Flicker Sensitivity Research Articles

Effects of auditory noise on the psychophysical detection of visual signals: Cross-modal stochastic resonance

Harper [D.W. Harper, Signal detection analysis of effect of white noise intensity on sensitivity to visual flicker, Percept. Mot. Skills 48 (1979) 791–798] demonstrated that the visual flicker sensitivity was an inverted U-like function of the intensity of different levels of auditory noise from 50 to 90 dB (SPL), without concomitant changes in the response bias. The aim of the present study was to extend these observations in the context of the stochastic resonance, a counterintuitive phenomenon in which a particular level of noise enhances the response of a nonlinear system to a weak input signal. We show psychophysical evidence in a yes–no paradigm for the existence of a stochastic resonance-like phenomenon in the auditory–visual interactions. We show that the detection of a weak visual signal was an inverted U-like function of the intensity of different levels of auditory noise. Nevertheless, for a strong visual signal the auditory noise acts in detriment of the ability of visual detection. Our results suggest that auditory noise could be employed in vision rehabilitation interventions in order to improve the detection of weak visual signals.

Temporal sensitivity in a hemianopic visual field can be improved by long-term training using flicker stimulation

Background: Blindness of a visual half-field (hemianopia) is a common symptom after postchiasmatic cerebral lesions. Although hemianopia severely limits activities of daily life, current clinical practice comprises no training of...

Visual function and dysfunction in early and late age-related maculopathy

Late age-related maculopathy (ARM) is responsible for the majority of blind registrations in the Western world among persons over 50 years of age. It has devastating effects on quality of life and independence and is becoming a major public health concern. Current treatment options are limited and most aim to slow progression rather than restore vision; therefore, early detection to identify those patients most suitable for these interventions is essential. In this work, we review the literature encompassing the investigation of visual function in ARM in order to highlight those visual function parameters which are affected very early in the disease process. We pay particular attention to measures of acuity, contrast sensitivity (CS), cone function, electrophysiology, visual adaptation, central visual field sensitivity and metamorphopsia. We also consider the impact of bilateral late ARM on visual function as well as the relationship between measures of vision function and self-reported visual functioning. Much interest has centred on the identification of functional changes which may predict progression to neovascular disease; therefore, we outline the longitudinal studies, which to date have reported dark-adaptation time, short-wavelength cone sensitivity, colour-match area effect, dark-adapted foveal sensitivity, foveal flicker sensitivity, slow recovery from glare and slower foveal electroretinogram implicit time as functional risk factors for the development of neovascular disease. Despite progress in this area, we emphasise the need for longitudinal studies designed in light of developments in disease classification and retinal imaging, which would ensure the correct classification of cases and controls, and provide increased understanding of the natural course and progression of the disease and further elucidate the structure-function relationships in this devastating disorder.

Low contrast vision function predicts subsequent acuity loss in an aged population: the SKI study

Can vision tests predict subsequent loss of acuity? The association between performance on several low contrast spatial vision measures, glare recovery, color discrimination, flicker sensitivity, stereopsis and ocular disease status at baseline and acuity loss 4.4 years later was examined in a large aged random sample with good initial acuity. In univariate analyses, several vision measures, retinal disease status and age were each significant predictors of subsequent acuity loss. In a multiple regression analysis, only low contrast spatial vision was a significant predictor, but the other vision measures, retinal disease status and age were not. For each doubling of low contrast spatial vision threshold at baseline, individuals were more than two times as likely to suffer subsequent significant visual acuity loss. Tests of low contrast spatial vision are strong predictors of significant subsequent visual acuity loss. These findings have implications for clinical trials, clinical management, and acceptance of these measures into clinical practice.

Adaptation from invisible flicker.

Human ability to resolve temporal variation, or flicker, in the luminance (brightness) or chromaticity (color) of an image declines with increasing frequency and is limited, within the central visual field, to a critical flicker frequency of approximately 50 and 25 Hz, respectively. Much remains unknown about the neural filtering that underlies this frequency-dependent attenuation of flicker sensitivity, most notably the number of filtering stages involved and their neural loci. Here we use the process of flicker adaptation, by which an observer's flicker sensitivity is attenuated after prolonged exposure to flickering lights, as a functional landmark. We show that flicker adaptation is more sensitive to high temporal frequencies than is conscious perception and that prolonged exposure to invisible flicker of either luminance or chromaticity, at frequencies above the respective critical flicker frequency, can compromise our visual sensitivity. This suggests that multiple filtering stages, distributed across retinal and cortical loci that straddle the locus for flicker adaptation, are involved in the neural filtering of high temporal frequencies by the human visual system.

