ACCOMMODATION FOR FLICKERING STIMULI

Abstract— A laser optometer was used to measure accommodative responses of three observers for sinusoidal gratings presented in Maxwellian view at optical distances ranging from 0 to 4 dioptres. Contrast of the stimuli was modulated spatially at 1.0, 4.2 and 6.5 cycles deg.^-1 (cpd), and temporally at six frequencies ranging from 3.0 to 40 Hz. Accommodation was consistently more accurate for the 4.2 cpd than for either the 1.0 or 6.5 cpd gratings. Furthermore, accommodative responsiveness for the 4.2 cpd was not affected by temporal modulation, while that for the other spatial frequencies improved monotonically as a function of temporal frequency. These results reinforce earlier reports that accommodation is most responsive for contrast of intermediate spatial frequencies and they indicate that stimulus flicker generally degrades accommodation for spatial contrast.     

A critical band of phase alignment for discrimination but not recognition of human faces

We investigated the processes underlying the discrimination and recognition of human faces as a function of spatial phase alignment to assess whether face processing can be understood in terms of the amplitude spectrum alone. Specifically, we varied the amount of aligned Fourier phase in different regions of the face frequency spectrum and argue that the properties of the underlying neural processes are best understood in terms of the number of phase alignments as opposed to octave bandwidths. Additionally, we observed performance differences for face discrimination tasks compared to face recognition tasks. For face recognition, our results show that a narrower range of phase alignment is needed for face frequencies near 9 cpf when compared to 3 and 27 cpf, thereby supporting the notion of a critical frequency for face recognition. However, for face discrimination where participants were required to discriminate between an average face and different unique faces along a face morph continuum, performance depended on a fixed signal-to-noise ratio of phase alignment within a contiguous range of face frequencies (termed critical band of phase alignments), regardless of the central face frequency of that range within the face frequency spectrum when compared to non-phase randomized control thresholds.     

Contrast sensitivity for oriented patterns in 1/f noise: contrast response and the horizontal effect

When observers detect an oriented, broadband contrast increment on a background of 1/f spatial noise, thresholds will be lowest for obliquely orientated stimuli and highest for horizontally oriented stimuli-an anisotropy termed the "horizontal effect." Here, we assessed what spatial frequencies within the broadband increment were relied on by observers in performing the original task and which spatial frequencies contribute to the anisotropic performance. We found that against a background of 1/f noise, contrast thresholds are lowest for content around 8 cycles per degree, and that at this spatial frequency a horizontal effect is seen which closely resembles the anisotropy observed in broadband masking. The magnitude of the horizontal effect decreased at lower and higher spatial frequencies. To allow for a fit to a standard "gain control" model of psychophysical contrast discrimination, threshold-versus-contrast (TvC) functions were measured for the 8-cpd noise broadband content against either an identical pattern (i.e., pedestal) or a broadband 1/f noise pattern, whose contrast was varied. Results and model application indicate that the threshold pattern for oriented noise around 8 cpd, and for oriented broadband content, is best explained as the result of an anisotropic contrast gain control process.     

Pattern masking: the importance of remote spatial frequencies and their phase alignment

To assess the effects of spatial frequency and phase alignment of mask components in pattern masking, target threshold vs. mask contrast (TvC) functions for a sine-wave grating (S) target were measured for five types of mask: a sine-wave grating (S), a square-wave grating (Q), a missing fundamental square-wave grating (M), harmonic complexes consisting of phase-scrambled harmonics of a square wave (Qp), and harmonic complexes consisting of phase-scrambled harmonics of a missing fundamental square wave (Mp). Target and masks had the same fundamental frequency (0.46 cpd) and the target was added in phase with the fundamental frequency component of the mask. Under monocular viewing conditions, the strength of masking depends on phase relationships among mask spatial frequencies far removed from that of the target, at least 3 times the target frequency, only when there are common target and mask spatial frequencies. Under dichoptic viewing conditions, S and Q masks produced similar masking to each other and the phase-scrambled masks (Qp and Mp) produced less masking. The results suggest that pattern masking is spatial frequency broadband in nature and sensitive to the phase alignments of spatial components.     

