[show abstract][hide abstract] ABSTRACT: Contrast sensitivity for face identification was measured as a function of image size to find out whether foveal and peripheral performance would become equivalent by magnification. Size scaling was not sufficient for this task, but when the data was scaled both in size and contrast dimensions, there was no significant eccentricity-dependent variation in the data, i.e. for equivalent performance both the size and contrast needed to increase in the periphery. By utilising spatial noise added to the images we found that in periphery information was utilised less efficiently and peripheral inferiority arose completely from lower efficiency, not from increased internal noise.
Vision Research 04/2001; 41(5):599-610. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Flicker sensitivities (1-30 Hz) in foveal, photopic vision were measured as functions of stimulus area with and without strong external white temporal noise. Stimuli were circular, sinusoidally flickering sharp-edged spots of variable diameters (0.25-4 degrees ) but constant duration (2 s), surrounded by a uniform equiluminant field. The data was described with a model comprising (i) low-pass filtering in the retina (R), with a modulation transfer function (MTF) of a form derived from responses of cones; (ii) normalisation of the temporal luminance distribution by the average luminance; (iii) high-pass filtering by postreceptoral neural pathways (P), with an MTF proportional to temporal frequency; (iv) addition of internal white neural noise (N(i)); (v) integration over a spatial window; and (vi) detection by a suboptimal temporal matched filter of efficiency eta. In strong external noise, flicker sensitivity was independent of spot area. Without external noise, sensitivity increased with the square root of stimulus area (Piper's law) up to a critical area (A(c)), where it reaches a maximum level (S(max)). Both A(c) and eta were monotonic functions of temporal frequency (f), such that log A(c) increased and log eta decreased linearly with log f. Remarkably, the increase in spatial integration area and the decrease in efficiency were just balanced, so A(c)(f)eta(f) was invariant against f. Thus the bandpass characteristics of S(max)(f) directly reflected the composite effect of the distal filters R(f) and P(f). The temporal equivalent (N(it)) of internal neural noise (N(i)) decreased in inverse proportion to spot area up to A(c) and then stayed constant indicating that spatially homogeneous signals and noise are integrated over the same area.
Vision Research 02/2000; 40(28):3841-51. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Contrast matching was performed with isoluminant red-green and s-cone gratings at spatial frequencies ranging from 0.5 to 8 c/deg. Contrast threshold curves were low-pass in shape, in agreement with previous findings. Contrast matching functions resembled threshold curves at low contrast levels, but became flat and independent of spatial frequency at high contrasts. Thus, isoluminant chromatic gratings exhibited contrast constancy at suprathreshold contrast levels in a similar manner as has been demonstrated for achromatic gratings.
Vision Research 02/2000; 40(16):2159-65. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: When masking one-dimensional gratings, the strongest masking effect is achieved by using one-dimensional spatial noise, which can be regarded as a special case of two-dimensional noise where the noise check height is equal to the grating height. The extent of spatial integration in the human visual system is limited, however. Hence, our aim was to investigate whether the effective height of noise checks of one-dimensional noise is similarly limited. We measured detection thresholds for vertical sinusoidal gratings with added spatial noise. The width of the noise checks remained constant, but their height increased until equal to the height of the image window which made noise one-dimensional. The contrast energy thresholds increased in direct proportion to increasing noise check height and the spectral density of noise, calculated by taking into account both the height and the width of the noise checks. The increase levelled off, however, after the critical noise check height (nyc). The critical noise check height in grating cycles changed as a function of spatial frequency (f) as nyc = 4.7 [1 + (1.4/f)2]-0.5. According to our results the effective height of noise checks was thus limited in accordance with studies on spatial integration, showing scale invariance above 1.4 c/deg.
Vision Research 06/1999; 39(10):1775-82. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: We studied spatial integration of Gaussian weighted cosine gratings (Gabor gratings) in contrast detection by using a two-alternative forced-choice method. The Gabor gratings, which were either complete or sharply truncated to a square shape, were presented either in the presence or absence of static noise. Contrast thresholds were determined for different truncation areas and different widths of the two-dimensional Gaussian weighting function. Contrast detection performance was found to depend only on the effective spatial spread of the stimuli. The spatial spread is a physical description of signal area, which takes into account the spatial inhomogeneity of the signal. The highest detection efficiencies were obtained when the spatial spread was small independently of whether the stimuli were sharply truncated or complete smooth Gabor gratings. The results show that the contrast detection mechanism takes into account the shape of the signal window and does not collect information outside the area of the signal.
