Orientation Discrimination Thresholds Research Articles

Different processing strategies underlie voluntary averaging in low and high noise

An underlying assumption of the external noise paradigm is that the same processing strategy operates whether the dominating noise source comes from the observer (i.e., internal) or the stimulus (i.e., external). Here, we challenged this noise-invariant processing assumption for a particular variant of the external noise paradigm--the voluntary averaging paradigm--where processing is characterized by the efficiency of averaging across the samples. The task consisted in discriminating the mean orientation of four distinctly perceived Gabors, and the external noise corresponded to orientation-jitter added to these Gabors. The averaging efficiencies for the four-sample case were measured by comparing discrimination thresholds of the average orientation of four Gabors to a baseline with a single Gabor. In high noise, orientation discrimination thresholds were better when 4 Gabors rather than 1 were presented, showing efficient averaging. But in absence of external noise, presenting four identically oriented Gabors rather than one did not improve performance, showing that subjects no longer averaged the individual estimates. We conclude that the averaging process operating in high noise does not operate in low noise because there is no reason to voluntarily average stimuli that appear identical. This conclusion implies that the processing strategy can change depending on the external noise level, an implication which violates the noise-invariant processing assumption underlying the averaging paradigm.

Regionally Specific Human GABA Concentration Correlates with Tactile Discrimination Thresholds

The neural mechanisms underlying variability in human sensory perception remain incompletely understood. In particular, few studies have attempted to investigate the relationship between in vivo measurements of neurochemistry and individuals' behavioral performance. Our previous work found a relationship between GABA concentration in the visual cortex and orientation discrimination thresholds (Edden et al., 2009). In the present study, we used magnetic resonance spectroscopy of GABA and psychophysical testing of vibrotactile frequency thresholds to investigate whether individual differences in tactile frequency discrimination performance are correlated with GABA concentration in sensorimotor cortex. Behaviorally, individuals showed a wide range of discrimination thresholds ranging from 3 to 7.6 Hz around the 25 Hz standard. These frequency discrimination thresholds were significantly correlated with GABA concentration (r = -0.58; p < 0.05) in individuals' sensorimotor cortex, but not with GABA concentration in an occipital control region (r = -0.04). These results demonstrate a link between GABA concentration and frequency discrimination in vivo, and support the hypothesis that GABAergic mechanisms have an important role to play in sensory discrimination.

Effects of speed, age, and amblyopia on the perception of motion-defined form

We determined the effect of dot speed on the typical and atypical development of motion-defined form perception. Monocular motion coherence thresholds for orientation discrimination of motion-defined rectangles were determined at slow (0.1 deg/s), medium (0.9 deg/s) and fast (5.0 deg/s) dot speeds. First we examined typical development from age 4 to 31 years. We found that performance was most immature at the slow speed and in the youngest group of children (4–6 years). Next we measured motion-defined form perception in the amblyopic and fellow eyes of patients with amblyopia. Deficits were found in both eyes and were most pronounced at the slow speed. These results demonstrate the importance of dot speed to the development of motion-defined form perception. Implications regarding sensitive periods and the neural correlates of motion-defined form perception are discussed.

Patterns and trajectories in Williams Syndrome: The case of visual orientation discrimination

Williams Syndrome (WS) is a developmental disorder typified by deficits in visuospatial cognition. To understand the nature of this deficit, we characterized how people with WS perceive visual orientation, a fundamental ability related to object identification. We compared WS participants to typically developing children (3–6 years of age) and typical adults in an orientation discrimination task with four stimulus types (small circular, large circular, collinear elongated and parallel elongated gratings). We measured orientation discrimination thresholds and the proportion of orthogonal errors (i.e., mirror-image reversal errors). We evaluated how these metrics (1) are modulated by stimulus condition, and (2) vary with chronological or mental age. We found that orientation perception in WS is comparable to that of typically developing children. Orientation discrimination thresholds were better for elongated gratings than circular gratings across all participant groups. For large circular gratings, the proportion of orthogonal errors was disproportionately greater in WS participants and typically developing 3–6 year old children than in typical adults. Moreover, we found that the ability to judge orientation in WS improves with increasing mental age, but not chronological age. These results suggest that orientation discrimination in WS is developmentally arrested, as opposed to abnormal or delayed.

