Directional Motion Sensitivity in Visual Snow Syndrome and Its Relation to Trailing-Type Palinopsia

Abstract

Purpose: Visual Snow Syndrome (VSS) is characterized by visual perceptual distortions, potentially linked to increased neural excitability and/or decreased inhibition in the visual cortex. If present, these putative physiological abnormalities may alter motion direction sensitivity. Trailing-type palinopsia (TTP), commonly associated with VSS, may further affect motion sensitivity. This study aimed to investigate the sensitivity to direction of motion and its dependence on stimulus size in patients with VSS using the Spatial Suppression paradigm.

Methods: We assessed motion duration discrimination thresholds for small (1 degree), medium (2.5 degrees), and large (12 degrees) high-contrast gratings in 23 patients with VSS and 27 healthy control participants. The Spatial Suppression Index (SSI) quantified size-dependent increases in duration thresholds. Visual Discomfort Questionnaire scores and VSS symptom ratings, including TTP, afterimages, photophobia, etc., were also collected.

Results: Patients with VSS reported higher visual discomfort and perceptual disturbances, but no group differences were found in duration thresholds or SSI. Notably, higher TTP scores were associated with lower duration thresholds, indicating a facilitatory effect of TTP on sensitivity to direction of motion.

Conclusions: Our findings indicate that when VSS is regarded as a unified diagnostic category, it is not associated with impaired motion direction sensitivity or abnormal center-surround suppression. However, our preliminary results suggest that an absence or presence of comorbid TTP has a qualitatively different effect on directional sensitivity in patients with VSS. The facilitatory effect of TTP on motion sensitivity provides insight into the functional concomitants of TTP, and warrants further exploration, as it may significantly influence experimental outcomes.

Visual Snow Syndrome (VSS) is a neurological disorder characterized by the persistent small flickering dots across the entire visual field that resemble tiny snowflakes or noise in a poorly tuned analog television (TV). In addition to the “visual snow,” individuals with VSS typically experience a variety of other visual and non-visual symptoms. The visual disturbances include photophobia, nyctalopia (night-blindness), palinopsia, enhanced entoptic phenomena, etc.^1,2 Among non-visual symptoms, most common are tinnitus, migraine, impaired concentration, fatigue, and anxiety.^3,4 

It has been suggested that VSS is associated with neuronal hyperexcitability, which leads to a perception of visual stimuli that do not exist.^5,6 Evidence for hyperexcitability comes from several sources. For example, people with VSS experience strong visual discomfort when viewing bright light or high-contrast visual patterns,⁷ a phenomenon that has been attributed to increased excitability in the visual pathways.⁸ The reduced phosphene threshold during transcranial magnetic stimulation (TMS) of the occipital cortex found in some individuals with VSS⁹ is also indicative of increased excitability. Other evidence pointing in the same direction includes shortened prosaccade latencies,¹⁰ lack of habituation of the visual evoked potentials,^9,11,12 and increased metabolism in the lingual gyrus.¹³ In addition, VSS is often comorbid with migraine and palinopsia, which are thought to be associated with altered excitation/inhibition (E/I) balance in the visual cortex.^14-17 Some authors have suggested that neural excitability in VSS is the result of deficient inhibitory processes.¹¹ However, a TMS study found normal magnetic suppression of perceptual acuity, indicating normal inhibition in the visual cortex in VSS.¹⁸ 

E/I imbalance in the visual cortex in patients with VSS may affect visual perception through altering center-surround antagonism, a phenomenon central to the ability to separate figure and/or motion from the visual background.^19,20 McKendrick et al.²¹ studied center-surround interactions in patients with VSS using center-surround contrast matching paradigm. Participants were presented with small moving gratings with and without an iso-orientated annular surround and compared the perceived contrast to an original reference. Participants with VSS showed less contrast suppression in the presence of the surround compared with controls. This effect may be due to weakened lateral inhibition, reduced feedback from the extrastriate areas, or atypical feedforward connections from the lateral geniculate nucleus of the thalamus. 

Another approach that is often used to study altered center-surround interactions across patients and age groups, but has not yet been applied in VSS, is the spatial suppression paradigm, originally proposed by Tadin et al.¹⁹ This approach exploits the observation that the duration of stimulation required to determine the direction of motion of high-contrast stimulus increases with stimulus size, reflecting the greater involvement of suppressive center-surround mechanisms. 

