserve account for healthy older bilinguals.