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Tim Gastrell

@tgastrell.bsky.social

106 followers 240 following 19 posts

Cognition and Computation 🧠 Tech enjoyer 💻 🥝➡️🦘 PhD candidate in Cog/Comp Neuroscience at The University of Queensland 🧠 Studying how the brain leverages experience to support sensory processing and decision-making under uncertainty.

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Tim Gastrell @tgastrell.bsky.social · 17/04/2026

5/n Inverted encoding models showed strong effects of motion coherence on representational fidelity during stimulus observation which carried over into the response period (top). Priors also enhanced stimulus representation (bottom), but only in the response period once the stimulus was gone! 🤯

Neural tuning to motion direction expressed as decoding accuracy stratified by A) stimulus precision and B) prior precision during the observation and response periods (note the difference y-axis scales). Vertical red lines and grey dashed lines indicate coherent motion onset, and offset, respectively. Black solid, dashed and dotted lines indicate cluster significance p < .05 of pairwise condition comparisons. For the purposes of presentation, each trace was smoothed using a temporal Gaussian filter (sigma = 4). All statistical comparisons were performed on the unsmoothed data.

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Tim Gastrell @tgastrell.bsky.social · 17/04/2026

3/n Estimation behaviour followed the textbook Bayesian pattern: Responses were biased towards expected motion directions and the magnitude of bias scaled with the relative precision of the stimulus and the prior. (panel C) Weak stimulus + strong informative prior = biggest bias.

Behavioural results for Experiment 2 (EEG + Behaviour). A) Trial-level error histograms for each level of stimulus precision. B) Response accuracy stratified by stimulus precision and prior precision. C) Response bias stratified by stimulus precision and prior precision. Bias expressed as mean signed-error magnitudes such that attraction to the central-tendency of the current prior is captured by a positive shift in the error distribution. D) Bias stratified by stimulus precision and time within the session grouped into three equal-sized bins. Here, time reflects the exposure participants had to each of the priors. For visualisation, the data used to construct boxplots were normalised to remove the between subjects’ variability (Cousineau, 2005). Inset bar plots indicate t-statistics and significance of mixed GLM parameter estimates. * = p < .05.

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Tim Gastrell @tgastrell.bsky.social · 17/04/2026

2/n Participants made continuous decisions about the direction of a dot-motion stimulus with varying levels of coherence. We manipulated prior expectation by sampling the target directions from von Mises distributions whose location and concentration parameters varied each block.

Overview of the task and the manipulation of stimulus and prior precision. A) Structure of a single trial from pre-fixation to response. Note: trial diagram is schematic only, colours and scale differ from actual design (see Methods). B) Stimulus precision was manipulated via motion coherence, corresponding to the precision of the sampling distribution of possible dot-directions on each frame. Zero corresponds to the global target motion direction. C) Priors for motion direction were manipulated via the block-level sampling distribution for global target motion directions. These distributions could either be completely uninformative (uniform) or contain some information about the probability of upcoming stimulus values. In this case zero corresponds to the central tendency of the block-wise sampling distribution/prior.

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Tim Gastrell @tgastrell.bsky.social · 20/06/2025

Excited to be presenting behavioral and modeling work looking at perceptual decision making under uncertainty with recently learned priors at the EPC/APCV joint meeting today at UNSW, Sydney! Catch me at 2:45pm in Gallery 1! @expsyanz.bsky.social #epc2025

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Tim Gastrell @tgastrell.bsky.social · 04/06/2025

New paper @ JOV! With Matt Oxner, Frank Schumann and David Carmel. 🧠 Perception of ambiguous motion is shaped by recent visual experience - but does this differ across the visual field? Yes! Full open access paper here (Gastrell et al., 2025, Journal of Vision): 🔗 doi.org/10.1167/jov.... 🧵

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Tim Gastrell @tgastrell.bsky.social · 30/11/2024

An absolute pleasure to present some new data at ACNS 2024 in (mostly) sunny Newcastle. Here we showed that human observers judgements of simple motion stimuli broadly follow precision-weighted inference principles. @acnsau.bsky.social

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