Cpd Gratings Research Articles

Pattern-Induced Visual Discomfort and Anxiety in Migraineurs: Their Relationship and the Effect of Colour.

In migraineurs, coloured lenses were found to reduce the visual stress caused by an aversive pattern known to trigger migraines by 70%, but do such patterns also produce a low-level anxiety/fear response? Is this response lessened by colour? We sought to investigate this in a study comprising a broad screening component followed by a dot-probe experiment to elicit attentional biases (AB) to aversive patterns. Undergraduate psychology students completed headache and visual discomfort (VD) questionnaires (N = 358), thereby forming a subject pool from which 13 migraineurs with high visual discomfort and 13 no-headache controls with low visual discomfort, matched on age and sex, completed a dot-probe experiment. Paired stimuli were presented for 500 ms: aversive achromatic 3 cpd square wave gratings vs control, scrambled patterns. These conditions were repeated using the colour that was most comfortable for each participant. VD was greater in the more severe headache groups. On all measures, the migraineurs were more anxious than the controls, and a positive relationship was found between VD and trait anxiety. The 3 cpd gratings elicited an aversive AB in the migraine group which was somewhat reduced by the use of colour, and this was not seen in the controls. The results suggest a new role for colour in reducing visual stress via anxiety/fear reduction.

Modulation of binocular rivalry with rapid monocular visual stimulation.

Rapid visual stimulation can increase synaptic efficacy by repeated synaptic activation. This long-term potentiation-like (LTP-like) effect can induce increased excitability in the human visual cortex. To examine the effect of rapid visual stimulation on perception, we tested the hypothesis that rapid monocular visual stimulation would increase the dominance of the stimulated eye in a binocular rivalry task. Participants (n=25) viewed orthogonal 0.5cpd gratings presented in a dichoptic anaglyph to induce binocular rivalry. Rivalry dynamics (alternation rate, dominance, and piecemeal durations) were recorded before and after 2min of rapid monocular stimulation (9Hz flicker of one grating) or a binocular control condition (9Hz alternation of the orthogonal gratings viewed binocularly). Rapid monocular stimulation did not affect alternation rates or piecemeal percept duration. Unexpectedly, the rivalry dominance of the stimulated eye was significantly reduced. A further experiment revealed that this effect could not be explained by monocular adaptation. Together, the results suggest that rapid monocular stimulation boosts dominance in the non-stimulated eye, possibly by activating homeostatic interocular gain control mechanisms.

Differences in early and late pattern-onset visual-evoked potentials between self- reported migraineurs and controls

Striped patterns have been shown to induce strong visual illusions and discomforts to migraineurs in previous literature. Previous research has suggested that these unusual visual symptoms to be linked with the hyperactivity on the visual cortex of migraine sufferers. The present study searched for evidence supporting this hypothesis by comparing the visual evoked potentials (VEPs) elicited by striped patterns of specific spatial frequencies (0.5, 3, and 13 cycles-per-degree) between a group of 29 migraineurs (17 with aura/12 without) and 31 non-migraineurs. In addition, VEPs to the same stripped patterns were compared between non-migraineurs who were classified as hyperexcitable versus non-hyperexcitable using a previously established behavioural pattern glare task. We found that the migraineurs had a significantly increased N2 amplitude for stimuli with 13 cpd gratings but an attenuated late negativity (LN: 400 - 500ms after the stimuli onset) for all the spatial frequencies. Interestingly, non-migraineurs who scored as hyperexcitable appeared to have similar response patterns to the migraineurs, albeit in an attenuated form. We propose that the enhanced N2 could reflect disruption of the balance between parvocellular and magnocellular pathway, which is in support of the cortical hyperexcitation hypothesis in migraineurs. In addition, the attenuation of the late negativity could reflect a top-down feedback mechanism to suppress visual processing of an aversive stimulus.

Pattern electroretinogram to detect glaucoma: comparing the PERGLA and the PERG Ratio protocols

