Contrast Discrimination Thresholds Research Articles

Poster Session I: Criterion effects in maximum likelihood difference scaling: Similar is not always the opposite of different.

Maximum Likelihood Difference Scaling (MLDS) is an efficient method of estimating perceptual representations of suprathreshold physical quantities (Maloney & Yang, 2003), such as luminance contrast. In MLDS, observers can be instructed to judge which of two stimulus pairs are more similar to one another, or which of the two pairs are more different from one another. If the same physical attributes are used for both the similar and dissimilar tasks, the two criteria should produce the same perceptual scales. We estimated perceptual scales for suprathreshold achromatic square patches. Increments and decrements on the mid-gray background were estimated separately. Observers judged which pair of stimuli were more similar in half of the sessions, and more different in the other half sessions. For most observers, the two tasks produced the same perceptual scales: a decelerating curve for increment contrasts and a cubic curve for decremental contrasts (cf. Whittle, 1992). These scales predicted forced-choice contrast discrimination thresholds for both increments and decrements. However, for a subset of observers, the 'more different' judgments produced scales that accelerated with contrast for both increments and decrements; these scale shapes do not predict their discrimination thresholds. Our results suggest that, even with these simple stimuli, observers in an MLDS experiment may attend to different aspects of the stimulus depending on the assigned task.

Perceptual Center-Surround Contrast Suppression in Adolescence.

Center-surround contrast suppression-typically induced when a center pattern is surrounded by another pattern with similar spatial features-is considered a perceptual analogue of center-surround neurophysiology in the visual system. Surround suppression strength is altered in a range of brain conditions affecting young people (e.g., schizophrenia, depression, migraine) and is modulated by various neurotransmitters. The early teen years are associated with neurotransmitter changes in the human visual cortex, which could impact on excitation-inhibition balance and center-surround antagonistic effects. Hence, we predict that early adolescence is associated with perceptual changes in center-surround suppression. In this cross-sectional study, we tested 196 students at every age from 10 to 17 years and 30 adults (aged 21-34 years) to capture the preteen, adolescent, and adult periods. Contrast discrimination thresholds were measured for a central, circular, vertical sinusoidal grating pattern (0.67° radius, 2 cyc/deg spatial frequency, 2 deg/s drift rate) with and without the surround (4° radius, otherwise same spatial properties as the center). Individual suppression strength was determined by comparing the perceived contrast of the target with and without the surround. After excluding unreliable data (7% of total), we found an effect of age on perceptual center-surround contrast suppression strength, F(8,201) = 2.30, P = 0.02, with weaker suppression in the youngest adolescents relative to adults (Bonferroni pairwise comparisons between adults vs 12-year-olds P = 0.01; adults vs 13-year-olds P = 0.002). Our data demonstrate different center-surround interactions in the visual system-a key building block for visual perception-in early adolescence relative to adulthood.

Stable abnormalities of contrast discrimination sensitivity in subthreshold depression: A longitudinal study.

Subthreshold depression (StD), as a subclinical state, is highly prevalent and increases the risk for developing major depressive disorder (MDD). Although several studies have reported deficits of contrast sensitivity in MDD patients, it is unclear whether individuals with StD could demonstrate deficits of contrast sensitivity and whether the deficits could remain stable over time. Here we used a contrast discrimination task (a suprathreshold task) and a contrast detection task (a near-threshold task) to compare contrast sensitivity of the StD group with that of matched non-depressed controls. For each task, a spatial four-alternative forced-choice method and a psychophysical QUEST procedure were used to measure contrast discrimination threshold or contrast detection threshold. Participants performed an initial assessment and a follow-up assessment 4 months later. Compared to the non-depressed controls, individuals with StD demonstrated reduced contrast discrimination sensitivity, not only at the initial assessment but also at the follow-up assessment, indicating a stable abnormality. Contrast discrimination thresholds at the initial assessment did not predict changes of depression symptom severity over time. For contrast detection sensitivity, there was no significant difference between the StD group and non-depressed controls. We concluded that contrast discrimination testing might provide a trait-dependent biomarker for depression.

