Cycles Deg Research Articles

Short-Term Memory for Speed

We have measured the retention of information about stimulus speed in visual short-term memory by measuring speed discrimination in a two-interval forced-choice task. We have also measured such discrimination in conditions where a ‘memory masker’ is presented during the interstimulus interval (ISI) in a fashion analogous to the experiment of Magnussen et al (1991 Vision Research31 1213 – 1219). Magnussen et al found that spatial frequency discrimination was disrupted when the mask had a spatial frequency that differed from the test spatial frequency by an octave or more. We have investigated the speed discrimination of 8 Hz, 1 cycle deg−1 drifting sine-wave gratings with the following drifting masks presented in the ISI: (i) 8 Hz 1 cycle deg−1, same direction as the test; (ii) 8 Hz, 8 cycles deg−1, opposite direction to the test; (iii) 8 Hz, 8 cycles deg−1, same direction as the test; (iv) 24 Hz, 3 cycles deg−1, same direction as the test. These masks were chosen to investigate whether the temporal frequency, the spatial frequency, the speed, or the direction of motion of the mask affected retention. We found that in none of these conditions was the discrimination of the test gratings impaired significantly. This pattern of results is therefore different from that found with spatial frequency discrimination and suggests that, whatever mechanism is responsible for the retention of information about speed, it is different from that responsible for the retention of information about spatial frequency.

Orientation Discrimination for Moving Isoluminant and Luminance Targets

Last year we presented orientation discrimination thresholds for stationary isoluminant red — green and luminance-defined stimuli as a function of spatial frequency and contrast (Wuerger and Morgan, 1995 Perception24 Supplement, 16). The horizontal profile of the stimuli was a Gabor function with a carrier frequency of either 0, 2, or 4 cycles deg−1. Observers made a binary decision (clockwise versus anticlockwise from vertical) and thresholds were defined as the standard deviation of their psychometric function. One main finding was that for carrier frequencies of 2 and 4 cycles deg−1, when stimuli of equal cone contrast are compared, orientation discrimination thresholds for red — green isoluminant stimuli do not differ significantly from the thresholds for luminance-defined stimuli. To further characterise the chromatic mechanisms involved in spatial vision, we assessed orientation discrimination thresholds for isoluminant red — green and luminance-defined Gabor stimuli as a function of the velocity of the moving target. When velocity is increased, orientation discrimination thresholds for isoluminant targets increase more rapidly than the thresholds for luminance targets of identical cone contrasts. We conclude that orientation discrimination is mediated by different chromatic mechanisms with different spatial and temporal sensitivities.

Simultaneously Judging Contrast in Different Orientation and Frequency Bands

Previous discrimination experiments suggest that suprathreshold contrast signals carried by orthogonally oriented gratings are combined when component frequencies are similar, but contrast signals are not combined across frequency bands regardless of orientation. We investigated neural and attentional processes underlying these findings in a concurrent-response paradigm. Test stimuli were composed of two superimposed sinusoidal gratings. In one condition, two 3 cycles deg−1 gratings were superimposed at orthogonal orientations to form plaids. In the other condition, the component gratings were 3 cycles deg−1 and 15 cycles deg−1, both vertical. In each condition, each component independently took one of two slightly different contrast values, combined all possible ways to create four stimuli. On each trial, one stimulus appeared for 1 s. Observers made two contrast discrimination judgments, one on the vertical (or low frequency) component, the other based on the horizontal (or high frequency) grating. Highly correlated response patterns and the collapse of two decision axes into nearly one confirmed that observers were unable to make independent judgments of contrast on orthogonally oriented components of similar frequency. Analyses suggested that decisions were based primarily on a single neural signal representing the sum or average of the two contrasts present. When cues to discrimination were in different frequency bands, observers showed a marked inability to perform the simultaneous judgment task, choosing idiosyncratic strategies to maximise performance. Analyses indicated that the contrast information was processed through separate and independent pathways, but that information from the two bands was not simultaneously available to the observer.