Perceptual learning for a pattern discrimination task

Our goal was to differentiate low and mid level perceptual learning. We used a complex grating discrimination task that required observers to combine information across wide ranges of spatial frequency and orientation. Stimuli were 'wicker'-like textures containing two orthogonal signal components of 3 and 9 c/deg. Observers discriminated a 15% spatial frequency shift in these components. Stimuli also contained four noise components, separated from the signal components by at least 45 degrees of orientation or approximately 2 octaves in spatial frequency. In Experiment 1 naive observers were trained for eight sessions with a four-alternative same-different forced choice judgment with feedback. Observers showed significant learning, thresholds dropped to approximately 1/3 of their original value. In Experiment 2 we found that observers showed far less learning when the noise components were not present. Experiment 3 found, unlike many other studies, almost complete transfer of learning across orientation. The results of Experiments 2 and 3 suggest that, unlike many other perceptual learning studies, most learning in Experiment 1 occurs at mid to high levels of processing rather than within low level analyzers tuned for spatial frequency and orientation. Experiment 4 found that performance was more severely impaired by spatial frequency shifts in noise components of the same spatial frequency or orientation as the signal components (though there was significant variability between observers). This suggests that after training observers based their responses on mechanisms tuned for selective regions of Fourier space. Experiment 5 examined transfer of learning from a same-sign task (the two signal components both increased/decreased in spatial frequency) to an opposite-sign task (signal components shifted in opposite directions in frequency space). Transfer of learning from same-sign to opposite-sign tasks and vice versa was complete suggesting that observers combined information from the two signal components independently.     

Spatial covert attention increases contrast sensitivity across the CSF: support for signal enhancement

This study is the first to report the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies when a target alone was presented in the absence of a local post-mask. We used a peripheral precue (a small circle indicating the target location) to explore the effects of covert spatial attention on contrast sensitivity as assessed by orientation discrimination (Experiments 1-4), detection (Experiments 2 and 3) and localization (Experiment 3) tasks. In all four experiments the target (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was presented alone in one of eight possible locations equidistant from fixation. Contrast sensitivity was consistently higher for peripherally- than for neutrally-cued trials, even though we eliminated variables (distracters, global masks, local masks, and location uncertainty) that are known to contribute to an external noise reduction explanation of attention. When observers were presented with vertical and horizontal Gabor patches an external noise reduction signal detection model accounted for the cueing benefit in a discrimination task (Experiment 1). However, such a model could not account for this benefit when location uncertainty was reduced, either by: (a) Increasing overall performance level (Experiment 2); (b) increasing stimulus contrast to enable fine discriminations of slightly tilted suprathreshold stimuli (Experiment 3); and (c) presenting a local post-mask (Experiment 4). Given that attentional benefits occurred under conditions that exclude all variables predicted by the external noise reduction model, these results support the signal enhancement model of attention.     

Ecologically valid combinations of first- and second-order surface markings facilitate texture discrimination

Natural scenes contain localized variations in both first-order (luminance) and second-order (contrast and texture) information. There is much evidence that first- and second-order stimuli are detected by distinct mechanisms in the mammalian visual system. However, in natural scenes the two kinds of information tend to be spatially correlated. Do correlated and uncorrelated combinations of first- and second-order stimuli differentially affect perception? To address this question we employed orientation-modulated textures in which observers were required to discriminate the spatial frequency of the texture modulation. The textures consisted of micropatterns defined as either local variations in luminance (first-order) or luminance contrast (second-order). Performance was robust with textures composed of only first-order micropatterns, but impossible with only second-order micropatterns. However, when the second-order micropatterns were combined with the first-order micropatterns, they enhanced performance when the two were spatially correlated, but impaired performance when the two were spatially uncorrelated. We conclude that local second-order information may enhance texture modulation discrimination provided it is combined with first-order information in an ecologically valid manner.     

