Biology subjects

Shapiro, K. L.

Publications and source records attributed to Shapiro, K. L..

9 recordsLinked to original sources

bioRxiv2026

Assessing the longevity of the Theta-induced Memory Effect (TIME)

Theta-phase synchronisation between visual and auditory rhythmically modulated sensory stimuli during encoding has been shown to enhance associative memory, an effect known as Theta-Induced Memory Effect (TIME). However, whether this advantage is short-lived or persists over time remains unknown. To address this, we examined the TIME effect in 52 participants randomly assigned to a short-delay group (immediate retrieval) or a long-delay group (retrieval after 24 hours). Electroencephalography (EEG) was recorded during incidental encoding of audio-video pairs. The luminance of the video clips and the amplitude of the sounds were modulated at theta frequency (4 Hz) and presented either in-synchrony (0{degrees} phase offset between audio and video) or out-of-synchrony (the sound 180{degrees} phase-shifted relative to the video). Source reconstruction was used to estimate the actual phase difference between visual and auditory cortices for every trial. At the behavioural level, no significant difference in memory accuracy was found between the synchronous and asynchronous conditions in either group. However, after reconstructing the actual phase difference at the source level, the TIME effect was replicated in the short-delay group, with significantly higher memory accuracy at the optimal reconstructed theta phase compared to out-of-phase. In contrast, the long-delay group showed a reversed pattern, with higher accuracy in the asynchronous condition, though this difference did not reach significance. These findings suggest that the TIME effect is time-limited and may not survive or even reverse after overnight consolidation.

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bioRxiv2025

Higher baseline alpha power is associated with faster responses in visual search

Visual search models have long emphasised that task-relevant items must be prioritized for optimal performance. While it is known that search efficiency also benefits from active distractor inhibition, the underlying neuronal mechanisms are debated. Neuronal alpha oscillations (7-14 Hz) have been associated with functional inhibition of cortical excitability, as well as distractor suppression in spatial attention and visual working memory tasks. We therefore hypothesised that alpha oscillations similarly support the deselection of distractors in visual search. Using Magnetoencephalography (MEG), we here show that high alpha power before the onset of a complex search display is associated with faster search performance. Crucially, we used a General Linear Model (GLM) approach to control for confounds between alpha power and task duration, ruling out that this result was merely driven by practice effects paired with increased fatigue over time. In addition to spontaneous oscillatory activity, we quantified the cortical excitability to colours of the search stimuli based on Rapid Invisible Frequency Tagging (RIFT) responses. In contrast to our initial hypothesis, increased pre-search alpha power did not correlate with the RIFT response, providing no direct evidence for feature-specific inhibition of distracting stimuli by alpha. Our findings challenge the traditional view of alpha oscillations reducing visual processing, showing instead that increased occipital alpha power can enhance performance in a visual task. We propose that the increase in alpha power may reflect increased top-down control supporting visual search

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bioRxiv2025

Pre-stimulus alpha power modulates trial-by-trial variability in theta rhythmic multisensory entrainment strength and theta-induced memory effect

Binding multisensory information into episodic memory depends partly on the timing of the hippocampal theta rhythm which provides time windows for synaptic modification. In humans, theta rhythmic sensory stimulation (RSS) enhances episodic memory when the stimuli are synchronised across the visual and auditory domain compared to when they are out-of-synchrony. However, recent studies show mixed evidence if the improvement in episodic memory is the result of modulating hippocampal theta activity. In the current study, we investigated whether pre-stimulus brain state could explain part of this variance in the neural and behavioural effects induced by the RSS, via recording participants brain activity with MEG during a multisensory theta RSS memory paradigm. Our findings suggest that pre-stimulus alpha power modulates entrainment strength in sensory regions, which in turn predicts subsequent memory formation. These findings suggest that for non-invasive brain stimulation tools to be effective it is crucial to consider brain-state dependent effects.