Classification images for contrast discrimination

Contrast discrimination measures the smallest difference in contrast (the threshold) needed to successfully tell two stimuli apart. The contrast discrimination threshold typically increases with contrast. However, for low spatial frequency gratings the contrast threshold first increases, but then starts to decrease at contrasts above about 50%. This behaviour was originally observed in contrast discrimination experiments using dark spots as stimuli, suggesting that the contrast discrimination threshold for low spatial frequency gratings may be dominated by responses to the dark parts of the sinusoid. This study measures classification images for contrast discrimination experiments using a 1 cycle per degree sinusoidal grating at contrasts of 0, 25%, 50% and 75%. The classification images obtained clearly show that observers emphasize the darker parts of the sinusoidal grating (i.e. the troughs), and this emphasis increases with contrast. At 75% contrast, observers almost completely ignored the bright parts (peaks) of the sinusoid, and for some observers the emphasis on the troughs is already evident at contrasts as low as 25%. Analysis using a Hammerstein model suggests that the bias towards the dark parts of the stimulus is due to an early nonlinearity, perhaps similar to that proposed by Whittle.

Magnocellular and Parvocellular Mediated Luminance Contrast Discrimination in Severe Alcohol Use Disorder

Severe alcohol use disorder (SAUD) is associated with widespread cognitive impairments, including low-level visual processing deficits that persist after prolonged abstinence. However, the extent and characteristics of these visual deficits remain largely undetermined, impeding the identification of their underlying mechanisms and influence on higher-order processing. In particular, little work has been conducted to assess the integrity of the magnocellular (MC) and parvocellular (PC) visual pathways, namely the 2 main visual streams that convey information from the retina up to striate, extrastriate, and dorsal/ventral cerebral regions. We investigated achromatic luminance contrast processing mediated by inferred MC and PC pathways in 33 patients with SAUD and 32 matched healthy controls using 2 psychophysical pedestal contrast discrimination tasks that promote responses of inferred MC or PC pathways. We relied on a staircase procedure to assess participants' ability to detect small changes in luminance within an array of 4 gray squares that were either continuously presented (steady pedestal, MC-biased) or briefly flashed (pulsed pedestal, PC-biased). We replicated the expected pattern of MC and PC contrast responses in healthy controls. We found preserved dissociation of MC and PC contrast signatures in SAUD but higher MC-mediated mean contrast discrimination thresholds combined with a steeper PC-mediated contrast discrimination slope compared with healthy controls. These findings indicate altered MC-mediated contrast sensitivity and PC-mediated contrast gain, confirming the presence of early sensory disturbances in individuals with SAUD. Such low-level deficits, while usually overlooked, might influence higher-order abilities (e.g., memory, executive functions) in SAUD by disturbing the "coarse-to-fine" tuning of the visual system, which relies on the distinct functional properties of MC and PC pathways and ensures proper and efficient monitoring of the environment.

Illusory Motion Perception Is Associated with Contrast Discrimination but Not Motion Sensitivity, Self-Reported Visual Discomfort, or Migraine Status.

PurposeAltered visual processing of motion and contrast has been previously reported in people with migraine. One possible manifestation of this altered visual processing is increased self-reported susceptibility to visual illusions of contrast and motion. Here, we use the Fraser–Wilcox illusion to explore individual differences in motion illusion strength in people with and without migraine. The motion-inducing mechanisms of the Fraser–Wilcox illusion are purported to be contrast dependent. To better understand the mechanisms of the illusion, as well as visual processing anomalies in migraine, we explored whether migraine status, susceptibility to visual discomfort, contrast discrimination, or motion sensitivity are related to quantified motion illusion strength.MethodsThirty-six (16 with aura, 20 without aura) people with migraine and 20 headache-free controls participated. Outcome measures were motion illusion strength (the physical motion speed that counterbalanced the illusory motion), motion sensitivity, and contrast discrimination thresholds (measured for each contrast pair that formed part of the illusory motion stimulus). Typical daily visual discomfort was self-reported via questionnaire.ResultsMotion illusion strength was negatively correlated with contrast discrimination threshold (r = –0.271, P = 0.04) but was not associated with motion sensitivity or migraine status. People with migraine with aura reported experiencing visual discomfort more frequently than the control group (P = 0.001). Self-reported visual discomfort did not relate to quantified perceptual motion illusion strength.ConclusionsIndividuals with better contrast discrimination tend to perceive faster illusory motion regardless of migraine status.

The Functional Field of View of Older Adults is Associated With Contrast Discrimination in the Magnocellular not Parvocellular Pathway.

