Test Flash Research Articles

Backward and forward masking associated with saccadic eye movement.

Visual masking effects on test flash thresholds were measured under real and simulated eye movement conditions to determine whether visual masking is primarily responsible for elevations in threshold that are sometimes associated with saccadic eye movements. Brief luminous flashes presented to the central retina before, during, and after saccades were masked by stimuli presented either pre- or postsaccadically. The amount and time course of masking were quantitatively dependent on stimulus parameters of intensity and temporal separation and were unaffected by eye movement parameters (amplitude, velocity, direction) as long as retinal stimulus conditions were constant. The duration of forward masking was longer than that of backward masking. When retinal conditions during saccades were mimicked while the eyes were held steady, masking interactions were identical to those obtained during real saccades. These results indicate that masking effects during saccades in ordinary environments are determined solely by the stimulus situation at the retina. Putative nonvisual, centrally originating saccadic suppression suggested by other authors is evidently not additive with visually determined masking during saccades.

Visual thresholds in dichromats and normals; the importance of Post-receptoral processes

Threshold-duration curves, for 10 protanopes, 8 deuteranopes and 13 normals were measured for 1°, 674 nm foveal test flashes on a 1000 td, 556 nm background. Deuteranope thresholds were found to be higher than normal by up to 0.5 log units; temporal integration for both protanopes and deuteranopes was only about 40% of normal. These results can be understood only if post-receptoral abnormalities of dichromats are considered, specifically a lack of red-green opponent-colour neurons. Differences in shape of the threshold-duration curves are related to the sustained or transient character of the response. The normal curve for 556 nm flashes is analysed in terms of two independent detection processes.

Punctate sensitivity of the blue-sensitive mechanism

Thresholds were measured for a tiny, brief, violet flash on a long wavelength, B cone-isolating background in foveal locations spaced only 4 or 5′ of arc apart. Large spatial variations in B cone sensitivity were found just beyond the foveal tritanopic area even though thresholds for the same wavelength test flash hardly varied at all across these same retinal locations when the flash was detected by G cones. The relative constancy of G cone threshold suggests that these spatial variations are intrinsic to the blue-sensitive mechanism and cannot be explained by prereceptoral filtering. The spatial variations in B cone sensitivity are consistent with physiological evidence that B cones are scarce in the retina. In one observer, it was possible to discern discrete peaks in sensitivity spaced roughly 10′ of arc apart. A model is described which takes optical spread and eye movements into account to show that these peaks may represent individual B cones (or clumps of B cones).

Large loss of visual sensitivity to flashed peripheral targets

Threshold measurements of flashed peripheral test spots are shown to be affected by continuing presentation. With repeated presentations of a tiny test flash, thresholds rose progressively, sometimes reaching more than 10 times the initial value. This large loss in sensitivity is not simply due to retinal light adaptation. Habituation to repetitive stimulation proceeds more rapidly with frequent flash presentation but is still conspicuous when flashes occur every 2–3 sec. With 15 min of rapid flash presentation, there is an initial sharp recovery in sensitivity after which there remains some loss of sensitivity even after 35 min. Habituation occurs within both rod and cone systems and transfers between them. The effect is specific to the size and spatial frequency, but not the orientation, of the habituating stimulus.

Reversed effect of adapting stimuli on visual sensitivity.

