Full-field Flicker Research Articles

Comparison of spectral measures of period doubling in the cone flicker electroretinogram

Under certain conditions, the full-field flicker electroretinogram (ERG) of the cone system can show period doubling, such that the shape of the ERG waveform alternates from cycle to cycle. The purpose of this study was to determine the relationship between stimulus temporal frequency and the amplitudes of the spectral components of the ERG that correspond to period doubling. ERGs were recorded from 10 visually normal subjects in response to full-field sinusoidal flicker presented at frequencies ranging from 12.5 to 100 Hz. Period doubling was apparent over the stimulus frequency range from 25 to 100 Hz and was quantified in terms of the amplitudes of spectral components of the ERG waveform that corresponded to half the stimulus frequency (f/2) and three and five times that frequency (3f/2 and 5f/2). At stimulus frequencies between 30 and 40 Hz, the amplitude of f/2 was significantly lower than either 3f/2 or 5f/2, which themselves did not differ significantly. At stimulus frequencies between 40 and 60 Hz, all three response components were equivalent in amplitude. At stimulus frequencies above 60 Hz, however, the amplitudes of 3f/2 and 5f/2 were reduced significantly compared to f/2. There is a frequency-dependent relationship among the spectral components of the flicker ERG that correspond to period doubling. The amplitude of f/2 underestimates the magnitude of period doubling at stimulus frequencies between 30 and 40 Hz, whereas the amplitudes of 3f/2 and 5f/2 underestimate the degree of period doubling at stimulus frequencies above 60 Hz. The explanation for the frequency dependence of these spectral components remains to be resolved.

Characteristics of period doubling in the human cone flicker electroretinogram

Electroretinogram (ERG) responses of the cone system to a flickering stimulus can exhibit a cyclic variation in amplitude. This phenomenon of synchronous period doubling has been attributed to a nonlinear feedback mechanism within the retina that alters response gain. The aim of the present study was to investigate intersubject variability in period doubling in the ERG of the human cone system, and to assess the implications of this variability for signal processing within the retina. Period doubling was examined in a group of 12 visually normal subjects, using sinusoidal full-field flicker and harmonic analysis of the ERG waveforms. For all subjects, the ERG responses to 32-Hz flicker (a frequency commonly used clinically) were characterized by a harmonic component at the stimulus frequency and at higher harmonics that were integral multiples of the stimulus frequency, as expected. In addition, six of the subjects showed period doubling at 32 Hz, characterized by harmonic components at integer multiples of a frequency that was half the stimulus frequency (the subharmonic). However, the subharmonic itself did not exceed the noise level. These findings suggest that the subharmonic is generated prior to or at the site that produces the nonlinear higher harmonics of the ERG response, and that a subsequent band-pass filter attenuates this subharmonic. Examination of harmonic components of the subjects' ERG waveforms at other stimulus frequencies, as well as a cycle-by-cycle analysis of the ERG waveforms, suggested that individual differences in period doubling may be due to intersubject variation in the strength of the hypothesized feedback signal and/or the time constant of its decay.

Flickering light increases retinal blood flow.

To examine the retinal blood flow in normal eyes before and during retinal stimulation by flickering light. A prospective cross-sectional study. PARTICIPANTS AND TESTING: Twenty-seven eyes of 27 normal subjects with a mean age +/- SD of 38 +/- 15 years (study I) and 21 eyes of 21 normal subjects with a mean age +/- SD of 46 +/- 17 years (study II) were examined with respect to capillary retinal blood flow and central retinal artery and central retinal vein blood flow velocities during flickering light stimulation. A luminance flicker light with a frequency of 8 Hz increased the neuronal activity of retinal ganglion cells. In study I, the retinal capillary blood flow was measured before and during flickering by scanning laser Doppler flowmetry (670 nm, Heidelberg Retina Flowmeter). In study II, the blood flow velocities in the central retinal artery and central retinal vein were examined by pulsed Doppler sonography. Change in blood flow velocities in the central retinal artery and vein and in retinal capillary blood flow after full-field flicker stimulation. In study I, measurements of blood flow during retinal flicker stimulation showed a significant increase in the mean value of blood flow +/- SD from 317 +/- 72 arbitrary units to 416 +/- 103 arbitrary units. The change was on average 46 +/- 19%. In study II, the systolic and end-diastolic blood flow velocities in the central retinal artery increased significantly (P < 0.0001): systolic, 9 cm/s to 15 cm/s (+62%); end-diastolic, 2.7 cm/s to 5.3 cm/s (+96%). In the central retinal vein, the systolic and end-diastolic blood flow velocities increased significantly (P < 0.0001): systolic, 4.3 cm/s to 6.7 cm/s (+56%); end-diastolic, 1.8 cm/s to 3.6 cm/s (+100%). The authors found no significant change in blood pressure and heartbeat frequency. Visual stimulation of the retina by flickering light strongly increased the juxtapapillary retinal capillary blood flow and central retinal artery blood flow velocity in normal eyes.

