Biology subjects

Samaha, J.

Publications and source records attributed to Samaha, J..

30 records · Page 2Linked to original sources

bioRxiv2021

Power spectrum slope confounds estimation of instantaneous oscillatory frequency

Oscillatory neural dynamics are highly non-stationary and require methods capable of quantifying time-resolved changes in rhythmic activity in order to understand neural function. Recently, a method termed frequency sliding was introduced to estimate the instantaneous frequency of oscillatory activity, providing a means of tracking temporal changes in the dominant frequency within a sub-band of field potential recordings. Here, the ability of frequency sliding to recover ground-truth oscillatory frequency in simulated data is tested while the exponent (slope) of the 1/fx component of the signal power spectrum is systematically varied, mimicking real electrophysiological data. The results show that 1) in the presence of 1/f activity, frequency sliding systematically underestimates the true frequency of the signal, 2) the magnitude of underestimation is correlated with the steepness of the slope, suggesting that, if unaccounted for, slope changes could be misinterpreted as frequency changes, 3) the impact of slope on frequency estimates interacts with oscillation amplitude, indicating that changes in oscillation amplitude alone may also influence instantaneous frequency estimates in the presence of strong 1/f activity; and 4) analysis parameters such as filter bandwidth and location also mediate the influence of slope on estimated frequency, indicating that these settings should be considered when interpreting estimates obtained via frequency sliding. The origin of these biases resides in the output of the filtering step of frequency sliding, whose energy is biased towards lower frequencies precisely because of the 1/f structure of the data. We discuss several strategies to mitigate these biases and provide a proof-of-principle for a 1/f normalization strategy.

neuroscience↗

bioRxiv2021

Pre-Stimulus Alpha-Band Phase Gates Afferent Visual Cortex Responses

The neurogenesis of alpha-band (8-13 Hz) activity has been characterized across many different animal experiments. However, the functional role that alpha oscillations play in perception and behavior has largely been attributed to two contrasting hypotheses, with human evidence in favor of either (or both or neither) remaining sparse. On the one hand, alpha generators have been observed in relay sectors of the visual thalamus and are postulated to phasically inhibit afferent visual input in a feedforward manner 1-4. On the other hand, evidence also suggests that the direction of influence of alpha activity propagates backwards along the visual hierarchy, reflecting a feedback influence upon the visual cortex 5-9. The primary source of human evidence regarding the role of alpha phase in visual processing has been on perceptual reports 10-16, which could be modulated either by feedforward or feedback alpha activity. Thus, although these two hypotheses are not mutually exclusive, human evidence clearly supporting either one is lacking. Here, we present human subjects with large, high-contrast visual stimuli that elicit robust C1 event-related potentials (ERP), which peak between 70-80 milliseconds post-stimulus and are thought to reflect afferent primary visual cortex (V1) input17-20. We find that the phase of ongoing alpha oscillations modulates the global field power (GFP) of the EEG during this first volley of stimulus processing (the C1 time-window). On the standard assumption21-23 that this early activity reflects postsynaptic potentials being relayed to visual cortex from the thalamus, our results suggest that alpha phase gates visual responses during the first feed-forward sweep of processing.

neuroscience↗

bioRxiv2021

Spontaneous neural oscillations influence behavior and sensory representations by suppressing neuronal excitability

The ability to process and respond to external input is critical for adaptive behavior. Why, then, do neural and behavioral responses vary across repeated presentations of the same sensory input? Spontaneous fluctuations of neuronal excitability are currently hypothesized to underlie the trial-by-trial variability in sensory processing. To test this, we capitalized on invasive electrophysiology in neurosurgical patients performing an auditory discrimination task with visual cues: specifically, we examined the interaction between prestimulus alpha oscillations, excitability, task performance, and decoded neural stimulus representations. We found that strong prestimulus oscillations in the alpha+ band (i.e., alpha and neighboring frequencies), rather than the aperiodic signal, correlated with a low excitability state, indexed by reduced broadband high-frequency activity. This state was related to slower reaction times and reduced neural stimulus encoding strength. We propose that the alpha+ rhythm modulates excitability, thereby resulting in variability in behavior and sensory representations despite identical input.

