[Assessment of a pupillometric method for the screening of glaucoma]. / Évaluation d'une méthode pupillométrique pour la détection du glaucome.

BACKGROUND: Glaucoma is a progressive optic neuropathy, remaining asymptomatic for a long time, which makes its early diagnosis difficult. Visual field testing is still the gold standard but is less than ideal. The goal of this study is to assess a pupillometric test, administered passively to the subject for one minute, to measure its sensitivity and specificity in the classification of healthy eyes and glaucomatous eyes, and to evaluate its tolerability compared to visual field testing. METHODS: Forty-five participants were included in this single-center, interventional, prospective study. They underwent 3 monocular pupillometric tests with light stimulation: 6 pupillary responses were recorded during full-field multifocal stimulation (performed twice) and pupillary hippus cycle study. RESULTS: Analysis of spectral power and pupillary measurements with full-field multifocal stimulation provides a 0.94 sensitivity and a 0.88 specificity, a virtually perfect discrimination for early stages of glaucoma. Analysis of pupil cycle time provides a 0.92 sensitivity and a 0.88 specificity for early stages. Acceptability of this test by patients is superior to visual field testing. CONCLUSION: These results show that data from our pupillometric recordings provide a good classification of healthy and glaucomatous eyes and must be confirmed on a larger population.

Shape and motion interactions at perceptual and attentional levels during processing of structure from motion stimuli.

This study uses a rapid-serial-visual-presentation (RSVP) paradigm to test the extent to which shape and motion direction can be independently accessed and processed during the perception of structure-from-motion (SFM) stimuli. Subjects reported the number of occurrences of shape or motion direction during RSVP sequences of 3D-SFM stimuli. Overall, performance was better for motion than shape. In the motion task, observers were less accurate when the motion direction was repeated revealing a repetition blindness (RB) effect. In addition, the repetition of shape, although irrelevant to the motion task, resulted in increased performance, without change in RB rate. In contrast, there was no RB at the group level in the shape task and the repetition of the irrelevant motion direction had no effect on the performance. A closer look at the data showed that observers fall in two statistically distinct groups for the shape task. Some observers (N = 6) show a repetition advantage (RA) while the others (N = 5) show a repetition blindness (RB) effect. No behavioral differences between groups could be found for the motion task. The implications of these results for models of SFM processing are discussed in the light of the type/token theory (N. Kanwisher, 2001).

Form constraints in motion binding.

Visual analyses of form and motion proceed along parallel streams. Unified perception of moving forms requires interactions between these streams, although whether the interactions occur early or late in cortical processing remains unresolved. Using rotating outlined shapes sampled through apertures, we showed that binding local motions into global object motion depends strongly on spatial configuration. Identical local motion components are perceived coherently when they define closed configurations, but usually not when they define open configurations. Our experiments show this influence arises in early cortical levels and operates as a form-based veto of motion integration in the absence of closure.

An intracellular study of space and time representation in primary visual cortical receptive fields.

In contrast with previous knowledge based on extracellular recordings, the recent development of intracellular techniques in vivo (sharp electrode or 'blind patch') ideally allows experimenters to analyze and dissect the contribution of feedforward and lateral connectivity in the functional expression of a synaptic 'integration field'. We will present recent data which demonstrate that the visual receptive field of cortical neurons described at the level of subthreshold synaptic events extends over much larger regions of the visual field than previously thought, and that the capacity of cells to amplify subthreshold responses depends on the immediate past history of their membrane potential. Our data suggest that visual cortical receptive fields should not be considered as a fixed entity but more as a dynamic field of integration and association. Two types of dynamics can be argued for: 1) the spatial structure of the minimal discharge field (defined by suprathreshold activation of the cell) can be profoundly reorganized at least during development and most probably during selective phases of learning under the control of activity-dependent mechanisms. Adaptive changes in visual responses are thought to reflect long-lasting potentiation and/or depression of synaptic efficacies conveying ON- and OFF-center information; and 2) during sensory processing, reconfiguration of synaptic weights may be achieved on a much faster time-scale and linked to nor-linear properties of the postsynaptic membrane as well as that of recruited networks. Association of information available in the central part of the receptive field (RF) and of input coming from the reputedly 'unresponsive' regions surrounding it, or arising simultaneously from different parts of the visual field, might be suppressive in certain cases and capable of boosting hidden responses in other cases, depending on the global stimulus configuration.