The neural underpinnings of surround suppression remain controversial, with evidence suggesting a complex interplay of feedforward, lateral, and feedback interactions within the recurrent excitatory-inhibitory networks.^22–26 Smaller stimuli, which are less likely to engage surround suppression, primarily elicit well-documented feedforward excitation and inhibition in the local visual circuitry. In contrast, larger stimuli elicit robust surround inhibition, involving feedback from higher-tier visual areas and additional mechanisms. As reviewed by Angelucci et al.,²² this complex process underscores the intricate nature of visual processing and the multifaceted interactions that underpin surround suppression in the visual cortex. 

Abnormalities in duration thresholds for the small and large stimuli may reflect alterations in different cortical mechanisms. Elevated duration thresholds for small high-contrast gratings have been observed in children with autism, a finding consistent with neuroimaging evidence suggesting increased receptive field sizes due to reduced local inhibition in their primary visual cortex – V1.^27,28 On the other hand, patients with major depression disorder during remission had reduced duration thresholds for large-size gratings, presumably reflecting a deficit in top-down inhibitory mechanisms.²⁹ In patients with Alzheimer's disease, thresholds were abnormally increased for small gratings and abnormally decreased for large gratings, suggesting deficits in both local and top-down inhibition.³⁰ Thus, changes in duration thresholds for small or large stimuli (or both) may, albeit indirectly, reflect the level of the visual hierarchy at which E/I balance is disrupted in individuals with VSS. 

In this study, we used the spatial suppression paradigm to investigate center-surround interactions in VSS. By investigating duration thresholds for the stimuli of different sizes we hoped to estimate different aspects of the E/I balance regulation in the visual cortex in patients with VSS. 

Our second aim was to differentiate the effects of the visual snow on duration thresholds from the effect of palinopsia. Palinopsia is marked by the persistence of visual images after a stimulus has disappeared, manifesting as the retention of images from stationary or moving objects. In contrast to physiologically normal “negative” afterimages, the images in palinopsia are almost always “positive,” that is, isochromatic to the original stimulus.¹⁵ According to recent studies, approximately 80% of patients with VSS experience palinopsia,^2,31 with approximately 45% presenting it as the trailing of moving objects.³¹ Although VSS often co-occurs with migraines, this prevalence is significantly higher than in patients with migraine without VSS, where palinopsia occurs in about 10% of cases.³² Palinopsia has also been linked to increased cortical excitability,^15,16 and/or attenuation of inhibitory controlling influences from higher-tier cortical areas,³³ but the extent to which neural alterations in palinopsia overlap with and influence VSS remains unclear. The spatial suppression paradigm focused on motion perception might be particularly informative regarding E/I imbalance in patients with VSS experiencing trailing-type palinopsia (TTP), a condition in which a moving object leaves behind a trail lasting several seconds. 

In summary, we hypothesized that if individuals with VSS exhibit an excitation/inhibition imbalance or a deficit in neural inhibition, this could influence their perception of visual motion in two distinct ways: (1) elevated duration thresholds for small visual stimuli, and/or (2) reduced spatial suppression. Furthermore, we anticipated these perceptual abnormalities to be more pronounced in individuals with VSS who experience TTP, a symptom that specifically engages visual networks involved in motion processing. 

Methods

Participants

Twenty-three patients recruited from an online community for people with VSS were included in the study. All patients were examined by a neurologist who collected information about their neurologic and visual symptoms. The examination was conducted by a board-certified neurologist who is also the author of this study (author A.A.). Subjects who acquired VS as a result of substance abuse were not included in this study. In 12 subjects, the VS was present from an early age (∼ <10 years). In 11 subjects, it appeared after the age of 14 years. Six of these 11 participants attributed the onset of VS to stress. All but two participants with VSS were medication-free. Two participants were taking antidepressants in low doses (venlafaxin, 75 mg per day, or escitalopram, 5 mg per day, both longer than 3 months). We also recruited 27 sex- and age-matched neurologically healthy control participants from the community. The characteristics of the subjects are presented in Table 1. 

Table 1.

 

Characteristics of the Participants

 

All subjects were asked to complete the Russian version of the Visual Discomfort Scale,³⁴ which assesses unpleasant somatic and perceptual side effects of viewing patterns. Participants were also asked to complete the Russian version State-Trait Anxiety Inventory.³⁵ 

The local ethics committee approved the study and all subjects gave written informed consent. 