neuroscience↗

bioRxiv2021

No Evidence for a Single Oscillator Underlying Discrete Visual Percepts

Theories of perception based on discrete sampling posit that visual consciousness is reconstructed based on snapshot-like perceptual moments, as opposed to being updated continuously. According to a model proposed by Schneider (2018), discrete sampling can explain both the flash-lag and the Frohlich illusion, whereby a lag in the conscious updating of a moving stimulus alters its perceived spatial location in comparison to a stationary stimulus. The alpha-band frequency, which is associated with phasic modulation of stimulus detection and the temporal resolution of perception, has been proposed to reflect the duration of perceptual moments. The goal of this study was to determine whether a single oscillator (e.g., alpha) is underlying the duration of perceptual moments, which would predict that the point of subjective equality (PSE) in the flash-lag and Frohlich illusions are positively correlated across individuals. Although our displays induced robust flash-lag and Frohlich effects, virtually zero correlation was seen between the PSE in the two illusions, indicating that the illusion magnitudes are unrelated across observers. These findings suggest that, if discrete sampling theory is true, these illusory percepts either rely on different oscillatory frequencies or not on oscillations at all. Alternatively, discrete sampling may not be the mechanism underlying these two motion illusions or our methods were ill-suited to test the theory.

neuroscience↗

bioRxiv2020

Spontaneous alpha-band amplitude predicts subjective visibility but not discrimination accuracy during high-level perception

Near-threshold perception is a paradigm case of perceptual reports diverging from reality - perception of an unchanging stimulus can vacillate from undetected to clearly perceived. Among the many factors that predict whether a stimulus will reach awareness, the amplitude of low-frequency brain oscillations - particularly in the alpha frequency band (8-13 Hz) - has emerged as a reliable predictor of trial-to-trial variability in perceptual decisions. Analysis grounded in signal detection theory suggest that strong prestimulus alpha oscillations diminish subjective perception without affecting the accuracy or sensitivity (d) of perceptual decisions. These results, coupled with recent studies on sensory responses, point to an inhibitory influence of alpha-band amplitude on early visuocortical activity. The findings to date have been based on simple, low-level visual stimuli, which warrant a focus on early visual processing. However, the physiology of alpha in higher-level visual areas is known to be distinct from early visual cortex, with evidence indicating that alpha amplitude in the inferior temporal (IT) cortex is excitatory (rather than inhibitory, as in early visual cortex). Here, we addressed the question of how spontaneous oscillatory amplitude impacts subjective and objective aspects of perception using a high-level perceptual decision task. Human observers completed a near-threshold face/house discrimination task with subjective visibility ratings while electroencephalograms (EEG) were recorded. Using a single-trial multiple regression analysis, we found that spontaneous fluctuations in pre-stimulus alpha-band amplitude were negatively related to visibility ratings but did not predict trial-by-trial accuracy. These results suggest that the inhibitory influence of prestimulus alpha activity in early visual cortex, rather than the excitatory influence of alpha in IT, comes to bias high-level perceptual reports. Our findings provide further evidence that ongoing alpha amplitude dissociates subjective and objective measures of visual perception.

neuroscience↗

bioRxiv2020

The positive evidence bias in perceptual confidence is not post-decisional

Confidence in a perceptual decision is a subjective estimate of the accuracy of ones choice. As such, confidence is thought to be an important computation for a variety of cognitive and perceptual processes, and it features heavily in theorizing about conscious access to perceptual states. Recent experiments have revealed a "positive evidence bias" (PEB) in the computations underlying confidence reports. A PEB occurs when confidence, unlike objective choice, over-weights the evidence for the chosen option, relative to evidence against the chosen option. Accordingly, in a perceptual task, appropriate stimulus conditions can be arranged that produce selective changes in confidence reports but no changes in accuracy. Although the PEB is generally assumed to reflect the observers perceptual and/or decision processes, post-decisional accounts have not been ruled out. We therefore asked whether the PEB persisted under novel conditions that eliminated two possible post-decisional accounts: 1) post-decision evidence accumulation that contributes to a confidence report solicited after the perceptual choice, and 2) a memory bias that emerges in the delay between the stimulus offset and the confidence report. We found that even when the stimulus remained on the screen until observers responded, and when observers reported their choice and confidence simultaneously, the PEB still emerged. Signal detection-based modeling also showed that the PEB was not associated with changes to metacognitive efficiency, but rather to confidence criteria. We conclude that once-plausible post-decisional accounts of the PEB do not explain the bias, bolstering the idea that it is perceptual or decisional in nature.