Temporal contrast sensitivity using full-field flicker test (Erlangen flicker test) in patients after penetrating keratoplasty.

The purpose of this study was to evaluate the feasibility of temporal contrast sensitivity testing using full-field flicker stimulation in patients after penetrating keratoplasty (PK) and to assess whether this method is influenced by postoperative corneal topographic changes. Forty-five patients (age 46.5+/-14.2, median 47 years) who had undergone PK and 194 age-matched controls were included in this study. The postoperative interval was 11.8+/-10.2 months (median 9 months). Patients with pre-existing glaucoma or any postoperative intraocular pressure elevation were excluded. The indications for PK were keratoconus in 54% of cases, Fuchs' dystrophy in 38% and stromal dystrophies in 8%. Temporal contrast sensitivity was determined with sinusoidal flickering light (37.1 Hz) of constant mean photopic luminance (10 cd/m(2)) presented in a full-field bowl with an increasing threshold strategy. Mean temporal contrast sensitivity did not differ between patients after PK (1.49+/-0.13, range 1.26-1.78, confidence interval 1.45-1.53) and controls (1.55+/-0.17, range 1.16-1.98, confidence interval 1.47-1.51). No significant correlation between temporal contrast sensitivity and visual acuity could be found in patients after PK or in normals ( r<0.2, P=0.3). In patients after PK, temporal contrast sensitivity was statistically independent of keratometric astigmatism ( r=0.3, P=0.7), topographic astigmatism ( r=0.3, P=0.4), spherical equivalent ( r=0.07, P=0.7), central corneal thickness ( r=-0.06, P=0.7) and time since operation ( r=-0.07, P=0.6). Temporal contrast sensitivity using full-field flicker stimulation seems to be feasible in patients after PK and does not depend on topographic changes of the cornea. The results indicate that the full-field flicker test may be helpful as a supplementary means of detecting early glaucoma caused by ocular hypertension in patients after PK.

Inner-retinal contributions to the photopic sinusoidal flicker electroretinogram of macaques. Macaque photopic sinusoidal flicker ERG.

The purpose of this study was to examine inner-retinal contributions to the photopic sinusoidal flicker ERG. ERGs were recorded from 5 anesthetized monkeys to sinusoidally modulated (100%, 0.5-120 Hz) red full field flicker at Lmean of 3.2 log phot td on a rod saturating blue background (3.7 log scot td; 3.0 log phot td) before and after intravitreal injections of tetrodotoxin (TTX) to block Na+-dependent spikes of retinal ganglion and amacrine cells, followed by N-methyl-D-aspartate (NMDLA) to suppress all activity of these cells. Recordings also were made after blocking bipolar (and horizontal) cell responses with L-2-amino-4-phosphonobutyric acid (APB) and 2-cis-piperidine-2,3-dicarboxylic acid (PDA) or 6-cyano-nitroquinoxaline-2,3-dione (CNQX). Control fundamental (F1) and second harmonic (F2) amplitudes were large and variable at temporal frequencies up to 2 Hz. At higher frequencies, Fl amplitude was minimal with a phase step at a frequency between 13 and 19 Hz and maximal at 27-33 Hz. F2 was minimal at 2-3 Hz and maximal at 6-8 Hz, again with a phase step near the minimum. TTX, or NMDLA, produced small changes in Fl that shifted the amplitude minimum to a lower and the maximum to a higher frequency. In contrast, F2 was more strongly affected; both the amplitude minimum (and phase step) and maximum were greatly attenuated, leaving a moderate response from 0.5 to 8 Hz, which then declined as frequency was increased to 30 HZ. After APB and PDA or CNQX, F1 decreased continuously with increasing frequency and F2 was generally much smaller. The nearly linear F1 phase plotwas consistent with the presence of a single mechanism (i.e. photoreceptors). Inner-retinal neurons contribute to the photopic sinusoidal flicker ERG. Whereas for F1, inner-retinal contributions are small relative to those from bipolar cells; for F2, they are equal or greater between 2and 16 Hz.