As we age, the functional field of view (FFOV) declines and these declines predict falls and motor vehicle accidents in older adults (Owsley, C. (2013). Visual processing speed. Vision Research, 90, 52-56. doi:10.1016/j.visres.2012.11.014). To increase understanding of possible causes of this decline, the current study explored whether the FFOV in older adults is associated with the sensitivity of the magnocellular and parvocellular sub-cortical pathways. Forty-four younger (M = 27.18, SD = 5.40 years) and 44 older (M = 72.18, SD = 5.82 years) adults completed an FFOV test and the steady- and pulsed-pedestal paradigms of Pokorny and Smith (Pokorny, J., & Smith, V. C. (1997). Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America. A, Optics, Image Science, and Vision, 14, 2477-2486. doi:10.1364/josaa.14.002477) as measures of magnocellular and parvocellular pathways, respectively. Older adults made more FFOV errors and had higher contrast discrimination thresholds in both the steady- and pulsed-pedestal paradigms, than younger adults. FFOV errors in the younger group were not related to contrast discrimination thresholds. In multiple regression, older group FFOV errors showed a strong unique association with contrast discrimination thresholds mediated via the magnocellular, but not the parvocellular pathway. We infer that reduced magnocellular pathway contrast sensitivity may contribute to reduced functional vision in older adults.

More than noise: context-dependent luminance contrast discrimination in a coral reef fish (Rhinecanthus aculeatus).

Achromatic (luminance) vision is used by animals to perceive motion, pattern, space and texture. Luminance contrast sensitivity thresholds are often poorly characterised for individual species and are applied across a diverse range of perceptual contexts using over-simplified assumptions of an animal's visual system. Such thresholds are often estimated using the receptor noise limited model (RNL). However, the suitability of the RNL model to describe luminance contrast perception remains poorly tested. Here, we investigated context-dependent luminance discrimination using triggerfish (Rhinecanthus aculeatus) presented with large achromatic stimuli (spots) against uniform achromatic backgrounds of varying absolute and relative contrasts. 'Dark' and 'bright' spots were presented against relatively dark and bright backgrounds. We found significant differences in luminance discrimination thresholds across treatments. When measured using Michelson contrast, thresholds for bright spots on a bright background were significantly higher than for other scenarios, and the lowest threshold was found when dark spots were presented on dark backgrounds. Thresholds expressed in Weber contrast revealed lower thresholds for spots darker than their backgrounds, which is consistent with the literature. The RNL model was unable to estimate threshold scaling across scenarios as predicted by the Weber-Fechner law, highlighting limitations in the current use of the RNL model to quantify luminance contrast perception. Our study confirms that luminance contrast discrimination thresholds are context dependent and should therefore be interpreted with caution.

Contrast discrimination under task-induced mental load

Task-induced mental load can potentially degrade visual performance, which could be important during tasks such as driving. In the natural world, most objects have visual contrasts that are supra-threshold, and so the ability to reliably distinguish the borders between these objects is related to contrast discrimination, rather than absolute contrast detection performance. This study investigated the effects of auditory task-induced mental load on contrast discrimination threshold across several spatial frequencies. Binocular contrast discrimination thresholds were measured in 14 participants at spatial frequencies of 0.25, 1, 4, 8, 14 cpd across pedestal contrast levels of 10, 16, 24, 38 and 60%. The task was repeated with an auditory 0-back task and 2-back task to impose a mental load, the magnitude of which was measured through subjective ratings and heart rate recording. A concurrent 2-back task significantly increased contrast discrimination thresholds across all spatial frequencies when compared to both the baseline and 0-back conditions. There was no significant effect of mental load on the slope of the Threshold vs Contrast function once potential ceiling effects in our staircase procedure were removed. We conclude that mental load induced by concurrent cognitive tasks can increase contrast discrimination thresholds. When evaluating an individuals’ performance under natural-world condition such as driving, the effects of task-induced mental load on contrast discrimination thresholds should be considered.

Auditory capture of visual motion: effects on perception and discrimination.

We asked whether the perceived direction of visual motion and contrast thresholds for motion discrimination are influenced by the concurrent motion of an auditory sound source. Visual motion stimuli were counterphasing Gabor patches, whose net motion energy was manipulated by adjusting the contrast of the leftward-moving and rightward-moving components. The presentation of these visual stimuli was paired with the simultaneous presentation of auditory stimuli, whose apparent motion in 3D auditory space (rightward, leftward, static, no sound) was manipulated using interaural time and intensity differences, and Doppler cues. In experiment 1, observers judged whether the Gabor visual stimulus appeared to move rightward or leftward. In experiment 2, contrast discrimination thresholds for detecting the interval containing unequal (rightward or leftward) visual motion energy were obtained under the same auditory conditions. Experiment 1 showed that the perceived direction of ambiguous visual motion is powerfully influenced by concurrent auditory motion, such that auditory motion 'captured' ambiguous visual motion. Experiment 2 showed that this interaction occurs at a sensory stage of processing as visual contrast discrimination thresholds (a criterion-free measure of sensitivity) were significantly elevated when paired with congruent auditory motion. These results suggest that auditory and visual motion signals are integrated and combined into a supramodal (audiovisual) representation of motion.