Perceptual Classification Images from Vernier Acuity Masked by Noise

Letting external noise rather than internal noise limit discrimination performance allows information to be extracted about the observer's stimulus classification rule. A perceptual classification image is the correlation over trials between the noise amplitude at a spatial location and the observer's responses. If, for example, the observer followed the rule of the ideal observer, the response correlation image would be an estimate of the ideal observer filter, the difference between the two unmasked images being discriminated. Perceptual classification images were estimated for a Vernier discrimination task. The display screen had 48 pixels deg−1 horizontally and vertically. The no-offset image had a dark horizontal line of 4 pixels, a 1 pixel space, and 4 more dark pixels. Classification images were based on 1600 discrimination trials with the line contrast adjusted to keep the error rate near 25%. In the offset image, the second line was one pixel higher. Unlike the ideal observer filter (a horizontal dipole), the observer perceptual classification images are strongly oriented. Fourier transforms of the classification images had a peak amplitude near 1 cycle deg−1 and an orientation near 25 deg. The spatial spread is much more than image blur predicts, and probably indicates the spatial position uncertainty in the task.

Functional Aspects of Border-Signalling Mechanisms

Multiple border-signalling mechanisms have been proposed in some models of object segregation. We investigated whether distinct functions of different border mechanisms could be isolated in human psychophysical tasks. The display was a low-spatial-frequency (1.5 cycles deg−1) sinusoidal grating. Two types of centre-surround stimuli were produced by (i) increasing or decreasing the mean luminance of a central patch of the grating to create a signed luminance border (LB), or (ii) by shifting the grating within the central patch 180° to create a sign-reversing border (PSB) without a change in mean luminance. Grating contrast was kept constant at 20%. Observers performed a spatial 2AFC task in each condition. The PSB stimulus served as the comparison stimulus. Test stimuli were either LB or PSB+LB. The mean luminance of the central test patches varied over trials to create border contrasts of −15% to 15% Michelson contrast. In separate sessions, subjects made four types of comparisons: which of the two central areas appeared (1) to differ more in depth from its surround; (2) to have greater modulation contrast of the sinusoid; (3) lighter/darker, (4) to have higher border contrast. Results depended strongly upon test border type and suggested that the mechanism responding to each has a distinct and different perceptual function, and the two may interact. Our results support the hypothesis that surface lightness is calculated by a sign-preserving mechanism, whereas a phase-insensitive mechanism is involved in percepts relating to segregation in depth.

Temporal and Spatial Frequency Tuning of the Flicker Motion Aftereffect

The motion aftereffect (MAE) was used to study the temporal-frequency and spatial-frequency selectivity of the visual system at suprathreshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (∼0.125 – 0.25 Hz). For slowly counterphasing test gratings (<∼2 Hz), the largest MAEs were found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 cycle deg−1). However, such narrow spatial frequency tuning was lost when the temporal frequency of the test grating was increased. The data suggest that MAEs are dominated by a single, low-pass temporal-frequency mechanism and by a series of band-pass spatial-frequency mechanisms at low temporal frequencies. At higher test temporal frequencies, the loss of spatial-frequency tuning implicates separate mechanisms with broader spatial frequency tuning.

The Effect of Surface Orientation on the Perception of Stereoscopic Corrugations

For most observers there is a pronounced orientational anisotropy in the perception of three-dimensional corrugated surfaces (Rogers and Graham, 1983 Science221 1409 – 1411; Bradshaw and Rogers, 1993 Perception22 Supplement, 117). Low-frequency corrugations which are oriented vertically have been found to have higher disparity modulation thresholds, the amount of perceived depth at suprathreshold levels is smaller, and typically they take longer to see than horizontally oriented corrugations. In the present experiments, the orientation of the corrugations was manipulated (from horizontal to vertical in 22.5 deg increments) to investigate the effect of surface orientation on both (i) a threshold detection task and (ii) a suprathreshold depth-matching task. The stimuli were 10 deg in diameter and were presented on two 12 inch monochrome monitors arranged to form a Wheatstone stereoscope. The surfaces were modulated in depth at four different corrugation frequencies (from 0.1 to 0.8 cycle deg−1 in octave steps). Thresholds were found to increase monotonically with increasing surface orientation from the horizontal: ∼2.5 arc s for horizontal corrugations to ∼10 arc s for vertical corrugations. The increase in thresholds was less marked for surfaces with higher corrugation frequencies. The rate of increase of threshold was greatest for surface orientations beyond 45°. A different pattern of results was found in the suprathreshold depth-matching task. Although the perceived depth in vertically oriented corrugations was significantly smaller (>50%) than for horizontally oriented corrugations, the largest amount of perceived depth was found for corrugated surfaces oriented at 45°. These results suggest that the disparity information used to process stereoscopic corrugations at threshold may be different from that used to process suprathreshold surfaces.