A mechanistic inter-species comparison of flicker sensitivity

The general validity of both the Rovamo [Vision Res. 39 (1999) 533] and Barten (Contrast sensitivity of the human eye, SPIE Optical Engineering Press, 1999), modulation transfer function models for describing flicker sensitivity in vertebrates was examined using published data for goldfish, chickens, tree shrews, ground squirrels, cats, pigeons and humans. Both models adequately described the flicker response in each species at frequencies greater than approximately 1 Hz. At lower frequencies, response predictions differed between the two models and this was due, in part, to dissimilar definitions of the role played by lateral inhibition in the retina. Modelled flicker sensitivity for a matched retinal illuminance condition enabled a direct inter-species comparison of signal processing response times at the photoreceptor level. The modelled results also quantified differences between species in post-retinal signal processing capability. Finally, the relationship between flicker frequency response curves and the perception of temporal signals in real visual scenes was examined for each species. It is proposed that the area under the flicker sensitivity function may offer a single “figure of merit” for specifying overall sensitivity to time signals in a species’ environment.

Modulation sensitivity of ganglion cells in peripheral retina of macaque

There is ample psychophysical evidence that flicker is more salient in the peripheral than the central visual field, but the physiological basis of this eccentricity-dependant change is unclear. Here, we compared responsivity to temporal modulation of ganglion cells in central and peripheral primate retina. Above 30 Hz modulation frequency, both magnocellular (MC) and parvocellular (PC) pathway cells are more responsive in peripheral retina. This suggests that an increase in high-frequency temporal responsiveness arises in outer retina before the MC and PC pathways diverge. In both central and peripheral retina, the critical fusion frequency of MC cells is higher than that of PC cells. This result is consistent with other evidence that psychophysical flicker sensitivity is mediated by the MC pathway.

Measuring and modelling the photopic flicker sensitivity of the chicken ( Gallus g. domesticus)

The photopic flicker sensitivity of the chicken was determined using an operant conditioning psychophysical technique. The results show both high- and low-frequency fall-off in the sensitivity response, which peaked around 15 Hz. Flicker sensitivity was determined for a range of stimulus luminance levels, and directly compared to human flicker response measured under similar stimulus conditions. At five luminance levels (10, 100, 200, 500 and 1000 cd/m 2), the overall chicken flicker sensitivity was found to be considerably lower than for humans, except at high frequencies. A greater degree of frequency tuning was also found in the chicken response. The critical flicker fusion values were either similar or slightly higher for chickens compared to humans (40.8, 50.4, 53.3, 58.2 and 57.4 Hz vs 39.2, 54.0, 54.0, 57.4 and 71.5 Hz respectively for humans and chickens for increasing stimulus luminance level). A recently proposed model for flicker sensitivity [Vision Research 39 (1999) 533], which incorporates low- and high-pass temporal filters in cascade, was found to be applicable to the chicken response. From this model, deductions were made concerning mechanisms controlling the transfer of temporal information.

Vision rehabilitation in homonymous hemianopia

During our investigation into the structure of different visual field defects, five patients with long-standing homonymous hemianopia showed an unexpected improvement in sensitivity on the hemianopic side of their visual field during repeated measurements. The testing paradigm was then used in a training program to improve the patients’ detection and recognition sensitivity. Training resulted in further improvement in two patients until flicker sensitivity became equal to that of the corresponding area of the normal hemifield; it improved in the other three, but did not become equal. One of our patients learned to recognize flickering and nonflickering letters at 20 degrees eccentricity. His recognition sensitivity became equal to that of the normal side after 13 laboratory sessions carried out during a 12-month period.There was also an improvement in the Goldmann visual fields after three years of training. Since there had been no change in the clinical findings or in the subjective experience of space on the hemianopic side in seven years after his accident, the improvement is likely to have resulted from the training with flickering stimuli. We present this case as a preliminary report on the reappearance of form vision in homonymous hemianopia during training seven years after a vascular accident.

Vision with a scanning laser display: comparison of flicker sensitivity to a CRT

Laser scanning displays allow for large depth-of-focus, reduced optical aberrations, and high luminance. However, scanned laser light images have no phosphor persistence. Psychophysical tests with a 3-color laser display showed that flicker thresholds were nearly equal with a laser display relative to the CRT when both were matched in luminance, resolution, frame rate, and similar chromaticity. The threshold ratio was 1.14 (range 0.7–1.6). Differences in flicker sensitivity between displays were significantly related to the absolute energy at the cornea irrespective of the subjective luminance match. Flicker sensitivity was not enhanced by a laser scanning display without image persistence.