The effect of flankers on three tasks in central, peripheral, and amblyopic vision

Using identical stimuli and methods, we assessed the effects of flankers on three different tasks, orientation discrimination, contrast discrimination, and detection, in central, peripheral, and amblyopic vision. The goal was to understand the factors that limit performance of a task in the presence of flankers in each of these visual systems. The results demonstrate that: (1) For unflanked targets, the losses in peripheral and amblyopic vision (relative to the normal fovea) are ordered, with the loss of unflanked contrast discrimination thresholds considerably smaller than those for either detection or orientation discrimination. (2) For flanked targets, in normal foveal vision and anisometropic amblyopia, the critical distance is more or less proportional to the target size, whereas in peripheral and strabismic amblyopic vision, the critical distance shows much less (or no) dependence on target size. (3) For the normal fovea, and anisometropic amblyopia, when the target is large (>≈0.2 deg) the amount of threshold elevation induced by flankers is low, increasing when the target is very small. On the other hand, for the periphery and the amblyopic eyes of most strabismic amblyopes, the elevation is large over the range of sizes tested. (4) In peripheral and strabismic amblyopic vision, remote flankers elevate orientation discrimination and contrast discrimination thresholds but not detection thresholds. Our results show clearly that the effects of flanks depend on both the task and the type of visual system. We conclude that in normal foveal vision and anisometropic amblyopia, the effects of flankers largely reflects a reduction in visibility and may be explained by masking. On the other hand, in peripheral vision and strabismic amblyopia, the effects of flankers on orientation discrimination and to a lesser extent contrast discrimination cannot be explained by simple masking and are due to crowding.

Reward Sharpens Orientation Coding Independently of Attention

It has long been known that rewarding improves performance. However it is unclear whether this is due to high level modulations in the output modules of associated neural systems or due to low level mechanisms favoring more “generous” inputs? Some recent studies suggest that primary sensory areas, including V1 and A1, may form part of the circuitry of reward-based modulations, but there is no data indicating whether reward can be dissociated from attention or cross-trial forms of perceptual learning. Here we address this issue with a psychophysical dual task, to control attention, while perceptual performance on oriented targets associated with different levels of reward is assessed by measuring both orientation discrimination thresholds and behavioral tuning functions for tilt values near threshold. We found that reward, at any rate, improved performance. However, higher reward rates showed an improvement of orientation discrimination thresholds by about 50% across conditions and sharpened behavioral tuning functions. Data were unaffected by changing the attentional load and by dissociating the feature of the reward cue from the task-relevant feature. These results suggest that reward may act within the span of a single trial independently of attention by modulating the activity of early sensory stages through a improvement of the signal-to-noise ratio of task-relevant channels.

Similar perceptual costs for dividing attention between retina- and space-centered targets in humans

Visual-spatial attention enhances the perception of behaviorally relevant stimuli. One issue that remains unclear is whether attention is preferentially allocated to stimuli that remain fixed in one reference frame (e.g., retina-centered), or whether it could be equally allocated to stimuli fixed in other frames. We investigated this issue by asking observers to covertly attend to sinusoidal gratings fixed in different reference frames and to discriminate changes in their orientation. First, we quantified orientation discrimination thresholds (ODTs) while subjects pursued a moving dot and either attended to a retina- or a space-centered grating. We then measured ODTs while subjects divided attention between the two gratings. We found that dividing attention proportionally increased ODTs for both target gratings relative to the focused attention condition. Second, we used the same stimulus configuration and conditions during a fixation task. Here, one grating was retina- and space-centered while the other moved in space and on the retina. Again, ODTs during divided attention proportionally increased for both gratings. These increases were similar to those measured during smooth pursuit. Our results show that humans can proportionally divide attention between targets centered in different reference frames during both smooth pursuit eye movements and fixations.