Attentional gain control during decision-making with multiple alternatives

The magnitude of sensory responses is enhanced when attention is directed to a relevant stimulus (i.e., sensory gain), and this gain modulation adequately accounts for attention-related behavioral improvements in simple two alternative-force-choice (2AFC) tasks (Itthipuripat et al., 2014a). However, other evidence suggests that sensory gain is not always sufficient to account for improved performance, and that attention may also influence post-sensory read-out mechanisms (Pestilli et al., 2011). Based on previous visual search studies (e.g., Palmer and Verghese, 2000), we reasoned that the number of competing items and alternative choices might be the key factor that determines how much sensory gain and efficient read-out contribute to behavioral performance. Fourteen subjects participated in 2AFC and 4AFC contrast discrimination tasks in which they reported the location that contained an incremental contrast within a target stimulus. This contrast increment target was embedded in an array of four competing stimuli flickering at different frequencies and contrasts. Subjects were cued either to the target-location (focused-attention) or all stimulus-locations (distributed-attention). Contrast discrimination thresholds were measured concurrently with electroencephalography (EEG). Steady-state visual evoked potentials (SSVEPs) were used as an index of sensory responses and late event-related potentials in occipital and frontal channels were used as indices of sustained attention and post-sensory decision processes, respectively. To our surprise, the amount of sensory gain measured by SSVEPs is comparable and sufficient to explain attention-related improvement in perceptual sensitivity in both 2 and 4AFC tasks. Similarly, sustained attention-related negative difference over occipital cortex is comparable across the two tasks. Finally, the pattern of late centrofrontal positive potentials suggests that the decision threshold in the 4AFC task is higher than that in the 2AFC task, consistent with the observed main effects of attention and task-type on decision times. Meeting abstract presented at VSS 2015

Flicker adaptation desensitizes the magnocellular but not the parvocellular pathway.

Anatomical and physiological studies show that in primates, visual information is conveyed through two parallel pathways, including the magnocellular (MC-) and parvocellular (PC-) pathways. However, the functional separation between the two pathways remains controversial and challenging. To resolve this, we show a psychophysical approach to desensitize the inferred MC-pathway of human observers independently of the inferred PC-pathway. The steady-pedestal and pulsed-pedestal paradigms that allow detection and discrimination to be mediated by only the inferred MC- or PC-pathway were used. Three observers (one male, aged 43 years, and two females, aged 33 and 62 years) adapted to either a steadily presented pedestal or a 2- or 10-Hz 50% contrast square-wave modulated luminance flicker. Contrast discrimination thresholds were measured following the flicker adaptation. Flicker adaptation reduces contrast detection and discrimination of the MC-pathway but not the PC-pathway, with larger MC losses from 10-Hz (∼ 0.28 log unit loss, P < 0.05 for all observers) than 2-Hz flicker (∼ 0.13 log unit loss, P < 0.05 for one or two observers depending on stimulus size). Further, our results show that the PC-pathway does not mediate the contrast detection threshold at the background luminance following MC-pathway desensitization. This study demonstrates the feasibility of independently manipulating sensitivity of the MC-pathway in human observers. Our paradigms provide powerful tools to independently investigate the perceptual functions in the MC- and PC-pathways. This could lead to a better understanding of the perceptual functions of these pathways.

Contextual effects on perceived contrast: figure-ground assignment and orientation contrast.

Figure-ground segregation is an important step in the path leading to object recognition. The visual system segregates objects ('figures') in the visual scene from their backgrounds ('ground'). Electrophysiological studies in awake-behaving monkeys have demonstrated that neurons in early visual areas increase their firing rate when responding to a figure compared to responding to the background. We hypothesized that similar changes in neural firing would take place in early visual areas of the human visual system, leading to changes in the perception of low-level visual features. In this study, we investigated whether contrast perception is affected by figure-ground assignment using stimuli similar to those in the electrophysiological studies in monkeys. We measured contrast discrimination thresholds and perceived contrast for Gabor probes placed on figures or the background and found that the perceived contrast of the probe was increased when it was placed on a figure. Furthermore, we tested how this effect compared with the well-known effect of orientation contrast on perceived contrast. We found that figure-ground assignment and orientation contrast produced changes in perceived contrast of a similar magnitude, and that they interacted. Our results demonstrate that figure-ground assignment influences perceived contrast, consistent with an effect of figure-ground assignment on activity in early visual areas of the human visual system.

