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| Research article summary (published 30 Dec 2002): |
Comparison at a distance.
Full Abstract
The visual system is known to contain hard-wired mechanisms that compare the values of a given stimulus attribute at adjacent positions in the visual field; but how are comparisons performed when the stimuli are not adjacent? We ask empirically how well a human observer can compare two stimuli that are separated in the visual field. For the stimulus attributes of spatial frequency, contrast, and orientation, we have measured discrimination thresholds as a function of the spatial separation of the discriminanda. The three attributes were studied in separate experiments, but in all cases the target stimuli were briefly presented Gabor patches. The Gabor patches lay on an imaginary circle, which was centred on the fixation point and had a radius of 5 deg of visual angle. Our psychophysical procedures were designed to ensure that the subject actively compared the two stimuli on each presentation, rather than referring just one stimulus to a stored template or criterion. For the cases of spatial frequency and contrast, there was no systematic effect of spatial separation up to 10 deg. We conclude that the subject's judgment does not depend on discontinuity detectors in the early visual system but on more central codes that represent the two stimuli individually. In the case of orientation discrimination, two naive subjects performed as in the cases of spatial frequency and contrast; but two highly trained subjects showed a systematic increase of threshold with spatial separation, suggesting that they were exploiting a distal mechanism designed to detect the parallelism or non-parallelism of contours.
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Author information
Author/s: Danilova, Marina V (MV); Mollon, John D (JD);
Affiliation: Visual Physiology Laboratory, I P Pavlov Institute of Physiology, Russian Academy of Sciences, nab. Makarova 6, 199034 St Petersburg, Russia. dan(-atsign-)pavlov.infran.ru
Journal and publication information
Publication Type: Journal Article; Multicenter Study; Research Support, Non-U.S. Gov't
Journal: Perception (Perception), published in England. (Language: eng)
Reference: 2003-; vol 32 (issue 4) : pp 395-414
Dates: Created 2003/06/05; Completed 2003/09/04; Revised 2006/11/15;
PMID: 12785480, status: MEDLINE (last retrieval date: 11/6/2008)
Sourced from the National Library of Medicine. Abstract text and other information may be subject to copyright.
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