Configural modulation of crowding

The ability to make judgments about a peripheral target stimulus can be impaired when the target is surrounded by flanking stimuli. This effect is called crowding. Crowding is often related to relatively simple low-level mechanisms that pool visual information over spatial location. We tested this hypothesis by determining the effect of stimulus configuration on crowding. We manipulated only the 'global' configuration, keeping constant the orientation, spatial frequency, and spatial phase of the elements making up the stimuli. We measured thresholds for contrast and orientation discrimination using a vertical Gabor patch as a target. The target was presented at an eccentricity of 5.7 degrees and was flanked on both sides by seven vertical Gabor patches. The spatial frequency of all stimuli was 3.3 cycles per degree. Contrast and orientation discrimination were both strongly impaired when the target was flanked on both sides by Gabors of equal length. However, when the flanks were either shorter or longer than the target, the impairment was largely reduced or even abolished, indicating that crowding was strongly modulated by stimulus configuration. Control experiments confirmed that this modulation was not due to a change in contrast energy with changing flank size or to reduced uncertainty about the target location. Further, crowding was reduced when the flanks were composed of small collinear Gabors that had the same orientation, spatial frequency, and spatial phase as the target. These results are not readily explained by a simple pooling account of crowding. Crowding seems to be weak whenever the target does not make up a coherent texture with the flanks, even when the `low-level' properties of the stimuli are kept constant

Orientation perception in Williams Syndrome: discrimination and integration

Abstract Williams Syndrome (WS) is a rare neurodevelopmental disorder, which stems from a genetic deletion on chromosome 7 and causes a profound weakness in visuospatial cognition. Our current study explores how orientation perception may contribute to the visuospatial deficits in WS. In Experiment 1, we found that WS individuals and normal 3–4 year olds had similar orientation discrimination thresholds and had similar prevalence of mirror-reversal errors for diagonal targets (±45 deg). In Experiment 2, we asked whether this immaturity in orientation discrimination would also be reflected in a task requiring integration of oriented elements. We found that sensitivities of WS individuals for detecting orientation-defined contours were higher than sensitivities of normal 3–4 year olds, and were not significantly different from sensitivities of normal adults. Together, these results suggest that orientation discrimination and orientation integration have different maturational trajectories in normal development and different susceptibilities to damage in WS. These may reflect largely separate visuospatial mechanisms.

Human orientation sensitivity during object perception

Orientation information is crucial for object perception, yet little is known about how it is encoded in complex images. The precision of orientation coding has been traditionally investigated using simple stimuli such as single Gabors. We developed an image manipulation technique that allows us to vary the complexity of an image using multiple Gabors. We also developed a paradigm to assess orientation discrimination thresholds in complex object images. In the present study we ask how well such measures capture the efficiency of coding orientation information during object perception. The relative fit of two models (Hard Threshold and Equivalent Noise) and their ability to explain changes in threshold with the addition of external noise are compared.

Direction-of-motion discrimination is facilitated by visible motion smear

Recent evidence indicates that motion smear can provide useful information for the detection and discrimination of motion. Further, it has been shown that the perception of motion smear depends critically on the density of dots in a random-dot (RD) stimulus. Therefore, in the present experiments, the contribution of perceived motion smear to direction-of-motion discrimination was evaluated using RD targets of different densities. Thresholds for direction-of-motion discrimination and the extent of perceived motion smear were determined for RD stimuli with densities of 1, 2, and 10 dots/deg(2), presented for 200 msec at a velocity of 4, 8, or 12 deg/sec. To evaluate the contribution of information about orientation from motion smear, thresholds for orientation discrimination were measured using parallel lines with the same length as the extent of perceived smear. Despite the opportunity for increased summation as RD density increases, our results indicate that direction-of-motion discrimination worsens. Because perception of motion smear is reduced with an increase in RD density, our results are consistent with a facilitation of direction-of-motion discrimination by visible motion smear.

TOD Performance Model for Staring Thermal Imager with Machine Vision

A triangle orientation discrimination threshold(TOD) performance theoretical model is derived for the staring thermal imager based on machine vision. Specifically, how to obtain the TOD curve based on machine vision is briefly described. The spatial frequency distribution of the triangle test pattern is first determined. The transform and response characteristics of the non-periodic triangle pattern and its background clutter through machine vision-based thermal imager are analyzed. The three-dimensional matched filter is adopted to characterize quantitatively the spatial and temporal integration of image enhancement algorithms to the output triangle pattern signal, various noise components and background clutter, and the signal-to-interference ratio (SIR) of the triangle pattern output image is derived for the staring thermal imager based on machine vision. Then, the TOD performance theoretical model is established by assuming that the output SIR is equal to the threshold SIR75% determined by the discrimination criteria of machine vision. Preliminary simulation and experimental results show that this theoretical model can give reasonable prediction of the TOD performance curve for staring thermal imagers based on machine vision.