Contrast-processing deficits in melanoma-associated retinopathy

PURPOSE: To evaluate the hypothesis that patients with melanoma-associated retinopathy (MAR) have a selective functional loss within the magnocellular (MC) pathway of the cone system, with sparing of parvocellular (PC) pathway function. METHODS: Two patients with MAR, ages 57 and 61 years, with normal Snellen visual acuity, participated in the study. Contrast sensitivity was measured at spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using two paradigms (steady pedestal and pulsed pedestal) designed to assess the functional integrity of the MC and PC pathways, respectively. Results in patients with MAR were compared with those in 10 visually normal observers, aged 23 to 57 years. RESULTS: Both patients with MAR showed a loss of contrast sensitivity compared to normal observers, but the pattern of loss differed for the two testing paradigms. For the steady-pedestal paradigm (presumed MC-pathway mediation), the patients' sensitivity loss was greatest at the lowest spatial frequency (0.25 cpd) and the sensitivity loss decreased systematically with increasing spatial frequency. For the pulsed-pedestal paradigm (presumed PC-pathway mediation), the sensitivity loss was greatest at an intermediate spatial frequency of 1 cpd. For both paradigms, the patients' sensitivities were within the normal range at the highest spatial frequency (8 cpd), consistent with their normal visual acuity. CONCLUSIONS: The contrast sensitivity deficits of patients with MAR under photopic conditions are not specific to the MC pathway, as proposed previously, but instead are related to the spatial frequency of the test target. The overall pattern of contrast sensitivity loss shown by the patients with MAR is consistent with the dysfunction at the level of the retinal bipolar cells that is presumed to underlie the MAR syndrome.     

Interaction of luminance and higher-order statistics in texture discrimination

Most studies of texture processing are based on textures in which individual pixel statistics are varied and spatial correlations are absent ("IID textures"), or textures in which spatial correlation structure is varied and luminance, or first-order, statistics are held constant. Here we jointly examine simple pixel statistics and fourth-order spatial correlation structure along the continuum of "even" and "odd" isodipole textures of Julesz, Gilbert and Victor [Julesz, B., Gilbert, E.N., & Victor, J.D. (1978). Biological Cybernetics, 31(3) 137-140], as well as their interactions. Absolute efficiency to detect either kind of statistical cue is low: approximately 0.05 for luminance statistics, and 0.004 for isodipole statistics. Above threshold, isodipole statistics must change by approximately four times the amount that pixel statistics must change to generate an equally salient texture. When pixel statistics and isodipole statistics are simultaneously varied, the two texture cues combine by probability summation and perceptual distances are approximately Euclidean. Superimposed on this picture are subtle foreground/background asymmetries that suggest properties of the visual mechanisms that are sensitive to these image statistics.     

Senescent changes in scotopic contrast sensitivity

Scotopic contrast sensitivity functions (CSFs) were measured for 50 observers between the ages of 20 and 88 years. Using a maximum-likelihood, 2-alternative, temporal forced-choice threshold-estimation algorithm, scotopic CSFs were measured at 7 spatial frequencies ranging from 0.2 to 3.0 cpd, with mean retinal illuminance equated for observers at -0.85 log scotopic Trolands. For each stimulus condition, eight cycles of a horizontal sinusoidal grating were presented within +/- 1 S.D. of a 2-D Gaussian-spatial envelope and within a 1-s Gaussian-temporal envelope. Stimuli were centered on the nasal retina along the horizontal meridian 6 degrees from the fovea. Scotopic CSFs were found to be low-pass. Statistically significant age-related declines in contrast sensitivities were found for spatial frequencies at or below 1.2 cpd. There was also a statistically significant decrease in the high frequency cut-off with age (P < 0.01). An explanation of these results in terms of optical factors is rejected, while the results are consistent with age-related changes in the magnocellular pathway.     