The visual cortical association field: a Gestalt concept or a psychophysiological entity?

The receptive field of a visual neurone is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. Intracellular recordings in cat area 17 show that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing (dominant excitation) responses decrease in strength for stimuli that are flashed at increasing distances away from the centre of the discharge field, while their onset latency increases. A detailed spatio-temporal analysis of these electrophysiological data shows that subthreshold synaptic responses observed in the 'silent' surround of cortical receptive fields result from the intracortical spread of activation waves carried by slowly conducting horizontal axons within primary visual cortex. They also predict that a perceptual facilitation may occur when feedforward activation produced by the motion signal in the retina travels in phase in the primary visual cortex with the visually induced spread of horizontal activation. A psychophysical correlate has been obtained in humans, showing that apparent motion produced by a sequence of co-linear Gabor patches, known to preferentially activate V1 orientation selective cells, are perceived by human observers as much faster than non co-linear sequences of the same physical speed.

Automatic access to object identity: attention to global information, not to particular physical dimensions, is important.

The authors examined whether, by attending to physical properties of objects, participants can prevent the activation of semantic information. Participants received a reference object followed by a display containing both a matching target and a distractor. In Experiments 1 and 2, participants attended to motion and to surface texture, respectively. Some evidence for the processing of semantic information occurred. This result contrasted with a previous study in which no evidence for semantic information processing was apparent in a color matching task (M. Boucart & G.W. Humphreys, 1994). In Experiment 3, pictures were used with outline contours composed of randomly distributed red and green dots, one color being overrepresented. Participants matched pictures according to the dominant color. Evidence for semantic processing emerged. The authors suggest that these results cannot be explained in terms of attention operating differently on separate physiological channels. Instead it is proposed that what is crucial in activating stored object representations is whether the global configuration of the picture is processed.

Recovery from contrast adaptation: effects of spatial and temporal frequency.

The time-course of the recovery from adaptation to drifting gratings was estimated as a function of the spatio-temporal characteristics of the stimulus. A new method was used, in which the response latencies for the detection of contrasts presented during the recovery were measured. An exponential function provides a good description of the recovery. The initial (i.e. at the beginning of the recovery period) and asymptotic values of this function depend on the temporal frequency but not on the spatial frequency of the adapting stimulus. The time constants increase with high spatial frequency and follow a U-shaped function of the temporal frequency of the adapting stimulus.

Motion integration with dot patterns: effects of motion noise and structural information.

To better understand how local motion detectors merge their responses so as to permit the global determination of objects' movements in the visual field, direction discrimination of performance was measured using a flexible class of moving dots--two sets of dots translating sinusoidally 90 deg out of phase along orthogonal axes. When dots' velocities are combined, a global motion along a circular trajectory emerges, clockwise or counter-clockwise depending on the sign of the phase lag. However, the results of the present experiments indicate that dot patterns are segregated into distinct, but interacting, streams when each dot motion can be accurately determined. In contrast, perceptual coherence of the global motion occurs when each local motion signal is "blurred" by a "motion noise". Direction discrimination performance then increases regularly with both noise amplitude and noise frequency, i.e., noise speed. Performance also increases when relative motion between dots is added. Testing different dot configurations indicates that performance is better for spatial arrangements that display structural properties (a square shape), as compared to overlapping random distributions. Interestingly, when the delay between stimulus onset and motion onset increases up to 300 msec, performance improves when dot patterns convey come form of structural organization but not when the dots are distributed at random. Relations of these results to existing models of motion integration are considered.

Veridical perception of global motion from disparate component motions.

Although it is in principle possible to determine the direction of motion of an object by combining the motion of its one-dimensional oriented contours (Fennema CL, Thompson WB. Comput. Graph. Image Processing 1979;9:301-315) there is still much debate on whether human observers can do so. The Intersection Of Constraint (IOC) rule proposed by Adelson and Movshon (Adelson EH, Movshon JA. Nature 1982;300:523-525), although compatible with the veridical object's motion, was challenged by recent psychophysical data obtained with type II plaids or lines moving behind apertures: perceived direction of motion is biased toward the vectorial average of the component motions, rather than in the direction predicted by the IOC rule. Since the velocity predicted by the vectorial rule is inconsistent with the physical velocity, its use leads to the puzzling prediction that the perceived position of a moving object becomes inconsistent with its actual position. In the present paper, the perceived path of a figure defined by its one-dimensional contours and moving behind apertures along a circular trajectory is compared with the discrepant predictions of the IOC and of the Vectorial model. The results show that the perceived path is close to veridical with these stimuli, therefore challenging the idea that the visual system uses a vector averaging rule.

