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We present a series of novel observations about interactions between flicker and motion that lead to three distinct perceptual effects. We use the term flicker to describe alternating changes in a stimulus’ luminance or color (i.e. a circle that flickers from black to white and visa-versa). When objects flicker, three distinct phenomena can be observed: (1) Flicker Induced Motion (FLIM) in which a single, stationary object, appears to move when it flickers at certain rates; (2) Flicker Induced Motion Suppression (FLIMS) in which a moving object appears to be stationary when it flickers at certain rates, and (3) Flicker-Induced Induced-Motion (FLIIM) in which moving objects that are flickering induce another flickering stationary object to appear to move. Across four psychophysical experiments, we characterize key stimulus parameters underlying these flicker-motion interactions. Interactions were strongest in the periphery and at flicker frequencies above 10 Hz. Induced motion occurred not just for luminance flicker, but for isoluminant color changes as well. We also found that the more physically moving objects there were, the more motion induction to stationary objects occurred. We present demonstrations that the effects reported here cannot be fully accounted for by eye movements: we show that the perceived motion of multiple stationary objects that are induced to move via flicker can appear to move independently and in random directions, whereas eye movements would have caused all of the objects to appear to move coherently. These effects highlight the fundamental role of spatiotemporal dynamics in the representation of motion and the intimate relationship between flicker and motion.

Processing speed, which can be measured behaviorally in various sensory domains, has been shown to be a strong marker of central nervous system health and functioning in adults. Visual temporal processing speed (measured via critical flicker fusion [CFF] thresholds) represents the maximum speed at which the visual system can detect changes. Previous studies of infant CFF development have been limited and inconsistent. The present study sought to characterize the development of CFF thresholds in the first year of life using a larger sample than previous studies and a repeated measures design (in Experiment 2) to control for individual differences. Experiment 1 ( n = 44 infants and n = 24 adults) used a cross-sectional design aimed at examining age-related changes that exist in CFF thresholds across infants during the first year of life. Adult data were collected to give context to infant CFF thresholds obtained under our specific stimulus conditions. Experiment 2 ( N = 28) used a repeated-measures design to characterize the developmental trajectory of infant CFF thresholds between three and six months of age, based on the results of Experiment 1. Our results reveal a general increase in CFF from three to four and one-half months of age, with a high degree of variability within each age group. Infant CFF thresholds at 4.5 months of age were not significantly different from the adult average, though a regression analysis of the data from Experiment 2 predicted that infants would reach the adult average closer to 6 months of age. Developmental and clinical implications of these data are discussed.

Uncomfortable images generally have a particular spatial structure, which deviates from a reciprocal relationship between amplitude and spatial frequency (f) in the Fourier domain (1/f). Although flickering patterns with similar temporal structure also appear uncomfortable, the discomfort is affected by not only the amplitude spectrum but also the phase spectrum. Here we examined how discomfort from flicker with differing temporal profiles also varies as a function of the mean light level and luminance contrast of the stimulus. Participants were asked to rate discomfort for a 17° flickering uniform field at different light levels from scotopic to photopic. The flicker waveform was varied with a square wave or random phase spectrum and filtered by modulating the slope of the amplitude spectrum relative to 1/f. At photopic levels, the 1/f square wave flicker appeared most comfortable, whereas the discomfort from the random flicker increased monotonically when the slope of the amplitude spectrum was shallow. This special status for the 1/f square wave condition was limited to photopic light levels. At the lower mesopic or scotopic levels, the effect of phase spectrum on the discomfort was diminished, with both phase spectra showing a monotonic change with the slope of the amplitude spectrum. We show that these changes cannot be accounted for by changes in the effective luminance contrast of the stimuli or by the responses from a linear model based on the temporal impulse responses under different light levels. However, discomfort from flicker is robustly correlated with judgments of the perceived naturalness of flicker across different contrasts and mean luminance levels.