The sensitivity of the eye to incremental stimuli normally decreases as the intensity of a steady background field increases. We examine here a range of conditions under which an added field anomalously increases sensitivity. If the threshold intensity for violet (A = 423 nm) test flashes is measured on a blue (μ 1 = 473 nm ) field of 10 9.7 quanta s -1 deg -2 and if then yellow (μ 2 = 575 nm) fields of increasing intensity are added to the background the threshold may fall by 0.3—0.4 logarithmic unit. The facilitation occurs whether yes/no or forced-choice procedures are used (experiments 1, 2), is abolished when the blue and yellow fields are presented dichoptically (experiment 5) and varies in magnitude with the duration of exposure to the composite adapting field (experiment 6). The intensity of the yellow field required for maximal facilitation ( a ) increases as the intensity of the blue field increases and ( b ) is typically that intensity required to render the composite field achromatic (experiments 1, 3). The results imply that the sensitivity of the psychophysically defined blue mechanism is not controlled merely by photons absorbed in the shortwavelength receptors. We hypothesize that signals originating in the short-wavelength receptors are confined to opponent-colour channels (assumption 1) and that any opponent-colour channel is most sensitive to input perturbations when at an intermediate value of its response range (assumption 6). We relate the latter principle to line-element analyses of colour discrimination. Variation of μ 2 allows us to estimate the action spectrum of the longwavelength input to the putative opponent channel (experiment 4). This spectrum resembles the photopic luminosity function, V; but for μ 1 > 600 nm the facilitation is less than for μ 1 « 575 nm. We discuss the implications of the present results for the π mechanisms of Stiles. If detection were by opponent channels, might Stiles’s field sensitivities be contaminated by variation in post-receptoral sensitivity when field wavelength was varied or when an auxiliary field was added? Although hindsight does suggest that the large and long targets used by Stiles would favour detection by opponent channels, an unappreciated advantage of the two-colour method may be that the monochromatic field desensitizes opponent channels and ensures that detection is via a nonopponent pathw ay whose (post-receptoral) sensitivity varies little in the course of field-sensitivity m easurem ents. Stiles’s own checks and other evidence suggest that π 4 and π 5 are little distorted by opponent processes and even for the blue mechanism, where the use of long-wavelength auxiliary fields would be likely to modulate the sensitivity of opponent channels, the relative field sensitivity for μ < 500 nm is probably that of the receptors. The present results do suggest that Stiles’s two-colour method can, with care, be extended to the study of chromatically opponent channels in the visual system.

Loss of color vision and Stiles' II1 mechanism in a patient with cerebral infarction.

A 70-year old man developed achromatopsia with bilateral loss of superior visual fields and an inability to recognize familiar faces (prosopagnosia). Ophthalmologic examination results were normal. Visual acuity was 20/25 in either eye. Computerized axial tomography of the brain revealed infarction of the inferior aspect of the temporal occipital cortex in both hemispheres. The patient's complaint that objects appeared only in shades of gray was supported by large errors made throughout the spectrum on the Farnsworth-Munsell 100 hue test and by matches over the entire red/green range on the Nagel anomaloscope. Although absolute scotopic and photopic thresholds were unremarkable, the increment thresholds to a 482-nm test on a red background increased monotonically as if the II1 mechanism were absent. In addition, the spectral sensitivity to large test flashes on an intense red background peaked in the middle rather than in the short-wave portion of the spectrum, as is normally found. We speculate that the chromatic channel is compromised. The patient's residual vision is mediated by a luminance channel that is subserved by the middle and long--but not the short--wave cone mechanisms.

Influence of background luminance on visual sensitivity during saccadic eye movements.

Detection thresholds were measured for a brief test flash projected on a uniform background before, during, and after saccadic eye movement. The amount and duration of threshold elevation during saccades was directly dependent on back-ground illumination; no significant elevations occurred at backgrounds of 2.0 log fl or less. Similar results were obtained during fixation when the backgrounds were "saccadically" displaced. An occipital evoked potential was recorded in association with both eye movements and background displacements at higher background luminances (no test flash). These results may indicate an activation of a selected population of neural elements - probably the "Y" channels - which occurs during saccades in illuminated environments and which renders the channels less responsive to additional, simultaneous, and appropriately structured stimuli.

Orientation-specific and luminance effects: interocular suppression of visual evoked potentials in man.

ABSTRACTThe interocular transfer of orientation‐specific effects was investigated using visual evoked potentials (VEPs). Monocular VEPs were obtained to either diffuse light or grids of 0° or 45° orientations flashed to one eye. The other eye continuously viewed either darkness, diffuse light or grids of 0°, 15°, 30°, or 45° orientations. Changing the continuous stimulus from darkness to diffuse light interocularly reduced the amplitude of a late VEP component (P200). Changing the orientation of the continuous stimulus primarily affected an earlier VEP component (N110). This interocular effect was orientation specific: the more similar the orientation of the flashed and continuously presented grids, the smaller the amplitude of the VEP components. The 1/2 bandwidths at 1/2 amplitude of the orientation tuning functions were 45° and 32° for the vertical and oblique test flashes respectively. This orientation‐specific interocular suppression was discussed in terms of electrophysiological and psychophysical data indicating binocular orientation channels and the interactions among these channels.

