Brown Bag Talk by Mr. Syaheed Jabar on 22 August


Speaker: Mr. Syaheed Jabar

Title: Orienting to probable stimuli increases speed, precision, and kurtosis. A study in perceptual estimation

Date: 22 August 2014, 1-2pm

Venue: AS4/02-08 (Psychology Department Meeting Room)


Stimulus probabilities affect detection performance. Rare targets, even in security or medical screenings, are missed more often than frequent ones. To minimize such probability-related costs, there is a need to understand how probability effects relate to perceptual processes. A previous experiment demonstrated that smaller errors were made when estimating orientations of exogenously-cued spatial Gabors. Exogenous cues might be biasing perceptual processing towards the features in the cued location, and enhancing the perceptual representations of the target. Here, the same “attentional” effects were replicated without the use of explicit cues. Instead, different location-orientation conjunctions occurred with different probabilities. Across different probability distributions, it was consistently observed that participants rapidly developed faster and more precise estimations for higher-probability tilts. This occurred despite participants not being instructed on the underlying probability distributions, despite participants not being able to indicate confidence differences, despite the probability distribution being complex, and despite probability differences being fine-grained. High-probability tilts were also consistently associated with a distribution of angular errors that were more kurtotic than for low-probability tilts. Mixture model analyses suggested that these kurtosis differences reflect a mix of ‘precise’ and ‘coarse’ estimations, with high-probability tilts being associated with more of the former. Additionally, near-vertical orientations were associated with an increased kurtosis, particularly when vertical tilts were probable. A neurobiological simulation further suggests that these observed perceptual effects are mathematically consistent with stimulus probability affecting the width and mixture of V1 population tuning functions. Similar to mechanisms underlying perceptual biases, these findings suggest that acquired information might be affecting neural sensitivity to result in better-encoded perceptual representations for high-probability tilts.

About the Speaker:

Syaheed received his B.Soc.Sci. (Hons.) degree majoring in Psychology from the National University of Singapore in 2013, where he was also in the University Scholars Programme. He is currently in the psychology (Cognitive Neuroscience) PhD program at the University of Waterloo, supervised by Dr. Britt Anderson. He is particularly interested in studying, and computationally modelling, perceptual and attentional effects. He prefers writing code to writing papers.

Print Friendly, PDF & Email

Leave a Reply

Skip to toolbar