All patients with VSS and 21 control participants rated a range of their visual symptoms on a 5-point Likert scale (1 – absence of the symptom, 2 – very rare, 3 – rare, 4 – often, and 5 – all the time). Table 2 summarizes the number of subjects who marked “4” or “5” on this scale for the symptoms most frequently associated with VSS. The presence of two of these symptoms was a necessary criterion for the diagnosis of VSS.¹ 

Table 2.

 

Visual Phenomena in Patients With VSS and Healthy Controls: Percent of Subjects Who Responded 4 or 5 (Frequently or Always) on the 5-Point Scale

 

The “object trailing” visual symptoms were of particular interest because of their potential relation to motion sensitivity assessed in our experimental paradigm. We therefore categorized palinopsia into two types: afterimages of static objects (type I) and trailing of moving objects (type II). All but 1 of the 8 patients who scored 4 or 5 on the type-II palinopsia also scored 4 or 5 on the type-I palinopsia, but 6 subjects who scored 4 or 5 on the type-I palinopsia (afterimages) scored lower than 4 on the type-II palinopsia (trailing). Among patients with type-II palinopsia, three had migraine with aura, two had migraine without aura, and three had no migraine. 

Stimuli and Paradigm

Visual stimuli were presented using PsychToolbox.³⁶ We used the approach similar to that described in our previous paper.³⁷ Stimuli and the experimental procedure are shown in Figure 1. Stimuli consisted of drifting vertical sinusoidal black and white gratings (1 cycle/degree, 4 degrees/sec). The size of the stimuli was controlled by a two-dimensional Gaussian envelope whose full-width at half-maximum was set to 1, 2.5, or 12 degrees for the small, medium, and large stimuli types. 

Figure 1.

 

Psychophysical experiment: stimuli (A) and schematic representation of the experimental paradigm. Figure is adapted from Ref. 38.

Participants sat at 60 cm distance from the monitor (Acer XF250Q, 24.5-inch′ W LED, 1920 × 1080 resolution, 240 hertz [Hz]). The monitor displayed a luminance of 1.2 cd/m² for a black screen and 212.4 cd/m² for a white screen, yielding a Michelson contrast of 99% for the gratings. A direction of motion (left or right) was determined randomly for each trial. Participants were instructed to make a two-alternative forced-choice response, indicating the perceived direction of motion (left or right) by pressing the corresponding arrow key on the keyboard, with no time constraints. The inter-trial interval was 500 ms. In the beginning of each trial, a central dot flickered at the screen for 500 ms followed by the stimulus presentation. The initial stimulus duration was set to 200 ms. The duration was further adjusted depending on a participant's response using three (one for a stimulus of each size) interleaved one-up two down staircases that converged on 71% correct performance. The duration changed in discrete time steps. The step was initially 12.3 ms and changed to 8.2 ms after the first 2 reversals and further to 4.1 ms after 2 more reversals. The session continued until all staircases completed nine reversals. Each subject had two sessions separated by a short break. In three participants (all with VSS), the data from the second session were lost due to a technical failure. 

For each stimulus size/staircase, the threshold was computed by averaging over the reversals, excluding the first two. The reversals followed by 12 or more correct responses were considered as not reflecting the true limit of perceptual capacity, because the probability of accidentally giving 12 correct responses in a row is very low (P = 0.00024). These reversals were also excluded from averaging. If there were less than three reversals left to calculate the threshold, the staircase was considered a non-converging staircase, and the threshold for the corresponding staircase was not calculated. The Spatial Suppression Index (SSI) was calculated based on the thresholds for the large and small gratings as follows:  

\begin{eqnarray*} SSI = {\rm{ }}log10\left( {Threshol{{d}_{Large}}} \right) - log10\left( {Threshol{{d}_{Small}}} \right). \end{eqnarray*}

 

Larger SSI values indicate stronger perceptual spatial suppression. 

Statistical Analysis

To assess reproducibility of duration thresholds between the two sessions we calculated intraclass correlations (ICCs) using the “irr” package in R (2-way mixed effects, absolute agreement). A mixed effects ANOVA was used to assess differences between groups. Duration thresholds were log-transformed to normalize the distributions. Spearman correlation coefficients were calculated to investigate the relationships between psychometric variables and visual perception scores, with Bonferroni correction applied for multiple comparisons to adjust the P values. The alpha level was set at P < 0.05. The data used for statistical analysis are available at https://ruspsydata.mgppu.ru/handle/123456789/180. 

