Frequency Doubling Illusion Research Articles

Magnocellular visual function in developmental dyslexia: deficit in frequency-doubling perimetry and ocular motor skills.

This study aimed to verify if patients with developmental dyslexia present deficits coherent with visual magnocellular dysfunction. Participants with confirmed diagnosis of developmental dyslexia (n=62; age range=8-25 years; mean age=13.8 years, standard deviation=3.9; 77% male) were compared to a control group with normal development, matched for age, sex, ocular dominance, visual acuity, and text comprehension. The frequency-doubling technology perimetry was used to evaluate the peripheral visual field contrast sensitivity threshold. The Visagraph III Eye-Movement Recording System was used to evaluate ocular motor skills during text reading. The developmental dyslexia group had significantly worse contrast sensitivity in the frequency-doubling technology, with strong effect size, than the matched control group. The developmental dyslexia group had more eyes classified in the impaired range of sensitivity threshold to detect frequency-doubling illusion than the control group. Moreover, the developmental dyslexia group had poorer ocular motor skills and reading performance, revealed by a difference in ocular fixations, regressions, span recognition, reading rate, and relative efficiency between groups. A significant correlation was found between contrast sensitivity and ocular motor skills. Participants with good relative efficiency had significantly better contrast sensitivity than participants with poor relative efficiency. The developmental dyslexia group presented a markedly worse performance in visual variables related to visual magnocellular function (i.e., frequency-doubling technology perimetry and ocular motor skills) compared with a matched control group. Professionals need to be aware of the importance of evaluating vision of individuals with developmental dyslexia beyond visual acuity and including in their assessments instruments to evaluate temporal processing, with contrast sensitivity threshold.

It is the egg, not the chicken; dorsal visual deficits present in dyslexia are not present in illiterate adults.

Some individuals with dyslexia demonstrate deficits in reading, visual attention, and visual processing which can be attributed to a functional failure of the magnocells in the visual system or in the dorsal visual pathway. The study examines the role of magno/dorsal function in dyslexic adults compared with normal, illiterate, and semi-literate readers. Coherent motion and coherent form were used in Experiment 1, and the frequency doubling illusion and static-gratings were used in Experiment 2. If a magno/dorsal deficit is demonstrated for dyslexic readers but not illiterate, semi-literate, and normal reading adults, then the deficit cannot be attributed to reading experience. Illiterate adults performed the same as normal and semi-literate readers in coherent motion and frequency doubling tasks, and all three groups performed better than the dyslexic readers. There was no difference between any of the groups in the coherent form or static grating tasks. Together, these studies show that illiterate and semi-literate adults do not demonstrate a magno/dorsal deficit that is a characteristic of some sufferers of dyslexia. Therefore, magno/dorsal deficits in dyslexia are unlikely to be a consequence of failing to learn to read but rather provides evidence to suggest a causal role for reduced visual magno/dorsal processing.

Magnocellular Deficit in Dyslexia: A Preliminary Analysis of Possible Patterns of Visual Field Deficit Tested with Frequency Doubling Illusion

Purpose: To identify distinguishing visual field (VF) deficiencies in dyslexic children by exploring responses to spatial frequency doubling illusions, characteristically involving the magnocellular visual pathway. Methods: Eighteen dyslexic subjects and nine controls [mean age 8.4 ± 1.4 years (range, 7-13 years)] were consecutively enrolled and underwent VF examination on a Frequency Doubling Technology (FDT) Visual Field Instrument. Among the 19 locations of the right eye (RE) and left eye (LE) VFs, 7 topographic patterns were identified: central (CE), superior-temporal (ST), and outer superior-nasal (OSN), inner superior-nasal (ISN), inner inferior-nasal (IIN), outer inferior-nasal (OIN), inferiortemporal (IT). The default settings included measurements of both eyes separately at all 19 VF locations, exploring the central 30-degree radius of eccentricity. For each pattern indicated the thresholds of contiguous 5 point clusters of the VF in both eyes were averaged: the 7 threshold averages obtained were compared for each eye between the control and dyslexic groups. Results: For RE and LE there was non-statistically significant difference between control and affected cases for all patterns except for CE, IIN, IT patterns in the left eye (P<0.05). The two groups were comparable for loss of fixation, false positive error, and false negative error. These data indicate that the dyslexic reader's left eye presents less sensitivity to FD illusions in almost all inferior retinal hemifield. Conclusion: Dyslexics analyzed using FD illusions may be less sensitive in specific sections of the VF. This topographic failure may help to confirm that a deficit in the magnocellular visual pathway is correlated to reading difficulties.