Context and Crowding in Perceptual Learning on a Peripheral Contrast Discrimination Task: Context-Specificity in Contrast Learning

Perceptual learning is an improvement in sensitivity due to practice on a sensory task and is generally specific to the trained stimuli and/or tasks. The present study investigated the effect of stimulus configuration and crowding on perceptual learning in contrast discrimination in peripheral vision, and the effect of perceptual training on crowding in this task. 29 normally-sighted observers were trained to discriminate Gabor stimuli presented at 9° eccentricity with either identical or orthogonally oriented flankers with respect to the target (ISO and CROSS, respectively), or on an isolated target (CONTROL). Contrast discrimination thresholds were measured at various eccentricities and target-flanker separations before and after training in order to determine any learning transfer to untrained stimulus parameters. Perceptual learning was observed in all three training stimuli; however, greater improvement was obtained with training on ISO-oriented stimuli compared to CROSS-oriented and unflanked stimuli. This learning did not transfer to untrained stimulus configurations, eccentricities or target-flanker separations. A characteristic crowding effect was observed increasing with viewing eccentricity and decreasing with target-flanker separation before and after training in both configurations. The magnitude of crowding was reduced only at the trained eccentricity and target-flanker separation; therefore, learning for contrast discrimination and for crowding in the present study was configuration and location specific. Our findings suggest that stimulus configuration plays an important role in the magnitude of perceptual learning in contrast discrimination and suggest context-specificity in learning.

Differential effects of alcohol on contrast processing mediated by the magnocellular and parvocellular pathways

This study investigated how acute alcohol intake affects contrast processing mediated by inferred magnocellular (MC) and parvocellular (PC) pathways. Achromatic contrast discrimination thresholds were measured in 16 young healthy participants using a steady-pedestal, pulsed-pedestal or pedestal-Δ-pedestal paradigm designed to favor the inferred MC or the PC pathway. Each participant completed two randomized sessions that included consumption of either 0.8 g/kg alcohol or a placebo beverage, with each session consisting of contrast discrimination measurements at baseline and at 60 min following beverage consumption. The results showed that, compared to placebo, alcohol significantly reduced MC contrast sensitivity and PC contrast gain but had no effect on PC contrast sensitivity for the majority of the participants; and did not alter MC contrast gain consistently across participants. The decrease in contrast gain in the PC pathway can be interpreted as a degradation of the postretinal signal-to-noise ratio, whereas the decrease of sensitivity in the MC pathway likely results from a change of cortical processing.

Cholinergic enhancement reduces orientation-specific surround suppression but not visual crowding

Acetylcholine (ACh) reduces the spatial spread of excitatory fMRI responses in early visual cortex and receptive field size of V1 neurons. We investigated the perceptual consequences of these physiological effects of ACh with surround suppression and crowding, two phenomena that involve spatial interactions between visual field locations. Surround suppression refers to the reduction in perceived stimulus contrast by a high-contrast surround stimulus. For grating stimuli, surround suppression is selective for the relative orientations of the center and surround, suggesting that it results from inhibitory interactions in early visual cortex. Crowding refers to impaired identification of a peripheral stimulus in the presence of flankers and is thought to result from excessive integration of visual features. We increased synaptic ACh levels by administering the cholinesterase inhibitor donepezil to healthy human subjects in a placebo-controlled, double-blind design. In Experiment 1, we measured surround suppression of a central grating using a contrast discrimination task with three conditions: (1) surround grating with the same orientation as the center (parallel), (2) surround orthogonal to the center, or (3) no surround. Contrast discrimination thresholds were higher in the parallel than in the orthogonal condition, demonstrating orientation-specific surround suppression (OSSS). Cholinergic enhancement decreased thresholds only in the parallel condition, thereby reducing OSSS. In Experiment 2, subjects performed a crowding task in which they reported the identity of a peripheral letter flanked by letters on either side. We measured the critical spacing between the targets and flanking letters that allowed reliable identification. Cholinergic enhancement with donepezil had no effect on critical spacing. Our findings suggest that ACh reduces spatial interactions in tasks involving segmentation of visual field locations but that these effects may be limited to early visual cortical processing.