Results

Visual Symptoms in Patients With VSS

Participants with VSS had a number of additional visual symptoms. Symptoms critical for the diagnosis of VSS¹ are listed in Table 2. Apart from VS, all of our VSS participants had high prominence of some of these critical symptoms (4 or 5 on the Likert scale) and all of them had additional visual symptoms (e.g. flashes of lights, swirls, or clouds with eyes closed, etc.). Patients with VSS had higher scores on the Visual Discomfort Scale than the control participants (t(46) = 6.2, P < 1e-6). 

Estimation of Duration Thresholds

The staircases failed to converge in five subjects in the first session and in two subjects in the second session, resulting in two non-converging staircases for the large grating, five non-converging staircases for the medium-sized grating, and two non-converging staircases for the small gratings. In seven of nine cases, the staircases failed to converge during the first session. If the staircase failed to converge in one of the sessions, the threshold was defined based on the results from another session, which was the case in seven subjects, all with VSS. 

Illusion of Reverse Motion

In 3 of 23 participants with VSS (13%) and in 3 of 27 control participants (11%), the large moving grating elicited a sustained illusion of reverse motion. This illusion was evident in both sessions and precluded assessment of duration thresholds for large-sized gratings. The original psychometric staircases for these participants are presented in the Supplementary Results. The illusion tended to disappear at long presentation times (approximately 1 second or more). None of the subjects demonstrated the reverse motion illusion in the case of the medium- or small-sized gratings. 

Reliability of Duration Thresholds Assessment

In both groups, ICCs suggested good reliability³⁹ for large gratings and moderate for small gratings, whereas it was moderate to poor reliability for the medium-size gratings (Table 3, Fig. 2).³⁸ This result is consistent with the results obtained using the same stimuli in another sample of healthy adult participants. 

Table 3.

 

Between-Sessions Infraclass Correlations^† of Duration Thresholds and 95% Confidence Intervals

 

Figure 2.

 

Scatterplots showing between-session consistency of the duration thresholds. The dotted line is the line of equality.

Effect of Session Order

Mixed ANOVA with the factors Size, Group, and Session revealed a significant Session effect (F(1,33) = 6.0, P = 0.02, partial eta-squared = 0.15) due to an improvement (decrease) in thresholds in the second session compared to the first, probably as a result of practice. The interactions of Session with other factors were not significant (all P > 0.4). 

When available, the individual thresholds were averaged across sessions for further analysis. Considering the improvement of performance with practice, we also presented the results of group comparison and correlations with visual-perceptual scores for the second session separately in the Supplementary Results. 

Group Comparisons

The medians and range values for duration thresholds are summarized in Table 4. No group differences in thresholds were found using the nonparametric Mann-Whitney U test. 

Table 4.

 

Duration Thresholds in VSS and Control Participants: Median Values and Ranges

 

To assess potential group differences in spatial suppression, we conducted a mixed-effects ANOVA with log-transformed duration thresholds as the dependent variable. Prior to analysis, we evaluated the distribution of individual log-transformed duration thresholds (Fig. 3) to confirm adherence to the ANOVA assumption of equal variances across groups. One participant with VSS exhibited an exceptionally high duration threshold for the small grating (239 ms). This individual (a male patient, 32 years old) was excluded from the ANOVA but retained for other analyses. After this exclusion, Levene's test for homogeneity of variances indicated no significant group differences in variances of duration thresholds (large: F(1,42) = 0.08, P = 0.78; medium: F(1,48) = 3.05, P = 0.09; and small: F(1,47) = 0.07, P = 0.79). 

Figure 3.

 

Duration thresholds in VSS and control participants. (A) Between-subject variability in the VSS and the control groups. (B) Mean duration thresholds and 95% confidence intervals for the subjects included in ANOVA.

ANOVA incorporated within-unit factor Size and between-unit factors Group, Gender, and their interaction (group × gender). Gender was included due to previous findings by Murray et al.⁴⁰ demonstrating that male patients generally exhibit lower duration thresholds than female patients. 

