Perceptual Latency Research Articles

The modulation of the flash-lag effect by voluntary attention.

In the flash-lag effect (FLE), a flashing object appears to lag behind a moving object when both happen to be physically aligned to each other. According to an earlier account of the FLE (Baldo and Klein 1995 Nature 378 565-566), this perceptual phenomenon would result from differential delays in the perceptual processing of moving and flashing stimuli, presumably involving attentional mechanisms. Here, we have attempted to demonstrate in a more convincing way the participation of voluntary attention as a major component of the FLE. In experiment 1 the observer's attentional set was induced by the spatial probability structure of the visual stimulus. A flashing dot (relative to which the location of a moving dot should be judged) was presented, in separate blocks, at fixed, alternating, or randomly chosen locations. The two former conditions, providing a higher spatial predictability, yielded a smaller FLE than the latter condition, which provided a lower spatial predictability of the flashing dot. In experiment 2 we employed a standard cueing procedure, in which a participant was instructed to shift covertly his/her attentional focus according to a symbolic cue. The cue indicated, with a validity of 80%, the visual hemifield at which the flashing dot would be presented. As predicted by our conceptual model, the mean magnitude of the FLE in the valid trials was significantly smaller than that found in the invalid ones. Therefore, both experiments provided strong evidence supporting the participation of voluntary attention in the FLE. Attentional mechanisms should be seen not as the primary cause of the FLE, but rather as an important modulatory component of a broader process whose spatiotemporal dynamics engenders the FLE and possibly other related phenomena. Even though we elected an account based on the influence of attention on perceptual latencies, our empirical findings are compatible with other theoretical models embraced by the current flash-lag controversy and should be accommodated by every attempt to explain this perceptual phenomenon.

Evidence against response bias in temporal order tasks with attention manipulation by masked primes.

Attending to a location shortens the perceptual latency of stimuli appearing at this location (perceptual latency priming). According to attentional explanations, perceptual latency priming relies on the speeded transfer of attended visual information into an internal model. However, doubts about the attentional origin have repeatedly been raised because efforts to minimize response bias have been insufficient in most studies. Five experiments investigated the contribution of a response bias to perceptual latency priming (judgment bias due to the two-alternative forced-choice method and due to the existence of the prime, criterion effects or second-order bias, sensorimotor priming). If any, only small response biases were found. The results thus support the attentional explanation.

The fluttering-heart illusion: a new hypothesis.

The fluttering-heart illusion is a perceived lagging behind of a colour target on a background of a different colour when the two are oscillated together. It has been proposed that the illusion is caused by a differential in the perceptual latencies of different colours (Helmholtz 1867/1962), a differential in rod-cone latencies (von Kries 1896) and rod-cone interactions (von Grünau 1975, 1976 Vision Research 15 431-436, 437-440; 16 397-401; see list of references there). The purpose of this experiment was to assess the hypothesis that the fluttering-heart illusion is caused by a differential in the perceived velocities of chromatic and achromatic motion. To evaluate this hypothesis, we tested observers possessing normal colour vision and deuteranopes. The perceived delay of a chromatic target relative to an achromatic target was measured as a function of background cone contrast and target colour. For observers with normal colour vision, the perceived delay of the chromatic target is greater in the L-S than the L-M testing conditions. The reverse is observed in deuteranope observers. We suggest that this is caused by the absence of an L-M opponent mechanism contributing to chromatic motion in deuteranopes. Greater background cone contrasts tended to yield smaller perceived delays in both normal and deuteranope observers, indicating that greater chromatic modulation decreases the perceived delay of the colour target. These results support the hypothesis that the fluttering-heart illusion can be explained by a differential in the perceived velocities of chromatic and achromatic motion.

Direct parameter specification of an attention shift: evidence from perceptual latency priming

In the direct parameter specification (DPS) mode of sensorimotor control, response parameters can be specified by stimuli that are not consciously perceived [Psychological Research/Psychologische Forschung 52 (1990) 207]. DPS is contingent on the current intentions. The invisible stimuli can be processed for the purposes of sensorimotor control only if they match the actual intentions, for example, share task-relevant features. The present experiments explore whether attentional capture by masked abrupt-onset stimuli is mediated via DPS. Participants judged which of two visual targets appeared first. Masked primes preceded one of the targets. The primes were either similar to the targets or not, in shape, or in color. Target-like (task-relevant), but not distractor-like (task-irrelevant), primes facilitated perceptual latencies of targets trailing at their positions. Thus, the latency effects resulted from DPS of an attention shift, rather than from bottom-up capture or from top-down search for dynamic features.