Effect of blur adaptation on contrast discrimination in emmetropes and myopes

Studies have investigated visual resolution and contrast detection following a period of degradation in the retinal image (blur adaptation) and have shown it can produce increases in visual resolution and changes in contrast sensitivity. However these studies only explored a limited range of spatial frequencies of sine wave gratings or tested visual resolution alone. The current study aimed to expand on previous work by investigating contrast discrimination and contrast threshold sensitivity before and after blur adaptation for a range of different spatial frequencies. In addition both myopes and emmetropes were tested to see if they responded in different ways to the blur adaptation. Contrast discrimination thresholds were measured (monocular) for a range of spatial frequencies of sine wave gratings (0.5 to 24 cycles per degree) before and after adaptation to +2D blur (45 minutes of adaptation) in four emmetropes and four myopes (mean refraction −6DS). The contrast pedestal was set to 10% and an adaptive 2AFC procedure was employed. Contrast sensitivity and high contrast visual acuity were also measured before and after adaptation. The results showed increases in contrast sensitivity and contrast discrimination at some spatial frequencies after adaptation while others were reduced. In particular, the myopes showed the largest increases in mid to high spatial frequencies (16–20 cycles per degree) while emmetropes displayed reduced sensitivity at the higher spatial frequencies (18–24 cycles per degree). Measurements of the optics before and after adaptation ruled out any optical changes being responsible. The results therefore point to different neural adaptation strategies in the refractive groups. Possible models of how the visual system responds to periods of blur are discussed.

Transcranial alternating stimulation in a high gamma frequency range applied over V1 improves contrast perception but does not modulate spatial attention

Spatial visual attention enhances information processing within its focus. Vision at an attended location is faster, more accurate, of higher spatial resolution, and has an enhanced sensitivity for fine changes. Earlier hypotheses suggest that the neuronal mechanisms of these processes are based on the interactions among different neuronal groups by means of cortical oscillations in the gamma range. The aim of the current study was to modulate these oscillations externally, using a new technique called transcranial alternating current stimulation (tACS). We investigated the effect of covert spatial attention within and outside its focus by probing contrast sensitivity and contrast discrimination at high resolution across the visual field of 20 healthy human subjects. While applying 40, 60, and 80 Hz tAC stimulation over the primary visual cortex (V1), subjects' contrast-discrimination thresholds were obtained using two different conditions: in the first condition we presented a black disc as a peripheral cue that automatically attracted the subject's attention, whereas there was no cue in the second condition. We found that the spatial profile of contrast sensitivity was not affected by the stimulation. Contrast-discrimination thresholds on the other hand decreased significantly during 60 Hz tACS, whereas there was no effect of 40 and 80 Hz stimulation. These results suggest that attention plays an important role in contrast discrimination based on V1 activities that are influences by gamma range tACS stimulation.

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.

Surround suppression in human V1 explains psychophysical lateral masking

Purpose: We used fMRI and psychophysics to test whether surround suppression in primary visual cortex (V1) can explain lateral masking. Methods: Observers performed a contrast discrimination task on a contrast-reversing sinusoidal grating target (1 c/d, 4 Hz), restricted to an annulus (4–7 deg eccentricity) both in the presence and absence of a full contrast lateral mask. We measured fMRI responses and psychophysical thresholds for each of several target contrasts, both with and without the lateral mask. V1 was identified using standard mapping procedures, and the analysis was restricted to the subregion of V1 responding to the target. Results: Contrast discrimination thresholds were increased in the presence of the lateral mask, in particular for low target contrasts. We used the psychophysical data to infer a nonlinear contrast response function, assuming that a fixed response difference is required for correct contrast discrimination. FMRI responses increased with increasing target contrast. In the presence of the lateral mask, responses were reduced by about a factor of two. The amount of suppression we observed in the fMRI responses was very similar to that inferred from the psychophysical data. Measurements in a control experiment ensured that the reduction in the fMRI signal reflected a neural suppresion and not some confounding effect of blood flow. In the control experiment, the lateral mask was presented with a temporal delay (375 ms after the target offset). This eliminated the psychophysical masking effect and eliminated most of the reduction in fMRI signal caused by the lateral mask. Conclusion: V1 is a plausible candidate for mediating the lateral masking observed behaviorally.