animal behavior and cognition↗

bioRxiv2019

Perceptual metacognition of human faces is causally supported by function of the lateral prefrontal cortex

Metacognitive visual awareness--the ability to know that one is having a particular visual experience--is thought to optimally guide behavior and help us accurately navigate our complex social environments. Yet the neural underpinnings of visual metacognition continue to be the subject of vigorous debate: While prior work identified correlations between perceptual metacognitive ability and the structure and function of lateral prefrontal cortex (LPFC), evidence for a causal role of this region in promoting metacognition is controversial--in fact, some question whether the prefrontal cortex plays any role in supporting conscious visual experiences. Moreover, whether LPFC function promotes metacognitive awareness of perceptual and emotional features of complex, yet ubiquitous socio-emotional face stimuli is unknown. Using model-based analyses following a causal intervention to LPFC in humans, we demonstrate that LPFC promotes metacognitive awareness of the orientation of emotional faces--although not of their emotional expressions. Effects were specific to metacognition, as LPFC perturbation did not alter stimulus discrimination performance. Collectively, these data support the causal involvement of the prefrontal cortex in metacognitive awareness, and indicate that the role of LPFC function in metacognition encompasses perceptual experiences of naturalistic social stimuli.

neuroscience↗

bioRxiv2018

Multiple mechanisms link prestimulus neural oscillations to sensory responses

Spontaneous fluctuations of neural activity may explain why sensory responses vary across repeated presentation of the same physical stimulus. To test this hypothesis, we recorded electroencephalography in humans during stimulation with identical visual stimuli and analyzed how prestimulus neural oscillations modulate different stages of sensory processing reflected by distinct components of the event-related potential (ERP). We found that strong prestimulus alpha- and beta-band power resulted in a suppression of early ERP components (C1 and N150) and in an amplification of late components (after 0.4 s). Whereas functional inhibition of sensory processing underlies the reduction of early ERP responses, we found that the modulation of non-zero-mean oscillations (baseline shift) accounted for the amplification of late responses. Distinguishing between these two mechanisms is crucial for the understanding of how internal brain states modulate the processing of incoming sensory information.

neuroscience↗

bioRxiv2018

Confidence boosts serial dependence in orientation estimation

In the absence of external feedback, a decision maker must rely on a subjective estimate of their decision accuracy in order to appropriately guide behavior. Normative models of perceptual decision making relate subjective estimates of internal signal quality (e.g. confidence) directly to the internal signal quality itself, thereby making it unknowable whether the subjective estimate or the underlying signal is what drives behavior. We constructed stimuli that dissociated human observers performance on a visual estimation task from their subjective estimates of confidence in their performance, thus violating normative principles. To understand whether confidence influences future decision making, we examined serial dependence in observers responses, a phenomenon whereby the estimate of a stimulus on the current trial can be biased towards the stimulus from the previous trial. We found that when decisions were made with high confidence, they conferred stronger biases upon the following trial, suggesting that confidence may enhance serial dependence. Critically, this finding was true also when confidence was experimentally dissociated from task performance, indicating that subjective confidence, independent of signal quality, can amplify serial dependence. These findings demonstrate an effect of confidence on future behavior, independent of task performance, and suggest that perceptual decisions incorporate recent history in an uncertainty-weighted manner, but where the uncertainty carried forward is a subjectively estimated and possibly suboptimal readout of objective sensory uncertainty.

neuroscience↗

bioRxiv2017

Correlated Individual Differences Suggest A Common Mechanism Underlying Metacognition In Visual Perception And Visual Short-Term Memory