Ophthalmic and Physiological Optics 06/1999; 19(3):242-52. · 1.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: An illusory bar emerges in a cleft between two opposing gratings. When the gratings rotated around the vertical axis in three-dimensional (3-D) space, the illusory bar was seen either (i) rotating with the inducing gratings or (ii) as a stationary and opaque tape located in front of gratings. This illusion seems to be caused by the different temporal dynamics of the illusion and its inducers, especially by the slower extinction rate for the illusory bar than its inducers. The illusion is a psychophysical demonstration of an illusory figure becoming spatially and temporally loose from its inducers, suggesting that they are processed separately in the brain. This indicates that illusory figures are not only by-products of normal vision but have their own important function.
[show abstract][hide abstract] ABSTRACT: We measured foveal flicker sensitivity with and without external added temporal noise at various levels of retinal illuminance and described the data with our model of flicker sensitivity comprising: (i) low-pass filtering of the flickering signal plus external temporal and/or quantal noise by the modulation transfer function (MTF) of the retina (R): (ii) high-pass filtering in proportion to temporal frequency by the MTF of the postreceptoral neural pathways (P): (iii) addition of internal white neural noise; and (iv) detection by a temporal matched filter. Without temporal noise flicker sensitivity had a band-pass frequency-dependence at high and medium illuminances but changed towards a low-pass shape above 0.5 Hz at low luminances, in agreement with earlier studies. In strong external temporal noise, however, the flicker sensitivity function had a low-pass shape even at high and medium illuminances and flicker sensitivity was consistently lower with noise than without. At low luminances flicker sensitivity was similar with and without noise. An excellent fit of the model was obtained under the assumption that the only luminance-dependent changes were increases in the cut-off frequency (fc) and maximum contrast transfer of R with increasing luminance. The results imply the following: (i) performance is consistent with detection by a temporal matched filter, but not with a thresholding process based on signal amplitude; (ii) quantal fluctuations do not at any luminance level become a source of dominant noise present at the detector; (iii) the changes in the maximum contrast transfer reflect changes in retinal gain, which at low to moderate luminances implement less-than-Weber adaptation, with a 'square-root' law at the lowest levels; (iv) the changes of fc as function of mean luminance closely parallels time scale changes in cones, but the absolute values of fc are lower than expected from the kinetics of monkey cones at all luminances; (v) the constancy of the high-pass filtering function P indicates that surround antagonism does not weaken significantly with decreasing light level.
Vision Research 03/1999; 39(3):533-50. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: We determined the foveal optical modulation transfer functions of the human eye (O) for pupil sizes of 1-8 mm by using two simple psychophysical techniques. O as a function of spatial frequency f could be described by exp[-(f/fc)n] at any pupil size in our data as well as in the data available in the literature [J. Physiol. (London) 186, 558 (1966); Opt. Acta 21, 395 (1974); Vision Res. 33, 15 (1993); J. Opt. Soc. Am. A 11, 246 (1994)]. When all these estimates of fc and n were pooled the parameters were found to depend on the pupil diameter as fc = 16.6 - 1.49p and n = exp(0.840/p - 0.318). This result indicates that at 1 mm O(f) is close to the diffraction-limited system.
Journal of the Optical Society of America A 10/1998; 15(9):2504-13. · 1.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: The purpose of the model presented in this paper is to explain the well known fact that perceived contrast becomes independent of optical low-pass and neural high-pass filtering as well as areal integration with increasing stimulus contrast. In the model we assume that perceived contrast is computed by two different parallel mechanisms. One of them integrates signal information across space to improve the signal-to-noise ratio and is affected by the optical low-pass and neural high-pass filtering. The other mechanism estimates external local contrast by using inverse filtering. These two factors are combined by a 'restoration' mechanism so that the first mechanism affects the perception of low contrast and the latter that of high contrast stimuli. The behaviour of the model was tested against experimental results obtained with normal human observers. At low contrast levels, contrast matching curves were similar in shape to the detection threshold curves both as a function of the spatial frequency and area of the grating. At high contrast levels, contrast matches became physically correct. The model described the experimental results accurately.