Retinal motion can modulate visual sensitivity. For instance, low contrast drifting waveforms (targets) can be easier to detect when abutting the leading edges of movement in adjacent high contrast waveforms (inducers), rather than the trailing edges. This target-inducer interaction is contingent on the adjacent waveforms being consistent with one another – in-phase as opposed to out-of-phase. It has been suggested that this happens because there is a perceptually explicit predictive signal at leading edges of motion that summates with low contrast physical input – a ‘predictive summation’. Another possible explanation is a phase sensitive ‘spatial summation’, a summation of physical inputs spread across the retina (not predictive signals). This should be non-selective in terms of position – it should be evident at leading, adjacent, and at trailing edges of motion. To tease these possibilities apart, we examined target sensitivity at leading, adjacent, and trailing edges of motion. We also examined target sensitivity adjacent to flicker, and for a stimulus that is less susceptible to spatial summation, as it sums to grey across a small retinal expanse. We found evidence for spatial summation in all but the last condition. Finally, we examined sensitivity to an absence of signal at leading and trailing edges of motion, finding greater sensitivity at leading edges. These results are inconsistent with the existence of a perceptually explicit predictive signal in advance of drifting waveforms. Instead, we suggest that phase-contingent target-inducer modulations of sensitivity are explicable in terms of a directionally modulated spatial summation.

When two flickering sources are far enough apart to avoid low-level motion signals, phase judgment relies on the temporal individuation of the light and dark phases of each source. The highest rate at which the individuation can be maintained has been referred to as Gestalt flicker fusion [Van de Grind, W. A., Grusser, O. -J., & Lunkenheimer, H. U. (1973). Temporal transfer properties of the afferent visual system. Psychophysical, neurophysiological and theoretical investigations. In R. Jung (Ed.), Handbook of sensory physiology (Vol. VII/3, pp. 431-573). Berlin: Springer, Chapter 7] and this has been taken as a measure of the temporal resolution of attention [Verstraten, F. A., Cavanagh, P., & Labianca, A. T. (2000). Limits of attentive tracking reveal temporal properties of attention. Vision Research, 40, 3651-3664; Battelli, L., Cavanagh, P., Intriligator, J., Tramo, M. J., Henaff, M. A., Michel, F., et al. (2001). Unilateral right parietal damage leads to bilateral deficit for high-level motion. Neuron, 32, 985-995]. Here we examine the variation of the temporal resolution of attention across the visual field using phase judgments of widely spaced pairs of flickering dots presented either in the upper or lower visual field and at either 4 degrees or 14 degrees eccentricity. We varied inter-dot separation to determine the spacing at which phase discriminations are no longer facilitated by low-level motion signals. Our data for these long-range phase judgments showed that temporal resolution decreases only slightly with increased distance from center of gaze (decrease from 11.4 to 8.9 Hz between 4 degrees to 14 degrees ), and does not differ between upper and lower visual fields. We conclude that the variation of the temporal limits of visual attention across the visual field differs markedly from that of the spatial resolution of attention.

This study examined the subjective disappearance of a visual object induced by a neighboring flickering ring (Experiments 1 and 2), a set of four flickering dots (Experiment 3), and apparent motion (Experiment 4) as flickering flankers. Observers were asked to report whether a target disappeared during 10 s of stimulus presentation. We used the proportion of disappearance as a measure of performance. Interestingly, subjective disappearance was rarely observed when flickering flankers were presented with a separation of less than 0.5 degrees from the target. However, disappearance was observed when dynamic random-dot patterns were presented with a separation of less than 0.5 degrees from the target border (Experiment 5). Our results indicate that the flicker of flankers near the target disturbs target adaptation or attentional inhibition, causing persistent target representation in higher-order object selection, and resulting in non-disappearance of the target.

When the two eyes are presented with sufficiently different stimuli, the stimuli will engage in binocular rivalry. During binocular rivalry, a subject’s perceptual state alternates between awareness of the stimulus presented to the right eye and that presented to the left eye. There are instances in which competition is not eye-based, but instead takes place between stimulus features, as is the case in flicker and switch rivalry (F&S). Here we investigate another such instance, interocular grouping, using a Diaz–Caneja type stimulus in conjunction with synchronous stimulus flicker. Our results indicate that stimulus flicker increases the total duration of interocularly bound percepts, and that this effect occurs for a range of temporal flicker frequencies. Furthermore, the use of contrast-inversion flicker causes a decrease of total dominance duration of the interocularly bound percepts. We argue that different flickering regimes can be used to differentially stimulate lower and higher levels of visual processing involved in binocular rivalry. We propose that the amount of interocularly combined pattern-completed percept can be regarded as a measure of the level at which binocular rivalry is resolved.