Postnatal development of flicker sensitivity in guinea pigs

Background: The retinal response to flickering stimuli (steady state ERG) recruits many retinal elements and is a sensitive indicator of early retinal dysfunction. This study reports the post‐natal maturation of the steady state ERG response in guinea pigs.Methods: The steady state ERG response to flickering stimuli (0.6 to 20 Hz) was recorded from dark adapted (more than 12 hrs) English Shorthair guinea pigs (n = 7) using flashes that produced rod and cone dominated responses. Temporal sensitivity functions and critical fusion frequencies (GFF) were derived over a range of ages from postnatal day (PND) 1 to 45.Results: Guinea pig rod and cone temporal sensitivity functions show shape characteristics and CFF similar to humans. Furthermore, the post‐natal development of the guinea pig temporal characteristics is also similar to that of humans—they are present at birth and mature rapidly post‐natally. The time‐course of CFF maturation is similar for rod and cone mediated responses.Conclusions: These data show that the temporal response and its maturation in the guinea pig retina is similar to that in humans. Therefore, we propose that the guinea pig is a particularly useful animal model to study retinal disease in early childhood.

DLP Cinema™ projection: A hybrid frame‐rate technique for flicker‐free performance

Abstract— Film is recorded at 24 Hz, which is sufficient to achieve the effect of motion but is well within the flicker sensitivity of the human‐visual system (HVS) and thus would result in severe flicker. To avoid this, film projectors project at twice this rate, using a 48‐Hz screen refresh rate. While this greatly mitigates flicker, projected film images still exhibit considerable flicker in bright scenes. DLP Cinema™ projection technology allows us to display images at any frame rate, and in practice we have been able to match the 48‐Hz refresh rate of film projectors. This paper describes a technique by which we take advantage of the fact that the HVS temporal sensitivity curve shows more sensitivity with bright content but much less sensitivity as content dims. This is done using the control versatility of the Digital Micromirror Device™ (DMD™), which allows independent control of every bit. The result is an overall image signal that is beyond the HVS temporal sensitivity curve, resulting in the complete removal of any visible flicker. This technique gives DLP Cinema™ projection its characteristic “solid” and “stable” appearance that standard film projection does not provide.

Multiple processes mediate flicker sensitivity

By systematically manipulating the luminance of a flickering spot and the area immediately surrounding it, we investigated why thresholds from flickering stimuli that cause a change in average luminance are elevated relative to those from stimuli with no luminance change. Threshold elevation resulted from local light adaptation and from temporal-frequency-specific interactions between the spot and its surround: at low frequencies, the contrast between the spot and the surround elevated thresholds, whereas at high frequencies, dark adaptation within the surround elevated thresholds. Our findings suggest that two common ways of determining temporal sensitivity may give markedly different outcomes.

Luminance flicker sensitivity in positive- and negative-symptom schizophrenia.

The aim of the present research was to investigate magnocellular and parvocellular channel disorders using luminance-flicker sensitivity in normal observers and a group with schizophrenia. The threshold sensitivity for a sine wave-modulated patch of achromatic flickering light in a gaussian envelope was measured as a function of its temporal frequency (1.0 Hz, 4.0 Hz, 8.0 Hz, 16.0 Hz, 32.0 Hz) and three space average luminance levels (mesopic 3.0 cd/m2, photopic 33.0 cd/m2 and 66.0 cd/m2). The Andreasen scales for the assessment of positive and negative symptoms in schizophrenia were used to classify subjects into subgroups with predominantly positive and negative symptoms. The results showed that there were no significant differences between the control and positive-symptom group in flicker sensitivity as a function of temporal frequency and luminance level, and there were no differences in flicker sensitivity between the three groups at 1.0 Hz at each of the three luminance levels. At 3.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz, 16.0 Hz and 32.0 Hz in comparison with the control and positive-symptom group. At 33.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz and 32.0 Hz, and at 66.0 cd/m2 they showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz and 32.0 Hz only in comparison with the control. It was concluded that the non-significant differences in flicker sensitivity in the positive-symptom group showed that the processing of temporal information in parvo- and magnocellular channels was unimpaired. Furthermore, the non-significant differences in flicker sensitivity at 1.0 Hz at each of the three luminance levels in the three groups provided evidence that functioning in parvocellular channels was unimpaired in the positive- and negative-symptom group. Finally, it was concluded that the significant reductions in flicker sensitivity at medium and high temporal frequencies in the negative-symptom group provided evidence for an impairment in magnocellular channels.

