Biology subjects

Salimpour, Y.

Publications and source records attributed to Salimpour, Y..

7 recordsLinked to original sources

bioRxiv2026

Cellular code for mnemonic pattern separation in the human hippocampus is revealed by false memories

Theoretical models propose that pattern separation, a computation thought to be implemented by the hippocampus, allows us to differentiate between familiar and novel items. However, whether pattern separation operates in the human hippocampus remains contested, with no established single-cell correlate. We recorded the single neuron activity of 3506 neurons in the human brain while 97 patients performed a recognition memory task in which novel images similar to previously seen images led to false memories and associated behavioral errors. We identified two kinds of memory selective neurons distributed across the brain: those responding differently to falsely recognized novel and correctly recognized familiar images in a manner compatible with pattern separation, and the other signaling the subject's choice. At the population level, these cells predicted mnemonic ground truth in the hippocampus and the decision in the pre-supplementary motor area, illustrating the progression from mnemonic signals to decisions. Removing pattern separation-signaling cells abolished the continuous memory strength gradient present in the hippocampus, suggesting a role of these cells in separating memories of different strength. These results establish a single cell correlate for mnemonic pattern separation in the human hippocampus and show its behavioral relevance in episodic memory.

neuroscience↗

bioRxiv2025

Stimulus-specific recruitment of human amygdala neurons predicts episodic memory encoding success

Controlling whether a given experience is encoded into long-term memory and thus later remembered is a crucial component of our memory system whose failure is often at the root of memory disorders. One brain area that takes part in controlling which experiences are remembered is the amygdala, but the mechanisms by which it does so remain poorly understood. Here we examined single-neuron activity and local field potentials as human participants performed recognition memory tasks with visual stimuli. Category-selective amygdala neurons exhibited elevated firing rates during encoding of later remembered items versus forgotten items. This subsequent memory effect was restricted to images of the preferred category of a given cell, was stronger and appeared earlier in the amygdala compared to the hippocampus, and did not depend on the valence and arousal of the stimuli. In contrast, category selective cells immediately upstream in the ventral temporal cortex did not exhibit a subsequent memory effect, highlighting specificity to the amygdala. Successful memory formation was accompanied by enhanced spike-field coherence between the activity of category cells in the amygdala and hippocampal field potentials. These findings, replicated in two large independent datasets with two different tasks, demonstrate that recruitment of stimulus-specific amygdala representations predicts episodic memory formation, particularly in the right amygdala. This data suggests category cells in the right amygdala as a cellular target for interventions to treat memory disorders in humans.

neuroscience↗

bioRxiv2025

Hippocampal stimulation reveals causal role of persistent neural activity in human working memory

Working memory (WM) enables the temporary maintenance and manipulation of information, supporting flexible, goal-directed behavior. While converging evidence suggests that persistent activity in the hippocampus and other areas of the brain contributes to WM storage, this link has not been examined by manipulating neural activity experimentally. Here, we combined simultaneous intracranial single-neuron recordings with focal electrical stimulation in the human hippocampus to test the role of memoranda-selective persistent neural activity for WM. Thirty patients with implanted hybrid depth electrodes performed a WM task with images as memoranda. Electrical stimulation (2 s, 50 Hz, 1 mA) was delivered to the hippocampus during the maintenance period on a subset of trials. Behaviorally, stimulation impaired WM performance, increasing response times especially in low-load conditions. Neuronally, stimulation reduced memoranda-selective activity in hippocampus and ventral temporal cortex (VTC), thereby disrupting content-specific neural representations of WM content. The extent of stimulation-induced change in firing rate mediated trial-by-trial impaired WM-related behavior, linking disrupted neural selectivity to impaired WM. At the population level, stimulation shifted neural trajectories farther from attractor states, consistent with degraded mnemonic fidelity. Together, these data provide evidence that selective persistent activity of individual neurons in hippocampus and VTC supports WM maintenance in humans. Our results demonstrate that hippocampal stimulation disrupts both single-neuron coding and population-level attractor stability, linking cellular mechanisms to behavior and highlighting the contributions of persistent activity to WM maintenance.

neuroscience↗

bioRxiv2024

Persistent activity during working memory maintenance predicts long-term memory formation in the human hippocampus

Working Memory (WM) and Long-Term Memory (LTM) are often viewed as separate cognitive systems. Little is known about how these systems interact when forming memories. We recorded single neurons in the human medial temporal lobe while patients maintained novel items in WM and a subsequent recognition memory test for the same items. In the hippocampus but not the amygdala, the level of WM content-selective persist activity during WM maintenance was predictive of whether the item was later recognized with high confidence or forgotten. In contrast, visually evoked activity in the same cells was not predictive of LTM formation. During LTM retrieval, memory-selective neurons responded more strongly to familiar stimuli for which persistent activity was high while they were maintained in WM. Our study suggests that hippocampal persistent activity of the same cell supports both WM maintenance and LTM encoding, thereby revealing a common single-neuron component of these two memory systems.

neuroscience↗

bioRxiv2024

Theta-Gamma Phase-Amplitude Coupling Supports Working Memory Performance in the Human Hippocampus

