Flicker Thresholds Research Articles

Directional and nondirectional coding of a spatio-temporal modulated stimulus

Contrast thresholds for apparent flicker and direction of movement were measured separately by using a subthreshold summation technique. The test stimulus consisted of a drifting grating superimposed on an identical grating moving in the opposite direction and set at different subthreshold contrasts. It was found that with increasing contrast of the subliminar grating that: (1) movement thresholds were increasing, (2) apparent flicker thresholds were slightly decreasing and (3) percentage of apparent flicker reports was increasing from 0 to 100. The distinction between flicker and movement thresholds was shown to be justified by two subsidiary experiments which showed that flicker is not to movement what movement is to flicker. A moving subliminar grating facilitates the detection of an objectively flickering grating whereas the reciprocal is not true. This might indicate separate detection of drifting and flickering gratings.

Human flicker sensitivity: two stages of retinal diffusion.

A well-known solution of the diffusion equation gives an exponential square-root function as the frequency response for a one-dimensional diffusion or transmission process. When two or more such processes are cascaded, the result is still an exponential square-root characteristic, but with a longer time constant. This seems to explain why flicker thresholds obey the Kelly-Veringa diffusion model at high frequencies, even though the psychophysically inferred diffusion process is much slower than the first stage of visual transduction measured by, for example, late receptor potentials. Two such stages in tandem are sufficient to account for the psychophysical data, because the psychophysical time constant is proportional to the square of the number of stages involved. In addition, the nonlinear behavior of flicker thresholds under intense light adaptation can be explained if the loss factor in the first stage is proportional to the amount of the photopigment bleached. Apparently the flicker thresholds are governed by first- and second-order retinal neurons.

Primate flicker sensitivity: psychophysics and electrophysiology.

A quantitative comparison is made between the psychophysical flicker response of man and similar data obtained electrophysiologically from the cones of macaque monkeys. When the psychophysical data are obtained from an eye that is strongly light-adapted, there is excellent agreement between the two sets of data at high frequencies. Under this condition, both kinds of data fit a distributed-parameter model, whose time constant also agrees with that derived from studies of the phosphenes elicited by electrical stimulation of the human eye. On the other hand, psychophysical data obtained with fully modulated stimuli (which minimally adapt the eye) yield a longer time constant for the same model. These results imply that the psychophysical flicker thresholds are normally controlled by a distributed filtering process that is proximal to the receptor stage. This slower, psychophysical process is evidently desensitized by intense adapting lights, so that the faster one that governs the electrophysiological responses can be detected.

No oblique effect in chromatic pathways*

The classical oblique effect appears in flicker thresholds for luminous-contrast gratings at all temporal frequencies, but is absent in flicker thresholds for chromatic (red/green) gratings that are coarse enough to be fully resolved by the opponent-color pathways. This result is consistent with the physiological finding that luminosity cells in the striate cortex are generally orientation selective, whereas opponent-color cells are not.

Analysis of visual modulation sensitivity: Two components in flicker perception

An analysis of human psychophysical flicker thresholds is developed from the increment threshold technique of Stiles (1939). Two independent detection components are required to account for all available modulation sensitivity data. These components are differentiated by properties corresponding to the difference between sustained and transient units in the cat retina.

Dynamic properties of vision—V: Perception lag and reaction time in relation to flicker and flash thresholds

In order to relate perception lag to flicker thresholds, both were measured by the same subjects under the same conditions. In a preliminary series of experiments, perception lag obtained by the three methods, double flash, eye and ear, and reaction time, were compared for foveal flashes at a dark background. The course of perceptive delay was measured as a function of the intensity of long flashes over a large intensity range. At medium intensities the results of the three methods agree. At low intensities, decreasing the intensity towards the threshold increases reaction time progressively faster than double-flash settings in the first decade above the threshold. The eye-and-ear method was not found feasible in this range. At high intensities the methods cannot be proved to give different results but the intensities are limited by disturbance of the adaptive state caused by the test flashes themselves. In a second series of experiments, reaction times and double-flash settings at a low and a high background level were compared. The flicker fusion characteristics of the same stimuli were determined by modulating them harmonically. The change of perception lag at the low background level was considerably greater than that at the high level on varying the flash intensity over a comparable range. The difference between the results of the two methods near the threshold is even more obvious with a background. Visual latency and flicker thresholds have been related on the basis of three hypothetical general system properties. The course of perception lag obtained by double-flash settings is in agreement with that calculated from the flicker fusion curve. Latency curves at different background levels can be found from a generalized function and two parameters which are characteristic for sensitivity and inertia and which can be found either from flicker or flash thresholds.