Orientation Discrimination with Macular Changes Associated with Early AMD

Age-related macular degeneration (AMD) is a condition that progressively reduces central vision in elderly individuals, resulting in a reduced capacity to perform many daily activities and a diminished quality of life. Recent studies identified clinical treatments that can slow or reverse the progression of exudative (wet) AMD and ongoing research is evaluating earlier interventions. Because early diagnosis is critical for an optimal outcome, the goal of this study is to assess psychophysical orientation discrimination for randomly positioned short line segments as a potential indicator of subtle macular changes in eyes with early AMD. Orientation discrimination was measured in a sample of 74 eyes of patients aged 47 to 82 years old, none of which had intermediate or advanced AMD. Amsler-grid testing was performed as well. A masked examiner graded each eye as level 0, 1, 2, or 3 on a streamlined version of the Age-Related Eye Disease Study (AREDS) scale for AMD, based on the presence and extent of macular drusen or retinal pigment epithelium (RPE) changes. Visual acuity in the 74 eyes ranged from 20/15 to 20/40, with no significant differences among the grading levels. Humphrey 10-2 and Nidek MP-1 micro-perimetry were used to assess retinal sensitivity at test locations 1 degrees from the locus of fixation. Average orientation-discrimination thresholds increased systematically from 7.4 degrees to 11.3 degrees according to the level of macular changes. In contrast, only 3 of 74 eyes exhibited abnormalities on the Amsler grid and central-field perimetric defects occurred with approximately equal probability at all grading levels. In contrast to Amsler grid and central-visual-field testing, psychophysical orientation discrimination has the capability to distinguish between eyes with and without subtle age-related macular changes.

Attention Alters Visual Plasticity during Exposure-Based Learning

It is generally believed that attention enhances the processing of sensory information during perception and learning. Here we report that, contrary to common belief, attention limits the degree of plasticity induced by repeated exposure to image features. Specifically, daily exposure to oriented stimuli that are not linked to a specific task causes an orientation-specific improvement in perceptual performance along the "exposed" axes. This effect is modulated by attention: human subjects showed a larger improvement in orientation discrimination when attention is directed toward the location where stimuli are presented. However, the capacity to perform discriminations away from the exposed orientation is enhanced when the exposure stimuli are unattended. Importantly, the improvement in orientation discrimination at the unattended location leads to a robust enhancement in the discrimination of complex stimuli, such as natural texture images, with orientation components along the exposed axes, whereas the improvement in orientation discrimination at the attended location exhibits only weak transfer to complex stimuli. These results indicate that sensory adaptation by passive stimulus exposure should be viewed as a form of perceptual learning that is complementary to practice-based learning in that it reduces constraints on generalization.

Orientation perception in Williams Syndrome: Discrimination and integration

Williams Syndrome (WS) is a rare neurodevelopmental disorder, which stems from a genetic deletion on chromosome 7 and causes a profound weakness in visuospatial cognition. Our current study explores how orientation perception may contribute to the visuospatial deficits in WS. In Experiment 1, we found that WS individuals and normal 3–4 year olds had similar orientation discrimination thresholds and had similar prevalence of mirror-reversal errors for diagonal targets (±45 deg). In Experiment 2, we asked whether this immaturity in orientation discrimination would also be reflected in a task requiring integration of oriented elements. We found that sensitivities of WS individuals for detecting orientation-defined contours were higher than sensitivities of normal 3–4 year olds, and were not significantly different from sensitivities of normal adults. Together, these results suggest that orientation discrimination and orientation integration have different maturational trajectories in normal development and different susceptibilities to damage in WS. These may reflect largely separate visuospatial mechanisms.

The effect of senescence on orientation discrimination and mechanism tuning

Accurately processing orientation information is a fundamental component of visual performance. Single-unit recordings have shown that the orientation tuning of individual neurons in macaque cortical areas V1 and V2 is reduced dramatically with age (M. T. Schmolesky, Y. Wang, M. Pu, & A. G. Leventhal, 2000; S. Yu, Y. Wang, X. Li, Y. Zhou, & A. G. Leventhal, 2006). These researchers suggest that losses in single-unit orientation selectivity result in declines in orientation discrimination and object recognition in older humans. Three experiments were conducted to determine whether human performance is affected by putative age-related changes in tuning of cortical neurons. Ten younger and ten older observers participated in this study. Experiment 1 demonstrated significant differences in the contrast sensitivity of the two age groups. Experiment 2 showed significant differences in orientation discrimination thresholds. However, when thresholds were plotted in terms of multiples of detection threshold, age-related differences were not observed. In Experiment 3, perceptual orientation tuning curves did not significantly differ in shape for younger and older subjects. As in Experiment 2, at any given contrast, there is a large difference in sensitivity between younger and older adults. This implies a model of orientation processing that allows the adult visual system to maintain consistent and reliable orientation information at the network and ultimately the perceptual level.

