Seeing light vs dark lines: psychophysical performance is based on separate channels, limited by noise and uncertainty

Stuart A. Wallis, Mark A. Georgeson, Puja Mehta

Research output: Unpublished contribution to conferencePosterpeer-review


Visual detection performance (d') is usually an accelerating function of stimulus contrast, which could imply a smooth, threshold-like nonlinearity in the sensory response. Alternatively, Pelli (1985 Journal of the Optical Society of America A 2 1508 - 1532) developed the 'uncertainty model' in which responses were linear with contrast, but the observer was uncertain about which of many noisy channels contained the signal. Such internal uncertainty effectively adds noise to weak signals, and predicts the nonlinear psychometric function. We re-examined these ideas by plotting psychometric functions (as z-scores) for two observers (SAW, PRM) with high precision. The task was to detect a single, vertical, blurred line at the fixation point, or identify its polarity (light vs dark). Detection of a known polarity was nearly linear for SAW but very nonlinear for PRM. Randomly interleaving light and dark trials reduced performance and rendered it non-linear for SAW, but had little effect for PRM. This occurred for both single-interval and 2AFC procedures. The whole pattern of results was well predicted by our Monte Carlo simulation of Pelli's model, with only two free parameters. SAW (highly practised) had very low uncertainty. PRM (with little prior practice) had much greater uncertainty, resulting in lower contrast sensitivity, nonlinear performance, and no effect of external (polarity) uncertainty. For SAW, identification was about v2 better than detection, implying statistically independent channels for stimuli of opposite polarity, rather than an opponent (light - dark) channel. These findings strongly suggest that noise and uncertainty, rather than sensory nonlinearity, limit visual detection.
Original languageEnglish
Publication statusUnpublished - 2008
Event12th Applied Vision Association Christmas Meeting - Aston University, Birmingham (UK), United Kingdom
Duration: 17 Dec 2007 → …


Other12th Applied Vision Association Christmas Meeting
Country/TerritoryUnited Kingdom
CityAston University, Birmingham (UK)
Period17/12/07 → …

Bibliographical note

Abstract published in Twelfth Applied Vision Christmas Meeting, Perception, 37(2), p.315, ISSN 0001-4966. If you have discovered material in AURA which is unlawful e.g. breaches copyright, (either yours or that of a third party) or any other law, including but not limited to those relating to patent, trademark, confidentiality, data protection, obscenity, defamation, libel, then please read our Takedown Policy and contact the service immediately.


  • visual detection performance
  • stimulus contrast
  • sensory response
  • psychometric functions
  • noise
  • uncertainty
  • sensory nonlinearity
  • visual detection


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