The prevalence effect in lateral masking and its relevance for visual search.

In stimulus displays with or without a single target amid 1,644 identical distractors, target prevalence was varied between 20, 50 and 80 %. Maximum gaze deviation was measured to determine the strength of lateral masking in these arrays. The results show that lateral masking was strongest in the 20 % prevalence condition, which differed significantly from both the 50 and 80 % prevalence conditions. No difference was observed between the latter two. This pattern of results corresponds to that found in the literature on the prevalence effect in visual search (stronger lateral masking corresponding to longer search times). The data add to similar findings reported earlier (Wertheim et al. in Exp Brain Res, 170:387-402, 2006), according to which the effects of many well-known factors in visual search correspond to those on lateral masking. These were the effects of set size, disjunctions versus conjunctions, display area, distractor density, the asymmetry effect (Q vs. O's) and viewing distance. The present data, taken together with those earlier findings, may lend credit to a causal hypothesis that lateral masking could be a more important mechanism in visual search than usually assumed.

Visual conspicuity: a new simple standard, its reliability, validity and applicability.

A general standard for quantifying conspicuity is described. It derives from a simple and easy method to quantitatively measure the visual conspicuity of an object. The method stems from the theoretical view that the conspicuity of an object is not a property of that object, but describes the degree to which the object is perceptually embedded in, i.e. laterally masked by, its visual environment. First, three variations of a simple method to measure the strength of such lateral masking are described and empirical evidence for its reliability and its validity is presented, as are several tests of predictions concerning the effects of viewing distance and ambient light. It is then shown how this method yields a conspicuity standard, expressed as a number, which can be made part of a rule of law, and which can be used to test whether or not, and to what extent, the conspicuity of a particular object, e.g. a traffic sign, meets a predetermined criterion. An additional feature is that, when used under different ambient light conditions, the method may also yield an index of the amount of visual clutter in the environment. Taken together the evidence illustrates the methods' applicability in both the laboratory and in real-life situations. STATEMENT OF RELEVANCE: This paper concerns a proposal for a new method to measure visual conspicuity, yielding a numerical index that can be used in a rule of law. It is of importance to ergonomists and human factor specialists who are asked to measure the conspicuity of an object, such as a traffic or rail-road sign, or any other object. The new method is simple and circumvents the need to perform elaborate (search) experiments and thus has great relevance as a simple tool for applied research.

Neural noise distorts perceived motion: the special case of the freezing illusion and the Pavard and Berthoz effect.

When a slowly moving pattern is presented on a monitor which itself is moved, the pattern appears to freeze on the screen (Mesland and Wertheim in Vis Res 36(20):3325-3328, 1996) even if we move our head with the monitor, as with a head mounted display (Pavard and Berthoz in Perception 6:529-540, 1977). We present a simple model of these phenomena, which states that the perceived relative velocity between two stimuli (the pattern and the moving monitor) is proportional to the difference between the perceived velocities of these stimuli in space, minus a noise factor. The latter reflects the intrinsic noise in the neural signals that encode retinal image velocities. With noise levels derived from the literature the model fits empirical data well and also predicts strong distortions of visually perceived motion during vestibular stimulation, thus explaining both illusions as resulting from the same mechanism.

How important is lateral masking in visual search?

Five experiments are presented, providing empirical support of the hypothesis that the sensory phenomenon of lateral masking may explain many well-known visual search phenomena that are commonly assumed to be governed by cognitive attentional mechanisms. Experiment I showed that when the same visual arrays are used in visual search and in lateral masking experiments, the factors (1) number of distractors, (2) distractor density, and (3) search type (conjunction vs disjunction) have the same effect on search times as they have on lateral masking scores. Experiment II showed that when the number of distractors and eccentricity is kept constant in a search task, the effect of reducing density (which reduces the lateral masking potential of distractors on the target) is to strongly reduce the disjunction-conjunction difference. In experiment III, the lateral masking potential of distractors on a target was measured with arrays that typically yield asymmetric search times in visual search studies (a Q among Os vs. an O among Qs). The lateral masking scores showed the same asymmetry. Experiment IV was a visual search study with such asymmetric search arrays in which the number of distractors and eccentricity was kept constant, while manipulating density. Reducing density (i.e., reducing lateral masking) produced a strong reduction of the asymmetry effect. Finally in experiment V, we showed that the data from experiment IV cannot be explained due to a difference between a fine and a coarse grain attentional mechanism. Taken together with eye movement data and error scores from experiment II and with similar findings from the literature, these results suggest that the sensory mechanism of lateral masking could well be a very important (if not the main) factor causing many of the well-known effects that are traditionally attributed to higher level cognitive or attentional mechanisms in visual search.