In the ultimate stage of the Adelson-Bergen motion energy model [Adelson, E. H., & Bergen, J. (1985). Spatiotemporal energy models for the perception of motion. Journal of the Optical Society of America, 2, 284-299], motion is derived from the difference between directionally opponent energies E(L) and E(R). However, Georgeson and Scott-Samuel [Georgeson, M. A., & Scott-Samuel, N. E. (1999). Motion contrast: A new metric for direction discrimination. Vision Research, 39, 4393-4402] demonstrated that motion contrast-a metric that normalizes opponent motion energy (E(L)-E(R)) by flicker energy (E(L)+E(R))-is a better descriptor of human direction discrimination. In a previous study [Rainville, S. J. M., Makous, W. L., & Scott-Samuel, N. E. (2002). The spatial properties of opponent-motion normalization. Vision Research, 42, 1727-1738], we used a lateral masking paradigm to show that opponent-motion normalization is selective for flicker position, orientation, and spatial-frequency. In the present study, we used a superposition masking paradigm and compared results to lateral masking data, as the two masking types activate local and remote normalization mechanisms differentially. Although selectivity for flicker orientation and spatial frequency varied across observers, bandwidths were similar across lateral and superimposed masking conditions. Additional experiments demonstrated that normalization signals are pooled over a spatial region whose aspect ratio and size are consistent with those of local motion detectors. Together, results show no evidence of remote normalization signals predicted by broadband inhibitory models [(e.g.) Heeger, D. J. (1992). Normalization of cell responses in cat striate cortex. Visual Neuroscience, 9, 181-197; Foley, J. M. (1994). Human luminance pattern-vision mechanisms: Masking experiments require a new model. Journal of the Optical Society of America A-Optics and Image Science, 11, 1710-1719] but support a local normalization process whose spatial properties are inherited from low-level motion detectors.

The effects of transient spatial attention on temporal resolution were recently studied and compared with attentional effects on spatial resolution. It was found that in contrast to the attentional enhancement of spatial resolution, transient attention impairs temporal resolution. To account for these findings a novel attentional mechanism was suggested. This attentional mechanism facilitates parvocellular neurons at the attended location, which in turn inhibit the activity of magnocellular neurons at the same location [Yeshurun & Levy, Psychol. Sci. 14 (3) (2003a) 225]. The goal of the present study was to replicate the attentional decrement in temporal resolution (Experiment 1), and perform direct tests of this `parvo facilitation––magno inhibition' hypothesis. The employment of isoluminant stimuli (Experiments 2a and 2b) or a red background (Experiment 3) ensured that the parvo system was the main system mediating performance. Consequently, any parvo–magno inhibitory processes elicited by the attentional mechanism should only have a minor effect on performance. As predicted, these manipulations either significantly attenuated or completely eliminated the attentional decrement in temporal resolution. These findings provide direct support to the hypothesis that attention favors parvocellular over magnocellular neurons.

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.

Psychophysical experiments with stimuli oscillating concurrently in colour and orientation revealed an apparently paradoxical dissociation between the perceived simultaneity of stimulus changes and the perceptual pairing of the events demarked by those changes. When subjects were required to report whether changes in colour and orientation were simultaneous, judgements were generally accurate within ±10 ms. When subjects were required to report which colour was paired predominantly with which orientation, judgements showed a systematic temporal bias of up to 50 ms in favour of colour. This dissociation between different temporal judgements concerning the same stimulus sequence is not predicted by any of the current models of binding in conscious vision. We propose an account of these data based on the temporal response properties of colour- and orientation-selective model neurons such that the perceived pairing of visual attributes is modelled as the cross-correlation of time-varying neural response profiles and thus reflects both neuronal latencies and the rate of rapid adaptation rather than simply the temporal pattern of responses to stimulus transitions.

We investigated the effect that spatially coincident luminance increments (luminance pedestals) have on flicker thresholds at several eccentricities and target sizes. Luminance pedestals elevated flicker amplitude-thresholds more when stimuli were presented eccentrically, both at low (4 Hz) and high (20 Hz) temporal frequencies. Altering the size of the eccentric stimulus failed to equate central and eccentric thresholds at all pedestal amplitudes. Comparisons with flicker thresholds at various background luminances suggests that the increase in luminance-pedestal flicker thresholds peripherally is due to increased suppressive rod–cone interactions, increased effectiveness of luminous contrast on edge-sensitive flicker mechanisms, as well as increased gain in the light adaptation response.