There was a strong effect of Size (F(2,78) = 120.9, epsilon = 0.80, P < 1e-5; partial eta-squared = 0.76), but no effect of Group (F(1,39) = 0.001, P = 0.97; partial eta-squared = 0.00) or Size × Group (F(2,78) = 0.08, epsilon = 0.80, P = 0.89; partial eta-squared = 0.00) interaction, indicating that the VSS and control groups, on average, had similar duration thresholds and exhibited equally strong spatial suppression. There was a significant main effect of Gender (F(1,39) = 8.8, P = 0.005; partial eta-squared = 0.18): the duration thresholds were shorter in male participants. Interactions of Gender with other factors were not significant (all P > 0.15). 

Thus, the ANOVA results revealed no significant differences between the VSS and control groups in duration thresholds or spatial suppression. Similarly, spatial suppression, as assessed by SSI, showed no significant group difference (MeanVSS = 0.42 ± 0.27; MeanControl = 0.46 ± 0.22; n = 20/23; t(42) = 0.45; Cohen's d = 0.14). 

Given the small sample size in our study, the lack of differences between groups should be interpreted with caution. The small effect sizes observed for both duration thresholds and SSI (all d ≤ 0.35, according to Cohen’s criteria) suggest that group differences in these parameters might still be detectable with a commonly recommended power of 0.8 (80%) in a sample size exceeding 150 participants—three times larger than our current sample. 

A previous study demonstrated that spatial suppression is increased in individuals experiencing migraine, including those with and without aura.⁴¹ However, the SSI in our 17 patients with VSS with migraine did not differ from that in the control participants (F(1,37) = 0.34, P = 0.57, partial eta-squared = 0.01). 

Relations Between Duration Thresholds and Visual Disturbances in People With VSS

Table 5 presents Spearman correlations between visual symptoms and performance on the psychometric task in patients with VSS. There was a strong correlation between scores on the Palinopsia-II scale (TTP) and duration thresholds for small and medium-size gratings (small: rho = 0.72, uncorrected P = 0.0004; medium: rho = 0.63, uncorrected P = 0.0012; both correlations survived Bonferroni correction for multiple comparisons). As illustrated in Figure 4, patients with VSS with TTP had lower duration thresholds compared to those patients with VSS who experienced less or no TTP symptoms. 

Table 5.

 

Spearman Correlation Between the Severity of Visual Disturbances and Visual Task Performance in Patients With VSS

 

Figure 4.

 

Duration thresholds for the gratings of different sizes in control participants and in participants with VSS, split by scores on palinopsia-II (trailing type) scale. The higher scores denote the greater severity of trailing-type palinopsia.

To investigate differences in duration thresholds between control participants and patients with VSS with and without TTP, we performed Kruskal-Wallis ANOVA with three groups: Control, TTP+ (patients with VSS with TTP scores 4–5), and TTP– (patients with VSS with TTP score 1). The pattern of group differences was consistent across all stimulus sizes (Fig. 5), highlighting the unexpected finding that TTP appeared to compensate for the attenuated motion direction sensitivity observed in patients with VSS without TTP. 

Figure 5.

 

Comparison of duration thresholds among healthy control participants, patients with VSS without trailing-type palinopsia (TTP–) and patients with VSS with trailing-type palinopsia (TTP+). ** P < 0.01, Mann-Whitney U test.

There was a significant effect of group on duration thresholds: χ² (df = 2) = 10.9, P = 0.004 for small; χ² (df = 2) = 10.9, P = 0.006 for medium; and χ² (df = 2) = 6.1, P = 0.046 for large stimuli. TTP– patients had higher thresholds than the healthy controls for small (Z = 2.75, P = 0.006, Cohen's r = 0.46), medium (Z = 2.78, P = 0.006, Cohen's r = 0.47), and large stimuli (Z = 2.78, P = 0.006, Cohen's r = 0.52). TTP+ patients had lower thresholds than TTP– for small (Z = 3.1, P = 0.0019, Cohen's r = 0.78), medium (Z = 2.68, P = 0.007, Cohen's r = 0.67), and large stimuli (Z = 2.71, P = 0.007, Cohen's r = 0.75). However, no significant differences were found between controls and TTP+ for small (Z = 1.04, P = 0.30, Cohen's r = 0.18), medium (Z = 1.10, P = 0.27, Cohen's r = 0.19), or large stimuli (Z = 0.28, P = 0.78, Cohen's r = 0.05). 

