'Saccadic suppression'- no need for an active extra-retinal mechanism.
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Motion Perception↗
Biomedical subjects
E Castet
Publications and source records attributed to E Castet.
Motion perception during saccadic eye movements.
During rapid eye movements, motion of the stationary world is generally not perceived despite displacement of the whole image on the retina. Here we report that during saccades, human observers sensed visual motion of patterns with low spatial frequency. The effect was greatest when the stimulus was spatiotemporally optimal for motion detection by the magnocellular pathway. Adaptation experiments demonstrated dependence of this intrasaccadic motion percept on activation of direction-selective mechanisms. Even two-dimensional complex motion percepts requiring spatial integration of early motion signals were observed during saccades. These results indicate that the magnocellular pathway functions during saccades, and that only spatiotemporal limitations of visual motion perception are important in suppressing awareness of intrasaccadic motion signals.
Acoustic Stimulation↗
The extrinsic/intrinsic classification of two-dimensional motion signals with barber-pole stimuli.
The perceived direction of different barber-pole stimuli was assessed by adjusting an arrow on the screen. The terminator ratio (TR: number of terminators moving along the long side divided by the number of terminators moving along the small side) was either one or three. In this latter case, the aperture orientation was either vertical or horizontal. The grating was either in the same plane as the aperture (intrinsic condition) or behind the aperture--the frame containing the aperture had a crossed disparity relative to the grating--(extrinsic condition). A nested design with 120 observers was used for the whole study. Five grating orientations were intermingled within any session. With a terminator ratio of three, the results depend strongly on the aperture's orientation. When the rectangular aperture is horizontal, the perceived direction of an intrinsic grating is horizontal (the typical barber-pole illusion), whereas it is only slightly biased towards orthogonal one-dimensional (1D) motion signals (Vp) in the extrinsic condition. When the aperture is vertical, the perceived direction in the intrinsic condition is largely biased toward Vp, and on average it is close to Vp in the extrinsic condition. In this latter case, however, analysing the distributions of responses shows that many responses do not lie around Vp but are clustered near vertical or horizontal. This motion capture depends on the grating's orientation. With a terminator ratio of one, motion capture is present in both the extrinsic and intrinsic conditions. Moreover, a global bias toward horizontal is observed: this horizontal bias is much larger in the extrinsic condition. Altogether, these results suggest that binocular disparity alone is a weak determinant of the extrinsic/intrinsic classification of two-dimensional (2D) motion signals compared to the occlusion cues provided by unpaired regions in binocular images. Second, truly extrinsic 2D motion signals are not suppressed but rather actively compete against each other to capture the 1D motion signals. This results in a perceptual multistability which is much stronger with extrinsic signals. Finally, given the inherent multistability of barber-pole stimuli, high-level factors can alter the strength of this competition and prime any of the 2D motion signals.
Discrimination, Psychological↗
Long-range interactions in the spatial integration of motion signals.
When a sinewave grating is moving within a cross-shaped aperture, a strongly multi-stable phenomenon is perceived. The percept switches between the coherence of an extended surface moving in a single direction and the segregation of two patterned strips sliding across each other in directions parallel to the branches of the cross. We studied how the balance between these two percepts is affected by the length of the arms and by the shape of their ends. We report here that human observers report the segregation into two surfaces more often when the branches of the cross are extended, and when the small sides of the arms are oriented parallel to the grating. Two kinds of early motion signals interact in the crossed barber-pole stimulus: (a) the signals extracted in the middle of the bars are ambiguous with regard to their direction, and usually would be interpreted as motion normal to the grating orientation; (b) the signals from regions where the grating is intersected by the borders of the aperture convey motion signals in direction of the border. Our results show that the global appearance of our display can be dramatically influenced by the reliability of motion signals located in small regions that may be separated by large distances. To explain this long-range effect, we tentatively propose the existence of a representation level situated between the extraction of low-level local signals and the final global percept. The postulated processing level is concerned with the segmenting of the entire image into surfaces that are likely to belong to the same object, even if they are not contiguous in space. This hypothetical mechanism involves the construction of coarse-scale 'patches' from the local motion signal distributions, each carrying a single velocity associated with a certain degree of reliability. Our experiments indicate that the probability of grouping together similar patches depends on their respective reliabilities.