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bioRxiv2024

Guided Visual Search is associated with a feature-based priority map in early visual cortex

Visual search models have long emphasised that task-relevant items must be prioritised for optimal performance. While it is known that search efficiency also benefits from active distractor inhibition, the underlying neuronal mechanisms are debated. Here, we used MEG in combination with Rapid Invisible Frequency Tagging (RIFT) to probe how neuronal excitability in early visual cortex is modulated during feature-guided visual search. Participants were instructed to indicate the presence or absence of a letter "T" presented amongst 16 and 32 "L"s. In the guided search condition, participants were informed about the colour of the "T" and could infer the colour of the irrelevant distractors. In the unguided search condition, the target colour was unknown. We found that guided search was associated with enhanced RIFT responses to the target colour, and decreased responses to the distractor colour compared to unguided search. These results conceptually replicated using both a conventional coherence approach, as well as with a General Linear Model approach based on a single-trial measure of the RIFT response. The present findings expand on previous reports based on electrophysiology and fMRI in humans and non-human primates by revealing that feature-guidance in visual search affects neuronal excitability as early as primary visual cortex.

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bioRxiv2023

Beta bursts correlate with synchronization of movements to rhythmic sounds

Accumulating evidence indicates transient beta bursts play an important role in the representation of temporal information and prediction. However, the role of beta bursts in sensorimotor synchronization (SMS) involving active interactions between motor and sensory systems to synchronize predictive movements to periodic events remains unclear. To answer this question, 15 participants were invited to complete a finger-tapping task whilst high-density EEG (128 channels) was recorded. Participants tapped with their right index finger in synchrony with 1 Hz and 0.5 Hz tone trains. In line with previous findings, we found a negative mean asynchrony between tone and tap time, i.e., taps preceded tones for both tone frequencies (1 and 0.5 Hz). In the EEG data, beta bursts were detected and their timing in relationship with tapping and auditory tracking was examined. Results revealed that beta bursts tracked tapping and were modulated by the low frequency phase of the tone frequency (i.e., 1 Hz or 0.5 Hz). Importantly, the locking of beta bursts to the phase of auditory tracking correlated with the behavioural variance on a single trial level that occurred while tapping to the tones. These results demonstrate a critical role for an interplay between beta bursts and low frequency phase in coordinating rhythmic behaviour.

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bioRxiv2022

Altering stimulus timing via fast rhythmic sensory stimulation induces STDP-like recall performance in human episodic memory

Animal studies suggest that the strength of synaptic modification depends on spike timing between pre- and post-synaptic neurons on the order of tens of milliseconds, which is termed spike-timing-dependent plasticity (STDP). However, evidence for STDP in human episodic memory is lacking. We investigated this using rhythmic sensory stimulation to drive visual and auditory cortices at 37.5 Hz with four phase offsets. Visual relative to auditory cued recall accuracy was significantly enhanced in the 90{degrees} condition since the visual stimulus led at the shortest delay (6.67 ms). This pattern was reversed in the 270{degrees} condition when the auditory stimulus led the shortest delay. Within cue modality, recall was enhanced when a stimulus of the corresponding modality led the shortest delay as compared to the longest delay (20 ms). Our findings provide novel evidence for STDP in human memory, which builds an important bridge from in-vitro studies in animals to human behaviour.

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bioRxiv2022

Visual awareness during the attentional blink is determined by representational similarity

Our visual perception seems effortless, but the brain has a limited processing capacity which curtails the amount of sensory information that can be brought into conscious awareness at any moment in time. A widely studied exemplar of this limitation is the attentional blink (AB), in which observers are unable to report the second of two rapidly sequential targets if it appears within 200-500 ms of the first. Despite the apparent ubiquity of the AB effect, its computational and neurophysiological underpinnings have remained elusive. Here we propose a simple computational model of temporal attention that unifies the AB with spatial and feature-based attention. We took a novel, integrative approach involving human psychophysics and functional brain imaging, along with neuronal recordings in mice to test this model. Specifically, we demonstrate that the AB only arises when visual targets have dissimilar representations in the brain but is absent when both targets have the same representation. Similarity in this context can be determined either by elementary features such as edge orientation, or by acquired, high-level factors such as numerical or alphabetical order. In this parsimonious model of the AB, attention to an initial target establishes a perceptual filter that is tuned to its unique representation in the brain. Subsequent items that match the filter remain available for conscious report, whereas those that do not match elude awareness altogether.