Classification images for detection, contrast discrimination, and identification tasks with a common ideal observer

We consider three simple forced-choice visual tasks--detection, contrast discrimination, and identification--in Gaussian white noise. The three tasks are designed so that the difference signal in all three cases is the same difference-of-Gaussians (DOG) profile. The distribution of the image noise implies that the ideal observer uses the same DOG filter to perform all three tasks. But do human observers also use the same visual strategy to perform these tasks? We use classification image analysis to evaluate the visual strategies of human observers. We find significantly different subject classification images across the three tasks. The domain of greatest variability appears to be low spatial frequencies [<5 cycles per degree (cpd)]. In this range, we find frequency enhancement in the detection task, and frequency suppression and reversal in the contrast discrimination task. In the identification task, subject classification images agree reasonably well with the ideal observer filter. We evaluate the effect of nonlinear transducers and intrinsic spatial uncertainty to explain divergence from the ideal observer found in detection and contrast discrimination tasks.     

Contrast adaptation in retinal and cortical evoked potentials: no adaptation to low spatial frequencies

Contrast adaptation occurs in both the retina and the cortex. Defining its spatial dependence is crucial for understanding its potential roles. We thus asked to what degree contrast adaptation depends on spatial frequency, including cross-adaptation. Measuring the pattern electroretinogram (PERG) and the visual evoked potential (VEP) allowed separating retinal and cortical contributions. In ten subjects we recorded simultaneous PERGs and VEPs. Test stimuli were sinusoidal gratings of 98% contrast with spatial frequencies of 0.5 or 5.0 cpd, phase reversing at 17 reversals/s. Adaptation was controlled by prolonged presentation of these test stimuli or homogenous gray fields of the same luminance. When adaptation and test frequency were identical, we observed significant contrast adaptation only at 5 cpd: an amplitude reduction in the PERG (-22%) and VEP (-58%), and an effective reduction of latency in the PERG (-0.95 ms). When adapting at 5 cpd and testing at 0.5 cpd, the opposite effect was observed: enhancement of VEP amplitude by +26% and increase in effective PERG latency by + 1.35 ms. When adapting at 0.5 cpd and testing at 5 cpd, there was no significant amplitude change in PERG and VEP, but a small effective PERG latency increase of +0.65 ms. The 0.5-cpd channel was not adapted by spatial frequencies of 0.5 cpd. The adaptability of the 5-cpd channel may mediate improved detail recognition after prolonged blur. The existence of both adaptable and nonadaptable mechanisms in the retina allows for the possibility that by comparing the adaptational state of spatial-frequency channels the retina can discern between overall low contrast and defocus in emmetropization control.     

Interocular differences in contrast and spatial frequency: effects on stereopsis and fusion

Anisometropia produces interocular differences in contrast and spatial frequency. The influence of these two parameters on Panum's fusional limit (PFL) and stereoscopic depth thresholds was investigated with sinusoidal gratings and one-dimensional band-pass-limited targets. Vertical fusion limits were unaffected by large interocular differences in contrast (40-10%) at two spatial frequencies (0.8 and 1.6 cpd). However, when tested with a low spatial frequency (0.8 cpd), stereothresholds increased 150% with an interocular difference in contrast as small as 50-25%. Stereoacuity was reduced less by differential contrast when tested with higher spatial frequencies (3.2 cpd). When tested with low spatial frequencies the stereothreshold was elevated more by reducing the contrast of one image than by equal contrast reductions of both ocular images. Stereothresholds appear to be elevated by binocular suppression evoked by interocular differences in contrast. Vertical as well as horizontal fusion limits decreased with increasing interocular size difference. Horizontal fusion limits fell off more gradually with increasing size difference than did vertical fusion limits, particularly at higher spatial frequencies (2.4 cpd). Similarly, stereothresholds increased with increasing interocular size differences. Changes in the fusion limit and stereothreshold that occur with interocular size differences are predicted from positional disparities between edge features rather than from differences in spatial frequency.     

