Natanael Antonio dos Santos

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.

Mesopic radial frequency contrast sensitivity function for young and older adults

The objective of the present study was to determine contrast sensitivity curves of concentric circular patterns with radial frequencies of 0.25, 0.5, 1.0, 2.0, and 4.0 cycles per degree in young and older adult volunteers. These parameters were also compared with sensitivity contrasts for sine-wave gratings. All participants had normal acuity vision and were free of identifiable ocular illness. Contrast sensitivity was measured in 6 young adults aged 19 to 23 years and 6 older adults aged 60 to 69 years using the psychophysical forced-choice method. In this paradigm the volunteers had to decide which of two stimuli contained the above radial frequencies at low contrast levels. The other neutral stimulus was gray with homogeneous luminance. We detected a decline in contrast sensitivity for older adults at all radial frequencies compared to young adults. Also, contrast sensitivity for sine-wave gratings at all measured frequencies was better, as predicted, for all young adults. Maximum sensitivities in the radial frequency contrast sensitivity function and contrast sensitivity function occurred at 0.25 and 0.5 cycles per degree, respectively, for both young and older adults. These results suggest age-related changes in the contrast sensitivity function for concentric symmetrical stimuli.

Função de Sensibilidade ao Contraste: Indicador da Percepção Visual da Forma e da Resolução Espacial

This work discusses the contrast sensitivity function (CSF) in three sections. The first describes some theoretical and experimental questions of the CSF related to basic and hard-working research. The second section discusses some elementary spatial stimuli used to characterize the CSF or the response of the human visual system. The third section shows some final considerations about CSF and spatial elementary stimuli. One of the goals of this paper is to discuss the CSF that together with visual acuity appear to form important indicators of the critical aspects of the visual perception of form and of the spatial resolution. The CSF normally is an estimate of the visibility of any pattern in function of the spatial frequency.

Comparison of angular frequency contrast sensitivity in young and older adults

The aim of the present study was to measure contrast sensitivity curves for angular frequencies in the range between 2 and 96 cycles/360 masculine in older human adult volunteers and to compare these measurements with the more usual contrast sensitivity functions for sine-wave gratings. All subjects were free of identifiable ocular disease and had normal acuity. We measured the contrast thresholds for young adults (N=6; age range, 20-26 years) and older adults (N=6; age range, 60-67 years) using the psychophysical forced-choice method. In this paradigm the volunteers had to choose the stimulus containing a test frequency at low contrast (e.g., either a sine-wave grating or an angular frequency stimulus), or another neutral stimulus at mean luminance (without any contrast). Older adults presented a loss in contrast sensitivity at high and medium angular frequencies compared to the young adults (i.e., from 8 to 96 cycles/360 degrees). Contrary to expectation, contrast sensitivity at low angular frequencies, i.e., 2 and 4 cycles/360 degrees, was better for the older group than for the younger group. On the other hand, contrast sensitivity for sine-wave gratings at 3 and 4 cpd was higher for young adults as expected. These results suggest age-related changes in the contrast sensitivity function for angular frequencies.

Percepção e Processamento Visual da Forma:Discutindo Modelos Teóricos Atuais

This work discusses some aspects about feature detectors and spatial frequency analysis models in five sections. The first section describes briefly historical aspects and the basic rationale idea of each model. The second section shows the neurophysiological and psychophysical evidences about spatial frequency selectivity. The third, the fourth and the fifth sections show controversial aspects about these models: spatial frequency versus feature detectors, global versus local filtering and nonlinear versus linear spatial processing, respectively. One of the goals of this paper is to discuss important aspects about perception and visual processing of form.

Contrast sensitivity to radial frequencies modulated by Jn and jn Bessel profiles

We measured human contrast sensitivity to radial frequencies modulated by cylindrical (Jo) and spherical (jo) Bessel profiles. We also measured responses to profiles of jo, j1, j2, j4, j8, and j16. Functions were measured three times by at least three of eight observers using a forced-choice method. The results conform to our expectations that sensitivity would be higher for cylindrical profiles. We also observed that contrast sensitivity is increased with the j n order for n greater than zero, having distinct orderly effects at the low and high frequency ends. For n = 0, 1, 2, and 4 sensitivity tended to occur around 0.8-1.0 cpd while for n = 8 and 16 it seemed to shift gradually to 0.8-3.0 cpd. We interpret these results as being consistent with the possibility that spatial frequency processing by the human visual system can be defined a priori in terms of polar coordinates and discuss its application to study face perception.

Processamento visual da forma em idosos: curvas de limiar de contraste para freqüências angulares e senoidais

The aim of this work was to measure contrast sensitivity curves for angular and spatial frequencies in the aged. We measured the contrast thresholds from young adults (20-26 years-old) and aged (60-67 years-old). All the participants had good visual acuity and were free from significant ocular pathology. The angular and spatial frequencies measured with a forced-choice psychophysic method were 2, 4, 8, 16, 24, 32, 48, 64 and 96 cycle/360o and 0.5, 1, 2, 3, 4, 6 and 9 cpd, respectively. The results showed a loss in contrast sensitivity for the aged at high and medium angular and spatial frequencies compared to the younger group. We still found that the contrast sensitivity of the aged group at low angular frequency is better than the younger group. These results are consistent with age-related changes in contrast sensitivity function.

Processamento visual da forma: evidências para canais múltiplos de freqüências angulares em humanos

O objetivo deste trabalho foi caracterizar e comparar curvas de resposta do sistema visual humano para filtros de frequencias a ngulares de 1e 4 ciclos/360o em coordenadas polares. Mensuramos 15 curvas para cada filtro com o paradigma de somacao de resposta de suprali miar ecom a participacao de 5 voluntarios adultos. Os resultados demonstraram somacoes maximas de limiar de contraste na frequencia d e teste dosfiltros angulares (1 e 4 ciclos/360

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

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).

Visual contrast sensitivity in patients with impairment of functional independence after stroke.

BackgroundStroke has been considered a serious public health problem in many countries, accounting for complex disorders involving perception, such as visual, cognitive and functional deficits. The impact of stroke on the visual perception of individuals with impairments in functional independence was investigated.MethodsWe measured changes in functional independence and visual function in 40 patients with stroke (M = 52.3, SD = 0.65) and 10 controls (M = 52.5, SD = 0.66). The patients were divided into four subgroups following the Barthel Index (Group A: 20–35, serious dependence; Group B: 40–55, moderate dependence; Group C: 60–95, mild dependence; and Group D: 100 points, independence). Visual function was evaluated using the Contrast Sensitivity Function (CSF). The contrast threshold was measured using a temporal, two-alternative, forced-choice psychophysical method.ResultsThe results show significant differences in CSF between healthy volunteers and patients with stroke (F (1.56) = 151.2, p < 0.001) for all frequencies (F (2.56) = 125.96, p < 0.001). The results also show that patients with low functional independence had lower contrast sensitivity than those with greater functional independence (F (3.56) = 344.82, p < 0.001).ConclusionsAn association exists between CSF and a worsening in the functional potential for performing daily living activities. Our results suggest that the CSF can be used as a diagnostic tool to analyze visual function associated with deficits in functional independence after stroke. These findings should be considered across the continuum of care for these patients.