Biology subjects

Naspi, L.

Publications and source records attributed to Naspi, L..

4 recordsLinked to original sources

bioRxiv2026

Dynamic shifts between pattern separation and reinstatement in human CA3 and dentate gyrus

Distinguishing similar memories is thought to require hippocampal pattern separation, the orthogonalization of overlapping neural representations. Here, high-resolution 7-Tesla fMRI during one-shot encoding of similar target-lure pairs demonstrated that pattern separation and reinstatement are modulated by stimulus domain, similarity, and hippocampal long-axis position. Univariate novelty responses during successful object and scene discrimination were found across medial temporal lobe cortex and hippocampal subfields. Multivariate neural pattern similarity was reduced for correctly rejected low-similarity lures across domains throughout all medial temporal regions, consistent with pattern separation. Increasing target-lure similarity resulted in more hippocampal than cortical pattern separation. An exception were high-similarity scene lures which elicited greater hippocampal pattern similarity, indicating reinstatement. In dentate gyrus and CA3, reduced pattern similarity predicted better object, increased similarity better scene memory, depending on hippocampal long-axis position. Pattern separation and reinstatement are therefore not fixed computational properties of specific subregions but shift dynamically during fast episodic learning.

neuroscience↗

bioRxiv2026

When knowledge interferes with perception: Neural mechanisms of the semantic amplification of visual false memory

Visual false memory refers to our tendency to falsely recognize novel stimuli that are visually similar to seen stimuli. Visual false memory also occurs when stimuli are meaningful, suggesting that semantic information interferes with the encoding of visual details. However, the neural mechanisms of this semantic interference effect are largely unknown. In the present fMRI study, participants were scanned while encoding visually similar fonts presented with words (word-fonts) or pseudowords (pseudoword-fonts), and later, when recognizing old, new similar (lures), and new dissimilar (novel) fonts displayed in the same meaningless letter string. We performed (1) representational similarity analysis (RSA) at encoding to identify visual, visuosemantic, and semantic representations associated with subsequent visual true and false font recognition, (2) encoding-retrieval similarity (ERS) analysis to assess their reinstatement during retrieval, and (3) mediational analyses to examine hippocampal contributions. The study yielded three main findings. First, visuosemantic representations supported true font recognition when stored in right fusiform gyrus, but false recognition of word-fonts when stored in the left fusiform gyrus. Second, mirroring this pattern, reinstatement in right fusiform gyrus was associated with true font recognition, whereas reinstatement in left fusiform gyrus was linked to false recognition of word-fonts. Finally, posterior hippocampal activation reduced false font memory mainly for pseudoword-associated fonts via decreased reinstatement in perceptual regions, while anterior hippocampal activity increased false memory of word-fonts via enhanced reinstatement in semantic regions. Taken together, these findings reveal how distinct hippocampal-cortical pathways differentially bias memory towards perceptual specificity or semantic generalization. Significance StatementFalse memories are often triggered by visual similarity, but this study shows that meaning encoded during learning can distort memory for visual details, even when retrieval cues are meaningless. Participants learned fonts associated with words or pseudowords and judged whether similar lure fonts, shown on a meaningless letter string, were seen before. Although behavioral performance was similar across conditions, brain imaging revealed a key dissociation: the left fusiform gyrus and anterior hippocampus promote semantic generalization that increases false recognition, whereas the right fusiform gyrus and posterior hippocampus support perceptual specificity that protects against it. These findings reveal how distinct hippocampal-cortical pathways differentially bias memory toward truth or illusion.

neuroscience↗

bioRxiv2022

Effects of aging on successful object encoding:Enhanced semantic representations compensate for impaired visual representations

Whereas episodic memory and visual processing decline substantially with healthy aging, semantic knowledge is generally spared. There is evidence that older adults can take advantage of their spared semantic knowledge to support their performance in episodic memory and visual tasks. Here, we used fMRI combined with representational similarity analyses (RSA) to examine how visual and semantic representations stored during encoding predict subsequent object memory. Young and older adults encoded images of objects during fMRI scanning and recalled these images while rating the vividness of their memories. After scanning, participants discriminated between studied images and similar lures. RSA based on a deep convolutional neural network and normative concept feature data was used to link patterns of neural activity during encoding to visual and semantic representations. The quality of visual representations was reduced in older adults, consistent with dedifferentiation, whereas the quality of semantic representations was enhanced in older adults, consistent with hyperdifferentiation. Despite dedifferentiation, visual representations stored in early visual cortex predicted later recall with high vividness in both young and older adults, with no age-related differences. In contrast, semantic representations in lingual and fusiform gyrus were associated with better subsequent object picture recall in older but not in young adults. This finding is consistent with evidence that older adults rely on semantic knowledge to compensate for cognitive deficits. Taken together, the results suggest that the age-related neural dedifferentiation for visual information in posterior regions might be partly counteracted by a boost on semantic representations in more anterior areas. Significance StatementPrevious research has shown that healthy aging tends to impair memory for individual events, visual processing, and other cognitive abilities but not semantic knowledge. We investigated the effects of aging on the quality of the information stored in the brain when viewing common objects and on how this information enables subsequent memory for these objects. Using fMRI combined with modeling of the stimuli, we found that visual information was degraded in older adults, but it was sufficient to support subsequent memory. In contrast, semantic information supported subsequent memory only in older adults. This is the first direct neuroscience evidence that older adults take advantage of spared semantic representations to boost their memory for individual events.

neuroscience↗

bioRxiv2021

Perceptual and Semantic Representations at Encoding Contribute to True and False Recognition of Objects

When encoding new episodic memories, visual and semantic processing are proposed to make distinct contributions to accurate memory and memory distortions. Here, we used functional magnetic resonance imaging (fMRI) and representational similarity analysis to uncover the representations that predict true and false recognition of unfamiliar objects. Two semantic models captured coarse-grained taxonomic categories and specific object features, respectively, while two perceptual models embodied low-level visual properties. Twenty-eight female and male participants encoded images of objects during fMRI scanning, and later had to discriminate studied objects from similar lures and novel objects in a recognition memory test. Both perceptual and semantic models predicted true memory. When studied objects were later identified correctly, neural patterns corresponded to low-level visual representations of these object images in the early visual cortex, lingual, and fusiform gyri. In a similar fashion, alignment of neural patterns with fine-grained semantic feature representations in the fusiform gyrus also predicted true recognition. However, emphasis on coarser taxonomic representations predicted forgetting more anteriorly in ventral anterior temporal lobe, left perirhinal cortex, and left inferior frontal gyrus. In contrast, false recognition of similar lure objects was associated with weaker visual analysis posteriorly in early visual and left occipitotemporal cortex. The results implicate multiple perceptual and semantic representations in successful memory encoding and suggest that fine-grained semantic as well as visual analysis contributes to accurate later recognition, while processing visual image detail is critical for avoiding false recognition errors. Significance StatementPeople are able to store detailed memories of many similar objects. We offer new insights into the encoding of these specific memories by combining fMRI with explicit models of how image properties and object knowledge are represented in the brain. When people processed fine-grained visual properties in occipital and inferior temporal cortex, they were more likely to be recognize the objects later, and less likely to falsely recognize similar objects. In contrast, while object-specific feature representations in fusiform predicted accurate memory, coarse-grained categorical representations in frontal and temporal regions predicted forgetting. The data provide the first direct tests of theoretical assumptions about encoding true and false memories, suggesting that semantic representations contribute to specific memories as well as errors.

neuroscience↗