Is Coarse-to-Fine Tuning in Object Recognition One of Size or Scale?

Subjects attempted to detect a target object specified by name (eg “chair”) in rapid serial visual presentation sequences, with each sequence consisting of 40 gray level, 72 ms images of common objects. On the 50% of the trials in which a target was present it was never in the first or last eight positions. In homogeneous sequences, the images were all of the same size (all large or all small differing by a factor of five) or scale (spatial frequency, SF, either all low-passed [2 cycles deg−1] or all high passed [10 cycles deg−1], in a 1.5 octave band). There was no difference in detection accuracy between the different sizes or the different scales. In the switched sequences, the target could differ in size or scale from all the other images in the sequence. A strong asymmetry now emerged in that small-size or high-passed targets were much more difficult to detect than when in homogeneous sequences, whereas the large-size or low-passed images were much easier! Is this coarse-to-fine tuning based on size or scale? When size was pitted against scale, in that targets could either be of small size and low SF or large size and high SF, the large-size objects were much easier to detect, with only a slight modulating effect of scale. Size — scale tuning thus operates on space (={size}) rather than SF, and requires some time for its implementation, though less than that required for the first eight images. Once implemented, the tuning can be completely efficient, as evidenced by the equivalent levels of performance in the homogeneous conditions.

The Binocular Summation of Chromatic Contrast

The level of binocularity possessed by mechanisms sensitive to chromatic contrast is still unclear. Recent studies of stereopsis and chromatic contrast have suggested that stereopsis is maintained at isoluminance, although the contrast sensitivity and disparity ranges of chromatic stereopsis mechanisms are reduced compared to luminance stereopsis mechanisms. Rose, Blake, and Halpern (1988 Investigative Ophthalmology and Visual Science29 283 – 290) hypothesised a link between binocular summation (ie the superiority of binocular detection over monocular detection) and stereopsis. Is this link maintained with heterochromatic isoluminant stimuli? To address this question, the binocular and monocular contrast thresholds for the detection of 0.5 cycle deg−1 Gabor patches were measured. The stimuli possessed different relative amounts of colour and luminance contrast ranging from isoluminance (red/green) to isochrominance (yellow/black) through intermediate values. It was found that, with these stimuli, binocular detection was well modelled by assuming independent mechanisms sensitive to chromatic contrast and luminance contrast. Furthermore, with isoluminant stimuli, levels of binocular summation were above those expected from probability summation between the eyes, thus providing evidence for binocular neural summation within chromatic detection mechanisms. Given that stereoscopic depth identification is impossible at contrast detection threshold with isoluminant heterochromatic stimuli, these results suggest that the link between stereopsis and levels of binocular neural summation may not be a particularly strong one. These results also provide clear evidence for the binocularity of chromatic detection mechanisms.

Strength of Motion Aftereffect Varies with Segregation of Test Field and Surround

It is widely accepted that the motion aftereffect (MAE) results from the adaptation of visual motion detectors. However, recent work suggests that how the effects of that adaptation are expressed (the nature of the perceived MAE) depends on the nature of the inducing and test fields. We investigated how the strength of the MAE varied with the nature of the boundary between the test field and the surround. The surround (18.5 deg wide × 13.5 deg high) to the adapting and test fields was an area of vertical square-wave grating of 0.7 cycle deg−1. During adaptation, vertical stripes of the same spatial frequency as the background moved horizontally at a speed of 2 deg s−1 for 14 s within a central rectangular window of 9.7 deg wide × 7.6 deg high. At the end of adaptation, one of six different test fields was presented in the central window. In three of these, the stationary test stripes were exactly aligned with the surrounding stripes, and in the other three they were offset by half a stripe width. For two of these conditions (one aligned, one offset), a black outline was drawn around the edge of the adapting window (and so was visible only where it crossed white areas), and for two others (one aligned, one offset) the outline was red, and so visible in its entirety. The strength of MAEs in twelve subjects was assessed both by ratings at an auditory signal which occurred 0.5 s after the end of adaptation and also by measurement of their durations. There was good agreement between these two measures. MAEs were significantly stronger on the offset than on the aligned test fields. The presence of an outline increased MAE strength compared with no outline, but these outline effects were much weaker than those of offsetting the test stripes from the surround. We suggest that the MAE depends in part on the presence of a visually separable test pattern to which motion may be allocated.