The effects of phase on the perception of 3D shape from texture: psychophysics and modeling

Two experiments are reported in which observers judged the apparent shapes of elliptical cylinders with eight different textures that were presented with scrambled and unscrambled phase spectra. The results revealed that the apparent depths of these surfaces varied linearly with the ground truth in all conditions, and that the overall magnitude of surface relief was systematically underestimated. In general, the apparent depth of a surface is significantly attenuated when the phase spectrum of its texture is randomly scrambled, though the magnitude of this effect varies for different types of texture. A new computational model of 3D shape from texture is proposed in which apparent depth is estimated from the relative density of edges in different local regions of an image, and the predictions of this model are highly correlated with the observers' judgments.     

Spatial frequency discrimination and detection characteristics for gratings defined by orientation texture

We describe evidence consistent with the proposal that the visual system contains a parallel array of size-tuned mechanisms sensitive to orientation texture-defined (OTD) form, and propose that the relative activity of these mechanisms determines spatial frequency discrimination threshold for OTD gratings. Using a pattern of short lines we measured spatial frequency discrimination thresholds for OTD gratings and luminance-defined (LD) gratings. For OTD gratings, the orientation of texture lines varied sinusoidally across the bars of the gratings, but line luminance was constant. For LD gratings, line orientation was constant, but line luminance varied sinusoidally across the bars of the grating. When the number of texture lines (i.e. spatial samples) per grating cycle was below about six, spatial sampling strongly affected both the spatial frequency discrimination and grating detection thresholds for OTD and LD gratings. However, when the number of spatial samples per grating cycle exceeded about six, plots of both discrimination threshold and detection threshold were different for OTD and LD gratings. For an OTD grating of any given spatial frequency, spatial frequency discrimination threshold fell as the number of samples per grating cycle was increased while holding texture line length constant: the lower limit was reached at six to ten samples per cycle. When we progressively increased the viewing distance (keeping the cycles per degree (cpd) constant), spatial frequency discrimination threshold reached a lower limit and increased thereafter. We propose that this minimum threshold represents a balance between opposing effects of the number of samples per grating cycle and the length of texture lines, and approaches the absolute physiological lower limit for OTD gratings. Spatial frequency discrimination was possible up to at least 7 cpd. Grating acuity for an OTD grating was considerably lower than the physiological limit for LD gratings, presumably because detectors of OTD form include a spatial integration stage following the processing of individual lines. For an LD grating, discrimination threshold fell as the number of samples per grating cycle was increased and asymptoted at six to ten samples per cycle. Spatial frequency discrimination thresholds for OTD and LD gratings were similar at low spatial frequencies (up to 3-4 cpd), but increased more steeply for OTD gratings at high spatial frequencies. For both OTD and LD gratings, discrimination threshold fell steeply as the number of grating cycles was increased from 0.5 to ca. 2.5 cycles, and thereafter decreased more slowly or not at all suggesting that, for both OTD and LD gratings, spatial frequency discrimination can be regarded as a special case of line interval or bar width discrimination. As orientation contrast was progressively increased, discrimination threshold for an OTD grating fell steeply up to about four to five times grating detection threshold, then saturated. This parallels the effect of luminance contrast on discrimination threshold for an LD grating.     

