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.

Subject Sammy

News

entry: 8.6.20

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

entry: 7.9.20

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 cortex

RC Lapate, J Samaha, B Rokers, BR Postle, RJ Davidson Communications Biology


entry: 6.7.20

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

entry: 3.17.20

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.

biorXiv link / data and code

entry: 12.11.19

"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!

entry: 9.2.19

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

entry: 7.10.19

New Paper in eLife!

https://elifesciences.org/articles/43620

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!

entry: 4.3.19

Chin/headrests are expensive! We made a step-by-step guide on how to make your own research-quality headrest for visual psychophysics for about $100 using computer monitor stands and some wood! [PDF]

entry: 2.10.19

Register now! The Northern California Consciousness Conference @ UC Davis March 15th: nccucdavis.org