The multiple-faces phenomenon: some investigate studies.
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Adolescent↗
Biomedical subjects
M L Simas
Publications and source records attributed to M L Simas.
Contrast sensitivity to angular frequency stimuli is higher than that for sinewave gratings in the respective middle range.
This study compares contrast thresholds for sinewave gratings, or spatial frequencies (1/CSF) with contrast thresholds for angular frequencies (1/aCSF) and for radial frequencies, or Jzero targets (1/rCSF). Observers had to differentiate between one of these frequency stimuli and a stimulus at mean luminance within a forced-choice procedure. All measurements were made with the same equipment, methods and subjects. Our results show higher sensitivity to, or lower thresholds for, angular frequencies when compared to either sinewave gratings or Jzero targets. Contrast values in arbitrary units, in the lower threshold range for angular frequencies, were about half those required to differentiate sinewave gratings from mean luminance in its most sensitive range.
Contrast Sensitivity↗
Inhibitory effects in the detection of 1 cpd jo targets superimposed to angular frequency stimuli or sinewave gratings.
Independence among channels processing different aspects of spatial information, including orthogonal stimuli, has been generally assumed in the literature. We tested independence between the processing of jo targets and the processing of either vertical sinusoidal gratings or angular frequency stimuli with suprathreshold summation. We found the detection of a jo target at 1 cpd to be affected in an inhibitory fashion by either background angular frequencies in the range of 3-96 cycles or sinewave gratings in the range of 0.8-3.0 cpd. These results demonstrate interactions both among orthogonal stimuli and among channels processing vertical sinewave gratings and jo target stimuli. Our discussion focuses on the hypothesis of frequency decomposition in polar coordinates.
Contrast Sensitivity↗
Inhibitory sidebands in multiple angular frequency filters in the human visual system.
The view of multiple spatial frequency channels to characterize spatial factors determining the processing of contrast and brightness information has been mostly based on studies using sine wave gratings. In a study of angular frequency filtering, we demonstrated the presence of multiple peaks reflecting inhibitory activity in the measurement of a 24-cycle angular frequency filter (Simas MLB & Dodwell PC (1990). Spatial Vision, 5: 59-74). We now report sensitivity functions for angular frequency filters at 2, 4, 9, 13, 16, and 47 cycles, and have included new data for 24 cycles using the same method and analogous equipment to allow comparisons with our previous study. The present findings consistently show inhibitory effects flanking the filter's specific test frequency and, in every case, much weaker or almost no inhibition occurs at the specific test frequency for the given angular frequency filter.
Contrast Sensitivity↗
Vanishing optotypes: is single presentation superior to chart exposure?
A study comparing single optotype presentation vs entire chart exposure (with spacings of either 1.5 or 3.0 times the optotype width) was conducted on 46 subjects divided into two groups in order to evaluate how the mode of presentation affects visual acuity measurements made with medium-pass filtered vanishing optotypes. There were no significant differences in either group between the two modes of presentation. However, in the individual cases in which a decrease in percent correct responses greater than 20% was observed in either mode of presentation, there was a slight tendency towards superiority of the entire chart exposure mode.
Adolescent↗
Angular frequency filtering: a basis for pattern decomposition.
Much insight has been gained into the nature of early vision by postulating that the system encodes patterns by means of channels, or filters, tuned to specific orientations and spatial frequencies. We argue that a different decomposition, in terms of radially and rotationally symmetrical filters, is equally plausible, both in terms of the geometry of the eye and the ecology of visual pattern recognition. We demonstrate experimentally some of the properties of angular frequency filtering.
Contrast Sensitivity↗