Phase-amplitude coupling (PAC) occurs in the human hippocampus during working memory and supports the contribution of the hippocampus in the maintenance of multiple items. Additionally, PAC has the potential to reveal the neural mechanisms underlying multi-item maintenance in the hippocampus by providing a putative architecture for multi-item representation. Theta and gamma range rhythms are prominent neuronal oscillations in the hippocampus. Studies on the role of theta frequency oscillation in local field potentials in human memory have shown mixed evidence for successful remembering. The role of gamma oscillatory activity in contributing to memory retrieval is not yet fully understood. They also interact with each other in the form of PAC during memory performance. This study aims to investigate the neurophysiological function of theta-gamma PAC in the human hippocampus during a multi-item working memory task and characterize its association with performance. Theta-gamma cross-coupling investigation in the electrocorticographic signals was performed from the hippocampus recording of ten epilepsy patients while they were engaged with the working memory task. The results show strong correlations between PAC levels and the subjects memory performance, but no correlation with theta and gamma power individually, specifically in the retrieval phase of a working memory task. These observations demonstrate the possible role of PAC in memory-related operations, suggesting a PAC-based neural mechanism for working memory in the hippocampus. Significance StatementThe findings from this study elucidate the crucial role of phase-amplitude coupling in the human hippocampus during working memory tasks, specifically in the maintenance of multiple items. By analyzing electrocorticographic recordings from epilepsy patients engaged in a working memory task, our research unveils a direct correlation between PAC levels and memory performance during the retrieval phase, which is not observed when analyzing theta and gamma oscillations individually. These findings suggest a theta-gamma coupling based mechanism within the hippocampus that facilitates working memory, offering new insights into the complex neural processes underlying memory encoding and retrieval. This advancement in understanding the neural architecture of memory not only contributes to the foundational knowledge of cognitive neuroscience but also opens avenues for developing targeted interventions for enhancing memory performance with translational application in treating memory-related neurological disorders.

neuroscience↗

bioRxiv2023

Phase-Amplitude Coupling Detection and Analysis of Human 2-Dimensional Neural Cultures in Multi-well Microelectrode Array in Vitro

Human induced pluripotent stem cell (hiPSC)- derived neurons offer the possibility of studying human-specific neuronal behaviors in physiologic and pathologic states in vitro. However, it is unclear whether these cultured neurons can achieve the fundamental network behaviors that are required to process information in the human brain. Investigating neuronal oscillations and their interactions, as occurs in cross-frequency coupling (CFC), is potentially a relevant approach. Microelectrode array culture plates provide a controlled framework to study populations of hiPSC-derived cortical neurons (hiPSC-CNs) and their electrical activity. Here, we examined whether networks of two-dimensional cultured hiPSC-CNs recapitulate the CFC that is present in networks in vivo. We analyzed the electrical activity recorded from hiPSC-CNs grown in culture with hiPSC-derived astrocytes. We employed the modulation index method for detecting phase-amplitude coupling (PAC) and used an offline spike sorting method to analyze the contribution of a single neurons spiking activities to network behavior. Our analysis demonstrates that the degree of PAC is specific to network structure and is modulated by external stimulation, such as bicuculine administration. Additionally, the shift in PAC is not driven by a single neurons properties but by network-level interactions. CFC analysis in the form of PAC explores communication and integration between groups of nearby neurons and dynamical changes across the entire network. In vitro, it has the potential to capture the effects of chemical agents and electrical or ultrasound stimulation on these interactions and may provide valuable information for the modulation of neural networks to treat nervous system disorders in vivo. SignificancePhase amplitude coupling (PAC) analysis demonstrates that the complex interactions that occur between neurons and network oscillations in the human brain, in vivo, are present in 2-dimensional human cultures. This coupling is implicated in normal cognitive function as well as disease states. Its presence in vitro suggests that PAC is a fundamental property of neural networks. These findings offer the possibility of a model to understand the mechanisms and of PAC more completely and ultimately allow us to understand how it can be modulated in vivo to treat neurologic disease.

neuroscience↗

bioRxiv2023

Control of working memory maintenance by theta-gamma phase amplitude coupling of human hippocampal neurons

Retaining information in working memory (WM) is a demanding process that relies on cognitive control to protect memoranda-specific persistent activity from interference. How cognitive control regulates WM storage, however, remains unknown. We hypothesized that interactions of frontal control and hippocampal persistent activity are coordinated by theta-gamma phase amplitude coupling (TG-PAC). We recorded single neurons in the human medial temporal and frontal lobe while patients maintained multiple items in WM. In the hippocampus, TG-PAC was indicative of WM load and quality. We identified cells that selectively spiked during nonlinear interactions of theta phase and gamma amplitude. These PAC neurons were more strongly coordinated with frontal theta activity when cognitive control demand was high, and they introduced information-enhancing and behaviorally relevant noise correlations with persistently active neurons in the hippocampus. We show that TG-PAC integrates cognitive control and WM storage to improve the fidelity of WM representations and facilitate behavior.

neuroscience↗