Ophthalmic and Physiological Optics 06/1998; 18(3):269-78. · 1.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Illusory figures, created by the visual system between visualizing real objects, are probably caused by processes designed to segregate objects from background. Support ratio--that is, the ratio between the physically specified and total triangle side length--has been suggested to be the main spatial determinant for suprathreshold perception of a Kanizsa-type illusion. To test this scale invariance hypothesis at threshold, illusory figure perception was studied by determining the effects of inducer size and distance at various exposure durations and fixation strategies on the frequency of seeing (FoS) an illusory Kanizsa triangle.
The effect of various support ratios was studied in the first experiment by varying the intercenter distance between constant-size inducers viewed at various distances. In the second experiment, the effects of various exposure durations and fixation strategies were investigated; and the third experiment repeated the second one, with backward masking to control the processing time. In the fourth experiment, the magnification of the stimulus configuration was varied, with a support ratio that had yielded 100% FoS in the first experiment, to study the range of scale invariance in illusory figure perception.
The support ratio was the main determinant for the perception of an illusory figure at various inducer sizes, exposure durations, and masking conditions when fixation was steady; FoS always increased from 0% to 100% with the support ratio of 0.30 to 0.37. However, free viewing, with and without masking, resulted in 100% illusory figure perception at all support ratios tested. Furthermore, when fixation was steady and support ratio and exposure duration were held constant, stimulus magnification reduced FoS from 100% to 0% at the smallest and largest stimulus sizes.
The support ratio seems to be the main spatial determinant for illusory figure perception. However, scale invariance in Kanizsa triangle perception broke down in the smallest and largest configurations, probably because of the limitations of visual acuity and spatial integration, respectively. Integration of information from several fixations enhances FoS at small support ratios, emphasizing the importance of the binding process between separate fixations for illusory figure perception.
[show abstract][hide abstract] ABSTRACT: We presented two tasks, spatial interval discrimination and displacement detection, simultaneously in the same location at various eccentricities. The subject was to solve (i) only the spatial interval task; (ii) only the displacement task; or (iii) both tasks simultaneously. With 500 msec stimulus duration, and using the method of spatial scaling, the E2 value (the eccentricity at which stimulus size has to be doubled to maintain performance level) was found to be 0.17-0.39 deg for spatial interval discrimination and 1.0-1.2 deg for displacement detection. These values remained unaffected whether the subject solved one task or two tasks simultaneously. This finding was confirmed using a shorter, 50 msec stimulus duration. As there is no interference between tasks, the mechanisms solving the tasks appear to be functionally independent i.e., operating in parallel at all eccentricities.
Vision Research 06/1997; 37(10):1261-70. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Human ability to perceive spatial stimuli declines with increasing eccentricity. To study this phenomenon with natural images, the authors applied the spatial scaling method by measuring the smallest detectable amount of geometric change in a human face at several eccentricities for a series of stimulus magnifications to find out whether performance could be made equal across the visual field simply by an appropriate enlargement.
The authors used a novel method to produce subtle changes to an image of a face. The smallest change recognized was determined using a two-alternative forced-choice method and expressed in terms of correlation sensitivity, the inverse of the correlation between the images that just could be discriminated.
The detection of changes in the facial features, presumably a spatially complex task, became equal across the visual field simply by an appropriate change of scale. The E2 value represents the eccentricity at which the foveal stimulus size must double to maintain performance at the foveal level. The E2 values, found to be 1.73 degrees to 2.45 degrees, were similar to our previously measured values for vernier acuity, orientation discrimination, and curvature detection and discrimination, obtained with the same method of spatial scaling.
The authors' results indicate that with adequate stimulus magnification, one is capable of detecting geometric changes in complex images such as face equally at the fovea and in the periphery. In this task, there seems to be no qualitative difference between the accuracy of foveal and peripheral processing.