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bioRxiv2021

Interaction between theta-phase and spike-timing dependent plasticity simulates theta induced memory effects

Rodent studies suggest that spike timing relative to hippocampal theta activity determines whether potentiation or depression of synapses arise. Such changes also depend on spike timing between pre- and post-synaptic neurons, known as spike-timing-dependent plasticity (STDP). STDP, together with theta-phase-dependent learning, has inspired several computational models of learning and memory. However, evidence to elucidate how these mechanisms directly link to human episodic memory is lacking. In a computational model, we modulate long-term potentiation (LTP) and long-term depression (LTD) of STDP, by opposing phases of a simulated theta rhythm. We fit parameters to a hippocampal cell culture study in which LTP and LTD were observed to occur in opposing phases of a theta rhythm. Further, we modulated two inputs by cosine waves with synchronous and asynchronous phase offsets and replicate key findings in human episodic memory. Learning advantage was found for the synchronous condition, as compared to the asynchronous conditions, and was specific to theta modulated inputs. Importantly, simulations with and without each mechanism suggest that both STDP and theta-phase-dependent plasticity are necessary to replicate the findings. Together, the results indicate a role for circuit-level mechanisms, which bridges the gap between slice preparation studies and human memory. Author SummaryLong-lasting changes in synaptic connectivity between neurons have been suggested to support learning and memory processes at the cellular level in the brain. Such synaptic modifications depend on synchronous activation of neurons, which leads to generate brain oscillations. Human memory studies focus on the relationships between brain oscillations and memory processes. Direct evidence on how the cellular mechanism links to human memory behaviour is lacking. To investigate the direct link between synaptic plasticity mechanisms and human memory formation, we built a computational neural network that implements two synaptic plasticity mechanisms, which are well-established in the rodents hippocampus. One mechanism shows that strengthening or weakening in synaptic connectivity depends on the phases of ongoing brain oscillation at theta frequency (4 - 8 Hz), which is a dominant signal in the hippocampus. The other mechanism suggests that synaptic modification depends on the precise timing of action potentials between two neurons. Our model successfully reproduces results from rodents, as well as several human episodic memory studies which demonstrated that human associative memory performance depends on phase synchronisation in theta frequency. These findings suggest a link between specific learning mechanisms at cellular level and human memory behaviour.

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bioRxiv2018

Single-trial Phase Entrainment of Theta Oscillations in Sensory Regions Predicts Human Associative Memory Performance

Episodic memories are rich in sensory information and often contain integrated information from different sensory modalities. For instance, we can store memories of a recent concert with visual and auditory impressions being integrated in one episode. Theta oscillations have recently been implicated in playing a causal role synchronizing and effectively binding the different modalities together in memory. However, an open question is whether momentary fluctuations in theta synchronization predict the likelihood of associative memory formation for multisensory events. To address this question we presented movies and sounds with their luminance and volume modulated at theta (4 Hz), with a phase offset at 0{degrees} or 180{degrees} with respect to each other. This allowed us to entrain the visual and auditory cortex in a synchronous (0{degrees}) or asynchronous manner (180{degrees}). Participants were asked to remember the association between a movie and a sound while having their EEG activity recorded. Associative memory performance was significantly enhanced in the synchronous (0{degrees}) compared to the asynchronous (180{degrees}) condition. Source-level analysis demonstrated that the physical stimuli effectively entrained their respective cortical areas with a corresponding phase offset. Importantly, the strength of entrainment during encoding correlated with the efficacy of associative memory such that small phase differences between visual and auditory cortex predicted a high likelihood of correct retrieval in a later recall test. These findings suggest that theta oscillations serve a specific function in the episodic memory system: Binding the contents of different modalities into coherent memory episodes.\n\nSignificance StatementHow multi-sensory experiences are bound to form a coherent episodic memory representation is one of the fundamental questions in human episodic memory research. Evidence from animal literature suggests that the relative timing between an input and theta oscillations in the hippocampus is crucial for memory formation. We precisely controlled the timing between visual and auditory stimuli and the neural oscillations at 4 Hz using a multisensory entrainment paradigm. Human associative memory formation depends on coincident timing between sensory streams processed by the corresponding brain regions. We provide evidence for a significant role of relative timing of neural theta activity in human episodic memory on a single trial level, which reveals a crucial mechanism underlying human episodic memory.

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