Flicker sensitivity as a function of target area with and without temporal noise

Flicker sensitivities (1–30 Hz) in foveal, photopic vision were measured as functions of stimulus area with and without strong external white temporal noise. Stimuli were circular, sinusoidally flickering sharp-edged spots of variable diameters (0.25–4°) but constant duration (2 s), surrounded by a uniform equiluminant field. The data was described with a model comprising (i) low-pass filtering in the retina ( R), with a modulation transfer function (MTF) of a form derived from responses of cones; (ii) normalisation of the temporal luminance distribution by the average luminance; (iii) high-pass filtering by postreceptoral neural pathways ( P), with an MTF proportional to temporal frequency; (iv) addition of internal white neural noise ( N i); (v) integration over a spatial window; and (vi) detection by a suboptimal temporal matched filter of efficiency η. In strong external noise, flicker sensitivity was independent of spot area. Without external noise, sensitivity increased with the square root of stimulus area (Piper's law) up to a critical area ( A c), where it reaches a maximum level ( S max). Both A c and η were monotonic functions of temporal frequency ( f), such that log A c increased and log η decreased linearly with log f. Remarkably, the increase in spatial integration area and the decrease in efficiency were just balanced, so A c( f) η( f) was invariant against f. Thus the bandpass characteristics of S max( f) directly reflected the composite effect of the distal filters R( f) and P( f). The temporal equivalent ( N it) of internal neural noise ( N i) decreased in inverse proportion to spot area up to A c and then stayed constant indicating that spatially homogeneous signals and noise are integrated over the same area.

Visual sensitivity to temporal modulations of temporal noise

The present endeavor is meant (a) to provide a direct comparison between first- and second-order temporal modulation and, by so doing, (b) to eliminate all spatial clues that might have contaminated previous assessments of the second-order temporal modulation transfer function (TMTF). The second aim was achieved by means of the temporal modulation of a purely temporal white noise, a stimulus used frequently in psychoacoustics but not used as yet in visual stimulation. Luminance and contrast temporal modulation thresholds were measured with a 2AFC staircase procedure. In the first case, the mean luminance of a spatially homogeneous, 30° field was modulated sinusoidally over time (first-order modulation). In the second case, the luminance of the same or of a 60° field was randomized over time at a rate of 150 Hz and this temporal white noise (the carrier) was modulated sinusoidally over time (second-order modulation). First-order thresholds reproduce the classical (large field) flicker sensitivity. Second-order thresholds (measured for the first time with purely temporal stimuli) are at least 100 times higher than first-order ones, display a low-pass characteristic (at least up to 0.5 Hz) and yield a critical fusion frequency (measured at 100% modulation) of ∼10 Hz. The data are in accord with other estimates of the TMTF of the second-order system and thus confirm the effective neutralization of the spatial cues present in these previous studies.

Blood glucose dependence of visual flicker threshold.

Published research indicated that fluctuations in blood glucose concentration (BGC) change the biochemical balance of the retina and may alter the sensitivity of retinal cells. Low-contrast flicker was used in this research to measure the changes in sensitivity of the retina accompanying shifts in BGC. In five subjects without diabetes and four subjects with diabetes, simultaneous measurements of flicker thresholds, using a two-alternative, forced choice psychometric test, and BGC, were made every 15 to 30 minutes for several hours, while BGC was allowed to fluctuate spontaneously. Flicker thresholds were found to rise and fall, generally tracking BGC in each subject. First-day flicker threshold measurements were used to "calibrate" the method for each subject, and subsequent days' measurements were used to infer blood glucose. The resulting inferred blood glucose values correlated with BGC measured from blood samples in the range of 70 mg/dL to 500 mg/dL with a maximum error of +/-33% in 97% of the measurements. The feasibility of using flicker sensitivity for monitoring blood glucose levels noninvasively is discussed. Improvements in the testing method can potentially reach +/-18% error margin with 1-minute measurements, according to computer simulations. Devices using a visual test would be inexpensive to manufacture and would need no disposable supplies. Planned research will assess the long-term stability of flicker threshold measurements and will determine the applicability of the technique in diabetic retinopathy, in hypoglycemia and in the presence of fatigue and inattention.

