Frequency Doubling Illusion Research Articles

Visual encoding mechanisms and their relationship to text presentation preference

This study was designed to investigate the importance of spatial encoding in reading, with particular emphasis on visuo-spatial encoding mechanisms. Thirty one school children participated in the first study in which they were measured on their ability to solve a centrally presented spatial encoding task, as well as their sensitivity to the frequency doubling illusion across the retina. We found that both spatial frequency doubling sensitivity and performance in the spatial encoding task were correlated with reading, however these tasks were unrelated to each other. Furthermore, frequency doubling sensitivity was correlated with contextual reading, but not single-word reading, while the central spatial encoding task was correlated with both reading tasks. These findings may have functional implications for text presentation preference. Accordingly, in Experiment 2 we demonstrated that children with poor FDT sensitivity read more accurately when words were presented singularly rather than in a whole-text format. In conclusion, we suggest that contextual reading may depend upon two separate and functionally distinct visual encoding mechanisms--one central, important for the spatial discrimination of letters within words, and the other, a spotlighting mechanism important for spatial localization within a body of text. While both mechanisms may constrain reading efficiency, neither mechanism enforces an absolute limit on reading ability.

Comparing multifocal frequency-doubling illusion, visual evoked potentials, and automated perimetry in normal and optic neuritis patients

Comparing multifocal frequency-doubling illusion, visual evoked potentials, and automated perimetry in normal and optic neuritis patients

Monocular and dichoptic masking effects on the frequency doubling illusion

The purpose of this paper was to investigate the effects of both monocular and dichoptic masking on the frequency doubling (FD) illusion, using both temporal and spatial masks. Monocular spatial tuning effects occurred around the fundamental FD spatial frequency of 0.25 cycles per degree (c/deg), whereas dichoptic spatial frequency tuning effects occurred at the doubled spatial frequency of 0.50 c/deg. Temporal tuning effects were observed at the FD temporal frequency of 25 Hz, in both monocular and dichoptic paradigms. These results suggest that the FD illusion is cortical in origin and is dominated by a flicker component.

Examination of mechanisms underlying the frequency-doubling illusion

Examination of mechanisms underlying the frequency-doubling illusion

Comparison of three tests using the frequency doubling illusion to diagnose glaucoma

Comparison of three tests using the frequency doubling illusion to diagnose glaucoma

Isolating the M(y)-cell response in dyslexia using the spatial frequency doubling illusion

The contribution of M(y)-cell activity within a framework of a magnocellular-deficit theory of dyslexia is currently unknown. Twenty-one dyslexic readers and 19 control readers were compared on their threshold detection for the frequency doubling illusion — an index of M(y)-cell activity, coherent motion, and a visual acuity task. The dyslexic group performed more poorly on detection of the frequency doubling illusion and coherent motion compared to the control group, but both groups performed comparably on the visual acuity task. The results from this study indicate that if a magno deficit exists in dyslexia, it may originate at a retinal level at least partly mediated by M(y)-cell abnormalities.

Employing following eye movements to discriminate normal from glaucoma subjects.

We recorded optokinetic nystagmus (OKN) to see if slow phase velocity, duration or other measures were affected by glaucoma. Drifting grating patterns that either weakly or strongly evoked the spatial frequency doubling illusion were employed. Analysis of 68 variables characterizing the OKN revealed that small subsets of these variables were good at discriminating normal from primary open angle glaucoma subjects. The variables were related to the regularity of following eye movements. Models including the best five variables selected in two different ways classified about 90% of subjects correctly. Impaired accuracy of eye movements suggests that glaucoma changes the signal to noise ratio available to the brain. The gross changes observed permit the use of electro-oculography or other simple methods in the clinic.

Testing for glaucoma with the spatial frequency doubling illusion

We examined the performance of tests for glaucoma based on the spatial frequency doubling (FD) illusion. Contrast thresholds for seeing the FD illusion in four large visual field regions were measured from 340 subjects who were tested up to seven times over 2 years. Median sensitivities of 91% at specificities of 95% were obtained. Test–retest variability for the worst hemifield thresholds averaged 2.22 db±0.09 S.E. for all tested groups, and significant progression was observed for glaucoma suspects over the seven visits, indicating that tests based on the FD illusion can detect diffuse early glaucomatous loss.

