Matthew Slayton

The application of transcranial magnetic stimulation (TMS) to lateral parietal cortex has shown promise in improving episodic memory in older adults with Mild Cognitive Impairment (MCI). Previous work has suggested that such improvements are achieved by activating hippocampus at a distance with TMS, though this explanation is incomplete. We hypothesized that the mnemonic benefits arise from an additional mechanism: the modulation of semantic representations. Nineteen participants with amnestic MCI received either active intermittent theta-burst stimulation (iTBS) to angular gyrus or control vertex stimulation over three consecutive days while viewing object stimuli and completing relational memory encoding tasks during fMRI, followed by conceptual and perceptual recognition memory tests. We found that active TMS (relative to control TMS) significantly modulated conceptual memory performance. Using Representational Similarity Analysis with semantic embeddings derived from a large language model, we examined how TMS affects neural representations in inferior parietal lobule and hippocampus. We found that TMS enhanced semantic representational strength in inferior parietal lobule and reduced representational strength in hippocampus. Surprisingly, both effects supported successful memory. Neural pattern similarity analyses suggested that reduced hippocampal similarity supported successful memory, perhaps by promoting pattern separation mechanisms. These findings demonstrate that parietal TMS modulates semantic processing in a region-specific manner, by strengthening semantic integration at the stimulation site while promoting representational differentiation in medial temporal regions. This work advances our mechanistic understanding of memory neuromodulation and has implications for the optimization of therapeutic interventions in age-related memory disorders. Significance Statement: Transcranial Magnetic Stimulation (TMS) applied to parietal cortex can improve memory in patients with Mild Cognitive Impairment (MCI), a population at high risk for Alzheimer’s Disease, yet the mechanisms underlying this benefit remain poorly understood. Using fMRI and Representational Similarity Analysis (RSA), we examined how TMS alters the neural representation of semantic stimulus information in parietal cortex and hippocampus during memory encoding. Our results show that TMS selectively modulates semantic representations at the stimulation site and in hippocampus, and that these representational changes predict memory improvement. These findings advance our mechanistic understanding of parietal memory neuromodulation and lay the groundwork for more targeted and effective TMS-based interventions for age-related memory disorders.

Recent work in vision sciences contends that objects carry an intrinsic property called memorability that describes the likelihood that an object can be successfully encoded and later retrieved from memory. It has been shown that object memorability is supported by semantic information, but the neural correlates of this relationship are largely unexplored. The present study explores these premises and asks whether neural correlates of object memorability can be accounted for by semantic dimensions. We combine three data sets: (1) feature norms for a database of ∼1000 natural object images, (2) normative conceptual and perceptual memory data for those objects, and (3) neuroimaging data from an fMRI study collected using a subset (n = 360) of those objects. We found that object-wise memorability elicits consistent brain activation across participants in key mnemonic regions, including the hippocampus and rhinal cortex, and that the variance in this neural activity is mediated by the semantic factors describing these images. We propose that the features of memorable images may be facilitating memory formation by more deeply engaging encoding processes.

Recent work in vision sciences contends that objects carry an intrinsic property, memorability, that describes the likelihood that an object can be successfully encoded and later retrieved from memory. It has been shown that object memorability is supported by semantic information, but the neural correlates involved are largely unexplored. The present study explores these premises and asks whether neural correlates of object memorability can be accounted for by semantic dimensions. To investigate these questions, we combine three datasets: (1) feature norms for a database of ∼1000 natural object images, (2) normative conceptual and perceptual memory data for those objects, and (3) neuroimaging data from an fMRI study collected using a subset (n=360) of those objects. We found that object-wise memorability elicits consistent brain activation across participants in key mnemonic regions (e.g., hippocampus and rhinal cortex), and that a substantial portion of the variance in this brain activity is mediated by the semantic factors describing these images. Regions with the strongest mediation effects are associated with sensory, motor, and visual processes, suggesting that the relationship between memorability and semantics may align with a sensory-functional account of the representation of concepts in the brain.

A key feature of the brain’s ability to tell time and generate complex temporal patterns is its capacity to produce similar temporal patterns at different speeds. For example, humans can tie a shoe, type, or play an instrument at different speeds or tempi—a phenomenon referred to as temporal scaling. While it is well established that training improves timing precision and accuracy, it is not known whether expertise improves temporal scaling, and if so, whether it generalizes across skill domains. We quantified temporal scaling and timing precision in musicians and non-musicians as they learned to tap a Morse code sequence. We found that non-musicians improved significantly over the course of days of training at the standard speed. In contrast, musicians exhibited a high level of temporal precision on the first day, which did not improve significantly with training. Although there was no significant difference in performance at the end of training at the standard speed, musicians were significantly better at temporal scaling—i.e., at reproducing the learned Morse code pattern at faster and slower speeds. Interestingly, both musicians and non-musicians exhibited a Weber-speed effect, where temporal precision at the same absolute time was higher when producing patterns at the faster speed. These results are the first to establish that the ability to generate the same motor patterns at different speeds improves with extensive training and generalizes to non-musical domains.

A key feature of the brain’s ability to tell time and generate complex temporal patterns is its capacity to produce similar temporal patterns at different speeds. For example, humans can tie a shoe, type, or play an instrument at different speeds or tempi—a phenomenon referred to as temporal scaling. While it is well established that training improves timing precision and accuracy, it is not known whether expertise improves temporal scaling. We quantified temporal scaling and timing precision in musicians and non-musicians as they learned to tap a Morse code sequence. We found that controls improved significantly over the course of days of training at the standard speed. In contrast, musicians exhibited a high level of temporal precision on the first day, which did not improve significantly with training. Although there was no significant difference in performance at the end of training at the standard speed, musicians were significantly better at temporal scaling—i.e., at reproducing the learned Morse code pattern at faster and slower speeds. Interestingly, both musicians and non-musicians exhibited a Weber-speed effect, where absolute temporal precision sharpened when producing patterns at the faster speed. These results are the first to establish that the ability to generate the same motor patterns at different speeds improves with extensive training and generalizes to non-musical domains.