Maximal oxygen uptake during cycling is reduced in moving environments; consequences for motion-induced fatigue.

In previous studies on physical fatigue during simulated ship movements, the apparent exhaustion of subjects after experimentation suggested that the traditional index of physical workload, oxygen consumption expressed as the percentage of peak oxygen consumption (VO2-peak) measured in a separate graded exercise test (GXT), underestimates workload in a moving environment. In these studies, the GXT was carried out in a stationary environment, as is standard practice. To explain the underestimation, it was hypothesized that VO2-peak might have been less if the GXT had been carried out in the moving environment. This paper reports on three experimental tests of this hypothesis, performed with a ship motion simulator and aboard a ship at sea. In all three experiments, VO2-peak was indeed significantly reduced when the GXT was carried out in the moving environment. Theoretical reasons for this phenomenon are discussed and investigated, but a clear explanation is still lacking.

Cognitive suppression of tilt sensations during linear horizontal self-motion in the dark.

On the basis of models of otolith functioning, one would expect that, during sinusoidal linear self-motion in darkness, percepts of body tilt are experienced. However, this is normally not the case, which suggests that the otoliths are not responsive to small deviations from the vertical of the gravito-inertial force vector acting on them. Here we show that this is incorrect. Subjects usually know on what kind of linear motion device they are (going to be) moved, having seen it prior to experimentation. This may result in a cognitive suppression of such otolith responses. In the present study, subjects were kept completely unaware of how they were moved and were asked to report on how they thought they moved. About 50% of the reports included tilt percepts almost immediately. It is concluded that this reveals the presence of otolith responsiveness to even small and short-lived deviations of the gravito-inertial force vector from verticality, a responsiveness which is suppressed when (prior) cognitions exist that the motion path is purely in the horizontal plane.

Working in a moving environment.

The present paper provides a review of research and theories concerning the question of how and why working in a moving environment may affect performance. It is argued that performance decrements can be expected to occur as a result of general factors or as a result of specific impairments of particular human skills. General effects happen when environmental motion, simulated or real, reduces motivation (due to motion sickness), increases fatigue (due to increased energy requirements), or creates balance problems. Specific effects of moving environments on task performance may only be expected through biomechanical influences on particular skills such as perception (interference with oculomotor control) or motor skills (such as manual tracking). There is no evidence for direct effects of motion on performance in purely cognitive tasks.

Human energy expenditure when walking on a moving platform.

The assumption that working on board ship is more strenuous than comparable work ashore was investigated in this study. Various physiological parameters (VO2, VCO2, VE and HR) have been measured to determine the energy expenditure of subjects walking slowly on a moving platform (ship motion simulator). Twelve subjects (eight men and four women) walked either freely on the floor or on a treadmill at a speed of 1 m x s(-1). Platform motion was either in a heave, pitch or roll mode. These three conditions were compared with a control condition in which the platform remained stationary. The results showed that during pitch and roll movements of the platform, the energy expenditure for the same walking task was about 30% higher than under the stationary control condition (3.6 J x kg[-1] x m[-1] vs 2.5 J x kg[-1] x m[-1], P < 0.05) for both walking on a treadmill and free walking. The heart rate data supported the higher energy expenditure results with an elevation of the heart rate (112 beats x min[-1] vs 103 beats x min[-1], P < 0.05). The heave condition did not differ significantly from the stationary control condition. Pitch and roll were not significantly different from each other. During all experimental conditions free walking resulted in a higher energy cost of walking than treadmill walking (3.5 J x kg[-1] x m[-1] vs 2.7 J x kg[-1] x m[-1], P < 0.05) at the same average speed. The results of this experiment were interpreted as indicating that the muscular effort, needed for maintaining balance when walking on a pitching or rolling platform, resulted in a significantly higher work load than similar walking on a stable or a heaving floor, independent of the mode of walking. These results explain in part the increased fatigue observed when a task is performed on a moving platform.