Magnocellular-dorsal pathway and sub-lexical route in developmental dyslexia.

Although developmental dyslexia (DD) is frequently associate with a phonological deficit, the underlying neurobiological cause remains undetermined. Recently, a new model, called “temporal sampling framework” (TSF), provided an innovative prospect in the DD study. TSF suggests that deficits in syllabic perception at a specific temporal frequencies are the critical basis for the poor reading performance in DD. This approach was presented as a possible neurobiological substrate of the phonological deficit of DD but the TSF can also easily be applied to the visual modality deficits. The deficit in the magnocellular-dorsal (M-D) pathway - often found in individuals with DD - fits well with a temporal oscillatory deficit specifically related to this visual pathway. This study investigated the visual M-D and parvocellular-ventral (P-V) pathways in dyslexic and in chronological age and IQ-matched normally reading children by measuring temporal (frequency doubling illusion) and static stimuli sensitivity, respectively. A specific deficit in M-D temporal oscillation was found. Importantly, the M-D deficit was selectively shown in poor phonological decoders. M-D deficit appears to be frequent because 75% of poor pseudo-word readers were at least 1 SD below the mean of the controls. Finally, a replication study by using a new group of poor phonological decoders and reading level controls suggested a crucial role of M-D deficit in DD. These results showed that a M-D deficit might impair the sub-lexical mechanisms that are critical for reading development. The possible link between these findings and TSF is discussed.

Spatial proximity rather than temporal frequency determines the wagon wheel illusion.

A rotating disk composed of alternating light and dark segments may give rise to the wagon wheel illusion: a perceptual reversal in rotation direction. Continuously illuminated (eg in daylight) as well as discretely presented (eg stroboscopic or computer-animated) versions of the illusion exist; here, we investigated the discrete version. Prominence of the illusion is commonly believed to depend on temporal frequency of rotation, but frequency effects have been unsystematic across previous experiments. Here, illusion strength is shown instead to lawfully depend on an attraction function of angular displacement between successive frames (experiments 1 and 2). We studied the illusion across a wider range of this factor than previously and as a result obtained unusually strong effects (up to 100% reversal). In two further experiments we showed that this is because the effect of the attraction function on the wagon wheel illusion strength is modulated by a perceived increase in the number of spokes of the wheel, a phenomenon generally known as the frequency doubling illusion. These factors combine to offer a unifying explanation of the wagon wheel illusion, at least under discrete presentation and possibly under continuous presentation conditions as well.

A phase mixing model for the frequency-doubling illusion

We introduce a temporal phase mixing model for a description of the frequency-doubling illusion (FDI). The model is generic in the sense that it can be set to refer to retinal ganglion cells, lateral geniculate cells, as well as simple cells in the primary visual cortex (V1). Model parameters, however, strongly suggest that the FDI originates in the cortex. The model shows how noise in the response phases of cells in V1, or in further processing of these phases, easily produces observed behavior of FDI onset as a function of spatiotemporal frequencies. It also shows how this noise can accommodate physiologically plausible spatial delays in comparing neural signals over a distance. The model offers an explanation for the disappearance of the FDI at sufficiently high spatial frequencies via increasingly correlated coding of neighboring grating stripes. Further, when the FDI is equated to vanishing perceptual discrimination between asynchronous contrast-reversal gratings, the model proposes the possibility that the FDI shows a resonance behavior at sufficiently high spatial frequencies, by which it is alternately perceived and not perceived in sequential temporal frequency bands.