Temporal parameters and time course of perceptual latency priming

Visual stimuli (primes) reduce the perceptual latency of a target appearing at the same location (perceptual latency priming, PLP). Three experiments assessed the time course of PLP by masked and, in Experiment 3, unmasked primes. Experiments 1 and 2 investigated the temporal parameters that determine the size of priming. Stimulus onset asynchrony was found to exert the main influence accompanied by a small effect of prime duration. Experiment 3 used a large range of priming onset asynchronies. We suggest to explain PLP by the Asynchronous Updating Model which relates it to the asynchrony of 2 central coding processes, preattentive coding of basic visual features and attentional orienting as a prerequisite for perceptual judgments and conscious perception.

Leading, but not trailing, primes influence temporal order perception: further evidence for an attentional account of perceptual latency priming.

Presenting a masked prime leading a target influences the perceived onset of the masking target (perceptual latency priming; Scharlau & Neumann, in press). This priming effect is explained by the asynchronous updating model (Neumann, 1982; Scharlau & Neumann, in press): The prime initiates attentional allocation toward its location, which renders a trailing target at the same place consciously available earlier. In three experiments, this perceptual latency priming by leading primes was examined jointly with the effects of trailing primes in order to compare the explanation of the asynchronous updating model with the onset-averaging and the P-center hypotheses. Experiment 1 showed that an attended, as well as an unattended, prime leads to perceptual latency priming. In addition, a large effect of trailing primes on the onset of a target was found. As Experiment 2 demonstrated, this effect is quite robust, although smaller than that of a leading prime. In Experiment 3, masked primes were used. Under these conditions, no influence of trailing primes could be found, whereas perceptual latency priming persisted. Thus, a nonattentional explanation for the effect of trailing primes seems likely.

Response offset: the first sign of change?

The nature of the relationship between the timing of neural activity (brain time) and the perceived timing of the events represented by that activity (event time) has long been a matter of debate among philosophers. It is tempting to assume a direct relationship between neuronal-onset latencies and corresponding perceptual latencies. However, the perceived timing of a change from stimulus A to stimulus B could, in principle, be based on either the offset of the response to stimulus A or the onset of the response to stimulus B – if indeed there is any direct relationship between brain time and event time at all.

Perceptual latencies to discriminate surface orientation in stereopsis.

The difference in sensitivity to stereoscopic surfaces oriented horizontally or vertically (the stereoscopic orientation anisotropy) can be redescribed as a difference in sensitivity to shear or compression transformations that relate the binocular images. The present experiment was designed to test this by dissociating the image transformation from the orientation of the surface. Surfaces were presented in isolation or in the presence of a surrounding frame that formed step and gradient discontinuities in the disparity field. Without discontinuities, observers required considerably more time to discriminate between surfaces differing in compression than between those differing in shear, irrespective of surface orientation. Disparity discontinuities facilitated the perception of the disparity gradients; minimum stimulus durations were reduced by over an order of magnitude when the reference frame was present. These results support the hypothesis that the disparity field is decomposed into different primitives during the recovery of depth and surface structure.

Visual attention and metacontrast modify latency to perception in opposite directions

In human observers, cue-induced visual attention (‘bottom-up’ transient focal attention) shortens the latency of perception. Metacontrast reduces the intensity of perception and can even obliterate it. We show that a close relationship exists between both, but that their effects are reversed: cue-induced visual attention not only shortens latency but also intensifies perception, and metacontrast not only lowers intensity of perception but also prolongs latency. A common neurophysiological mechanism for both is possible. Indirect evidence suggests that this could be a subthreshold modulation of neuronal thresholds by de- and hyperpolarization.

Ecologically invalid monocular texture leads to longer perceptual latencies in random-dot stereograms.

Two experiments were conducted to explore Gillam and Borsting's (1988, Perception 17 603-608) report that uncorrelated monocular texture facilitates stereopsis by shortening the latency to see depth in random-dot stereograms. Experiment 1 used stereograms similar, in pattern but not disparity, to Gillam and Borsting's with monocular texture present or absent. A third condition, where monocular texture was dissimilar to the binocular panels and background, was also used. We were unable to generalize the findings of Gillam and Borsting for a depth step of 6 min of arc to a larger depth step of 24 min of arc. That is, we observed no significant difference in latencies between the conditions with monocular texture absent and present at a disparity of 24 min of arc. We found latencies to be significantly longer in the monocular-texture-different condition than the monocular-texture-absent condition, however. We account for this, ad hoc, by arguing that the monocular-texture-different stereogram depicts a rare or 'accidental' visual scenario. This account was supported by the results of experiment 2 which showed that stereograms depicting accidental views yielded longer latencies than those depicting generic views. We conclude that the ecological validity of monocular texture must also be considered when assessing the effects of monocular texture on stereopsis.