Do Monocular Tilt and Spatial Frequency Aftereffects Induce the Binocular Perception of Inclination and Slant?

Prolonged viewing of a set of tilted lines can affect the perceived orientation of a second set of lines with a different physical orientation (tilt aftereffect). Similarly, prolonged viewing of a set of lines of a particular spatial periodicity can affect the perceived periodicity of a second set of lines with a different physical periodicity (spatial frequency shift aftereffect). We investigated whether a binocular difference resulting from monocular tilt or spatial frequency aftereffects could induce the perception of 3-D inclination or slant respectively. Observers adapted to monocular patterns (5 deg in diameter) arranged in a vertical ‘dumbbell’ configuration in dichoptic alternation. The adapting patterns differed in either orientation (±6.25° or ±11.25°) or in spatial frequency (±0.5 or ±0.75 octaves) from a test surface comprising vertical lines at 4 cycles deg−1. The period of adaptation was 3 min. Observers judged whether the test surfaces appeared to (i) form a convex or concave hinge in depth (after adaptation to tilt), or (ii) to slant in opposite directions about a vertical axis (after adaptation to periodicity). Using a relative slant/inclination judgment may be more sensitive than depth matching or nulling of a single surface (eg Sloane and Blake, 1987 Perception & Psychophysics42 569 – 575). Our results suggest that (i) differences in perceived periodicity in separate monocular images do not induce the impression of stereoscopic slant (confirming the results of Sloane and Blake) and (ii) differences in perceived orientation in separate monocular images do not induce an impression of stereoscopic inclination.

Reaction Time to Gratings: A Re-Examination

We have examined the relationship between the reaction time for detecting a sinusoidal grating stimulus and the stimulus variables of spatial frequency, contrast, window size, and uncertainty with respect to spatial frequency. Detection was measured in a two-alternative spatial-forced-choice procedure. The stimuli were horizontal cosine gratings windowed spatially by two-dimensional Gaussians. Spatial frequency was varied from 0.7 to 6.5 cycles deg−1 and contrast was varied from 0.054 to 0.673. The standard deviation of the Gaussian window was fixed in some conditions and the number of cycles presented in each window covaried with spatial frequency. In other conditions, window size was varied, along the vertical axis only, to hold the number of cycles constant. Contrasts were always randomly intermixed, but frequencies were intermixed in some conditions and blocked in others. We confirm previous findings that reaction time increases as spatial frequency increases and decreases as contrast increases. We also confirm and extend the proposal of Rudd that reaction time closely approximates a single function of the product of contrast and the square of the grating period. We consider the implications of these findings for the nature of the physiological mechanisms which govern reaction time.

Do Monocular Tilt and Spatial Frequency Aftereffects Induce the Binocular Perception of Inclination and Slant?

Prolonged viewing of a set of tilted lines can affect the perceived orientation of a second set of lines with a different physical orientation (tilt AE). Similarly, prolonged viewing of a set of lines of a particular spatial periodicity can affect the perceived periodicity of a second set of lines with a different physical periodicity (spatial frequency shift AE). We investigated whether a binocular difference resulting from monocular tilt or spatial-frequency aftereffects could induce the perception of 3-D inclination or slant, respectively. Observers adapted to monocular patterns (5 deg in diameter) arranged in a vertical ‘dumbbell’ configuration in dichoptic alternation. The adapting patterns differed in either orientation (±6.25° or ±11.25°) or in spatial frequency (±0.5 or ±0.75 octaves) from a test surface comprising vertical lines at 4 cycles deg−1. The period of adaptation was 3 mins. Observers judged whether the test surface appeared to (i) form a convex or concave hinge in depth (after adaptation to tilt) or (ii) to slant in opposite directions about a vertical axis (after adaptation to periodicity). Using a relative slant/inclination judgment may be more sensitive than depth matching or nulling of a single surface (eg Sloane and Blake, 1987 Perception & Psychophysics42 569 – 575). Our results suggest that (i) differences in perceived periodicity in separate monocular images do not induce the impression of stereoscopic slant [confirming the results of Sloane and Blake (1987)] and (ii) differences in perceived orientation in separate monocular images do not induce an impression of stereoscopic inclination.