Perceptual bistability with counterphase gratings.

Suprathreshold counterphase modulated gratings induce a bistable percept of drift or flicker. It is argued that these perceptual alternations might provide a new means for the investigation of directional selective mechanisms. The prevalance of either of the two perceptions has been studied as a function of the spatio-temporal characteristics of the stimulus and compared with: (1) the spatio-temporal contrast sensitivity surface for counterphase modulated gratings; (2) the motion/counterphase sensitivity ratio. Drift perception elicited by suprathreshold counterphase gratings attains a maximum for 8 c/deg, 12 Hz stimuli and decreases for any other experimental condition. For spatial frequencies below 1 c/deg, or temporal frequencies below 2 Hz, only flicker perception is reported. These phenomenal experiences do not show any systematic dependence on the involuntary eye movements of the observer. Comparison with the threshold measurements does not support their explanation in terms of the transient-sustained dichotomy, nor does it allow for a straightforward equivalance between the spatio-temporal characteristics of direction-selective mechanisms at threshold and at suprathreshold levels. It is suggested that the balance between flicker and motion is the perceptual outcome of the competition between lower and higher order motion detectors.

The influence of terminators on motion integration across space.

Individual motion measurements are inherently ambiguous since the component of motion parallel to a homogeneous translating edge cannot be measured. Numerous models have proposed that the visual system solves this ambiguity through the integration of motion measurements across disparate contours. To examine this proposal, subjects observed a translating diamond through four stationary apertures. Since the diamond's motion could not be determined from any single contour, motion integration across contours was required to determine the diamond's direction of motion. We demonstrate that observers have difficulty accurately integrating motion information across space. Performance improved when the diamond stimulus was presented at 7 degrees eccentricity, through jagged apertures, or at low contrast. Taken together, these results imply that integration across space is more likely when the motion of contour terminators is less salient or reliable.

Effects of a benzodiazepine, lorazepam, on motion integration and segmentation: an effect on the processing of line-ends?

Previous studies have shown that the perceptual integration of component motions distributed across space is inhibited whenever segmentation cues, such as line-ends, are salient. Herein, we investigate to what extent enhanced inhibition induced by lorazepam, a benzodiazepine facilitating the fixation of GABA on GABAA receptors, modifies the balance between motion integration and motion segmentation at the behavioural level. Motion integration was tested in 16 healthy volunteers taking a single and oral dose of either placebo or lorazepam (0.038 mg kg-1). The stimulus consisted of an outlined diamond presented behind four, otherwise invisible, apertures and translating along a circular trajectory (Lorenceau & Shiffrar (1992). Vision Research, 32, 263-273). Under these conditions, recovering the global diamond direction requires the integration of the component motions available within each aperture. The observers were asked to discriminate the global, clockwise or counter-clockwise, diamond direction under difficult--at high luminance contrasts--or easy--at low luminance contrasts--conditions. Overall, reaction times and error rates increased in the lorazepam group as compared to the placebo group, suggesting strong non-specific effects. However, the changes in performance in the lorazepam group are not homogeneous across conditions, suggesting that lorazepam also induces specific effects that modulate the integration/segmentation balance. Additional experiments performed with visible apertures or visible diamond vertices indicate that the effects of lorazepam are unlikely to reflect a deficit of motion processing or motion integration mechanisms since performance is only slightly impaired in the lorazepam as compared to the placebo group under these conditions. These results suggest that lorazepam might specifically modulate the saliency of line-ends, presumably because processing these features involves inhibitory mechanisms using GABA as a neuromediator, and in turn modify the balance between motion integration and segmentation.

Effects of a static textured background on motion integration.