Effect of Contrast, Stimulus Density, and Viewing Distance on Multifocal Steady-State Visual Evoked Potentials (MSVs)

We investigated the effects of image contrast, stimulus density, and viewing distance upon a multifocal steady-state visual evoked potential (MSV) method. Fourteen adults with normal vision (mean age = 27.0 ± 6.6 years; 6 males) participated in the study. Each of the stimulus regions of the multifocal ensembles presented a contrast modulated grating, displaying spatial and temporal frequencies that evoke the spatial frequency doubling illusion. All subjects were tested at five contrasts: 0.06, 0.11, 0.22, 0.45, and 0.89; viewed at 16, 32, and 128 cm. A multivariate linear model estimated factors for each stimulus region, recording channel, number of stimuli (9 or 17 regions), and sex; and covariates for contrast, and octaves of viewing distance, and age. The responses per unit area for the 17-region display were significantly larger than for the 9-region display (P < 10⁻¹²). The contrast-response function could be described by a power law with exponent 0.068 (P < 0.008). The effect of viewing distance was small but significant: response amplitude dropped by -0.17 ± 0.03 dB per octave of viewing distance (P < 10⁻⁶), or 10% over 8 octaves. The response per unit area indicated that cortical folding diminishes responses to larger stimuli. Viewing distance did not greatly affect response amplitude. This suggested that we can use similar, but scaled, stimuli to study central and peripheral disease. The rapidly saturating contrast responses imply that there would be nothing lost from testing at contrasts as low as 20% given that higher, saturating contrasts might mask visual field defects.

Author Response: Frequency-Doubling Technology and Parasol Cells

Author Response: Frequency-Doubling Technology and Parasol Cells

On the Relationship between Magnocellular Pathway and Automatic Attentional Orienting: Evidences from Developmental Dyslexia

Although developmental dyslexia (DD) is frequently associate to a linguistic deficit, the underlying neurobiological cause remains unclear. One prominent vision science hypothesis suggests a specific deficit in magnocellular (M) pathway in DD. However, a recent hypothesis proposes that M-deficits contribute to the etiology of phonological decoding ability (i.e., new word and nonword reading), impairing selectively the attentional orienting process onto letter string. Thus, a specific M-pathway impairment in association with an attentional orienting disorder in DD children with phonological decoding deficit was specifically predicted by this neurobiological hypothesis. In the present study we investigated the M-pathway and attentional orienting in 18 dyslexic (10 with phonological decoding deficit) and in 29 chronological age and IQ matched normally reading children by measuring dynamic stimuli sensibility (i.e., spatial frequency doubling illusion) and the time-course of automatic covert attention (i.e., target detection effect at different SOAs of a non-predictive and peripheral spatial cue), respectively. The results showed a specific deficit of the M-pathway task in dyslexic with phonological decoding deficit. More importantly, the same group of dyslexics with M-deficit presented a sluggish attentional orienting: attentional facilitation was present at longer cue-target SOAs compared with normal readers. These results highlight that a M-deficit linked to a parietal-attentional dysfunction might impair the phonological decoding mechanisms that are critical for reading acquisition.

Multifocal frequency-doubling pattern visual evoked responses to dichoptic stimulation

To examine the feasibility of a multifocal visual evoked potential (mfVEP) binocularly, using a variant of the multifocal frequency-doubling (FD) pattern-electroretinogram (MFP). Stimuli were presented in both monocular and dichoptic conditions at eight visual field locations/eye. The incommensurate stimulus frequencies ranged from 15.45 to 21.51 Hz. Five stimulus conditions differing in spatial frequency and orientation were examined for three viewing conditions. The resulting 15 stimulus conditions were examined in 16 normal subjects who repeated all conditions twice. Several significant independent effects were identified. Response amplitudes were reduced for dichoptic viewing (by 0.85 times, p<4 x 10(-11)); offset by increases in responses for between eye differences of one octave of spatial frequency: lower (1.15 times, 0.1 cpd); higher (1.29 times, 0.4 cpd), both p<1.8 x 10(-7). Crossed orientations produced significant effects upon response phase (p=0.023) but not amplitude (p=0.062). The results indicated that dichoptic evoked potentials using multifocal frequency-doubling illusion stimuli are practical. The use of crossed orientation, or differing spatial frequencies, in the two eyes reduced binocular interactions. The results indicate a method wherein several spatial or temporal and frequencies per visual field region can be tested in reasonable time using a multifocal VEP using spatial frequency-doubling stimuli.

