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Visual categorization of simple stimuli

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Stimuli that vary along quantitative (or ‘prothetic’) continuua can be categorized on the basis of how much neural activity they elicit. Loud and bright stimuli elicit more activity than quiet and dim stimuli. Stimuli that vary qualitatively (either along metathetic continuua or between prothetic continuua) cannot be categorized on this basis. It is conceivable that our brains contain homunculi who read tiny signs attached to each fibre that describe their neurons’ preferred stimuli, but contemporary theorists believe category information to be inherent in the cerebral positions of active neurons. This helps explain why stimulus preferences vary systematically in the cortex, forming multi-dimensional ‘maps’ of position, orientation, and possibly other attributes such as spatial frequency, binocular disparity, and chromaticity. Nonetheless, neurons and channels that can be distinguished on the basis of the stimuli they prefer are still called ‘labelled lines’. Various psychophysical methods exist for quantifying channel selectivity and deciding whether those channels qualify as labelled lines. The focus of my talk will be a new model for the 2×2FC paradigm, in which observers must both detect and identify (or categorize) modulations in a visual stimulus. Heretofore, results from this paradigm have either been interpreted in lieu of any models (e.g. ‘labelled lines are implied when threshold for identification equals threshold for detection’) or easily falsifiable high-threshold theories of detection. My new model is based on signal-detection theory, and accommodates a wide range of relationships between detection and identification.

This talk is part of the Craik Club series.

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