Fixation, Ocular↗
Visual search for a tilted target: tests of spatial uncertainty models.
We report that spatial cueing of a parafoveal target in the presence of distractors enhances orientational acuity for that target. When no distractors were present, orientation thresholds were in the range 1-4 degrees. For long exposure times, distractors increased threshold by the amount predicted from a winner-takes-all spatial uncertainty model. For short (100-msec) exposures followed by a random dot mask, the rise in threshold with distractors was considerably greater than that predicted from spatial uncertainty. For brief exposures the effect of distractors was greater when the target and distractors were spatially crowded rather than widely spaced. Adding a tilt to the distractors in the opposite direction to the target increased thresholds still further. Cueing the target with a spatial pointer decreased the effect of distractors, even when they were crowded. We suggest that when attention cannot be appropriately focused, discrimination is carried out by a relatively coarse texture analyser, which averages over several elements, and that focused attention permits the analysis of the target over a smaller area of space.
Attention↗
Perception of moving lines: interactions between local perpendicular signals and 2D motion signals.
An oblique line translating vertically behind a horizontal rectangular aperture is perceived as moving in the horizontal direction, i.e., in the line-ending's direction. When a feature is added on the line, and thus provides a vertical unambiguous motion signal, the line's perceived direction is still horizontal. In parallel, the feature appears to slide obliquely along the line (Wallach, 1935). We first show that this finding which we call "the sliding effect" is robust and easy to replicate for different orientations of the rectangular aperture (up to about 20 deg from vertical). This effect also occurs with an invisible circular aperture. In this case, using an adjustment task, observers have great difficulty extracting the actual direction of the feature (a gap or a dot on the line). Instead, a systematic bias towards the direction of "sliding" is observed. This misperception of the feature's velocity is markedly reduced or even suppressed when the circular aperture is outlined or when a visible circle is drawn around this invisible aperture. The line's perceived direction is always roughly orthogonal to the line's orientation regardless of the presence of a visible circular outline. This latter result is important because it shows that the feature on the line does not disambiguate the perpendicular signals extracted along the contour, even in conditions where this feature's motion is almost correctly perceived. Altogether, these results suggest that line-endings are not used by the visual system in the same way as a feature on the line when it comes to determining the line's perceived direction.
Female↗
The aperture problem in stereopsis.
Stereoacuity was determined for gratings and gabor patches as their orientation was varied. Acuity was constant for 1 c/deg and 2 c/deg gratings over the range 0-80 deg when it was expressed as positional shift at right angles to the grating orientation. The same was true of an 8 c/deg elongated gabor patch. However, the threshold disparity at right angles to the major axis of an oriented gaussian patch rose as it was tilted away from the vertical. Also, thresholds for a circularly symmetrical gaussian patch rose steeply with the angle of the disparity away from the horizontal. Disparities of the centroids of 4 c/deg gabor patches could be detected equally well at angles of 0 and 70 deg, independently of the angle of the carrier grating. The data indicate a variety of rules for stereoscopic matching. Large-field gratings are matched by detecting orthogonal phase shifts, or alternatively phase shifts along the horizontal axis. Smaller patches of grating are matched by their centroids, independently of their angle. Small gaussian patches are difficult to match in any direction other than along the horizontal axis. The difference between gaussian patches and 4 c/deg gabor patches with the same envelope argues against the involvement of a second-order rectifying non-linearity preceding matching.
Depth Perception↗
Apparent speed of type I symmetrical plaids.
The apparent speed of plaids made up of two gratings having the same spatial frequency and the same speed was evaluated (symmetrical type I). The plaids were moving vertically as defined by the intersection-of-constraints (IOC) rule with a mean duration of 300 msec. The comparison-stimulus was a horizontal line moving vertically. The main goal of the study was to test the effect of the spatial frequency of the Distortion Product (DP). Here the DP velocity is identical to the IOC velocity. The main effect is that reducing the DP spatial frequency from 4 to 1 c/deg decreases apparent speed. A smaller effect is due to the speed of the components: when this speed becomes relatively smaller than the IOC speed, apparent speed of the plaid decreases. Finally, the DP temporal frequency seems to determine the upper limit (about 16 Hz) beyond which the plaid appears as a non-rigid moving and flickering pattern.
Differential Threshold↗
Stereoscopic depth perception at high velocities.