Shifts in perceived position following adaptation to visual motion

Where do we perceive an object to be when it is moving? Nijhawan [1] has reported that if a stationary test pattern is briefly flashed in spatial alignment with a moving one, the moving element actually appears displaced in the direction in which it is moving. Nijhawan postulates that this may be the result of a mechanism that predicts the future position of the moving element so as to compensate for the fact that the element will have moved position from the time at which the light left it to the time at which the observer becomes aware of it (as a result of the finite time taken for neural transmission). There is an alternative explanation of this effect, however. Changes in the stimulus presentation could affect perceptual latency [2], and therefore the perceived position if in motion (as suggested for the Pulfrich pendulum effect [3,4]). In other words, if the flashed probe of the Nijhawan demonstration takes longer to reach perceptual awareness than the moving stimulus, the latter will appear to be ahead of the probe. Here, I demonstrate an alternative way of testing this hypothesis. When an illusory movement is induced (via the motion aftereffect) within a stationary pattern, it can be shown that this also produces a change in its perceived spatial position. As the pattern is stationary, one cannot account for this result via the notion of perceptual lags.

Multiple Visual Latency

It has long been known that a dark visual stimulus is seen later than a bright one, with a delay up to several 10s of milliseconds. Systematic studies of various phenomena demonstrating this delay have revealed that the perceptual latency decreases monotonically as the stimulus intensity increases. Because latencies measured by psychological methods and cortical evoked responses are very similar to electroretinogram latencies, it has become a common belief that there is little in the intensity-dependent latency function that cannot be explained by retinal processes. In this study, we report evidence that there is no one absolute visual delay common to the whole visual system, but rather that the delay varies considerably in different perceptual subsystems. The relative visual latency was found to be considerably shorter in the task involving detecting the direction of movement than in other perceptual tasks that presume visual awareness of the beginning or temporal order of visual events.

Multiple Visual Latency

It has long been known that the time taken to detect a dim visual stimulus is longer than that to detect a bright one, with a relative delay of up to several tens of milliseconds. Systematic studies of various phenomena demonstrating this delay have revealed that the perceptual latency decreases monotonically as the stimulus intensity increases. Because latencies measured by psychological methods and cortical evoked responses are very similar to electroretinogram latencies, it has become a common belief that there is little in the intensity-dependent latency function that cannot be explained by retinal processes. We report evidence that there is not one absolute visual delay common to the whole visual system, but that the delay varies considerably in different perceptual subsystems. The relative visual latency was found to be considerably shorter in a movement-discrimination task than in other perceptual tasks which presume visual awareness of the onset of visual events, or of their temporal order.

Assessing the Role of Vergence Changes in the Perception of Random-Dot Stereograms by Using Open-Loop Control of Vergence

The fact that the 3-D shape of surfaces depicted by random dot stereograms can take several seconds or even tens of seconds to appear has been attributed to the failure to make appropriate vergence changes [B Julesz, 1971 Foundations of Cyclopean Perception (Chicago, IL: University of Chicago Press)]. Alternatively, the long latencies could be a consequence of the processing time needed to match the disparate images. To distinguish between these possibilities we measured perceptual latencies in a situation in which vergence changes had no effect on retinal disparities. To do this, horizontal eye movements were recorded with the aid of close-fitting scleral search coils in both eyes and the difference signal used to shift horizontally the two halves of a random-dot stereogram by equal and opposite amounts. When the amount of shift was equal to the magnitude of the vergence change, changes of vergence had no effect on the pattern of disparities—open-loop vergence. Three observers were presented with a sequence of stereograms depicting both ‘simple’ surfaces (a single square lying in front of the surround) and ‘complex’ surfaces, including spirals, ‘wedding cakes’, and saddle shapes under both normal and open-loop conditions. Under open-loop conditions, the complete 3-D shape was never perceived when the disparity range of the stereogram was large (>40 min arc), demonstrating the necessity of vergence changes, but the 3-D structure of ‘complex’ surfaces did build up over a period of several seconds indicating a separate disparity processing limitation.

Simple reaction time and perception of temporal order: dissociations and hypotheses.

Simple motor reaction time and judgment of temporal order are commonly recognized as two methods for estimation of perceptual latency. Unfortunately, the results obtained by the methods under the same conditions do not agree. We review hypotheses attempting to explain the disagreement. Although some of these seem to be promising, no one at present could be fully accepted.