Transient sensitization by a contrast flash

A large, annular contrast-flash that raises thresholds for a test spot under some conditions can lower theresholds under other conditions. Incremental thresholds for a 2.5° test flash were measured upon a 10° adapting field. When an annulus, 2.5–9°, was flashed shortly before or shortly after presentation of the test spot, threshold was higher than when the annulus was not presented. When the test spot and annulus were flashed simultaneously, however, test threshold was lower with the annulus than without. This transient sensitization occurs over a limited range of annulus and adapting field intensities. Under appropriate conditions, the effect exceeds 0.5 log units.

Detection of long-duration, long-wavelength incremental flashes by a chromatically coded pathway

This paper reports tests of the hypothesis that a 200 msec 667 nm, foveal test flash, presented upon a steady, 8° background, is detected by a pathway whose sensitivity is monotonically related to a linear functional of photoreceptor quantum catch. The measurement conditions used in this paper are the same as those used by Stiles in his π-mechanism analysis. The hypothesis is tested by measuring increment-threshold curves for various background wavelengths and for various mixtures of two backgrounds. The results clearly reject the hypothesis. We propose two models that are consistent with the measurements reported here as well as measurements—using a 10 msec test flash—reported previously. We then discuss related empirical and theoretical work.

Test of the identity of the site of blue/yellow hue cancellation and the site of chromatic antagonism in the π 1 pathway

The foveal increment threshold for a 425 nm test flash upon monochromatic and bi-chromatic fields exhibits four distinct features: (1) super-additivity of the effects of approximately π 1 equated long- and short-wavelength fields; (2) upward deviation of the threshold from the Weber line on bright bluish fields; (3) cancellative sub-additivity of long- and short-wavelength fields whose mixture is approximately in blue/yellow equilibrium; (4) the “limited conditioning effect” of long-wavelength fields, i.e. the π 3 plateau. A one-pathway, “two-site” theory proposed by Pugh and Mollon, in which the adaptation at the first site is controlled by the short-wavelength cones alone, and the second site is chromatically opponent, can give a good account of these features. We have investigated the relationship of the opponent site in the π 1π 3 pathway to the site of blue/yellow hue cancellation. We measured blue/yellow equilibria for continuously viewed, 10° fields composed of mixtures of a short-wavelength and a long-wavelength component; increment thresholds for a 425 nm flash were then measured upon these and other field mixtures. Lights in blue/yellow equilibrium were found always to yield minimum second-site adaptation in the π 1π 3 pathway.

A field-additive pathway detects brief-duration, long-wavelength incremental flashes

This paper reports tests of the hypothesis that a 10 msec, 667 nm, foveal test flash, presented upon a steady, 8° background, is detected by a pathway whose sensitivity is monotonically related to a linear functional of photoreceptor quantum catch. The hypothesis is tested by (1) measuring incrementthreshold curves for various background wavelenghts, and (2) measuring threshold upon mixtures of two backgrounds with different wavelengths. The data are consistent, within measurement error, with the hypothesis. We argue, on grounds of plausibility, that the pathway's sensitivity to the 10 msec test flash is controlled not by an arbitrary linear functional, but by the quantum catch of a single class of photoreceptors.