Contrast amplification in global texture orientation discrimination

We show that adding a low-contrast texture stimulus that is far below its own detection threshold to an ambiguously oriented high-contrast texture can produce an easily perceived global orientation. When such a low-contrast (e.g., 0.1%) test texture and a high-contrast (e.g., 2%) amplifier texture are interleaved, the effective strength for global orientation detection closely approximates the product of the two contrasts. Therefore, adding two ambiguous textures, an amplifier texture at 5x its threshold contrast for global orientation discrimination and a test texture at 1/5x its threshold contrast, produces threshold global orientation discrimination, that is, 5x amplification of the below-threshold test texture. The observed 5x amplification factors are larger than facilitation effects reported in other pattern tasks. Amplification is 11x when orientation discrimination thresholds are compared to absolute detection thresholds. For second-order textures, maximum contrast amplification is about 2.5x. A contrast gain control model is presented that accounts for 90% of the variance in observed d' for texture patterns of differing geometries, exposure durations, and component contrasts. In the model, very low-contrast orientations are represented by power functions of their contrasts, with an exponent greater than two. As the contrast of an amplifier texture increases beyond about 4%, feed-forward gain control exerted by the amplifier ultimately nullifies the amplification effect and produces masking.

Foveal and extra-foveal orientation discrimination

Performance can often be made equal across the visual field by scaling peripherally presented stimuli according to F = 1 + E/E (2) where E (2) is the eccentricity at which stimulus size must double to maintain foveal performance levels. Sally and Gurnsey (Vision Res 43:1375-1385, 2003 and Vision Res 44:2719-2727, 2004) have previously shown that estimates of E (2) for orientation discrimination are significantly larger (i.e., less spatial scaling is required) at stimulus contrasts near detection threshold than at contrasts well above detection threshold. To examine the nature of this effect parametrically we measured orientation discrimination thresholds at 0 degrees and 10 degrees eccentricity for three levels of Michelson contrast (3, 12 and 48%) and three stimulus length-to-width aspect ratios (36.4, 9.1 and 2.3) for a range of line sizes (0.19 degrees -36 degrees visual angle). On average, E (2) values decreased as stimulus contrast decreased, consistent with the previous results of Sally and Gurnsey (Vision Res 43:1375-1385, 2003 and Vision Res 44:2719-2727, 2004). It is proposed that contrast reductions have a proportionally larger effect on small orientation-selective units than large ones and thus produce a greater rightward shift of acuity functions (orientation threshold vs. size) at the fovea than in the periphery. This explains why less spatial scaling is required to equate foveal and peripheral acuity functions at low contrasts than at high contrasts.

The effects of aging on orientation discrimination

The current experiments measured orientation discrimination thresholds in younger (mean age ≈ 23 years) and older (mean age ≈ 66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3 c/deg) that differed in orientation by 12 deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5 deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155–162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023–1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812–815).

Orientation repulsion and attraction in alignment perception

Orientation masking induces changes of discrimination thresholds and perceived orientation. Studies on alignment discrimination of Vernier stimuli concentrated on masking induced changes of discrimination thresholds, without considering possible changes of perceived orientation and/or alignment of the two-line segments. Measuring both parameters in an orientation discrimination task, we confirmed a standard repulsion effect between a single line target and a mask grating that co-varied with elevated orientation discrimination thresholds. Masking a Vernier stimulus in an alignment discrimination task, we observed a strong misperception of alignment that was accompanied with elevated alignment discrimination thresholds. Orientation masking on perceived orientation and alignment of a Vernier stimulus revealed orientation repulsion and attraction that depended on the spatio-orientation configuration of the superimposed stimuli. Control of task-dependent effects confirmed that our observed pattern of results was independent of attentional or cognitive demands.