Spectral sensitivity of the modulation-sensitive mechanism of vision

Spectral sensitivities were measured by a procedure of flicker threshold. The shape of the spectral sensitivity curve depends systematically on both the frequency and percentage of temporal modulation. The variations in spectral sensitivity are consistent with two different sets of deLange characteristics, one set applicable to short and medium wavelengths, the second applicable to long wavelengths.

Spatial interactions between the red- and green-sensitive colour mechanisms of the human visual system

Spatial interactions in the visual system were studied psychophysically by measuring the effects of (a) subliminal annuli on the increment threshold of a central test spot and (b) suprathreshold annuli on the flicker threshold of a central test. Because previous evidence indicates that interactions of type (a) occur at a level before the optic chiasma, while those of type (b) occur more centrally (e.g. Fiorentini, Bayly and Maffei, 1972), the present investigation was designed to test interactions at two levels in the visual system. The visual stimuli were selected to stimulate individual spectral classes of colour mechanisms, and in particular interactions between red- and green-sensitive channels of foveal vision were investigated. For both types of measurement interactions between centre and surround regions of receptive fields occurred only between like spectral response mechanisms, whereas interactions within the centre of receptive fields also occurred between the different spectral mechanisms. These conclusions are discussed in relation to those of other investigations.

Studies of spectral sensitivity as measured by a procedure of flicker threshold.

Subjects adjusted the radiance of monochromatic stimuli, modulated at constant frequencies, to produce just-detectable flicker. Experiment I showed that spectral sensitivity determined at frequencies 20-35 Hz matches CIE photopic luminous efficiency; higher frequencies (40-45 Hz) yielded reduced sensitivity to long wavelengths. Experiment II showed that increasing intensity alone narrows spectral sensitivity, whereas increasing frequency broadens it. Experiment III isolated three modulation-sensitive mechanisms by chromatic adaptation; these mechanisms have spectral sensitivities similar to Stiles' π1 π4, and π5.

On neural inhibition, contrast effects and visual sensitivity

Through a reinterpretation of the literature, as well as original experiments, it is shown that bright annular surrounds tend to increase incremental thresholds obtained on dimmer center conditioning fields, but tend to decrease incremental thresholds when the center field is equal to or brighter than the annulus. This is true for white and red flashes with similarly colored surrounds and centers, but green flashes on red conditioning fields are not affected in the same way. It is also shown that flicker threshold modulation is affected by annular surrounds in the same way as simple ΔI′s. An explanation is offered to account for the effects that annular surrounds have on center ΔI′s. Some important implications of this explanation are: (1) the fact that visual thresholds often seem unrelated to field brightness does not imply the existence of different threshold and brightness mechanisms; and (2) the traditional procedure of using visual thresholds to assess the inhibitory effects of interretinal or intraretinal illumination is logically unsound.

Dynamic properties of vision—I. Experimental relationships between flicker and flash thresholds

Flicker thresholds of harmonically modulated light with variable frequency and flash thresholds of square flashes of variable duration are compared quantitatively. The experiments were carried out with the same subjects under the same conditions and with the same equipment over an intensity range of five decades. Characteristic quantities for sensitivity and inertia are defined for both types of measurement. Sensitivity S for flicker and sensitivity F for flashes are found to have a constant ratio over the entire background intensity range. The product of the cut-off frequency with flicker and the critical duration with flashes is also found to be constant.

MEASURING FLICKER THRESHOLDS IN THE BUDGERIGAR1

A method of measuring thresholds in animals is described and illustrated in the case of flicker fusion in the budgerigar. After training with reinforcement for pecking at a high-frequency light (115 Hz) and nonreinforcement for pecking at a low-frequency light (20 Hz), subjects were given threshold trials and reinforcement trials mixed randomly in equal number. In threshold trials (no reinforcement), the target began flashing at 115 Hz and decreased in flash rate with each peck until the subject stopped responding. During reinforcement trials, the target continued to flash at 115 Hz, and responses were reinforced on a variable-ratio schedule. Flicker thresholds obtained from two birds showed a linear relation to the logarithm of intensity in accordance with the Ferry-Porter law.

Diffusion model of linear flicker responses.

Recent cascaded-integrator models do not fit the sine-wave flicker thresholds as well as we might wish, but neither does the Ferry–Porter law. In fact, the Ferry–Porter function is not physically realizable as a linear model. By modifying it to yield realizable responses like those of the cascaded integrator, we obtain a much simpler model, which appears to be a special case of the photochemical diffusion mechanism proposed by Ives and more recently by Veringa. This model is a good fit, not only to the flicker data, but also to human phase-shift measurements obtained by the phosphene method. We infer that receptor-cell properties probably control the high-frequency linear filtering of flicker waveforms.