Temporal processing in very low vision

Visual temporal frequency discrimination (TFD) capacities of four adults with very low vision (VLV) were assessed. Full-field flicker was generated by placing a 5×5 red LED array against a translucent eyepatch that served as a diffuser. Temporally modulated phase-randomized sine wave stimulus pairs (standard and test frequencies) were presented monocularly using a 2IFC procedure. Discrimination was tested at standard frequencies ranging between 0.75 and 57 Hz; TFD difference thresholds (?F) for the ten standard frequencies were estimated from maximum likelihood fits of Weibull functions. ?F was a constant or nearly constant proportion of F throughout the assessed range. These results show conclusively that TFD judgements can be rendered by those with simulated and actual VLV. Temporal coding of full-field light modulation has the potential to optimize a neglected sensory channel in individuals with VLV.

Steady-state visual evoked potentials and travelling waves

Objective: The amplitude and phase of the steady-state visual evoked potential (SSVEP) is sensitive to cognition and attention but the underlying mechanism is not well understood. This study examines stimulus evoked changes in the SSVEP phase topography and the putative role of travelling waves. Methods: Eighteen subjects viewed a central-field checkerboard and full-field flicker stimulus temporally modulated at the peak alpha rhythm frequency. EEG was recorded from 10 midline scalp sites and the bipolar SSVEP obtained from differences between adjacent electrodes. Results: The SSVEP phase comprised either progressive variations consistent with travelling waves or a phase reversal consistent with standing waves. The checkerboard pattern elicited travelling wave patterns in 14 subjects with estimated phase velocities ranging from 7 to 11 m/s after correcting for folded cortex. The flicker stimulus elicited phase reversals in 9 subjects, suggesting standing waves. Six subjects demonstrated a phase topography specific to the stimulus with travelling wave patterns associated with the checkerboard and standing wave patterns associated with the flicker. Conclusions: These differences suggest the emergence of travelling and standing waves under different spatial configurations of visual input to the cortex and that wave phenomena contribute to the spatiotemporal dynamics of the SSVEP.

Synchronous period-doubling in flicker vision of salamander and man.

Periodic flashes of light have long served to probe the temporal properties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every other flash of light. In this regime, retinal ganglion cells of the salamander fire spikes on alternating flashes. Neurons across the entire retina are locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of nonlinear feedback reproduces the phenomenon. Pharmacological studies indicate that the critical feedback interactions require only cone photoreceptors and bipolar cells. Analogous period-doubling is observed in the human visual system. Under bright full-field flicker, the electroretinogram (ERG) shows a regime of period-doubling between 30 and 70 Hz. In visual evoked potentials from the occiput, the subharmonic component is even stronger. By analyzing the accompanying perceptual effects, we find that retinal period-doubling begins in the periphery of the visual field, and that it is the cause of a long mysterious illusory flicker pattern.

The Full-field Flicker Test in Early Diagnosis of Chronic Open-angle Glaucoma

To evaluate whether the full-field flicker test, a psychophysical test employing full-field flickering stimuli to measure temporal contrast sensitivity, can detect glaucomatous optic nerve damage in patients with increased intraocular pressure and glaucomatous optic disk abnormalities but normal visual fields. Temporal contrast sensitivity was determined with a sinusoidally flickering light (frequency, 37.1 Hz) of constant mean photopic luminance (10 cd/m2) presented in a full-field bowl of 58-cm diameter. The prospective study included three groups of individuals: the "preperimetric" glaucoma group of 80 patients with increased intraocular pressure, glaucomatous optic disk abnormalities, and normal visual fields; the "perimetric" glaucoma group of 56 glaucomatous patients with increased intraocular pressure and glaucomatous changes of the optic disk and visual field; and the control group of 96 normal subjects. Temporal contrast sensitivity was significantly (P < .001) lower in the two glaucoma groups than in the control group. In the preperimetric glaucoma group, 34% of the patients (27/ 80) were recognized by the full-field flicker test at a specificity of 99%. For all study subjects, temporal contrast sensitivity decreased significantly (P < .001) with decreasing neuroretinal rim area, enlarging peripapillary atrophy, and diminishing retinal nerve fiber layer visibility. The full-field flicker test can detect glaucomatous optic nerve damage in patients with increased intraocular pressure, glaucomatous optic disk abnormalities, and normal visual fields. Considering its feasibility, simplicity, quick performance, and low costs, the full-field flicker test may be helpful in clinics and in screening examinations as a supplement to glaucoma diagnosis.