[show abstract][hide abstract] ABSTRACT: Contrast detection performance is known to be better for single component sinusoidal gratings than for sums of gratings at different orientations. A recent study Rovamo et al. (1994) (Investigative Ophthalmology and Visual Science, 35, 2611-2619) showed that spatial integration is less effective for multiple orientation component than for single component gratings. This suggests an explanation that the size of a spatial integration window depends on the orientation contents of the stimulus. To test this hypothesis we designed a computational detection model and tested it against new experimental data. The model generates a cross-correlation template, the extent of which is limited both in the spatial and spatial frequency domain. The template is a copy of the band-pass filtered signal weighted by a spatial window function. The spatial window function, which limits spatial integration, decreases with increasing orientation range of the stimulus. The experimental stimuli were composed of side-by-side located square shaped, one cycle, grating patches. The range of either grating orientations or phases within the patches as well as the number of patches in a stimulus were varied. We also measured detection efficiency for Bessel Jo images as a function of area. Human spatial integration became considerably weaker with increasing orientation range. The increasing phase range also reduced detection efficiency to some extent. Supporting the idea of the varying size of the spatial integration window, the computational model explained the orientation, phase, and Bessel Jo data well.
Vision Research 05/1997; 37(8):1025-32. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Foveal flicker sensitivity at 0.5-30 Hz was measured as a function of the spectral density of external, white, purely temporal noise for a sharp-edged 2.5 deg circular spot (mean luminance 3.4 log phot td). Sensitivity at any given temporal frequency was constant at low powers of external noise, but then decreased in inverse proportion to the square root of noise spectral density. Without external noise, sensitivity as function of temporal frequency had the well-known band-pass characteristics peaking at about 10 Hz, as previously documented in a large number of studies. In the presence of strong external noise, however, sensitivity was a monotonically decreasing function of temporal frequency. Our data are well described (goodness of fit 90%) by a model comprising (i) low-pass filtering by retinal cones, (ii) high-pass filtering in the subsequent neural pathways, (iii) adding of the temporal equivalent of internal white spatiotemporal noise, and (iv) detection by a temporal matched filter, the efficiency of which decreases approximately as the power -0.58 of temporal frequency.
Vision Research 01/1997; 36(23):3767-74. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using an 8 mm pupil, 2AFC-method, and 2 x 2 deg2 grating at 2 c/deg we measured contrast sensitivity as a function of integrated radiance for a series of interference filters with peak wavelengths at 400-700 nm. Irrespective of the radiance level, contrast sensitivity was highest when wavelength was at and around 550 nm. It decreased towards longer and shorter wavelengths, reflecting the variation of the probability of quantal catch with light wavelength. When contrast sensitivity functions plotted in double logarithmic coordinates were shifted horizontally by multiplying the integrated radiances of each filter by an appropriate scaling factor, the functions superimposed onto a single curve. Contrast sensitivity at lower levels of relative radiance (R) increased in proportion to square root of R, obeying DeVries-Rose law, but at higher levels contrast sensitivity was constant, obeying Weber's law. Scaling factors plotted as a function of wavelength provided an estimate of V(lambda) quite similar to the standard 2 deg photopic spectral-luminosity function of CIE 1924.
Vision Research 10/1996; 36(17):2675-80. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Visual search for a target, orthogonal to distractors (nontargets) in orientation, has been shown to be independent of the number of distractors (set size). This finding has been thought to indicate that the search occurs spatially in parallel and without capacity limitations. The current study was designed to test whether an orientation difference of 90 degrees between the target and distractor gratings would produce a set-size effect when performance was measured at contrast threshold, that is, whether the threshold contrast at which the target was detected among distractors increased as a function of the number of distractors. The second question studied was whether signal-position uncertainty could explain the possible set-size effect.
The observer searched for a horizontal Gabor patch target in a two-interval, forced-choice task. In the search condition, the target patch was among seven vertical distractor Gabors, all positioned along an isoeccentric circle. The number of possible display locations monitored by the observer varied, and, before each block, he was informed which locations were relevant. In the single-element condition, the target appeared alone, but the number of possible target locations varied as above.
In both conditions, the contrast thresholds almost doubled when the number of possible target locations increased from 1 to 8.
Even though the orientation difference between target and distractors was maximal, a set-size effect was found. The effect could be explained by positional uncertainty.