To detect early glaucoma, Porciatti and Ventura suggested in 2004 the pattern electroretinogram (PERG) protocol "PERGLA" with the following features: (1) skin electrodes, (2) steady-state reversal (15 rps) of horizontal 1.6 cpd gratings and (3) Fourier-based analysis. We compared this to our "PERG Ratio" protocol which uses (1) corneal (DTL) electrodes, (2) nearly the same reversal rate, but 2 check sizes of 0.8° and 16° and (3) Fourier-based analysis using the ratio of amplitudes to the 2 check sizes. We examined 16 eyes with glaucoma (age 64 ± 7 years) and a control group (n = 13, age 59 ± 8 years). Responses to all stimuli were simultaneously recorded with both electrode types using PERGLA-type gratings and checkerboards as necessary for the PERG Ratio. The median intra-individual test-retest coefficient of variation in normals pooled across stimuli for skin was 10.0 (95 % CI 1-85 %) and for DTL 9.95 (0.5-49 %), and they are thus nearly identical. The amplitudes obtained from skin were on average 30 % of those with DTL; the skin's signal-to-noise ratio (SNR) was 66 % of DTL electrodes. Glaucoma detection was assessed via receiver operating characteristics (ROCs). Using skin electrodes, ROC area-under-curve (AUC) was 72/76/72 % using gratings (PERGLA), checks or the PERG Ratio, respectively. Using DTL electrodes, the ROC areas were 60/67/77 %; the last value corresponds to the PERG Ratio protocol. Our results suggest that skin electrodes are a valid alternative to corneal electrodes: their advantages being no direct eye contact and smaller normal amplitude range compared to DTL; disadvantages: amplitude reduced to 30 % of DTL and SNR ratio reduced to 66 % of DTL. Efficacy in detecting glaucoma was a little higher with the PERG Ratio protocol (ROC AUC: 77 %, PERGLA: 72 %), but not significantly so.

Impaired Speed Perception but Intact Luminance Contrast Perception in People With One Eye

It is generally accepted that early visual deprivation from monocular enucleation (ME; the surgical removal of one eye) results in intact spatial vision. Yet, motion perception studies in this population yield inconsistent findings. Here, we investigated speed and luminance contrast perception in a group of ME individuals. Twelve ME participants (mean age = 24 years; mean age at enucleation = 24 months) and 17 controls (mean age = 25 years) viewing binocularly (BV) and monocularly (MV) completed a series of speed discrimination and luminance contrast detection and discrimination tasks. Stimuli consisted of 0.5 cpd vertical sine wave gratings varying in speed (3.8°/s-24°/s) or luminance contrast (0%-78%). A second set of luminance contrast tasks with 4 cpd gratings teased apart any spatial frequency effects. The ME group exhibited elevated speed discrimination thresholds compared with BV (P = 0.001) and MV (P = 0.027) controls, but intact luminance contrast discrimination (P = 0.530). Notably, both ME and MV groups displayed elevated luminance contrast detection thresholds compared with the BV group (Ps ≤ 0.006). However, the ME group exhibited slightly lower thresholds compared with MV controls for all 4 cpd tasks. Our data indicate a disruption in the development of speed perception, but not luminance contrast perception with monocular enucleation. These data highlight the importance of receiving healthy binocular vision during postnatal development for the maturation of cortical regions associated with motion processing.

Color and Spatial Frequency Are Related to Visual Pattern Sensitivity in Migraine

To assess the potential for particular colors to alleviate visual discomfort when people with migraine view repetitive geometric or striped patterns. Visual stimuli, such as flicker, glare, or stripes, can trigger migraine and headache. They can also elicit feelings of discomfort and aversion. There are reports that color can be used to decrease the experience of discomfort and reduce migraine frequency. Five sets of striped patterns (3, 12 cycles per degree [cpd]) were created using cardinal colors tailored to selectively stimulate the early visual pathways: achromatic (black/white), tritan (black/purple, black/yellow), protan/deutan (black/red, black/green). All had the same high luminance contrast (0.9 Michelson contrast). Twenty-eight migraine (14 migraine with aura, 14 migraine without aura) and 14 control participants rated the discomfort and described the distortions seen in these patterns. They were also assessed for visual migraine/headache triggers, contrast sensitivity, color vision, acuity, stereopsis, visual discomfort from reading, and dyslexia. In the migraine groups, a comparable number of illusions were seen with the 3 and 12 cpd achromatic gratings, whereas in the control group the greatest number was seen with the 3 cpd grating. In the migraine groups only, all 4 colors reduced, to some extent, the number of illusions and 2 decreased the discomfort, particularly for the 12 cpd gratings. There were significant group differences for contrast sensitivity, reported visual migraine/headache triggers, and the visual discomfort scale. There were a few significant correlations between the different measures, notably between the achromatic visual discomfort measures and reports of visual migraine triggers. Color, independent of luminance or particular color contrasts, can have therapeutic effects for people with visually triggered migraine as it can reduce the number of perceived illusions when viewing stripes or text. The effect was not color-specific and was greatest for the 12 cpd gratings. Given the significant associations between the achromatic discomfort measures and reports of visual triggers, and the lack of significant associations between the chromatic discomfort measures and reports of visual triggers, further research is recommended to explore the potential to reduce the number of visually triggered migraines with color in addition to alleviating visual discomfort.

