Radial frequency stimuli and sine-wave gratings seem to be processed by distinct contrast brain mechanisms

Brazilian Journal of Medical and Biological Research

M.l.b. Simas¹  R.m.t.b.l. Nogueira¹  N.a. Santos² 

[1] ,Universidade Federal de Pernambuco Departamento de Psicologia Laboratório de Percepção VisualRecife PE ,Brasil

关键词: Sine-wave gratings;    Spatial frequency;    Radial frequency;    Bessel functions;    Multiple channels;    Contrast sensitivity;   

DOI  :  10.1590/S0100-879X2005000300013

来源: SciELO

【 摘 要 】

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).

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