For SSI, the effect of group was not significant (χ² (df = 2) = 2.4, n = 37, P = 0.30, η2 = 0.067), indicating that spatial suppression was equally robust in patients with VSS compared to controls, regardless of the presence or absence of TTP. 

Notably, whereas the duration thresholds in patients with VSS were related to TTP scores, they remained within the range of variation observed in control subjects. Specifically, participants without TTP had thresholds overlapping with the higher end of the normal range, while those with TTP aligned with the lower end. The only exception was a patient with VSS without TTP (scored “2” on the trailing-type palinopsia scale), who had an extremely high perceptual duration threshold for small-size gratings (see Fig. 3). 

Given the significant effect of gender on duration thresholds (with shorter thresholds in male subjects), we examined the male-to-female ratio in the groups of subjects who scored “5” and “1” on the TTP scale. Among the patients who scored “5,” there were two male patients and two female patients. Among the patients who scored “1,” there were three male patients and five female patients. We also calculated correlations with TTP scores separately for male patients and female patients. All correlations were negative and, in the case of small gratings, significant in both male patients and female patients (P < 0.05, uncorrected for multiple comparisons, see Supplementary Results). 

Discussion

In patients with VSS, a putative imbalance between excitation and inhibition in visual cortical areas may alter visual motion processing, differently affecting the time required to identify a direction of motion of large and small stimuli (i.e. motion duration thresholds). To investigate this possibility, we measured the duration thresholds for drifting gratings of different sizes in people with VSS and neurologically healthy control participants. Contrary to our hypothesis, when analyzed as a cohort, patients with VSS exhibited no significant differences from the healthy controls in motion duration discrimination thresholds or their stimulus size dependence. Intriguingly, TTP generally enhanced motion direction discrimination irrespective of stimulus size. 

Animal studies have shown that increasing stimulus size results in activation of the “far surround” of neurons in the primary visual cortex (V1), which in turn leads to suppression of neuronal activity via processes within area MT⁴² and/or excitatory feedback from the higher-order cortical areas to V1 inhibitory circuitry.^22,43 The increase in suppressive interactions limiting neuron sensitivity may explain why the time required to discern the direction of motion of a high-contrast iso-oriented patterned stimulus (grating) increases as a function of stimulus size.¹⁹ Participants with VSS exhibited a normal spatial suppression effect, characterized by a worsening of motion duration thresholds with increasing grating size (see Fig. 3). Additionally, an equal number of individuals in VSS and control groups (3 in each group) exhibited the reverse motion illusion which has been previously reported in case of brief motion of large high-contrast stimuli and was explained by surround suppression effects.⁴⁴ In sum, these findings suggest that VS symptoms are not necessarily associated with abnormal center-surround suppression caused by the motion of large stimuli. 

This result contradicts the findings of McKendrick et al.,²¹ who observed reduced center-surround contrast suppression in individuals with VSS. The discrepancy between these results could be attributed to differences in experimental paradigms and/or participant variability. Unlike the contrast suppression task used by McKendrick et al., our task may place greater demand on the magnocellular subdivision of the visual pathway, which is more sensitive to motion and may be relatively unaffected in VSS. In addition, the small sample size in McKendrick's study (16 patients with VSS and 18 control participants) cannot entirely rule out the possibility of sampling bias, which can occasionally skew the distribution of individual measures of surround suppression. Future studies with larger and more diverse samples are needed to clarify these differences. 

Although 74% of our VSS participants had migraine, their typical spatial suppression distinguished them from patients with migraine without VSS. Battista and colleagues found heightened spatial suppression in individuals with migraine, which they attributed to an enhanced top-down feedback signaling.⁴¹ These authors suggested that this enhanced feedback leads to exaggerated surround inhibition, compensating for the hyperexcitability in V1 circuitry. The difference between our results and those of Battista et al. suggests that migraine with and without VS may have a different neural basis. Although our findings of normal suppressive effect of increasing stimulus size in patients with VSS do not rule out potential alterations in excitatory or inhibitory signaling during motion processing, they suggest that the overall balance between excitatory and inhibitory influences in VSS is preserved. 