The Pattern Glare Test: a review and determination of normative values

Pattern glare is characterised by symptoms of visual perceptual distortions and visual stress on viewing striped patterns. People with migraine or Meares-Irlen syndrome (visual stress) are especially prone to pattern glare. The literature on pattern glare is reviewed, and the goal of this study was to develop clinical norms for the Wilkins and Evans Pattern Glare Test. This comprises three test plates of square wave patterns of spatial frequency 0.5, 3 and 12 cycles per degree (cpd). Patients are shown the 0.5 cpd grating and the number of distortions that are reported in response to a list of questions is recorded. This is repeated for the other patterns. People who are prone to pattern glare experience visual perceptual distortions on viewing the 3 cpd grating, and pattern glare can be quantified as either the sum of distortions reported with the 3 cpd pattern or as the difference between the number of distortions with the 3 and 12 cpd gratings, the '3-12 cpd difference'. In study 1, 100 patients consulting an optometrist performed the Pattern Glare Test and the 95th percentile of responses was calculated as the limit of the normal range. The normal range for the number of distortions was found to be <4 on the 3 cpd grating and <2 for the 3-12 cpd difference. Pattern glare was similar in both genders but decreased with age. In study 2, 30 additional participants were given the test in the reverse of the usual testing order and were compared with a sub-group from study 1, matched for age and gender. Participants experienced more distortions with the 12 cpd grating if it was presented after the 3 cpd grating. However, the order did not influence the two key measures of pattern glare. In study 3, 30 further participants who reported a medical diagnosis of migraine were compared with a sub-group of the participants in study 1 who did not report migraine or frequent headaches, matched for age and gender. The migraine group reported more symptoms on viewing all gratings, particularly the 3 cpd grating. The only variable to be significantly different between the groups was the 3-12 cpd difference. In conclusion, people have an abnormal degree of pattern glare if they have a Pattern Glare Test score of >3 on the 3 cpd grating or a score of >1 on the 3-12 cpd difference. The literature suggests that these people are likely to have visual stress in everyday life and may therefore benefit from interventions designed to alleviate visual stress, such as precision tinted lenses.

The relationship between temporal phase discrimination ability and the frequency doubling illusion

The frequency doubling illusion (FDI) occurs when a low spatial frequency sinusoidal grating is modulated at high temporal frequencies--its apparent spatial frequency increases. A recent study suggests that this illusion is perceived due to a frequency-dependent loss of temporal phase encoding ability. We sought to elucidate the relationship between temporal phase encoding and the FDI by exploring the spatiotemporal characteristics of temporal phase discrimination (TPD) thresholds using a novel stimulus comprising three grating patches presented simultaneously in a triangular pattern. A reference grating was presented superiorly, and six degrees below two gratings (one a copy of the reference) were each randomly presented in one of two fixed positions. The odd grating had abutting regions of spatial half-cycles with alternate half-cycles locked in temporal phase. The temporal phase difference between adjoining half-cycles was varied between 0 degrees and 180 degrees via QUEST staircase--subjects had to identify which lower stimulus appeared different from the reference grating. TPD thresholds were measured for 0.25, 0.50, and 2.20 cpd stimulus at six temporal frequencies (1 to 28 Hz) at 2x, 4x, and 8x orientation identification contrast thresholds. For all subjects, thresholds were variable at low contrasts. At higher contrasts, TPD thresholds increase for 0.25 and 0.50 cpd gratings with increasing flicker rate. These data support the idea that frequency-dependent loss of temporal phase encoding ability could possibly underlie the FDI.

Frequency doubling illusion VEPs and automated perimetry in multiple sclerosis

We examined frequency doubling (FD) illusion based automated perimetry (FDT) and dichoptic FD multifocal visual evoked potentials (FDmfVEPs) in Normal and multiple sclerosis (MS) subjects. Contrast thresholds were determined at 17 visual field locations using an FDT perimeter. The stimuli presented to each location were 0.25 cpd gratings presented with rapid (25 Hz) counterphase flicker and thus displayed the spatial FD illusion. Dichoptic mfVEPs were recorded by concurrently stimulating eight regions/eye with FD stimuli presented at 95% contrast. Recordings were obtained from 27 Normal subjects, 26 MS patients who had experienced Optic Neuritis (MSON) and 24 MS patients without a history of ON (MSNON). The FDT thresholds showed enhanced contrast sensitivity for MSON patients (P < 0.0001) but not for MSNON patients. Response amplitudes for the central four regions of the mfVEP stimulus were reduced in both patient groups (P < 0.005). A classification model based upon the FDT thresholds performed at a specificity of 96% for a sensitivity of 97% in MSON patients, but the accuracy (simultaneously largest sensitivities and specificities) was poor (approximately 60%) in MSNON patients. Discriminant models based on the FDT thresholds and FDmfVEPs were able to diagnose more that 90% MSON patients but performed poorly for MSNON patients.