[show abstract][hide abstract] ABSTRACT: When noise checks are small, spatial noise mimics the effect of white noise in grating detection in the sense that contrast energy threshold is directly proportional to the spectral density of noise and the physical signal-to-noise ratio at threshold thus remains constant. We investigated how the size and shape of noise checks affect the masking properties of noise by using vertical and polar-circular gratings embedded in the spatial noise. The noise check shape and area was varied in both horizontal and vertical dimensions. For polar-circular gratings, noise mimicked the effect of white noise when both the noise check width and height were below a critical size. For vertical gratings there was a critical noise check width, but not critical noise check height. Thus, when the noise check side length was at or below the critical size across the grating bars, and at or below the stimulus size along the grating bars, contrast energy threshold was proportional to the spectral density of noise, calculated by multiplying the noise check area by the r.m.s contrast of noise squared both for one- and two-dimensional check noises.
Vision Research 02/1996; 36(2):271-9. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: We extended the contrast detection model of human vision to temporal integration by taking into account the effect of exposure duration on contrast sensitivity for stationary gratings. The extended model thus comprised: (i) low-pass filtering due to the optical modulation transfer function of the eye; (ii) high-pass filtering (lateral inhibition) due to the neural modulation transfer function of the visual pathways; (iii) addition of internal neural noise; and (iv) detection by a local matched filter whose efficiency for gratings decreased with increasing area and exposure duration. To test the model we measured binocular contrast sensitivity in foveal photopic vision as a function of exposure duration and area for sinusoidal gratings with equiluminous surround at spatial frequencies of 0.25-16 c/deg. In agreement with the model, contrast sensitivity at all grating areas first increased in proportion to square root of t when exposure duration (t) was shorter than critical duration. Thereafter the increase saturated and contrast sensitivity became independent of exposure time. Critical exposure duration was found to be independent of grating area but increased with spatial frequency. Similarly, at all exposure durations contrast sensitivity first increased in proportion to square root of A when grating area (A) was smaller than critical area. Thereafter the increase saturated and contrast sensitivity became independent of area. Critical area was found to be independent of exposure duration but decreased with increasing spatial frequency. The extended model explained 95-97% of the total variance of our contrast sensitivity data at the spatial frequencies studied. Our results also mean that spatial and temporal integration processes are mutually independent and thus area and time are separable variables in the detection of stationary gratings.
Vision Research 09/1995; 35(16):2339-46. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: We studied the effect of spatial location and orientation uncertainty on r.m.s. contrast sensitivity with and without external spatial noise in peripheral and foveal vision.
In the first experiment we used a small circular cosine grating with randomized location embedded in spatial noise and exposed for 33 to 533 ms in peripheral vision. In the second experiment, performed with and without noise, we varied the randomization range of stimulus location in a foveal search task with free eye movements and an unlimited exposure time. In the third experiment we used a vertical cosine grating exposed for 500 ms and varied the randomization range of aperture orientation in the fovea with and without noise.
Uncertainty of spatial location had no effect on r.m.s. contrast sensitivity in the periphery or fovea. However, sensitivity decreased with increasing randomization range of aperture orientation in the fovea.
Uncertainty of spatial location had no effect because the accuracy of positional information is inherently poor in peripheral vision, whereas in the fovea the effect of location uncertainty was compensated for by searching eye movements. Randomization of aperture orientation reduced contrast sensitivity in the fovea because in this case the effect of randomization could not be compensated for.
Optometry and Vision Science 07/1995; 72(6):387-95. · 1.90 Impact Factor
[show abstract][hide abstract] ABSTRACT: We measured thresholds for the perception of blue under chromatic adaptation to white, green, yellow or red at the eccentricities of 0-70 deg in the temporal visual field of four subjects. We used a series of stimulus sizes at each eccentricity, without a prior assumption of any peripheral size-scaling factor. The CIE 1976 UCS (u',v') chromaticity coordinates corresponding to blue perception were subtracted from the chromaticity coordinates of the adaptation field in order to obtain the threshold differences (du',dv') in chromaticity coordinates. Spatial scaling factors for the perception of blue were obtained by non-linear regression (E2 + 5 deg) refers to the eccentricity at which stimulus diameter had to be doubled in order to maintain performance found at the eccentricity of 2.5 deg. E2 for the perception of blue tint varied from 1.2 to 36 deg depending on the state of chromatic adaptation and subject. For the perception of blue tint in yellow three subjects and for the perception of blue tint in red one subject had no spatial scaling factor that would make performance independent of eccentricity. Thus, spatial scaling does not always work.
Vision Research 04/1995; 35(5):589-90. · 2.14 Impact Factor