Sinusoidal flicker characteristics of the color-sensitive mechanisms of the eye

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Flicker fusion phenomena.

The high-frequency temporal behavior of the human visual system has been shown to have some of the properties of a linear low-pass filter. For such a system it is appropriate to consider a repetitive stimulus as having separable Fourier harmonic components. The direct-current component or average luminance is important in that it sets the adaptation level. It is therefore convenient to keep it constant when varying other stimulus parameters, such as frequency or wave form. Of the alternating-current components, only the fundamental is important at high frequencies, the higher harmonics being relatively more attenuated. Any linear low-pass filter system responds in a predictable way to sinusoidal stimulation, whether continuous or of short duration. In the case of the visual system, predictable behavior is found at high frequencies, and it leads to discovery of hitherto unobserved pseudoflash and real flash phenomena. Measurement of a new characteristic time is suggested. At low frequencies the use of half and whole-sinusoidal flashes leads to the discovery of some interesting relations with flicker thresholds, but these remain for future discussion.

Flicker threshold shifts as a function of frequency of interposed stimulation: the local adaptation phenomenon.

Flicker threshold shifts as a function of frequency of interposed stimulation: the local adaptation phenomenon.

Impaired Flicker Detection in Visual Fields Subserved by Non-Damaged Hemispheres

Visual temporal acuity (flicker detection) throughout the visual field has been found to be a more sensitive indicant of brain dysfunction than standard visual field examinations. This finding may be largely due to non-specific and indirect effects of brain injury upon temporal resolving processes. If there were evidence for impaired flicker thresholds in visual fields subserved by uninjured cortex, the hypothesis of non-specific effects would be supported. The flicker detection thresholds for left and right visual fields at the 30° circle were compared for 50 non-brain-damaged S s, 18 left hemisphere damaged and 16 right hemisphere damaged S s. The brain-injured S s, who had temporal, parietal and frontal lobe damage, were selected on the basis of having clear-cut evidence for unilateral damage only, according to the best available neurological ratings. They manifested no visual field abnormalities by standard perimetry in the fields subsumed by the “unimpaired” hemisphere. The brain-damaged S , regardless of location of lesion, had a significant impairment in the perimetrically “spared” field. The impairment was about half that found in the field subserved by the injured cortex. Discussion of the possible effect of interrupted cross-hemispheric cortical and cortical-lower brain center reverberating circuits, emphasized the complexity of “central” factors in flicker perception. The sensitivity of measures of visual temporal acuity appears to be due to their reflecting the generalized dimension of arousal level as well as specific and focal deficits in visual functions.

Human visual responses to sinusoidally modulated light

Sinusoidally modulated light, even at low percentages of modulation, gives rise to occipital responses, sometimes large. They may reproduce rather well the input sine wave in shape and in amplitude, especially between 9 and 15 c/sec for diffuse large field illumination and above 35 c/sec for strong illumination with small fields. The responses between 9 and 15 c/sec may be partly described by assuming a linear filter action. Resonance phenomena are observed, in which different types of stimulation yield compatible results. Responses and spontaneous activity seem to be additive to a considerable degree, as are the responses to stimulation of each eye. In the 9–15 c/sec frequency range, where the responses exhibit mainly the fundamental frequency, cancellation effects are found with out-of-phase stimulation of both eyes. That the large distortions found may be caused in early stages of the visual process, and are essential, is discussed. Responses have been recorded at many frequencies when the light was seen as fused by the subjects. Although subjective flicker threshold curves bear no clear relation to either the size or shape of the responses, it is hoped that the recordings of evoked responses with sinusoidally modulated light, combined with the techniques presented by Clynes et al. (1964), will be a valuable supplement to the current methods of investigation.

THE ATTENUATION CHARACTERISTICS OF THE VISUAL SYSTEM DETERMINED BY MEASUREMENTS OF FLICKER THRESHOLD, BRIGHTNESS AND PUPILLOMOTOR EFFECT OF MODULATED LIGHT.

The attenuation characteristics of the human visual system have been determined by the measurement of flicker threshold and of the brightness and pupillomotor effect of modulated light.

Fourier treatment of some experiments in visual flicker.

This report analyzes some experiments in visual flicker recently described by Forsyth and Brown. The assumption that only the first Fourier component of their stimuli is at the flicker threshold, the other components being below threshold, provides a model that accurately predicts their data. Hence, contrary to the conclusions drawn by Forsyth and Brown, no new nonlinear property of the visual system has been demonstrated.