The ability to accurately perceive the speed of moving objects is one of many visual functions that decline with age. One factor that may contribute to this is a deterioration in temporal processing speed. At present, there is a dearth of information concerning how this may occur in the central nervous system, particularly in the visual cortex. Thus, in the present study, we investigated the neural basis of speed and temporal processing in areas 17 and 18 of visual cortex in young and aged rats using either a moving bar of light or a series of flashing lights. Our results showed that the mean preferred speed of a moving bar of light was significantly reduced in aged as compared to young animals. We also found that cells recorded from young animals were able to entrain to a higher frequency of flashing light stimuli than those recorded from aged animals. In addition, we found no age-related differences between cortical fields. These results suggest an age-related difference in temporal processing speed at the level of visual cortex.

The blue-blindness (tritanopia) of the human foveola normally goes unnoticed but can be directly visualized by having observers view a flickering, monochromatic, short-wavelength field. The blue scotoma appears as a tiny dark spot in central vision, the visibility of which depends upon the wavelength of the field and the temporal frequency of modulation. Comparisons of fading times as a function of flicker frequency for the blue scotoma, foveal afterimages and optically stabilized images indicate a common time course, consistent with the hypothesis that perceptual filling-in of the foveal blue scotoma reflects the operation of neural processes similar to those involved in fading and regeneration of stabilized images.

Using the probed-sinewave paradigm, we explore the differences between increment and decrement probes across a range of frequencies (approx. 1–19 Hz). In this paradigm, detection threshold is measured for a small test probe presented on a large sinusoidally flickering background (at eight different phases). Probe thresholds are very similar for increment and decrement probes, but there is a very small (and systematic) difference: increment thresholds are usually slightly higher relative to decrement thresholds during the part of the cycle when the background's intensity is increasing. Although Wilson's (1997, Vis. Neuro., 14, 403–423) model substantially underestimates the size of this difference, it predicts some phase dependency. However, the existence of On- and Off-pathways in Wilson's model is not important for these predictions. A recent model by Snippe, Poot, and van Hateren (2000, Vis. Neuro., 17, 449–462) may be able to predict this result by using explicit contrast-gain control rather than separate On- and Off-pathways. Auxiliary experiments measuring the perceived polarity of the probe provide a further argument suggesting that separate On- and Off-pathways are not useful in explaining increment and decrement probe thresholds.

To evaluate the pattern of losses associated with glaucomatous injury in patients with progressing glaucoma, functional losses were examined in 14 patients with progressing glaucoma using tests for which detection should be selectively mediated by one of three psychophysical mechanisms. Red-on-white increments, blue-on-white increments and critical flicker frequency were used to isolate the responses of the red–green chromatic mechanism, the blue-on chromatic mechanism, and the high-frequency flicker achromatic mechanism. For our 3.1° circular stimuli, chromatic defects were found in a greater number of the patients with glaucoma than were achromatic defects. We evaluated these defects in terms of two existing hypotheses: preferential loss and reduced redundancy. The greater sensitivity to glaucomatous injury of chromatic tests, compared to achromatic tests, found in this and other studies and the apparent discrepancy between anatomical and psychophysical studies can be parsimoniously explained by differences in cortical summation of ganglion cell responses for the chromatic and achromatic pathways.

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 degrees ) 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 eta. 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 eta were monotonic functions of temporal frequency (f), such that log A(c) increased and log eta decreased linearly with log f. Remarkably, the increase in spatial integration area and the decrease in efficiency were just balanced, so A(c)(f)eta(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.

A spot that flickers at 16 Hz between two luminance levels (on a grey surround) has an appearance of metallic lustre, which we call ‘monocular lustre’. Binocular and monocular lustre were measured in comparable conditions by a rating procedure, and both were reported only when the light and dark values of the flickering (or binocularly fused) spot straddled the surround luminance, so that the spot was alternately brighter and darker than the surround. We attribute lustre to competition between ON and OFF visual pathways.

Interocular transfer of the motion aftereffect (MAE) has been extensively investigated for the purpose of analysing the binocularity of the underlying motion mechanism. Previous studies unanimously reported that the transfer of the classical static MAE is partial, but there is a controversy as to whether the transfer of the flicker MAE (MAE measured using counterphase gratings) is partial or perfect. To gain insight into the discrepancy between studies, we investigated whether the interocular transfer of the flicker MAE is influenced by the MAE measurement method, retinal eccentricity and attention. Our results showed that the transfer was perfect or nearly so when the MAE duration was measured in the central visual field with observers paying attention to the adaptation stimulus, but the transfer was partial when the MAE nulling strength was measured, when the MAE duration was measured in the peripheral visual field, or when the observers’ attention was distracted by a secondary task. These results not only resolve discrepancies between previous studies, but also suggest that the flicker MAE reflects adaptation at multiple stages in the hierarchical architecture of motion processing.