Orientation Discrimination at Isoluminance

Colour and form are important attributes of the objects in our visual environment. We tested the hypothesis that colour and orientation are processed independently in the visual system. Orientation perception for stimuli defined by luminance is characterised by a decrease in thresholds with increasing stimulus contrast, and by a strong oblique effect: stimuli along the horizontal or vertical axes are discriminated more easily. We determined orientation discrimination thresholds for stationary, slow (1 Hz) or fast (8 Hz) moving sine-wave gratings (1 cycle deg−1) defined by luminance or isoluminant (red — green) contrast. In a 4AFC paradigm, three of four stimulus patches were identical and the fourth differed in orientation, contrast, or in both. When we measured orientation discrimination thresholds as a function of stimulus contrast, thresholds decreased for all stimuli with increasing contrast. At all temporal frequencies the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings. On a cone-contrast metric, thresholds for stationary and slowly moving stimuli were consistently lower for isoluminant compared to luminance stimuli. For fast-moving stimuli orientation thresholds were similar for both kinds of gratings. For both types of stimuli a marked oblique effect was observed. To characterise processing of contrast and orientation completely, we measured simultaneous thresholds for contrast and orientation. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating similar rules for combining differences in contrast and orientation. We conclude that processing of isoluminant and luminance stimuli undergoes the same neural processing at least for the low spatial frequencies used here.

Evoked Magnetic Field Elicited by Motion and Motion Aftereffect

Motion aftereffect (MAE) is a negative aftereffect caused by prolonged viewing of visual motion: after gazing at a moving grating for a while, a stationary image will appear to move in the opposite direction (Ashida and Osaka, 1995 Vision Research35 1825). Evoked magnetic field (magnetoencephalogram: MEG) was measured on a human subject observing visual motion and MAE. Magnetic evoked field (80 averagings) was measured from 37 points over occipital and parietal areas (Magnes SQUID biomagnetometer, BTi) during watching a horizontally moving sinusoidal grating with low spatial frequency (2 cycles deg−1 with 5 Hz: motion condition) and immediately after stopping the moving grating (MAE condition). Dipole estimates based on equal magnetic field contour suggest that the main loci subserving visual motion and MAE appear to be the surrounding region over occipital and parietal areas in the human brain. Further analysis is now underway. In general, this appears to be in good agreement with another study using fMRI-based MAE measures [Tootell et al, 1995 Nature (London)375 139] in which a clear increase in activity in these areas was observed when subjects viewed MAE.

Bloch's Law for Contrast Increment Detection: The Effects of Pedestal Contrast and Light Level

The duration over which contrast detection improves (Bloch's regime) decreases with increasing light level and is often thought to reflect the temporal characteristics of the visual system. There is also some evidence to suggest that the temporal characteristics of the visual system might also change with increasing contrast level (M A Georgeson, 1987 Vision Research27 765 – 780). Here we compare temporal summation for stimuli presented on a blank field or on a high contrast background. On each trial a test grating was presented for X ms with the use of a spatial-alternate forced-choice procedure. The test grating (2 cycles deg−1) was presented superimposed on a similar pedestal grating which was also present for 500 ms prior to and after the test grating. Pedestal contrasts of 0% and 32% were tested at mean luminance levels of 150 cd m−2 and 1.5 cd m−2. The results show that both increasing light level and increasing contrast level resulted in smaller temporal summation times. In the current conditions both these effects approximately halve the summation time such that for a stimulus of low light level and of low pedestal contrast the summation time was ∼60 ms; low light, high contrast ∼30 ms; high light, low contrast ∼30 ms; and high light, high contrast ∼15 ms. The results imply that the temporal response of the visual system quickens with increasing contrast.