Low-Spatial-Frequency Channels and the Spatial Frequency-Doubling Illusion

This study examined the number and nature of spatiotemporal channels in the region where the frequency-doubling (FD) illusion would be expected to occur at eight locations spanning the central 30 degrees of the visual field. The probability of seeing the FD illusion was examined in 17 subjects. Stimuli were presented at 5 octaves of low spatial frequencies, at each of seven flicker frequencies in the range 5.65 to 27.95 Hz. In a single trial, subjects matched the apparent spatial frequency of the flickering test pattern using a two-alternative, forced-choice method. Thirteen subjects were examined for stimuli presented at contrast 0.95. Three or four subjects were examined at each of the contrasts 0.2, 0.4, and 0.8. A factor analysis was conducted on the psychometric functions, quantifying the number and possible spatiotemporal tuning of neural channels present. At contrast 0.95, three factors were able to explain 79.3% of the total variance in the psychometric responses to the 35 test conditions. This simple form of three broad spatiotemporal channels was also found at the other contrasts and in different subjects. The factor scores showed differential distribution of the factors onto the eight different visual field locations. Thus the expression of the three channels differed somewhat across the visual field. The results support earlier reports, that there are several low-spatial-frequency channels below 1 cyc/deg in the periphery. The results may have implications for the FDT and matrix perimeters.

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 under Scotopic Illumination and in Peripheral Vision

The authors sought to determine whether frequency-doubling illusion (FDI) could be perceived under scotopic illumination at central and peripheral retinal locations. For comparison, perception of the FDI at the central and peripheral retina under photopic illumination was also evaluated. Five subjects matched the apparent spatial frequency of counterphase flickering sinusoidal gratings with stationary sinusoidal gratings presented foveally and out to 20 degrees eccentricity under photopic and scotopic illumination conditions. Two spatial frequencies (0.25 and 0.50 cpd) were used at four temporal frequencies (2, 8, 15, and 25 Hz). Subsequent experiments explored the range of spatial and temporal frequency stimulus conditions under which the scotopic FDI might be observed. Under scotopic illumination conditions, the apparent spatial frequency of eccentrically presented 0.25- and 0.50-cpd flickering gratings gradually increased as a function of flicker frequency and approaches "doubling" at 15 Hz. Under photopic conditions, the apparent spatial frequency of 0.25-cpd flickering at 25 Hz was approximately doubled in all four primary meridians at central and peripheral eccentricities. The final experiment showed that the spatiotemporal range under which the scotopic FDI could be seen was similar to the photopic illumination condition reported earlier. Scotopic FDI is similar to photopic FDI at the central and the peripheral retina. This suggests that similar mechanisms are responsible for generating the illusion under both photopic and scotopic illumination conditions.

Spatial structure of the frequency doubling illusion

We sought to determine (1) the perceived spatial frequency of flickering sinusoidal gratings as a function of temporal frequency at contrasts down to detection threshold levels, and (2) the influence of two different matching criteria on reported size matches to identify changes in the apparent spatial structure introduced by counterphase flicker. To determine if spatial structure was observable at detection threshold contrasts, we simultaneously measured contrast detection and orientation–identification thresholds of flickering gratings. We then measured the apparent size of flickering grating bars at sub-threshold to supra-threshold contrast levels using two matching criteria: (1) by adjusting the spatial frequency of a stationary comparison grating and (2) by adjusting the distance between two parallel thin lines. For all subjects, contrast detection and orientation–identification thresholds were similar, indicating that sufficient spatial structure is observable at detection threshold to make judgements of local spatially defined features. With increasing flicker rate, the apparent periodicity of low (0.25 and 0.50 cpd) spatial frequency test gratings increased. At detection threshold contrast levels and below, all observers reported difficulty in performing the match and results were variable. Size matches using stationary gratings were consistently smaller than those made using the line targets, which is suggestive of distortion in perceived spatial structure at the nodal region of flickering gratings.