Blue on yellow full field flicker test

Blue on yellow full field flicker test

P 448 Blue on yellow full field flicker test

P 448 Blue on yellow full field flicker test

Quick Full-Field Flicker Test in Glaucoma Diagnosis

The value of a full-field flicker stimulus in glaucoma diagnosis was tested. In 69 normals, 60 ocular hypertension, and 50 manifest primary openangle glaucoma patients, the temporal contrast (flicker) thresholds were tested with a sinusoidally flickering white light of constant mean photopic luminance (10 candela/m) presented in a full-field bowl. A modulatable xenon-arc lamp was used, and only one frequency (37.1 Hz) was tested. Luminances were corrected according to pupil widths. No significant age relationships of flicker thresholds were found in normal eyes. Forty (80%) of glaucoma eyes were recognized as pathologic at a specificity of 93%. In the glaucoma group, significant correlations of flicker thresholds were found with measurements of neuroretinal rim areas of the optic disc (linear r = -0.61, nonparametric r = -0.59, p = 0.001) and with perimetric mean defects (linear r = 0.74, nonparametric r = 0.66, p = 0.0001). Because of its high sensitivity and the significant correlation with perimetric losses and neuroretinal rim areas, the full-field flicker test can be used as a quick (5 min per eye) additional examination in the diagnosis of glaucoma.

Fractal analysis of steady-state-flicker visual evoked potentials: feasibility.

Electrophysiological models of visual evoked potential recording have assumed that response variability is caused predominantly by random noise added to a true steady signal. Since neuronal geometry has a fractal structure, neural activity may demonstrate deterministic nonlinear dynamics, i.e., chaos. We recorded several-minute time-series traces of the visual evoked potential magnitude in response to full-field flicker from three glaucoma patients and one normal subject. When plotted in phase space, the steady-state response derived from a lock-in amplifier shows an apparent so-called strange attractor (extended nonrepeating loops) rather than the pattern expected from a signal-plus-noise model (a fuzzy dot). The fractal dimension of this attractor may be a more sensitive indicator of early optic-nerve damage than are visual evoked potential latency or amplitude measures.

Electrophysiological assessment of visual deficit in glaucoma

Steady-state visual evoked potentials (VEP(S)) in response to contrast reversed vertical grating patterns and uniformly flickering fields were recorded in patients with glaucoma and in controls. In steady-state VEP(S), responses to temporal and spatial aspects of visual stimuli are combined, which makes the assessment of visual function dependent on their interaction. Fourier analysis of pattern reversal VEP waveforms showed marked deficits in temporal processing of spatial contrast for glaucoma patients. The degree of glaucomatous damage, as manifested by greater optic nerve atrophy and visual field loss, was correlated with progressive disruption of the VEP waveform and progressive failure in temporal processing of spatial contrast patterns. The VEP(S) in response to uniform full-field flicker showed no deficit.

Dichoptic temporal frequency differences do not lead to binocular rivalry.

The influence of temporal modulation on binocular rivalry was examined in two ways. First, an uncontoured field viewed by one eye had its luminance sinusoidally modulated at 4 Hz (full-field flicker). The other eye viewed a similar field that flickered at different temporal frequencies ranging from 5 to 16 Hz. Second, orthogonal sinusoidal dichoptic gratings were phase shifted so as to duplicate the range of temporal-frequency disparities presented with full-field flicker. Three subjects recorded exclusive visibility of the rival fields over 60-sec trials. Full-field flicker produced very few reports of rivalry, and was described as resembling “visual beats” or as an intermediate flicker rate. Usual amounts of rivalry were reported between orthogonal sine-wave gratings; the field flickering at low rates dominated less. These results were essentially confirmed in a second experiment, designed to minimize individual differences in response criteria by requiring subjects to respond to disappearances of small, dichoptic, nonrival markers superimposed on the rival fields. The markers disappeared in concert with episodes of rivalry suppression of the gratings, although disappearances were slightly briefer than supression durations. Disappearances of markers on full-field flicker were rare, extremely brief, and could be attributed to Troxler’s fading rather than rivalry. The absence of rivalry between temporally disparate full-field flicker suggests that rival interactions within transient mechanisms are minimal.