Apparent fineness of stationary compound gratings

Patterns consisting of the sum of a sinusoidal grating and its second spatial harmonic have an apparent spatial fineness, or periodicity, that is about halfway between the two component spatial frequencies. There are also phase dependent modulations of the apparent fineness about the mean fineness shift. Covariance between individuals’ phase dependent fineness shifts indicates the presence of four spatial phase channels. The apparent fineness effects, and the putative phase channels, may both be a product of a local, linear, analysis of spatial frequency content. Illusory second harmonics, as generated in the spatial frequency doubling illusion, also change apparent fineness.

Testing for glaucoma with the frequency-doubling illusion in the whole, macular and eccentric visual fields.

The frequency doubling (FD) illusion is the basis for new diagnostic methods for glaucoma. The FD illusion is seen when low spatial frequency grating patterns are contrast modulated at high rates. The present experiments examined which spatial frequencies might be optimal and whether high flicker rates are required. We determined contrast thresholds for the following: W1, a wide-field 0.25 c/deg grating at 27 Hz contrast reversal; W2, as W1 but no flicker; MAC, 27 Hz, 4 c/deg grating presented to the central 4 degrees; and E1 to E7, seven spatial frequencies in the range 0.063-0.813 c/deg, 27 Hz, presented in a 5 degrees aperture at 15 degrees (nasal) eccentricity. W1 was the best predictor of glaucoma. Of the eccentrically presented stimuli, E6 (0.688 c/deg) was the best predictor of glaucoma while the lower spatial frequencies performed less well. Only MAC was significantly age-dependent.

Clinical classification of glaucomatous visual field loss by frequency doubling perimetry

Purpose: To determine whether the frequency doubling perimeter (FDP) can grade glaucomatous visual function loss in a clinically relevant manner. Sinusoidal gratings <1 cpd that undergo counterphase flicker >15 Hz appear to have twice as many bands of light, a phenomenon referred to as the “frequency doubling illusion.” Evidence suggests that this psychophysical effect is mediated in part by large-diameter ganglion cells, which are reported to be lost early in the glaucomatous disease process. A portable, commercially available FDP has already demonstrated high diagnostic potential for glaucoma screening. Methods: Sixty-four eyes of 42 glaucomatous patients and 22 eyes of 14 normal subjects were evaluated by means of both frequency doubling perimetry and Humphrey perimetry. A clinical scoring algorithm modeled after the Hodapp-Parrish-Anderson criteria for scoring Humphrey visual field defects was derived for the FDP at the halfway point of the study, and all participants were reassessed with this algorithm upon its completion. Results: FDP mean and pattern deviation showed strong linear correlations with Humphrey 30-2 mean deviation (R = 0.75; P < .0001) and corrected pattern standard deviation values (R = 0.64; P < .0001). Despite this, neither global index could consistently categorize the graded glaucomatous visual fields in a manner consistent with the Hodapp-Parrish-Anderson criteria. The new FDP scoring algorithm did provide good segregation (73% precise parity, 93% parity within one Humphrey grade). Conclusions: Sixteen-zone frequency doubling perimetry can segregate glaucomatous visual field loss into pathologic categories approximating those obtained with Humphrey 30-2 perimetry by means of a formula modeled after the Hodapp-Parrish-Anderson criteria.

Figure–ground discrimination from temporal information

The separation of figures from ground is achieved by the visual system based on differences in features such as luminance, colour, depth, orientation, texture or motion. Temporal information, namely phase differences between groups of spatially homogeneous points, can also lead to a clear discrimination of an object shape. The time difference needed to separate figure and ground is around 5 ms over a range of temporal frequencies between 1.3 Hz and 30 Hz, both for sharply focused and for blurred points. These short delays are clearly below the temporal integration time of the visual system. The results have implications for theories on temporal binding and object recognition.

Nonlinear visual responses to flickering sinusoidal gratings.

Over a range of high temporal and low spatial frequencies, counterphase flickering gratings evoke the so-called frequency-doubling illusion, in which the apparent brightness of the grating varies at twice its real spatial frequency. The form of the nonlinearity that causes this second-harmonic distortion of the visual response was determined by a cancellation technique. The harmonic distortion can be measured as a function of amplitude (or contrast) by adding to the flickering grating a real, nonflickering, double-frequency component with the amplitude and phase required to cancel the illusory second harmonic. Harmonic distortion curves obtained in this way imply that the nonlinearity is of the form /s/p, where s is the stimulus pattern (without its dc component) and p is close to 0.6. If p = 1, or if the absolute value is not taken, this expression predicts distortion curves that differ significantly from the experimental results. Hence neither rectification nor compression alone is sufficient to account for the second-harmonic distortion; both are required.