Using the method of limits, we measured spatial and temporal vision in 15 patients, aged 4–28 years, who had been monocularly deprived of patterned visual input during infancy by a dense cataract. All patients showed losses in both spatial and temporal vision, with greater losses in spatial than in temporal vision. Losses were smaller when there had been more patching of the non-deprived eye. The results indicate that visual deprivation has smaller effects on the neural mechanisms mediating temporal vision than on those mediating spatial vision.

We measured foveal flicker sensitivity with and without external added temporal noise at various levels of retinal illuminance and described the data with our model of flicker sensitivity comprising: (i) low-pass filtering of the flickering signal plus external temporal and/or quantal noise by the modulation transfer function (MTF) of the retina (R); (ii) high-pass filtering in proportion to temporal frequency by the MTF of the postreceptoral neural pathways (P); (iii) addition of internal white neural noise; and (iv) detection by a temporal matched filter. Without temporal noise flicker sensitivity had a band-pass frequency-dependence at high and medium illuminances but changed towards a low-pass shape above 0.5 Hz at low luminances, in agreement with earlier studies. In strong external temporal noise, however, the flicker sensitivity function had a low-pass shape even at high and medium illuminances and flicker sensitivity was consistently lower with noise than without. At low luminances flicker sensitivity was similar with and without noise. An excellent fit of the model was obtained under the assumption that the only luminance-dependent changes were increases in the cut-off frequency (fc) and maximum contrast transfer of R with increasing luminance. The results imply the following: (i) performance is consistent with detection by a temporal matched filter, but not with a thresholding process based on signal amplitude; (ii) quantal fluctuations do not at any luminance level become a source of dominant noise present at the detector; (iii) the changes in the maximum contrast transfer reflect changes in retinal gain, which at low to moderate luminances implement less-than-Weber adaptation, with a ‘square-root’ law at the lowest levels; (iv) the changes of fc as function of mean luminance closely parallels time scale changes in cones, but the absolute values of fc are lower than expected from the kinetics of monkey cones at all luminances; (v) the constancy of the high-pass filtering function P indicates that surround antagonism does not weaken significantly with decreasing light level.

The Talbot-Plateau law fails for flicker detected by the short-wavelength-sensitive (S) cones: a 30-40 Hz target, flickering too fast for the flicker to be resolved, looks more yellow than a steady target of the same average intensity. The color change, which is produced by distortion at an early compressive nonlinearity, was used to reveal a slightly bandpass S-cone temporal response before the distortion site and a lowpass response after it. The nonlinearity is probably a 'hard' nonlinearity that arises because the S-cone signal is limited by a response ceiling, which the mean signal level approaches and exceeds as the S-cone adaptation level increases. The nonlinearity precedes the combination of flicker signals from all three cone types.

When an high frequency grating of high retinal contrast is presented intermittently by modulating its contrast at constant average luminance, observers experience uniform field flicker, even if the grating is too fine to be resolved. For long and middle wavelength cones, this contrast-modulation flicker can be seen for fringe periods as small as the diameter of a cone [MacLeod & He (1993). Nature, 361, 256-258], implying no substantial neural spatial integration prior to the nonlinear site. We now report that the short-wavelength cone system, despite its greater spatial integration than the other cone systems, can generate contrast-modulation flicker at spatial frequencies as high as 50 cycles/deg, a value comparable with that of the other cone systems in the same retinal area. Spatial resolution at the nonlinear site is in all cases apparently limited by the size of the cones. Likewise, little temporal filtering (in the range up to 18 Hz) precedes the S-cone nonlinearity. This suggests that the reduced S-cone system sensitivity for rapid flicker is due to postreceptoral limitations.

