Visual contrast perception in visual snow syndrome reveals abnormal neural gain but not neural noise.

Visual snow syndrome is a neurological condition characterized by a persistent visual disturbance, visual snow, in conjunction with additional visual symptoms. Cortical hyperexcitability is a potential pathophysiological mechanism, which could be explained by increased gain in neural responses to visual input. Alternatively, neural noise in the visual pathway could be abnormally elevated. We assessed these two potential competing neural mechanisms in our studies of visual contrast perception. Cortical hyperexcitation also occurs in migraine, which commonly co-occurs with visual snow syndrome. Therefore, to determine whether the effect of visual snow syndrome can be distinguished from interictal migraine, we recruited four participant groups: controls, migraine alone, visual snow syndrome alone and visual snow syndrome with migraine. In the first experiment, we estimated internal noise in 20 controls, 21 migraine participants and 32 visual snow syndrome participants (16 with migraine) using a luminance increment detection task. In the second experiment, we estimated neural contrast gain in 21 controls, 22 migraine participants and 35 visual snow syndrome participants (16 with migraine) using tasks assessing sensitivity to changes in contrast from a reference. Contrast gain and sensitivity were measured for the putative parvocellular and 'on' and 'off' magnocellular pathways, respectively. We found that luminance increment thresholds and internal noise estimates were normal in both visual snow syndrome and migraine. Contrast gain measures for putative parvocellular processing and contrast sensitivity for putative off magnocellular processing were abnormally increased in visual snow syndrome, regardless of migraine status. Therefore, our results indicate that visual snow syndrome is characterized by increased neural contrast gain but not abnormal neural noise within the targeted pathways.

Impacts of older age on the temporal properties of collinear facilitation.

Collinear facilitation is a visual phenomenon by which the contrast detection threshold of a central target is reduced (facilitation) when placed equidistant between two high-contrast flankers. The neural mechanisms underpinning this phenomenon originate from feed-forward lateral facilitation between cell layers in V1 (slower) and feedback facilitation from extrastriate visual areas to V1 (faster). The strength of these contributions has been explored in younger adults by presenting the central target and flankers at varying timing offsets. Here, we investigated the effects of older age on collinear facilitation with flankers presented in sync, before, and after target onset, to allow the inference of any characteristic effect of older age on feed-forward and feedback facilitatory mechanisms. Seventeen older and 19 younger observers participated. Our data confirms previous findings of an age-related reduction in facilitation when flankers and target occur at synchrony, but no age difference was found at other timings. Marked interindividual variability in facilitation for the different flanker onset timings was present, which was repeatable within individuals. Further research is required to ascertain the mechanistic underpinnings for different facilitation profiles between individuals. Longitudinal study across an individual's life span is needed to determine whether an individual's facilitation profile changes with age.

Age-related changes in auditory and visual interactions in temporal rate perception.

We investigated how aging affects the integration of temporal rate for auditory flutter (amplitude modulation) presented with visual flicker. Since older adults were poorer at detecting auditory amplitude modulation, modulation depth was individually adjusted so that temporal rate was equally discriminable for 10 Hz flutter and flicker, thereby balancing the reliability of rate information available to each sensory modality. With age-related sensory differences normalized in this way, rate asynchrony skewed both auditory and visual rate judgments to the same extent in younger and older adults. Therefore, reliability-based weighting of temporal rate is preserved in older adults. Concurrent presentation of synchronous 10 Hz flicker and flutter improved temporal rate discrimination consistent with statistically optimal integration in younger but not older adults. In a control experiment, younger adults were presented with the same physical auditory stimulus as older adults. This time, rate asynchrony skewed perceived rate with greater auditory weighting rather than balanced integration. Taken together, our results indicate that integration of discrepant auditory and visual rates is not altered due to the healthy aging process once sensory deficits are accounted for, but that aging does abolish the minor improvement in discrimination performance seen in younger observers when concordant rates are integrated.

Improving Understanding of Visual Snow by Quantifying its Appearance and Effect on Vision.