Motion sickness: only one provocative conflict?

In reviewing the various forms of motion sickness, the classic sensory rearrangement theory has been redefined by demonstrating that only one type of conflict is necessary and sufficient to explain all different kinds of motion sickness. A mathematical description is provided from the summarizing statement that "All situations which provoke motion sickness are characterised by a condition in which the sensed vertical as determined on the basis of integrated information from the eyes, the vestibular system and the nonvestibular proprioceptors is at variance with the subjective vertical as expected from previous experience."

Contributions of roll and pitch to sea sickness.

The purpose of this study was to test the traditional assumption that sea sickness is uniquely provoked by heave motion characteristics, with pitch and roll movements being ineffective. In an experiment with a ship motion simulator, subjects were exposed to pitch and roll motions in combination with rather weak heave motions that have no motion sickness-inducing potential. Very high levels of motion sickness were observed (with a motion sickness rating scale) in almost 50% of our subjects. In three control experiments, it was shown that these heave motions, when presented separately, indeed have no motion sickness-inducing potential and that pitch and roll motions presented alone or in combination with each other have only a very small motion sickness-inducing potential. These results indicate that pitch and roll when combined with small heave motions, which in themselves are not sickness provoking, produce more motion sickness than claimed by the classic models. This suggests that in models on motion sickness, pitch and roll should be combined in a nonlinear fashion with heave and that such models will remain rather crude if they do not include a description of the vestibular contribution to motion sickness.

A puzzling percept of stimulus stabilization.

A visual illusion is reported which comprises the following: when a monitor with a moving constant velocity grating is swayed in front of a subject, the grating may be perceived as freezing or decelerating on the screen. This percept appears to depend on the magnitude and direction of the retinal grating velocity, relative to that of the monitor. Various possible explanations are rejected. It is concluded that the illusion shows a resemblance to the phenomenon of "motion capture" but that it has certain new characteristics that require an explanation.

Object motion perception during ego-motion: patients with a complete loss of vestibular function vs. normals.

Object motion perception was assessed in avestibular patients and normal controls. Two experiments were conducted, in which subjects were required to assess the motion of a visual stimulus with respect to earth. In the first experiment, we measured the velocity at which a briefly presented (200 ms) grating was perceived as earth fixed, while the subject maintained fixation on a visual target fixed relative to the body, during whole-body yaw rotation (VOR suppression). In this experimental setup, the influence of the semicircular canal signals on object motion perception was evaluated. The avestibular patients judged the grating to be stationary with respect to earth, when it was moving at the same velocity as their body, whereas for normal controls, the grating was perceived as stationary when it moved at a velocity slower than their body motion, but greater than zero. The difference between the two subject groups was significant, and showed the strong contribution of the vestibular system to object motion perception. Similarly, a measurement of the velocity at which a grating was perceived as stationary was obtained during smooth pursuit eye movements. In this experiment the contribution of the efference copy of the oculomotor signal and proprioceptive signals to object motion perception were assessed. As with the first experiment, the normal controls displayed a more veridical sense of object motion perception than the patients, although the difference was only just significant. We suggest that the difference could be an adaptive change in the patients perception of motion, which allows them to reduce the effects of oscillopsia.

Visual and nonvisual contributions to perceived ego-motion studied with a new psychophysical method.

