Biology subjects

Gillette, K.

Publications and source records attributed to Gillette, K..

3 recordsLinked to original sources

bioRxiv2026

Understanding the mechanisms of lateral parietalmemory modulation in Mild Cognitive Impairment

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 StatementTranscranial Magnetic Stimulation (TMS) applied to parietal cortex can improve memory in patients with Mild Cognitive Impairment (MCI), a population at high risk for Alzheimers 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.

neuroscience↗

bioRxiv2025

Local and distributed information coding in the ventral stream

Neuroscience is replete with evidence that cognitive representations are distributed across many cortical regions. Yet, the scale and content of such distributed processing is unclear. Do findings of widespread information coding suggest a large-scale "forest" of regions interacting to represent information or instead imply a multitude of small-scale trees, processing information as localized modules. To investigate this distinction, we used visual and semantic representational analysis of fMRI data from 60 participants viewing everyday objects in multiple task contexts, and we examined the relationships between regions in terms of information coding. We demonstrate that coding of visual content in the occipital lobe is overwhelmingly modular, such that different occipital structures show limited coordination and tend to encode information redundantly. By contrast, the coding of semantic content in the inferior temporal lobe involves a high degree of coordination between regions, which optimize their coding to collectively represent a large semantic space with minimal redundancy between regions. No other brain area - neither the parietal nor prefrontal cortices - shows the preference for large-scale coding seen in the inferior temporal lobe. Taken together, these results outline a framework of how the ventral stream transitions from small-scale to large-scale coding as information progresses from visual to semantic representations. Significance statementHow does the brain convert incoming signals into usable information? Many studies have investigated this question by attempting to clarify which brain regions encode what information (e.g., V4 encodes color information). We instead aimed to shed light on the degree of coordination among information coding regions. We find that the visual-to-semantic transition as information flows anteriorly in the occipitotemporal cortex is accompanied by a shift from modular to distributed coding. That is, occipital regions encode perceptual information relatively independently with redundancy, while inferior temporal lobe regions cooperate to most efficiently represent a large space of semantic information. By leveraging ideas from information theory, our work introduces coding scale as a new dimension for understanding the architecture of information coding.

neuroscience↗

bioRxiv2024

Cortico-hippocampal interactions underlie schema-supported memory encoding in older adults

Although episodic memory is typically impaired in older adults (OAs) compared to young adults (YAs), this deficit is attenuated when OAs can leverage their rich semantic knowledge, such as their knowledge of schemas. Memory is better for items consistent with pre-existing schemas and this effect is larger in OAs. Neuroimaging studies have associated schema use with the ventromedial prefrontal cortex (vmPFC) and hippocampus (HPC), but most of this research has been limited to YAs. This fMRI study investigated the neural mechanisms underlying how schemas boost episodic memory in OAs. Participants encoded scene-object pairs with varying congruency, and memory for the objects was tested the following day. Congruency with schemas enhanced object memory for YAs and, more substantially, for OAs. FMRI analyses examined how cortical modulation of HPC predicted subsequent memory. Congruency-related vmPFC modulation of left HPC enhanced subsequent memory in both age groups, while congruency-related modulation from angular gyrus (AG) boosted subsequent memory only in OAs. Individual differences in cortico-hippocampal modulations indicated that OAs preferentially used their semantic knowledge to facilitate encoding via an AG-HPC interaction, suggesting a compensatory mechanism. Collectively, our findings illustrate age-related differences in how schemas influence episodic memory encoding via distinct routes of cortico-hippocampal interactions.

neuroscience↗