The view of the world from different perspectives provided by the two eyes is used by the human visual system to compute the relative distances and solid shapes of objects. However, the traditional theory of binocular disparity takes little account of the fact that a moving target will stimulate many different sets of disparate points in the two eyes with a range of temporal delays. Here we show that stereoacuity for periodic grating is not degraded by velocities of up to 640 degrees s-1 provided that they do not move at a greater rate than 30 cycles s-1. The minimum detectable spatial phase difference between the eyes was equivalent to a spatial phase difference of about 5 degrees and an interocular temporal delay as small as 450 microseconds. We suggest that stereopsis for moving targets is accomplished by neurons having a spatial-temporal phase shift in their receptive fields between the eyes.
Depth Perception↗
Apparent speed of sampled motion.
Perceived speed was measured for stimuli moving unidirectionally in apparent motion with different sampling steps. The stimuli were displayed at successive locations for very brief durations (on-time = 1 msec). The basic result is an elevation of apparent speed produced by increasing the sampling step. This speed-up effect is maximal at low speeds (2 deg/sec), then progressively decreases with higher speeds until it disappears at medium velocities (8 deg/sec). In addition, the speed-up observed at low speeds declines when the ontime is gradually increased from 1 msec to larger values, the largest one corresponding to "staircase motion". These results are consistent with models assuming that speed-encoding is based on an antagonistic comparison of the activity in two broadly tuned temporal filters (low-pass and band-pass). The high temporal frequencies introduced by motion-sampling would activate the band-pass filter relatively more and would thus produce an overestimation of apparent speed.
Humans↗
Effect of the ISI on the visible persistence of a stimulus in apparent motion.
The persistence of briefly flashed stimuli undergoing a horizontal apparent motion is assessed as a function of the temporal interval (inter-stimulus interval or ISI) between successive locations. The main result is that the duration of persistence is increased when the ISI is reduced (within the range 1-15 msec). An increase of persistence also occurs when the spatial separation (delta chi) between successive presentations of the moving stimulus becomes larger, a well established result which is replicated here. In both cases, the elevation of persistence suggests that inhibitory processes, which are assumed to underlie the persistence-suppression, have become less efficient. According to the data, it seems that the spatio-temporal parameters of motion, and not the speed as such, are responsible for the strength of inhibition. Namely, optimal inhibition, and thus suppression, would need a minimum amount of time to take place, and would improve with proximity (i.e. with smaller delta chi). Finally, a persistence-suppression decrease is observed when the angular size of the flashed stimuli is reduced (i.e. when higher spatial frequencies become more predominant). A model of transient-on-sustained inhibition accounts well for these results.
Afterimage↗
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.
Differential Threshold↗
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.
Afterimage↗
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.
Computers↗
Temporal dynamics of motion integration for the initiation of tracking eye movements at ultra-short latencies.
The perceived direction of a grating moving behind an elongated aperture is biased towards the aperture's long axis. This "barber pole" illusion is a consequence of integrating one-dimensional (1D) or grating and two-dimensional (2D) or terminator motion signals. In humans, we recorded the ocular following responses to this stimulus. Tracking was always initiated at ultra-short latencies (approximately 85 ms) in the direction of grating motion. With elongated apertures, a later component was initiated 15-20 ms later in the direction of the terminator motion signals along the aperture's long axis. Amplitude of the later component was dependent upon the aperture's aspect ratio. Mean tracking direction at the end of the trial (135-175 ms after stimulus onset) was between the directions of the vector sum computed by integrating either terminator motion signals only or both grating and terminator motion signals. Introducing an elongated mask at the center of the "barber pole" did not affect the latency difference between early and later components, indicating that this latency shift was not due to foveal versus peripheral locations of 1D and 2D motion signals. Increasing the size of the foveal mask up to 90% of the stimulus area selectively reduced the strength of the grating motion signals and, consequently, the amplitude of the early component. Conversely, reducing the contrast of, or indenting the aperture's edges, selectively reduced the strength of terminator motion signals and, consequently, the amplitude of the later component. Latencies were never affected by these manipulations. These results tease apart an early component of tracking responses, driven by the grating motion signals and a later component, driven by the line-endings moving at the intersection between grating and aperture's borders. These results support the hypothesis of a parallel processing of 1D and 2D motion signals with different temporal dynamics.
Eye Movements↗