Previous work shows that the red-green (RG) detection mechanism is highly sensitive, responding to equal and opposite long-wave (L) and middle-wave (M) cone contrast signals. This mechanism mediates red-green hue judgements under many conditions. We show that the RG detection mechanism also receives a weak input from the short-wave (S) cones that supports the L signal and equally opposes M. This was demonstrated with a pedestal paradigm, in which weak S cone flicker facilitates discrimination and detection of red-green flicker. Also, a near-threshold +S cone flash facilitates detection of red flashes and inhibits green flashes, and a near-threshold —S cone flash facilitates detection of green flashes and inhibits red flashes. The S contrast weight in RG is small relative to the L and M contrast weights. However, a comparison of our results with other studies suggests that the strength of the absolute S cone contrast contribution to the RG detection mechanism is 1/4 to 1/3 the strength of the S contribution to the blue-yellow (BY) detection mechanism. Thus, the S weight in RG is a significant fraction of the S weight in BY. This has important implications for the ‘cardinal’ color mechanisms, for it predicts that for detection or discrimination, the mechanisms limiting performance do not lie on orthogonal M-L and S axes within the equiluminant color plane.

The changes in apparent brightness or color, induced into a test spot by a surround, can be greatly enhanced either by flickering the test spot between two luminances, or by binocularly fusing a pair of test spots of different luminances. Simultaneous contrast, in which a white surround makes a grey spot look darker, is greatly enhanced if the spot (not the surround) flickers between black and white. Colour contrast is likewise enhanced by chromatic flicker: on a blue surround, a grey spot looks slightly yellowish, but a yellow/blue flickering spot looks strongly yellow. Temporal successive contrasts, or negative afterimages, are also enhanced by flickering the test field. The negative afterimage of a half-white, half-black rectangle looked dark grey and light grey when projected on a grey test field, but it looked almost black and almost white when projected on a test field that flickered between black and white. Coloured negative afterimages were also enhanced by projecting them on a chromatic flickering test field. We examined the combination rules for pairs of luminances which were presented either successively as flicker or else dichoptically (and fused binocularly). The brightness averaging functions for spatial increments (light spots) on dark surrounds were quasi-linear for binocular fusion but quadratic for flicker. For spatial decrements (dark spots) on white surrounds, the brightness averaging functions were strongly nonlinear winner-take-all for both binocular fusion and flicker. We also found temporal analogues of Fechner's [(1860). Elements of psychophysics. New York: Holt, Rinehart, Winston, 1966] paradox and Levelt's [(1965). British Journal of Psychology, 56, 1-13] dichoptic contour effect. We conclude that the visual rules for combining luminance excursions, whether in flicker or binocular fusion, favour disproportionately the spot with the higher contrast.

Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HFP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09 1 and 0.03 1 , compared with ratios in the range of 0.6 1 to 10 1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on at white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.

Progressive cone dystrophy (CD) is usually marked in the initial stages by reduced visual acuity, color vision deficiency and alterations in the photopic electroretinogram, while morphological alterations can be very mild; in some forms rods are affected in a later stage as well. We examined 40 patients with progressive cone dystrophy to determine the extent of functional losses in the cone system with psychophysical tests. A great variety of visual acuity and fundus alterations was found. Myopia was present in 74% of the patients. An autosomal dominant pattern of inheritance predominated (32%). No prevalence of gender was found. The age of onset ranged between 10 and 30 yr. All patients had progression of their symptoms. The total error score in color arrangement tests, the saturated Farnsworth Panel D-15 and the Farnsworth-Munsell 100-hue test, was pathologic with a predominance of confusions along the tritan and scotopic axis. Especially if visual acuity was below 0.5, color vision defects increased, but color vision defects were also found in patients with normal visual acuity. A general decrease of sensitivity in all three cone mechanisms was observed in measurements of spectral sensitivity. Moreover, cone-cone interaction as tested by transient tritanopia measurements was usually disturbed. In the dark adaptation function the threshold of the cone branch was usually elevated. These tests provide a good means to ascertain the correct diagnosis in early stages of the disease and to monitor progression in patients suffering from cone dystrophy.

The time course for the development of the temporal contrast sensitivity function in humans is uncertain. Some studies indicate that temporal contrast sensitivity is immature in infants. However, earlier work suggests that critical flicker fusion is adult-like by 2 months. We traced the development of temporal contrast sensitivity to uniform field flicker in 2-, 3- and 4-month-old infants using a modified preferential-looking technique that employed a rating scale. Two-month-old infants exhibited highest sensitivities at 1 and 2 Hz. Three- and 4-month infants exhibited peak sensitivity at 4 and 8 Hz, respectively. Overall temporal contrast sensitivity increased with age and the peak frequency shifted toward higher temporal frequencies. Using this paradigm, no infant subjects showed responses to 32 Hz, the highest temporal frequencies tested.