We measured human frequency response functions for seven angular frequency filters whose test frequencies were centered at 1, 2, 3, 4, 8, 16 or 24 cycles/360º using a supra-threshold summation method. The seven functions of 17 experimental conditions each were measured nine times for five observers. For the arbitrarily selected filter phases, the maximum summation effect occurred at test frequency for filters at 1, 2, 3, 4 and 8 cycles/360º. For both 16 and 24 cycles/360º test frequencies, maximum summation occurred at the lower harmonics. These results allow us to conclude that there are narrow-band angular frequency filters operating somehow in the human visual system either through summation or inhibition of specific frequency ranges. Furthermore, as a general result, it appears that addition of higher angular frequencies to lower ones disturbs low angular frequency perception (i.e., 1, 2, 3 and 4 cycles/360º), whereas addition of lower harmonics to higher ones seems to improve detection of high angular frequency harmonics (i.e., 8, 16 and 24 cycles/360º). Finally, we discuss the possible involvement of coupled radial and angular frequency filters in face perception using an example where narrow-band low angular frequency filters could have a major role.

Função de Sensibilidade ao ContrasteA sensibilidade ao contraste é definida na literatura como a recíproca da quantidade mínima de contraste necessária para detectar uma grade de uma freqüência espacial específica (Cornsweet, 1970). Em outras palavras, a FSC é, por definição, o inverso da curva de limiar de contraste (1/FSC). Logo, a sensibilidade ao contraste está relacionada à diferença existente entre picos e depressões das ondas. Essa diferença traduz o valor de contraste requerido pelo sistema visual humano, SVH, para distinguir um estímulo de freqüência espacial daquele com um campo uniforme de luminância média. Assim, o sistema visual, SV, possui alta sensibilidade quando um padrão precisa de pouco contraste para ser detectado ResumoEste trabalho discute a função de sensibilidade ao contraste, FSC, em três seções. A primeira descreve alguns aspectos teóricos e experimentais da FSC relacionados à pesquisa básica e aplicada. A segunda discute alguns estímulos espaciais utilizados para caracterizar a FSC ou a resposta do sistema visual humano. A terceira apresenta algumas considerações finais sobre a FSC e estímulos espaciais elementares. Um dos objetivos deste trabalho é discutir a FSC que junto com a acuidade visual parecem formar os principais indicadores dos aspectos críticos da percepção visual da forma e da resolução espacial. A FSC geralmente estima a percepção de qualquer padrão em função de sua freqüência espacial. Palavras-chaves: Percepção visual da forma; acuidade visual; função de sensibilidade ao contraste; resolução espacial; freqüência espacial. Contrast Sensitivity Function: Indicator of the Visual Perception of Form and of the Spatial Resolution AbstractThis 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. Keywords: Visual perception of form; visual acuity; contrast sensitivity function; spatial resolution; spatial frequency. contraste de grades (ou qualquer padrão) como uma função de sua freqüência espacial. Modelo de Canais Múltiplos e a Função de Sensibilidade ao ContrasteDesde que foi proposto por Campbell e Robson (1968), a abordagem de canais múltiplos tem sido utilizada para caracterizar o processamento visual da forma. Esta abordagem assume que o SVH responde seletivamente a freqüências espaciais, ou seja, o SVH processa simultaneamente diferentes faixas estreitas de freqüências espaciais (Blakemore & Campbell, ...

Cigarette smoke is a complex chemical mixture, involving health-damaging components such as carbon monoxide, ammonia, pyridine, toluene and nicotine. While cognitive functions have been well documented in heavy smokers, spatial vision has been less characterized. In the article, we investigated smoking effects through contrast sensitivity function (CSF), a rigorous procedure that measures the spatial vision. Data were recorded from 48 participants, a group of non-smokers (n = 16), a group of chronic and heavy cigarette smokers (n = 16) and deprived smokers (n = 16); age range 20–45 years. Sinewave gratings with spatial frequencies ranging from 0.25 to 20 cycles per degree were used. All subjects were free from any neurological disorder, identifiable ocular disease and had normal acuity. No abnormalities were detected in the fundoscopic examination and in the optical coherence tomography exam. Contrary to expectations, performance on CSF differed between groups. Both smokers and deprived smokers presented a loss of contrast sensitivity compared to non-smokers. Post-hoc analyses suggest that deprived smokers were less sensitive at all spatial frequencies. These results suggest that not only chronic exposure to cigarette compounds but also withdrawal from nicotine affected spatial vision. This highlights the importance of understanding diffuse effects of smoking compounds on visual spatial processing.

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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 J 0 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 J 0 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.

We measured human contrast sensitivity to radial frequencies modulated by cylindrical (J o ) and spherical (j o ) Bessel profiles. We also measured responses to profiles of j o , j 1 , j 2 , j 4 , j 8 , and j 16 . 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.

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