Electroretinographic and psychophysical measures of cone spectral mechanisms using the two-color threshold technique

The two-color threshold pradigm of Stiles (1939) and a spatially alternating bar pattern stimulus were used to derive electrophysiological and psychophysical spectral sensitivity curves from the human retina. The test wavelength was kept constant at 600 nm and the background wavelength was varied from 450 to 650 nm. Three experiments were carried out with two color normal subjects. In the first experiment the original stimulus configuration of Stiles, a 1°, 500 msec test flash on a 10° background, was used to determine increment threshold curves. In the second experiment, ERGs were recorded with a bar pattern test field with a stripe width of 30′ alternating at a rate of 8Hz superimposed on a steady background. The diameter of both test and adaptation field was 15°. In the third experiment psychophysical thresholds were determined with the same stimulus conditions used to record the ERG. Stiles π 5 mechanism provides a good fit to the spectral sensitivity curves derived from the increment threshold ( ΔI/I) functions.

Stiles's II5 color mechanism: tests of field displacement and field additivity properties.

The II5 color mechanism was isolated in two observers with a 667 nm, 200 ms, 1 deg foveal test flash. Stiles's field displacement law and the law of field additivity were tested with increment thresholds upon monochromatic and bichromatic adapting fields. The data of both observers obey the displacement law and exhibit no systematic deviations from additivity. Our results cannot reject the hypothesis that the first log unit of adaptation observed under II5-isolation conditions is determined exclusively by the quantum catch rate of the long-wavelength-sensitive cones.

Lateral interactions in human cone dark adaptation.

1. The course of cone dark adaptation after exposure to a strong bleaching light depends on the size of the bleached region. Threshold for brief, tiny test flash centred in the bleached region is elevated more, and recovery is retarded by a small bleach. This effect has its parallel in the sensitization effect observed with steady backgrounds. 2. Previous results, that a similar sensitization effect is not observed in rod dark adaptation, are confirmed. 3. This sensitization effect in cone dark adaptation does not transfer binocularly, and is unaffected by pressure blinding during the bleaching exposure. 4. Threshold following a small bleach may be lowered by adding a steady annular background to the region surrounding the bleached patch. Conversely, bleaching the area surrounding a small, steady background can lower threshold for a test flash centred on the background. 5. These interactions between backgrounds and bleaches may be explained if bleaches produce long-lasting signals from neurones in the bleached area, which then lead into a spatially opponent stage of processing. 6. It is likely that the persisting signals come from the cone receptors, since the Bunsen-Roscoe Law (intensity-time reciprocity) holds for small bleaches as well as large, for durations up to about 3 sec.

Two definitions of persistence in visual perception.

It is well known that the perception of a visual stimulus does not terminate with the offset of that stimulus, but decays with time. Although a variety of specific methods have been developed to study this phenomenon, these methods have not always yielded consistent results. We believe that some of the discrepancies in the literature on visual persistence have arisen because different researchers have based their estimates of persistence on qualitatively different features of the visual residual. Efron (1970) asked subjects to temporally align the onset of a click with the offset of a brief flash. Persistence, defined as the interval between flash offset and onset of the indexing click, was found to decrease with increasing flash intensity. Bowen, Pola, and Matin (1974) asked subjects to adjust the temporal placement of a probe flash so that it was coincident with the apparent offset of a spatially separated test flash. Persistence was defined as the interval between test flash offset and onset of the indexing flash. Again, measured persistence decreased with increasing test stimulus intensity. The same result has been reported by Hansteen (1971) using a procedure resembling that of Bowen et al. In a somewhat different approach, Haber and Standing (1969) presented a repetitive cycle in which a stimulus was followed by a blank interval. Subjects were asked to adjust the duration of the blank interval between successive stimulus presentations so as to produce a discontinuity. Once again, persistence, defined by the duration of the blank interval between successive stimuli, declined with increases in the intensity of the test stimulus. Other researchers, however, using different procedures, have reported that stimulus intensity has the opposite effect on persistence. In one of a series of experiments, Sakitt (1976) asked observers to temporally align the onset of a click with the disappearance of the residual (icon) of a briefly exposed letter array. Her measure of persistence, the temporal inter-

The membrane current of single rod outer segments.