For small stimuli that do not extend beyond the population receptive field (less than approximately 1 degree of visual angle for the central visual field⁴⁵), the directional sensitivity is primarily determined by local excitatory and inhibitory processes.²² Thus, a direct interpretation of the fact that the perceptual duration thresholds for small gratings (see Fig. 3) are normal in VSS is that the VS illusion, a symptom that was characteristic of all individuals with VSS, is unlikely to be due to a general inhibitory deficit in the early visual cortex. This interpretation is consistent with the finding of normal luminance increment thresholds for small stationary gratings in patients with VSS in a luminance detection task.⁷ Indeed, reduced lateral inhibition within the classical receptive fields in V1 is known to decrease discrimination accuracy for moving stimuli⁴⁶ and alters contrast sensitivity.^47,48 Recent results on normal contrast adaptation in VSS³¹ are also compatible with normal functioning of inhibitory neurons, at least of one of their major class – parvalbumin containing neurons, which is critically important for depressing adaptation.⁴⁹ 

Although duration thresholds in patients with VSS were normal at the group level, they correlated negatively with the severity of comorbid TTP symptoms (see Table 5). Specifically, greater TTP severity was associated with relatively lower (improved) duration thresholds, suggesting that higher TTP symptom intensity may enhance directional sensitivity. Interestingly, none of the patients with VSS who demonstrated the illusion of reverse motion during the presentation of large gratings had TTP: all three of them scored “1” on the 5-point Likert scale. Notably, eight patients with VSS without TTP (TTP–) exhibited higher duration thresholds for all stimuli compared to both TTP+ patients and controls (see Fig. 5). This suggests a common factor impairing motion direction discrimination across all stimulus sizes (see Ref. 50 for discussion of one possible factor). 

Given the lack of prior studies on directional sensitivity in TTP, either in conjunction with or independent of VSS, our findings regarding the link between TTP and motion duration thresholds, although preliminary, are particularly intriguing and merit further investigation. The neural mechanisms underlying the positive aftereffect of motion in TTP remain largely unknown. Regardless of the precise underlying mechanism, the sustained visual representation following the disappearance of a moving object may contribute to reduced motion duration thresholds for drifting gratings, effectively enhancing perceived exposure time. Remarkably, Brooks and colleagues⁷ found a trend toward increased visual image persistence (measured through prolonged temporal integration threshold in the missing element task) in patients with VSS as compared with patients with migraine without VSS. Being a trend in a whole VSS group, increased temporal integration window appears to be a significant finding in VSS patients with TTP, as seen in the figure 5A in Ref. 7. It is conceivable that increased persistence of visual image in TTP counterbalances the relatively worsened duration thresholds observed in VSS without TTP, potentially mitigating the effects of local E/I imbalances. 

Our exploratory analysis of TTP’s effect on motion direction discrimination, based on a small sample, provides preliminary findings that merit further investigation. To better understand the role of trailing afterimages in motion perception among patients with VSS with comorbid palinopsia, more extensive research is essential. Because trailing afterimages are distinct from visual snow, future studies should investigate motion duration thresholds and spatial suppression in larger, well-defined groups: patients with VSS with palinopsia, those without, and individuals with palinopsia but no visual snow symptoms. This approach will help elucidate the complex relationships between these visual phenomena and their influence on motion perception. 

To advance this research, incorporating direction discrimination tasks with varying contrast or noise levels and extended exposure times (e.g. Ref. 51) could yield valuable insights. Coupled with the exposure duration-dependent thresholds used in the current study, this approach would offer a more thorough assessment of visual motion sensitivity in patients with VSS and palinopsia, enhancing our understanding of their altered motion perception. 

Conclusions

In conclusion, our study suggests that visual snow syndrome, when considered as a unified diagnostic entity without subgroup distinctions, is not associated with deficits in motion direction discrimination or abnormalities in center-surround suppressive interactions. However, the presence or absence of trailing-type palinopsia appears to have a qualitatively different effect on motion discrimination sensitivity in patients with VSS. Although this finding is preliminary and based on exploratory analyses, it underscores the importance of accounting for this comorbidity in neurophysiological and clinical models of VSS. The presence of trailing-type palinopsia may substantially impact experimental outcomes and should be carefully addressed in future research. 

Acknowledgments

Supported by Moscow State University of Psychology and Education, Program “Prioritet 2030” (project No. 125012400732-5). 

Disclosure: T.S. Obukhova, None; T.A. Stroganova, None; A.R. Artemenko, None; A.V. Petrokovskaia, None; E.V. Orekhova, None