Resolution of binocular rivalry: Perceptual misbinding of color

Is neural binding of color and form required for perception of a unified colored object? Individual cells selectively tuned to both color and orientation are proposed to moot the binding problem. This study reveals perceptual misbinding of color, thereby revealing separate neural representations of color and form followed by a subsequent binding process. Low luminance-contrast, rivalrous chromatic gratings were presented dichoptically. Each grating had alternating chromatic and gray stripes (e.g., red/gray in the left eye, green/gray in the right eye). Observers viewed the two rivalrous, 2 cpd gratings for 1 min. The duration of exclusive visibility was measured for four percepts: left-eye stimulus, right-eye stimulus, fusion of the two colors, or a two-color grating (e.g. a red/green grating). The percept of a two-color grating (misbinding) was observed with Michelson luminance contrast in the grating up to 20%. In general, for a given level of luminance contrast either misbinding (low luminance contrast) or color mixture (high luminance contrast) was observed, but not both of them. The perceived two-color gratings show that two rivalrous chromaticities are both represented neurally when color and form are combined to give a unified percept. "Resolution" of competing chromatic signals from the two eyes is not restricted to color dominance and color mixture. The transition from misbinding to color mixture by increasing luminance contrast shows that luminance edges have an important role in correct localization of color.

How is a grating detected on a narrowband noise masker?

Thresholds were measured for detecting 4 cpd gratings added to maskers consisting of nine sinusoidal components spanning 1 octave around the signal frequency. Phases of all mask components were randomized on every presentation. To assess their importance, contrast differences were either rendered unreliable by introducing contrast jitter between-intervals, or eliminated by equating contrast energy within the octave band across intervals and trials. The deleterious effects of contrast jitter and the similarity of grating detection and contrast discrimination thresholds argues that contrast cues are being used. Those cues are not the only ones available, because contrast jitter has less than the expected effect, and equating contrast energy only raises threshold a few dB. Computer simulations reveal that there is sufficient information in several spatial pattern cues to support detection performance.

Nasotemporal overlap at the retinal vertical meridian: Investigations with a callosotomy patient

To evaluate nasotemporal overlap at the retinal vertical midline, we asked a callosotomy patient to compare the orientation of square wave gratings that were presented in his opposing visual fields. The gratings were horizontally or vertically oriented and had spatial frequencies ranginf rom 1 to 8 cycles per degree (cpd). Retinal stabilization ensured the gratings remained properly lateralized during sustained presentations. In accord with previous investigations, when the gratings were presented for only 200 msec, or their medial edges were 2° from the vertical meridian, performance was generally at chance. However, when presentations lasted 2 sec and the medial edges of the gratings were 1° from the vertical meridian, above chance performance was obtained. Accuracy rates were highest with 2 and 4 cpd gratings, and dropped at 1 and 8 cpd. Unexpectedly, the subject performed significantly better when the gratings were offset vertically from each other than when both were displayed on the horizontal meridian. Since this improvement did not occur when gratings were presented horizontally aligned above or below the horizontal meridian, it must be attributed to the relative offset between the gratings. The data suggest a narrow zone of nasotemporal overlap at the retinal vertical midline where very limited visual information is encoded by crossed temporal and uncrossed nasal retinal ganglion cells. An experiment in which only one grating in a pair was close to the vertical meridian indicates that this zone may be more pronounced in the nasal hemiretina.

Aging, background luminance, and threshold-duration functions for detection of low spatial frequency sinusoidal gratings.

This study examined the temporal summation properties of the aging visual system under a wide range of background luminances. Contrast thresholds for 0.5 cpd gratings for 12 younger (mean age 20.6 years) and 12 older (mean age 71.6 years) observers in good ocular health were measured for 6 stimulus durations (from 10 to 1000 ms) under 4 background luminances (from 0.44 to 249.50 cd/m2). Two-alternative forced-choice staircase method was used. We found significant main effects of age, background luminance, and stimulus duration on the contrast threshold. There were no age-related differences in the shape of the threshold-duration functions under all background luminances, although the shape changed significantly with background luminance. Age-related differences in contrast threshold were significant under the two lowest, but not under the two highest, background luminances. Age-related alteration in visual sensitivity may be attributed to sensitivity (amplitude) losses but not to changes in temporal characteristics of the visual system.

ACCOMMODATION FOR FLICKERING STIMULI

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