Cognitive and Computational Neuroscience at UCSC
Welcome to the Samaha Lab
Our lab uses psychophysics and computational modeling in concert with tools from cognitive neuroscience to measure and manipulate the human brain. We seek to understand the neural basis of visual consciousness, attention, perceptual decision making, metacognition, and working memory, and have a particular interest in the role of neural oscillations in these domains.
We are housed within the Department of Psychology at the University of California, Santa Cruz.
Two new lab pre-prints on neural oscillations and sensory brain activity
In "Pre-Stimulus Alpha-Band Phase Gates Afferent Visual Cortex Responses" led by lab Ph.D. student Wei Dou, we find that the phase of ongoing alpha-band oscillations modulated the afferent cortical response to a visual stimulus, as captured by EEG responses taken during the time window of the C1 event-related potential component. We take this as evidence for a thalamic inhibition model of alpha, consistent with animal models of alpha that posit generators of phasic inhibition in the LGN.
In "Spontaneous neural oscillations influence behavior and sensory representations by suppressing neuronal excitability" collaborator Luca Iemi and colleagues analyzed electrocorticography data recorded across many brain areas to ask how oscillatory amplitude modulates broad-band gamma activity in response to visual and auditory stimuli. We report that alpha and beta power inversely correlates with spontaneous and stimulus-evoked broad-band gamma activity - an effect that mediated reaction times in a discrimination task. Interestingly, a pattern classifier trained to discriminate different stimuli found that ongoing alpha power modulated the magnitude of the responses, leading to higher classifier confidence, but no change in classifier accuracy.
Spontaneous neural oscillations influence behavior and sensory representations by suppressing neuronal excitabilityL Iemi, L Gwilliams, J Samaha, R Auksztulewicz, YM Cycowicz, JR King, VV Nikulin, T Thesen, W Doyle, O Devinsky, CE Schroeder, L Melloni, S Haegens biorXiv
New pre-print suggests that a unified visual sampling rate does not explain both the Flash-lag and Fröhlich effect.
In this project led by Ph.D. student Audrey Morrow, we find that individual differences in the magnitude of two motion illusions - the flash-lag and Fröhlich effect - are not correlated. Prior theory has suggested that the two illusions are driven by discrete sampling in the visual system, which has been suggested to occur at the rate of the alpha-band oscillations. However, the finding that these two illusions are not correlated indicates that they are produced by different mechanisms. Whereas discrete sampling may still be correct, this null effect suggests that different oscillation frequencies could underlie the illusions or that oscillations are not the neural implementation of discrete sampling at all. Alternatively, discrete sampling may not drive these illusions and alternative accounts, for example, based on motion extrapolation and masking may explain the flash-lag and Fröhlich effect, respectively.
No evidence for a single oscillator underlying discrete visual perceptsA Morrow, J Samaha biorXiv
Pre-registered report on reconstructing working memory representations under different states of prioritization
Led by Quan Wan in the Postle lab, we show, across two experiments (one pre-registered) that a stimulus held in working memory (WM) but in an unprioritized state (e.g., when attending to a different working memory item) is represented in a pattern of neural activity that is "inverted" compared to when that same item is prioritized. This suggests that unprioritized WM items are maintained in active patterns of neural activity, as opposed to being 'silent', but in a qualitatively distinct neural code, as opposed to simply being weaker. Great work by Quan and team!
Open access link below!
Tracking stimulus representation across a 2-back visual working memory task. Quan Wan, Ying Cai, Jason Samaha and Bradley R. Postle. Royal Society Open Science
New paper on the role of PFC in perceptual metacognition
There is growing debate around the role of the prefrontal cortex (PFC) in sensory metacognition. In this paper, lead by UCSB prof. Regina Lapate, we add new evidence in support of a causal role of PFC to human metacognition by perturbing the dorsal-lateral PFC with MRI-guided transcranial magnetic stimulation (TMS). TMS to dlPFC was specifically found to impair metacognitive awareness (the extent to which subjective visibility ratings discriminate between task performance) without changing task performance. This was all done in the context of a face discrimination task, thereby extending prior results into the domain of higher-level perception of socially-relevant stimuli. Open access link below!
Perceptual metacognition of human faces is causally supported by function of the lateral prefrontal cortexRC Lapate, J Samaha, B Rokers, BR Postle, RJ Davidson Communications Biology
New TiCS review on neural oscillations and perceptual decision-making
We reviewed decades of evidence linking spontaneous fluctuations in the amplitude of low-frequency neural oscillations as well as more recent experiments using signal detection theory frameworks. We suggest that weak prestimulus alpha amplitude enhances early sensory responses, which causes sensory activity to surpass detection and confidence/visibility criteria more frequently. However, the global nature of such spontaneous excitability fluctuations means that sensory activity reflecting all decision options are equally enhanced so that discriminability (or d') is unchanged, leading to a dissociation between subjective awareness (detection, visibility/confidence) and objective performance (accuracy/sensitivity). Special thanks to co-authors Luca Iemi, Saskia Haegens, and Niko Busch!
link to pdf of paper
New preprint: The positive evidence bias in perceptual confidence is not post-decisional
We investigated at what stage of processing the "positive evidence bias" (PEB) in human confidence occurs. We find that the bias is not explained by post-decisional mechanisms (e.g., by assuming that additional evidence accumulates after the perceptual decision is made). This suggest the PEB emerges in perception itself or during the decision process.
"Confidence Database" paper accepted in Nature Human Behavior
We are thrilled to be part of a new resource for the community working on confidence and decision making. As part of a multi-site initiative led by Doby Rahnev, a new paper describing a database of confidence experiments with over 8,700 participants has just been accepted at Nature Human Behavior. We contributed some of our datasets on perceptual estimation and confidence in perception and short-term memory.
Data and code are here (https://osf.io/s46pr/)
Pre-print is here (https://psyarxiv.com/h8tju)
The database is still accepting experiments. Consider contributing!
STEM Diversity Poster Session
Undergraduate researchers Lleymi Martinez (right) and Olenka Graham Castaneda (left) present their work in the lab looking at neural correlates of subjective confidence in motion perception. We used a novel paradigm that controls for objective accuracy in motion direction discrimination but leads to changes in subjective confidence. Under these conditions, it seems like late components of the event-related potential - which have previously been associated with confidence under non-performance matched conditions - still tracks subjective reports of confidence!
2019 UCSC STEM Diversity poster faire
New Paper in eLife!
Iemi et al find that prestimulus neural oscillations have opposite effects on the early and late visual-evoked response. We found suppression of the C1 and N1 ERP component by prestimulus alpha- and beta-band power, as predicted by functional inhibition theories of low-frequency oscillations. Interestingly, however, the same prestimulus oscillations led to enhanced late (>200 ms) ERP components, which we can explain by a baseline shift mechanism (caused by modulation of oscillations with non-zero mean). We hope this paper adds to the discussion about the functional role of alpha/beta oscillations and the neural generators of ERPs!