We studied how the visual system integrates locally ambiguous velocities into global unambiguous coherent motion in the presence or absence of a textured background. Line drawings of complex figures were presented through invisible (i.e. same luminance and hue as the background) circular apertures such that only straight line segments were visible. These figures were either presented against a uniform background or embedded in static textures made of similar line segments in such a way that figures cannot be detected if they remain static. Under our experimental conditions, the figures translated clockwise or counterclockwise along a circular path and observers were required to discriminate the global direction of motion. Because of the aperture problem, a single moving segment cannot disambiguate the global direction of the figures and integration across multiple line segments is therefore necessary to perform the task. We found that with figures at high contrast, the presence of a texture enhanced direction discrimination, while direction discrimination of figures at low contrast was impaired by the presence of the texture. These paradoxical effects of a static texture were further tested by manipulating the relative contrast between figures and texture, the motion onset asynchrony (the delay between stimulus onset and motion onset or MOA), the density, the orientation and the distribution of texture elements. The effects of the texture, either facilitation or suppression, increase with texture contrast. Accuracy improves with MOA and decreases with texture density. In general, at high figure contrast, accuracy is better whenever referents are present in the image. We suggest that facilitation by the texture at high figure contrast is accounted for by reduced salience of segmentation cues such as line terminators and increased accuracy of local velocity measurements. On the other hand, decreased performance at low figure contrast may reflect lateral suppression of the responses to motion signals by the texture.

Increased motion linking across edges with decreased luminance contrast, edge width and duration.

Accurate interpretations of image require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images.

The inverse intensity effect is not lost with stimuli in apparent motion.

The inverse relationship between the visible persistence of a briefly presented stimulus and its intensity is well established for static displays. However, with non-static displays, this relationship is only partially reported by previous studies. In order to clarify this topic, we investigated the effect of luminance on the visible persistence of a stimulus in apparent motion. Assuming that persistence duration is a normally distributed random variable, we studied whether the mean persistence of a stimulus could be systematically varied by varying its luminance. Our paradigm permits evaluation of this effect without changing the temporal interval between two successive presentations of the stimulus, thus avoiding the potential influence of this latter factor on persistence. Our results show that the inverse intensity effect still occurs at each of the successive locations of a stimulus in apparent motion. In addition, we provide evidence that increasing the spatial separation between the successive presentations, and decreasing the background luminance, result both in longer persistence duration. Altogether, these findings favour the hypothesis that persistence is actively suppressed by inhibitory interactions between adjacent neural zones.

Motion integration across differing image features.

To interpret the projected image of a moving object, the visual system must integrate motion signals across different image regions. Traditionally, researchers have examined this process by focusing on the integration of equally ambiguous motion signals. However, when the motions of complex, multi-featured images are measured through spatially limited receptive fields, the resulting motion measurements have varying degrees of ambiguity. In a series of experiments, we examine how human observers interpret images containing motion signals of differing degrees of ambiguity. Subjects judged the perceived coherence of images consisting of an ambiguously translating grating and an unambiguously translating random dot pattern. Perceived coherence of the dotted grating depended upon the degree of concurrence between the velocities of the grating terminators and dots. Depth relationships also played a critical role in the motion integration process. When terminators were suppressed with occlusion cues, coherence increased. When dots and gratings were presented at different depth planes, coherence decreased. We use these results to outline the conditions under which the visual system uses unambiguous motion signals to interpret object motion.

Different motion sensitive units are involved in recovering the direction of moving lines.

We studied direction discrimination for lines moving obliquely relative to their orientation. Manipulating contrast, length and duration of motion, we found systematic errors in direction discrimination at low contrast, long length and/or short durations. These errors can be accounted for by a competition between ambiguous velocity signals originating from contour motion processing units and signals from line terminator processing units. The dynamic of this competition can be described by a simple model involving two different classes of processing units with different contrast thresholds, different integration time constants and different levels of response saturation.

Perceived speed of moving lines depends on orientation, length, speed and luminance.

In this study, the perceived speed of a tilted line translating horizontally (for a duration of 167 msec) is evaluated with respect to a vertical line undergoing the same translation. Perceived speed of the oblique line is shown to be underestimated when compared to the vertical line. This bias increases: (1) when the line is further tilted, (2) with greater line lengths, (3) with lower contrasts, and finally (4) with a speed of 2.1 deg/sec as compared to a higher speed of 4.2 deg/sec. These results may be accounted for by considering that two velocity signals are used by the visual system to estimate the speed of the line: the translation of this line (this signal does not depend on the line's orientation) and the motion component normal to the line (this signal depends on orientation). We suggest that these two signals are encoded by different types of units and that the translation signal is specifically extracted at the line endings. We further suggest that these signals are integrated by a weighted average process according to their perceptual salience. Other interpretations are considered at the light of current models dealing with the two-dimensional integration of different velocity signals.