Radial frequency stimuli and sine-wave gratings seem to be processed by distinct contrast brain mechanisms
Braz. j. med. biol. res
;
38(3): 419-430, mar. 2005. ilus, graf
Article
in English
| LILACS
| ID: lil-394794
RESUMO
An assumption commonly made in the study of visual perception is that the lower the contrast threshold for a given stimulus, the more sensitive and selective will be the mechanism that processes it. On the basis of this consideration, we investigated contrast thresholds for two classes of stimuli sine-wave gratings and radial frequency stimuli (i.e., j0 targets or stimuli modulated by spherical Bessel functions). Employing a suprathreshold summation method, we measured the selectivity of spatial and radial frequency filters using either sine-wave gratings or j0 target contrast profiles at either 1 or 4 cycles per degree of visual angle (cpd), as the test frequencies. Thus, in a forced-choice trial, observers chose between a background spatial (or radial) frequency alone and the given background stimulus plus the test frequency (1 or 4 cpd sine-wave grating or radial frequency). Contrary to our expectations, the results showed elevated thresholds (i.e., inhibition) for sine-wave gratings and decreased thresholds (i.e., summation) for radial frequencies when background and test frequencies were identical. This was true for both 1- and 4-cpd test frequencies. This finding suggests that sine-wave gratings and radial frequency stimuli are processed by different quasi-linear systems, one working at low luminance and contrast level (sine-wave gratings) and the other at high luminance and contrast levels (radial frequency stimuli). We think that this interpretation is consistent with distinct foveal only and foveal-parafoveal mechanisms involving striate and/or other higher visual areas (i.e., V2 and V4).
Full text:
Available
Index:
LILACS (Americas)
Main subject:
Photic Stimulation
/
Visual Perception
/
Contrast Sensitivity
Limits:
Adult
/
Humans
Language:
English
Journal:
Braz. j. med. biol. res
Journal subject:
Biology
/
Medicine
Year:
2005
Type:
Article
/
Project document
Affiliation country:
Brazil
Institution/Affiliation country:
Universidade Federal da Paraíba/BR
/
Universidade Federal de Pernambuco/BR
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