Simulation of Cortex Visual Cells for Texture Segmentation: Foveal and Parafoveal Projections

The objective of this work is to simulate visual cortical cells, their sensitivities to frequencies and orientations, and their part in texture segmentation. The simulation of these cells is realised through band-pass, oriented filters (Gabor filters), and multiresolution image decomposition. By this means, the filter sensitivities represent cell sensitivities to preferred orientations according to their frequency and orientation bandwidths, and multiresolution represents the different band frequencies. For texture analysis and segmentation, overlaying of band-pass filters is necessary to completely cover the Fourier domain. A continuous sensitivity to frequency and orientation is achieved by the filters overlapping and consequently by their interpolation. We used here four octave frequency bands from 1 to 16 cycles deg−1 and six orientations per band. The results obtained for texture segmentation with these parameters are very promising (up to 97% recognition rate) [Guérin-Dugué and Palagi, 1994 Neural Processing Letters1(1) 25 – 29]. The images analysed cover a multitude of different domains such as psychophysical tests and natural textures of different roughness. In order to create a cortical cell representation closer to neurophysiological data, and to improve texture segmentation results, we represent cell sensitivities by their foveal and parafoveal projections [R L DeValois, K K DeValois, 1988 Spatial Vision (Oxford: Oxford Science Publications)]. Cells receiving projections from the foveal zone are modeled by five octave frequency bands (from 0.5 to 16 cycles deg−1) and six orientations. Cells receiving projections from the parafoveal zone have the same sensitivities but are modeled by four octave frequency bands (from 0.5 to 8 cycles deg−1). By using these two different resolutions, preliminary tests have shown the capability of detecting textured regions by the parafoveal projection and localisation of boundaries by the foveal projection.

Spatial Filter Combination in Human Pattern Vision: Channel Interactions Revealed by Adaptation

Above threshold, two superimposed sinusoidal gratings of the same spatial frequency (eg 1 cycle deg-1), of equal moderate contrast (eg C1 = C2 = 6%), and with orientations of +/- 45 degrees, usually look like a compound structure containing vertical and horizontal edges (ie a blurred checkerboard). These feature orientations are very different from the dominant filter orientations in a wavelet-type (eg simple-cell) transform of the stimulus, and so present a serious challenge to conventional models of orientation coding based on labelled linear filters. Previous experiments on perceived structure in static plaids have led to the view that the outputs of tuned spatial filters are combined in a stimulus-dependent way, before features such as edges are extracted. Here an adaptation paradigm was used to investigate the cross-channel interactions that appear to underlie the spatial-filter-combination process. Reported are two aftereffects of selective adaptation: (i) adaptation to a 1 cycle deg-1 plaid whose component orientations are intermediate to those in a 1 cycle deg-1 test plaid 'breaks' perceptual combination of the components in the test plaid; (ii) adapting to a 3 cycles deg-1 plaid whose component orientations match those in a 1 cycle deg-1 test plaid facilitates perceptual combination of the components in the test plaid. The results are taken as evidence that spatial channels remote from those most responsive to a test plaid play a crucial role in determining whether the test plaid segments or coheres perceptually.

Evidence of spatial and temporal channels in the correlational structure of human spatiotemporal contrast sensitivity.

1. The statistical correlation of detection thresholds for pairs of stimuli should be higher for stimuli detected by the same mechanism than for stimuli detected by different mechanisms--a property that can be used to probe the visual mechanisms that underlie detection. 2. Correlation of contrast sensitivities for pairs of spatiotemporal stimuli is approximately a linear function of spatial or temporal frequency separation in octaves. Using the slope of this function as an index of neural processing gave results consistent with: more spatial mechanisms than temporal; more spatial mechanisms at low temporal frequencies than at high; and at least two temporal mechanisms active at spatial frequencies up to 22.6 cycles deg-1. 3. This method of analysing sensitivity data is insensitive to experimental conditions and applicable to any sensory detection task mediated by tuned channels. In addition to being applicable to psychophysical sensitivity measurements, it may also be useful in analysing some kinds of electrophysiological measurements that pool the responses from many active mechanisms (such as evoked potentials).

Retinal and cortical evoked responses to chromatic contrast stimuli. Specific losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis.

It is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits (Ishihara). Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/(red + green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and green-black). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg-1) modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However, the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast, suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour, suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.