The Contribution of Visual Sensitivity, Phonological Processing, and Nonverbal IQ to Children's Reading

It has been suggested that the differences observed for dyslexic readers compared to normal readers on tasks measuring visual sensitivity may simply be the result of differences between the two groups in general cognitive ability and/or attentional engagement. One common way to accommodate this proposal is to match normal and dyslexic readers on IQ. However, an explicit test of this suggestion is to take normal and dyslexic readers who differ on IQ—where IQ would be expected to explain reading ability—and determine if visual sensitivity can still account for reading skill, even when IQ is taken into account. In this study we explored the relative contributions of nonverbal IQ, visual sensitivity as measured by sensitivity to the frequency doubling illusion, and phonological and irregular word reading to reading ability. Visual sensitivity explained a significant amount of variance in reading ability, over and above nonverbal IQ, accounting for 6% of the unique variance in reading ability. Moreover, visual sensitivity was related primarily to irregular word reading rather than to nonsense word decoding. This study demonstrates that low-level visual sensitivity plays an intrinsic role in reading aptitude, even when IQ differences between normal and dyslexic readers are contrived to maximize the contribution of IQ to reading skill. These results challenge the suggestion that impaired visual sensitivity may be epiphenomenal to poor reading skills.

The Contribution of Visual Sensitivity, Phonological Processing, and Nonverbal IQ to Children's Reading

It has been suggested that the differences observed for dyslexic readers compared to normal readers on tasks measuring visual sensitivity may simply be the result of differences between the two groups in general cognitive ability and/or attentional engagement. One common way to accommodate this proposal is to match normal and dyslexic readers on IQ. However, an explicit test of this suggestion is to take normal and dyslexic readers who differ on IQ—where IQ would be expected to explain reading ability—and determine if visual sensitivity can still account for reading skill, even when IQ is taken into account. In this study we explored the relative contributions of nonverbal IQ, visual sensitivity as measured by sensitivity to the frequency doubling illusion, and phonological and irregular word reading to reading ability. Visual sensitivity explained a significant amount of variance in reading ability, over and above nonverbal IQ, accounting for 6% of the unique variance in reading ability. Moreover, visual...

P30.38 Pattern pulse versus frequency doubling illusion: Sensitivities and specificities in optic neuritis patients

P30.38 Pattern pulse versus frequency doubling illusion: Sensitivities and specificities in optic neuritis patients

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.

Independent patterns of damage within magno-, parvo- and koniocellular pathways in Parkinson's disease

Sensory deficits have been documented in Parkinson's disease, in particular within the visual domain. However, ageing factors related to the brain and to neural and non-neural ocular structures could explain some of the previously reported results, in particular the claimed impairment within the koniocellular pathway. This study addressed visual impairment attributable to the magno- (luminance), parvo- (red-green) and koniocellular (blue-yellow) pathways in a population of Parkinson's disease patients. To avoid potentially confounding factors, all subjects underwent a full neurophthalmological assessment which led to exclusion of subjects with increased intraocular pressure, diabetes even in the absence of retinopathy, and ocular abnormalities (from a total of 72 patients' eyes, 12 were excluded). Both parvo- and koniocellular pathways were studied by means of contrast sensitivity (CS) measurements along protan, tritan and deutan axes and also by fitting chromatic discrimination ellipses using eight measured contrast axes. Magnocellular function was assessed, using stimuli that induce a frequency doubling illusion, in 17 locations in the fovea and periphery. Achromatic (luminance modulation) thresholds were significantly higher in Parkinson's disease both in foveal and peripheral locations. A significant impairment was observed along protan and deutan axes, but only marginally along the tritan axis. These results were corroborated by a significant elongation of chromatic discrimination ellipses in our Parkinson's disease group. Correlation analysis showed that achromatic and chromatic CS measures were independent, which implies that multiple visual pathways are affected independently in Parkinson's disease. Magnocellular impairment was significantly correlated with age and disease stage, in contrast to the measured chromatic deficits. We conclude that in Parkinson's disease, independent damage occurs in the early magno- and parvocellular pathways. Furthermore, traditional koniocellular probing strategies in Parkinson's disease may be confounded by ageing factors, which may reconcile the previously reported controversial findings concerning chromatic impairment in Parkinson's disease.

New methods to detect multiple sclerosis

This paper describes multifocal visual evoked potentials (mfVEPs) recorded to different levels of temporal sparseness. It presents the usefulness and diagnostic value of mfVEPs in multiple sclerosis (MS) and optic neuritis (ON). The paper also discusses the usefulness of frequency doubling (FD) illusion and the effect of binocularity in Normal and MS study groups.