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Comparison at a distance
Marina V Danilova1, John D Mollon
1Visual Physiology Laboratory, I P Pavlov Institute of Physiology, Russian Academy of Sciences, nab. Makarova 6, 199034 St Petersburg, Russia. dan@pavlov.infran.ru
Perception
|June 6, 2003
Summary
Human visual perception can compare non-adjacent stimuli, with performance for spatial frequency and contrast unaffected by separation. Orientation discrimination, however, improves with training, suggesting specialized mechanisms for contour analysis.
Area of Science:
- Visual perception
- Human psychophysics
- Computational neuroscience
Background:
- The visual system possesses mechanisms for comparing adjacent visual stimuli.
- It remains unclear how comparisons are made between stimuli separated in the visual field.
Purpose of the Study:
- To empirically investigate human observers' ability to compare two stimuli separated in the visual field.
- To determine the effect of spatial separation on discrimination thresholds for spatial frequency, contrast, and orientation.
Main Methods:
- Psychophysical experiments using briefly presented Gabor patches.
- Stimuli were presented on an imaginary circle (5 deg radius) centered on the fixation point.
- Procedures ensured active comparison of two stimuli per presentation, avoiding template matching.
Main Results:
- For spatial frequency and contrast, discrimination thresholds showed no systematic effect of spatial separation up to 10 degrees.
- For orientation, naive subjects showed no effect of separation, but trained subjects exhibited increased thresholds with greater separation.
Conclusions:
- Visual judgments for spatial frequency and contrast likely rely on central codes representing individual stimuli, not early visual discontinuity detectors.
- Orientation discrimination in trained individuals suggests the involvement of a specialized distal mechanism for detecting contour parallelism or non-parallelism.