Relative number of long- and middle-wavelength-sensitive cones in the human fovea.

Flicker photometric measurements yield spectral sensitivity curves that are well fitted by sums of the spectral sensitivity curves of long-wavelength-sensitive (L) cones and middle-wavelength-sensitive (M) cones if the L cones are given twice the weight of the M cones. This result has been interpreted as implying that L cones are more numerous than M cones but is also consistent with a different numerical ratio, say, 1:1, and with the assignment of greater weight to the L cone input than to the M cone input by the mechanism subserving flicker photometry. Measurements of temporal sensitivity are presented for lights that modulate the inputs of either only the L cones or only the M cones. Sensitivity to modulation of the L cones is approximately twice that of modulation of the M cones at approximately 30 Hz, but that advantage disappears at approximately 2 Hz. Thus flicker sensitivity is equivocal with regard to cone numerosity. Electrophysiological, anatomical, and psychophysical evidence is reviewed, with particular weight placed on the statistics of color appearance of small, brief, monochromatic lights and on increment thresholds measured on the same observers. It is concluded that, in the central fovea, the ratio of L:M cone numbers is close to unity and may not be so variable as is usually supposed.

Flicker sensitivity and cardiovascular function in healthy middle-aged people.

To establish normative relations between measures of visual function and cardiovascular variables that are important for age-related disease, including various forms of glaucoma. Foveal flicker sensitivities, resting blood pressures and heart rates, and intraocular pressures were measured in 18 individuals aged 40 to 68 years. All subjects had 20/20 or better visual acuity in the test eye and no evidence of eye disease or glaucoma suspicion on clinical evaluation and medical history. No subjects were using medication to lower blood pressure. Flicker sensitivity was measured by increasing the illuminance of a fully modulated 20-Hz test stimulus until flicker was perceived. Two test-background stimulus combinations were used: a 570-nm ("yellow") test on a predominantly long-wavelength ("magenta") background and a 580-nm ("yellow") test on a 580-nm ("yellow") background. The illuminance of the yellow background was dimmer than that typically used for short-wavelength automated perimetry, whereas the illuminance of the magenta background was greater. The 2 flicker sensitivity measures were distinguished by the strong dependence of the magenta background measure on the ratio of mean arterial blood pressure to heart rate. Log flicker sensitivity on this background generally could be modeled as a linear combination of age, intraocular pressure, and ratio of mean arterial blood pressure to heart rate. The optimal model accounted for 84% of the variance (R = 0.92) from all but 2 outlying individuals. After age and intraocular pressure effects were partialed out, an increasing ratio of mean arterial blood pressure to heart rate was strongly associated with decreasing flicker sensitivity. Reduced cardiovascular function impacts the ability of the normal visual system to adapt and regulate flicker sensitivity. Elevated intraocular pressure and increased age reduce flicker sensitivity relatively uniformly across a range of stimulus conditions. Because the ratio of mean arterial blood pressure to heart rate equals total peripheral vascular resistance multiplied by cardiac stroke volume, and because total peripheral resistance is determined largely at the arterioles, it is likely that even modest changes in arteriolar function are associated with measurable alterations of visual function. Arch Ophthalmol. 2000;118:1049-1055

Detection of flickering edges: absence of a red–green edge detector

Kelly ((1975) Science, 188, 371–372) showed that a centrally-fixated, contrast-reversing edge has a very different effect on the detection of luminance and red–green flicker. Red–green flicker sensitivity was ∼3-fold greater for a uniform field than for a ‘split’ field with the two sides flickering out-of-phase. Just the opposite effects were observed for luminance flicker — the split field yielded a 7-fold advantage over the uniform field at 2 or 4 Hz and a 3-fold advantage at 12 Hz. Contrary to Kelly, we find that the split field offers only a very small advantage of 40% for luminance flicker at 2 Hz and virtually no advantage at 4 Hz and above. Kelly’s chromatic results are surprising since one might expect that the larger color difference (or step) across the central edge would aid chromatic discrimination rather than strongly suppressing sensitivity. We show that the central chromatic edge only weakly impairs detection. Further results show that the two sides of the chromatic split field are detected essentially independently by red or green ‘blob’ detectors, which do not take advantage of the color difference across the edge. This has a remarkable implication: when wavelength discrimination is measured with a bipartite field whose two side are slowly modulated in opposite directions, then one side may be deleted with little adverse effect.