The effect of a moderate dose of alcohol on the traffic hazard perception profile of young drink-drivers.

Hazard perception latency has been identified as one source of individual differences in road accidents, but alcohol's effects on hazard perception has not been addressed thoroughly. Furthermore, individuals convicted of driving while impaired (DWI), in comparison with other drink-drivers, have been found to possess a poor driving record, suggesting that they may also respond poorly to hazards. Therefore, this research studied young drivers across the spectrum of drink-driving practices, from non drink-drivers to DWI offenders. It examined alcohol's effects on their hazard perception profile, including aspects of both driving skill (hazard perception latency) and driving style (the perceived level of risk in hazards). Thirty-two subjects aged 18-25 years underwent two experimental conditions in a counterbalanced design: no alcohol and moderate alcohol. Alcohol was found to affect both driving skill and driving style. With a 0.05% BAC subjects took longer to detect hazards and responded to them in a more abrupt manner and these effects were particularly pronounced for DWI offenders. The results also supported a distinction between active hazards (hazards arising from the driver's own actions) and passive hazards (hazards arising from the actions of other road users). Irrespective of their drink-driving practices, subjects perceived active hazards as less dangerous than passive hazards. Furthermore, compared with other drink-drivers, DWI offenders perceived less risk during passive hazards (with a 0.05% BAC) and active hazards (when sober). It is suggested that these effects may underlie, at least in part, the increase in accident risk associated with impaired driving.

Perceptual latency and complex random-dot stereograms.

Naive observers of random-dot stereograms depicting complex surfaces often find that they require several tens of seconds before the impression of depth emerges. With practice, however, perception times often decrease markedly: perceptual learning occurs. Current explanations of these effects were assessed in two experiments. In the first experiment the perception times of naive observers for random-dot stereograms which depicted the same complex shape but contained different ranges of disparity were measured. In the second experiment the minimum times required by experienced observers to perceive a given complex shape in stereograms that contained different ranges of disparity were determined. Perception times for the naive observers were all very fast (<3 s) and showed no evidence of perceptual learning. There was no effect of disparity range on perception times in either experiment. It was found that very large-disparity (80 min arc) complex stereograms could be perceived quickly, even by naive observers. It is concluded that the long initial latencies previously reported are not due to surface complexity nor to the range of disparities present. Other factors. such as dot size, dot density, and the correlation of the stereo images, appear to be important determinants of efficient stereoscopic performance when viewing complex random-dot stereograms.

Effect of low and moderate doses of alcohol on driving hazard perception latency and driving speed

Both driving speed and speed of detection of potentially hazardous events while driving have been found to correlate positively with accident rates across individuals. Alcohol ingestion is also known to increase risk of a traffic accident. This paper reports two double-blind, placebo-controlled studies: one on the effect of alcohol on driving speed and the other on the effect of alcohol on time taken to detect potential traffic hazards. Moderate drinkers aged between 30 and 55 took part. Each subject underwent three experimental conditions on separate days: no alcohol, low alcohol (0.025% BAC) and moderate alcohol (0.05% BAC). The order of conditions was counterbalanced. The moderate alcohol dose increased mean time taken to respond to hazards (2.5 s in no alcohol condition compared with 3.2 s in moderate alcohol condition) but did not affect mean driving speed (indexed by time taken to travel sections of a fixed route; 19.3 s in no alcohol compared with 19.0 s in moderate alcohol). The results support the view that at least part of the excess risk of accident associated with alcohol ingestion is attributable to an increase in the time taken to respond to traffic hazards.

Variations du temps normal de réponse du patient pendant la mesure du champ visuel

The perceptual latency (PL) to detect stimuli during visual field measurement is not a usual routine test. An automated perimeter ('moniteur ophtalmologique') was used to measure the central 30 degrees of vision with static targets in 50 normal subjects. The visual field was divided into four areas. The purpose of this study was to compare the PL in each area. An increase in the PL was observed, the nasal superior area being longer than the inferior temporal area. It seems that this psychophysical test is transmitted by a specific class of retinal ganglion cells: Y cells. We believe that PL could be a tool for detecting retinal ganglion cell damage.

Selective directional sensitivity in visual motion perception

We present two experiments demonstrating that: (i) the latency of perception of the position of a small visual target moving towards the fovea is shorter than that of the same target moving away from the fovea; (ii) the reaction time (RT) to onset of motion of the same type of target is also shorter when it moves towards the fovea; and (iii) the RT to onset of motion away from the fovea may be shorter when larger, textured stimuli are employed. The relation of the findings to the existence of two systems for visual motion information processing and to recent neurophysiological findings is discussed.