Purpose: Visual snow is the hallmark of the neurological condition visual snow syndrome (VSS) but the characteristics of the visual snow percept remain poorly defined. This study aimed to quantify its appearance, interobserver variability, and effect on measured visual performance and self-reported visual quality. Methods: Twenty-three participants with VSS estimated their visual snow dot size, separation, luminance, and flicker rate by matching to a simulation. To assess whether visual snow masks vision, we compared pattern discrimination thresholds for textures that were similar in spatial scale to visual snow as well as more coarse than visual snow, in participants with VSS, and with and without external noise simulating visual snow in 23 controls. Results: Mean and 95% confidence intervals for visual snow appearance were: size (6.0, 5.8-6.3 arcseconds), separation (2.0, 1.7-2.3 arcmin), luminance (72.4, 58.1-86.8 cd/m2), and flicker rate (25.8, 18.9-32.8 frames per image at 120 hertz [Hz]). Participants with finer dot spacing estimates also reported greater visibility of their visual snow (τb = -0.41, 95% confidence interval [CI] = -0.62 to -0.13, P = 0.01). In controls, adding simulated fine-scale visual snow to textures increased thresholds for fine but not coarse textures (F(1, 22) = 4.98, P = 0.036, ηp2 = 0.19). In VSS, thresholds for fine and coarse textures were similar (t(22) = 0.54, P = 0.60), suggesting that inherent visual snow does not act like external noise in controls. Conclusions: Our quantitative estimates of visual snow constrain its likely neural origins, may aid differential diagnosis, and inform future investigations of how it affects vision. Methods to quantify visual snow are needed for evaluation of potential treatments.

Exploring the Phenotype and Possible Mechanisms of Palinopsia in Visual Snow Syndrome.

Purpose: Palinopsia (persistent afterimages and/or trailing) is a common but poorly understood symptom of the neurological condition visual snow syndrome. This study aimed to collect a phenotypical description of palinopsia in visual snow syndrome and probe for abnormalities in temporal visual processing, hypothesizing that palinopsia could arise from increased visibility of normal afterimage signals or prolonged visible persistence. Methods: Thirty controls and 31 participants with visual snow syndrome (18 with migraine) took part. Participants completed a palinopsia symptom questionnaire. Contrast detection thresholds were measured before and after brief exposure to a spatial grating because deficient contrast adaptation could increase afterimage visibility. Temporal integration and segregation were assessed using missing-element and odd-element tasks, respectively, because prolonged persistence would promote integration at wide temporal offsets. To distinguish the effects of visual snow syndrome from comorbid migraine, 25 people with migraine alone participated in an additional experiment. Results: Palinopsia was common in visual snow syndrome, typically presenting as unformed images that were frequently noticed. Contrary to our hypotheses, we found neither reduced contrast adaptation (F(3.22, 190.21) = 0.71, P = 0.56) nor significantly prolonged temporal integration thresholds (F(1, 59) = 2.35, P = 0.13) in visual snow syndrome. Instead, participants with visual snow syndrome could segregate stimuli in closer succession than controls (F(1, 59) = 4.62, P = 0.04, ηp2 = 0.073) regardless of co-occurring migraine (F(2, 53) = 1.22, P = 0.30). In contrast, individuals with migraine alone exhibited impaired integration (F(2, 53) = 4.44, P = 0.017, ηp2 = 0.14). Conclusions: Although neither deficient contrast adaptation nor prolonged visible persistence explains palinopsia, temporal resolution of spatial cues is enhanced and potentially more flexible in visual snow syndrome.

Audiovisual Temporal Perception in Aging: The Role of Multisensory Integration and Age-Related Sensory Loss.

Within each sensory modality, age-related deficits in temporal perception contribute to the difficulties older adults experience when performing everyday tasks. Since perceptual experience is inherently multisensory, older adults also face the added challenge of appropriately integrating or segregating the auditory and visual cues present in our dynamic environment into coherent representations of distinct objects. As such, many studies have investigated how older adults perform when integrating temporal information across audition and vision. This review covers both direct judgments about temporal information (the sound-induced flash illusion, temporal order, perceived synchrony, and temporal rate discrimination) and judgments regarding stimuli containing temporal information (the audiovisual bounce effect and speech perception). Although an age-related increase in integration has been demonstrated on a variety of tasks, research specifically investigating the ability of older adults to integrate temporal auditory and visual cues has produced disparate results. In this short review, we explore what factors could underlie these divergent findings. We conclude that both task-specific differences and age-related sensory loss play a role in the reported disparity in age-related effects on the integration of auditory and visual temporal information.