The perception of ego-velocity (PEV) during sinusoidal linear acceleration (otolith stimulation) is investigated with a psychophysical method proposed by Wertheim (1990;1). This method is based on the threshold for object motion perception measured during ego-motion. In a first experiment, PEV was measured both with and without illumination of the experimental room. It was shown that, for the particular frequency and amplitude of ego-motion used, PEV in the light approached physical ego-velocity, whereas in darkness PEV was significantly smaller than physical ego-velocity. In a second experiment the PEV was measured again without illumination of the experimental room. It was found that in response to a sinusoidal ego-velocity stimulus, PEV was sinusoidal as well with a gain of 0.8 and a phase-lead of 4 degrees at 0.15 Hz.

'When is VISION asked too much'?

The last two decades a shift took place from substitutional/compensatory training to utilisation of residual vision regarding rehabilitation of the visually impaired. Some of the visually impaired are able to use their visual perception nearly as complete as normal seeing people in spite of a severe visual disability. On the other hand, people with nearly normal functions can be severely visually handicapped. To illustrate this, two cases are presented. The first case is a man, aged 47 years, with a juvenile macular degeneration on both eyes. In spite of a very low visual acuity of less then 0.05, he finished an university education and he is able to maintain himself very well in a leading position in a scientific environment, by using adequate low vision devices. Also for his leisure activities, as photography and speed skating, he relies upon visual perception. The second case is a woman, aged 30 years, with nearly normal visual functions, who is not able to read for longer periods caused by conflicting information from the body- and eye movements, and the visual input. This causes sickness during reading. She is unable to use books for her study and is working with recordings on tape. The results of a comprehensive visual assessment will be related to the specific low vision devices and its use.

Motion thresholds of briefly visible stimuli increase asymmetrically with age.

During a pursuit eye movement made across a stationary stimulus, that stimulus is often perceived as moving slightly in the direction opposite to the eyes (Filehne illusion). The illusion is generally thought to increase in strength when the stimulus is made visible only briefly. In two experiments the illusion was indeed observed with young subjects. However, with older subjects brief stimulus presentations yielded a strong inverted Filehne illusion (the stimulus appeared to move in the same direction as the eyes). This age dependency of the Filehne illusion is caused by an increase of only the threshold for stimulus motion in the direction opposite to the eyes. No such effect happens with the threshold for stimulus motion in the same direction as the eyes. These findings can be explained if we assume that with increasing age it takes more time to properly register retinal image velocity within the perceptual system.

The Aubert-Fleischl paradox does appear in visually induced self-motion.

An experiment was set up to investigate the possible influence of oculomotor activity on experienced speed of circular vection. With the standard lined inner wall of an optokinetic drum as stimulus, we found that subjects, sequentially exposed to periods with or without fixation point, experienced an increment in speed of circular vection when the eyes were kept stationary as compared to when optokinetic nystagmus occurred. In a control condition, however, where the influence of optokinetic nystagmus vs fixed gaze on the speed of circular vection was measured separately, the effect was not significant. These findings might explain a discrepancy found in the literature.

Angular velocity, not temporal frequency determines circular vection.

This paper shows that the experienced speed of circular vection depends on stimulus speed, not on stimulus temporal frequency. But why would anyone think the contrary? The point is that many modelers in the field of motion perception believe that perceived speed is determined by temporal frequency. Moreover, the optokinetic behaviour of the fly is said to be dependent on the temporal frequency, not the speed, of the stimulus pattern (Reichardt, 1987). It was the aim of the present experiment to test the notion that the experienced speed of circular vection is proportional to stimulus velocity information, which is carried by the temporal and the spatial characteristics of light.

An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited.

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.

The perception of object motion during smooth pursuit eye movements: adjacency is not a factor contributing to the Filehne illusion.

During smooth pursuit eye movement performance often an illusory motion of background objects is perceived. This so called Filehne illusion has been quantified and explored by Mack and Herman [Q. J.exp. Psychol. 25, 71-84 (1973); Vision Res. 18, 55-62 (1978)]. According to them two independent factors contribute to the Filehne illusion: (1) a subject relative factor, viz. the underregistration of pursuit eye movements by the perceptual system, and (2) an object relative factor, viz. adjacency of the pursued fixation point and the background stimulus. The evidence of the present experiment supports the former but rejects the latter as a contributing factor. Instead of the concept of adjacency, an alternative theoretical extension of the subject relative factor is offered.