1. Outer segments of individual rods in the retina of the toad, Bufo marinus, were drawn into a glass pipette to record the membrane current. 2. Light flashes evoked transient outward currents. The peak response amplitude was related to flash intensity by a Michaelis equation with half-saturating intensity about 1 photon mum-2. 3. The saturating response amplitude ranged up to 27 pA and corresponded closely to complete suppression of the steady inward current present in darkness. 4. For a given cell the saturating response amplitude varied linearly with the length of outer segment within the pipette. This is consistent with a uniform density of light-sensitive channels and negligible gradient of membrane potential along the outer segment. 5. Responses to bright flashes never showed the relaxation from an initial peak seen previously in intracellular voltage recordings, suggesting that the conductance change responsible for the relaxation does not occur in the outer segment. 6. Responses to local illumination of only the recorded outer segment were very similar to those obtained with diffuse light at the same intensity, indicating that peripheral rods made little contribution to the responses. 7. The spectral sensitivity of 'red' rods was consistent with a retinal1-based pigment with lambda max = 498 +/- 2 nm. 8. The kinetics of the response were consistent with four stages of delay affecting action of the internal transmitter. Responses were faster at the basal end of the outer segment than at the distal tip. 9. Background light reduced the sensitivity to a superposed dim test flash and shortened the time course of the response, indicating that adapting light modifies the kinetics and gain of the transduction mechanism within the outer segment. 10. Responses to dim lights exhibited pronounced fluctuations which are attributed in the succeeding paper (Baylor, Lamb & Yau, 1979) to the quantal nature of light.

Local foveal inhibitory effects of global peripheral excitation.

Global excitation produced by oscillating a peripheral square-wave grating back and forth through one-half cycle inhibits the visibility of an incremental test flash only when the flash is presented in the foveal region of the visual field. This finding is discussed in the context of the neurophysiological periphery effect and shift-effect and their possible role in saccadic suppression.

Similar threshold functions from contrasting neural signal models

Three neural signal models of increment threshold detection are compared. All assume that the criterion for threshold is the attainment of a critical, minimum neural signal (or difference between two neural signals), and that the signal due to a test flash of intensity λ in the absence of a background light is λ (λ + σ) (where σ is the semi-saturation constant). The models differ in the manner in which a background light of intensity θ is assumed to affect the signal. One model (due to Alpern, Rushton, and Torii, 1970a, b, c) assumes that the test flash signal, λ (λ + σ) , is attenuated by the multiplicative factor θ D (θ + θ D) (where θ D is a constant interpreted as sensory noise); another model specifies that the test flash signal is simply reduced (by subtraction) by the amount θ (θ + K) ( K a constant). One main result of this paper is that in the absence of pigment bleaching, these two models imply indistinguishable increment threshold functions. Further, a necessary and sufficient condition for each model guaranteeing the absence of saturation with steady backgrounds is found to be empirically satisfied. A third model is considered where the background field is assumed both to contribute to the neural signal and simultaneously to attenuate it (via a gain change). These assumptions are closely related to theoretical accounts of color induction and color perception. Though this model needs further investigation, it appears to be in better accord with actual increment threshold data than the others.

Transient threshold increase due to combined changes in direction of propagation and plane of polarization

A transient brightness increase occurs after a sudden change in direction of polarization of an otherwise steady retinal illuminance of 4.3 log td for all pupil entry positions except the point for which sensitivity is maximal (Stiles-Crawford I maximum). Simultaneously with this subjective change in brightness we find that the threshold energy of a superimposed test flash is raised. The threshold increase just after the transient change in direction of polarization is used as a measure of the magnitude of the effect. The effect is greater if the light enters the pupil more eccentrically. The increment threshold increase following a transient in direction of polarization was found to be comparable in magnitude to the change in increment threshold following a change in direction of propagation of the light. The magnitude of the effect in the case of simultaneous changes in direction of propagation and plane of polarization exceeds the magnitude of the effect for each change taken alone, regardless of the polarization properties and direction of polarization of the test flash. Enlarging the change in direction of propagation raises the magnitude of the effect. These facts bear on the controversy concerning mechanisms underlying the Stiles-Crawford I and Transient Stiles-Crawford Effects: is receptor disarray important, or are essentially single receptor collective mechanisms (modal patterns) responsible for one or the other or for both effects? The results support the disarray hypothesis less than they do the modal pattern hypothesis.