Adaptive behavior depends on the ability to accurately introspect about ones own performance. Whether this metacognitive ability is supported by the same mechanisms across different tasks has thus far been investigated with a focus on correlating metacognitive accuracy between perception and long-term memory paradigms. Here, we investigated the relationship between metacognition of visual perception and metacognition of visual short-term memory (VSTM), a cognitive function thought to be more intimately related to visual processing. Experiments 1 and 2 required subjects to estimate the perceived or remembered orientation of a grating stimulus and rate their confidence. We observed strong positive correlations between individual differences in metacognitive accuracy between the two tasks. This relationship was not accounted for by individual differences in task performance or average confidence, and was present across two different metrics of metacognition and in both experiments. A model-based analysis of data from a third experiment showed that a cross-domain correlation only emerged when both tasks shared the same task-relevant stimulus feature. That is, metacognition for perception and VSTM were correlated when both tasks required orientation judgments, but not when the perceptual task was switched to require contrast judgments. In contrast to previous results comparing perception and long-term memory, which have largely provided evidence for domain-specific metacognitive processes, the current findings suggest that metacognition of visual perception and VSTM is supported by a domain-general metacognitive architecture, but only when both domains share the same task-relevant stimulus feature.

neuroscience↗

bioRxiv2016

Prestimulus alpha-band power biases visual discrimination confidence, but not accuracy

Oscillations in the alpha-band (8-13 Hz) of human electroencephalographic (EEG) recordings are thought to reflect cortical excitability. As such, the magnitude of alpha power prior to the onset of a near threshold visual stimulus has been shown to predict stimulus detectability. Mechanistically, however, non-specific increases in visual-cortical excitability should result in amplified signal as well as amplified noise, leaving actual discriminability unchanged. Using a two-choice orientation discrimination task with equally probable stimuli, we found that discrimination accuracy was unaffected by fluctuations in prestimulus alpha-band power. Decision confidence, on the other hand, was strongly negatively correlated with prestimulus alpha power. This finding constitutes a clear dissociation between objective and subjective measures of visual perception as a function of prestimulus cortical excitability. This dissociation is predicted by models of perceptual confidence under which the balance of evidence in favor of each choice drives objective performance but only the magnitude of evidence in favor of the chosen stimulus drives subjective reports, suggesting that human perceptual confidence can be suboptimal.

neuroscience↗

bioRxiv2016

Electrophysiological correlates of phosphene perception induced by stimulation of human occipital and parietal cortex

Magnetic stimulation (TMS) of human occipital and posterior parietal cortex can give rise to visual sensations called phosphenes, but neural correlates of phosphene perception preceding and succeeding stimulation of both areas are unknown. Using near-threshold TMS with concurrent electroencephalography (EEG) recordings, we uncover oscillatory brain dynamics that covary, on single trials, with the perception of phosphenes following occipital and parietal TMS. Prestimulus power and phase predominantly in the alpha-band (8-13 Hz) predicted occipital TMS phosphenes, whereas higher frequency beta-band (13-20 Hz) power (but not phase) predicted parietal TMS phosphenes. TMS evoked responses related to phosphene perception were similar across stimulation sites and were characterized by an early (200 ms) posterior negativity and a later (>300 ms) parietal positivity in the time domain and an increase in low-frequency (~5-7 Hz) power followed by a broadband decrease in alpha/beta power in the time-frequency domain. These correlates of phosphene perception closely resemble known electrophysiological correlates of conscious perception using near-threshold visual stimuli and speak to the possible early onset of visual consciousness. The differential pattern of prestimulus predictors of phosphene perception suggest that distinct frequencies reflect cortical excitability within different cortical regions, and that the alpha-band rhythm, long thought of as a general index of cortical inhibition, may not reflect excitability of posterior parietal cortex.\n\nSignificance statementAlpha-band oscillations are thought to reflect cortical excitability and are therefor suggested to play an important role in gating information transmission across cortex. We directly probe cortical excitability in human occipital and parietal cortex and observed that whereas alpha-band dynamics indeed reflect excitability of occipital areas, beta-band activity was most predictive of parietal cortex excitability. Differences in the state of cortical excitability predicted perceptual outcomes, which were manifest in both early and late patterns of evoked activity, shedding light on the neural correlates of consciousness. Our findings prompt revision of the notion that alpha activity reflects inhibition across all of cortex and suggests instead that excitability in different regions is reflected in distinct frequency bands.

neuroscience↗