Visual reaction time of cats to different spatial frequencies

If physiological mechanisms similar to cat Y and X cells explain faster detection of low spatial frequencies by humans, then cats should show the same effect. We have tested this prediction by determining the visual reaction time of cats over a range of spatial frequencies and contrasts by training them to respond quickly when a vertical sine-wave grating was presented. At 50% contrast, the cat's visual reaction time increased monotonically from 0.25-2.0 cpd (cycle/deg). At every spatial frequency tested, the cat's reaction time increased monotonically as contrast decreased. By determining contrast threshold (70% detection) at each spatial frequency, it was possible to determine reaction times for different spatial frequencies at equal physical contrasts and equal "threshold equivalent" contrasts. Some of the cat's faster detection of low spatial frequencies was due to sensitivity differences and some was not. To determine if faster detection of low spatial frequencies was based upon Y cells, we took advantage of the fact that Y cells show a strong peripheral effect while X cells do not. Low and high spatial frequencies were detected in the presence of a flickering (7 Hz) or steady (70 Hz) surround. Surround frequency had no effect upon reaction times to 2.0 cpd but the flickering surround increased reaction times to 0.25 cpd. These results indicate that, in cats, rapid detection of low spatial frequencies is by Y cells and slower detection of high spatial frequencies is by X cells.     

Relating spatial and temporal orientation pooling to population decoding solutions in human vision

Spatial pooling is often considered synonymous with averaging (or other statistical combinations) of local information contained within a complex visual image. We have recently shown, however, that spatial pooling of motion signals is better characterized in terms of optimal decoding of neuronal populations rather than image statistics (Webb et al., 2007). Here we ask which computations guide the spatial and temporal pooling of local orientation signals in human vision. The observers’ task was to discriminate which of two texture patterns had a more clockwise global orientation. Standard textures had a common orientation; comparison textures were chosen independently from a skewed (asymmetrical) probability distribution with distinct spatial or temporal statistics. We simulated observers’ performance using different estimators (vector average, winner-takes-all and maximum likelihood) to decode the orientation-tuned activity of a population of model neurons. Our results revealed that the perceived global orientation of texture patterns coincided with the mean (or vector average read-out) of orientation signals accumulated over both space and time. To reconcile these results with our previous work on direction pooling, we varied stimulus duration. Perceived global orientation was accurately predicted by a vector average read-out of orientation signals at relatively short stimulus durations and maximum likelihood read-out at longer durations. Moreover, decreasing the luminance contrast of texture patterns increased the duration of the transition from a vector average to maximum likelihood read-out. Our results suggest that direction and orientation pooling use similar probabilistic read-out strategies when sufficient time is available.     

The effects of display time and eccentricity on the detection of amplitude and phase degradations in textured stimuli

The amplitude and phase spectra of an image contain important information for perception, and a large body of work has investigated the effects of manipulating these spectra on the recognition or classification of image content. Here, we use a novel means of investigating sensitivity to amplitude and phase spectra properties, testing the ability of observers to detect degradations of the spectral content of synthetic images of textured surfaces that are broadband in the frequency domain. The effects of display time and retinal eccentricity on sensitivity to these two manipulations are compared using stimuli matched for difficulty of detection. We find no difference between the time courses for the detection of degradation in the two spectra; in both cases, accuracy rises above chance when display times are greater than 80 ms. Increasing retinal eccentricity to 8.7°, however, has a significantly stronger effect on the accuracy of detecting degradations of the amplitude spectrum than of the phase spectrum. Further, sensitivity to phase randomization that is restricted to low spatial frequencies is greater in the periphery (at 8.7° eccentricity) than in the fovea. These last two results imply that the fovea and periphery are specialized for the processing of phase spectrum information in distinct spatial frequency bands.     

Spatial frequency and visual discomfort

Images created from noise filtered to have an approximately 1/f amplitude spectrum were altered by adding excess energy concentrated at various spatial frequencies. The effects of this manipulation on judgements of visual discomfort were studied. Visual noise with a 1/f amplitude spectrum (typical of natural images) was judged more comfortable than any image with a relative increase in contrast energy within a narrow spatial frequency band. A peak centred on 0.375-1.5 cycles/degree of spatial frequency was consistently judged as more uncomfortable than a peak at a higher spatial frequency. This finding was robust to slight differences in eccentricity, and when stimuli were matched for perceived contrast across spatial frequency. These findings are consistent with the idea that deviation from the statistics of natural images could cause discomfort because the visual system is optimised to encode images with the particular statistics typical of natural scenes.