Perception of acceleration in motion-in-depth with only monocular and both monocular and binocular information

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Resumen: Observers are often required to adjust actions with objects that change their speed. However, no evidence for a direct sense of acceleration has been found so far. Instead, observers seem to detect changes in velocity within a temporal
   Psicológica (2003), 24, 93-108. Perception of acceleration in motion-in-depth with onlymonocular and both monocular and binocularinformation Joan López-Moliner * , Alejandro Maiche y Santiago EstaúnUniversitat Autònoma de Barcelona   Observers are often required to adjust actions with objects that change theirspeed. However, no evidence for a direct sense of acceleration has beenfound so far. Instead, observers seem to detect changes in velocity within atemporal window when confronted with motion in the frontal plane (2Dmotion). Furthermore, recent studies suggest that motion-in-depth isdetected by tracking changes of position in depth. Therefore, in order tosense acceleration in depth a kind of second-order computation would haveto be carried out by the visual system. In two experiments, we show thatobservers misperceive acceleration of head-on approaches at least within theranges we used [600-800 ms] resulting in an overestimation of arrival time.Regardless of the viewing condition (only monocular or monocular andbinocular), the response pattern conformed to a constant velocity strategy.However, when binocular information was available, overestimation washighly reduced. In many actions requiring adjustment to moving objects, the observeris often confronted with targets that change their speed 1 . In physics, the rateof change in speed is defined as acceleration and the situation of constantvelocity is a particular case. This conception raised the issue of whether ornot visual acceleration has the same status as in physics. That is to say, if the visual system is tuned to acceleration and we perceive constant velocity just as the case of null acceleration. Pioneer studies showed that humansrespond to smoothly accelerated motion as if the velocities were constantbut they could detect high rates of changes in speed (see Gottsdanker 1956,for a review). However, these experiments failed to shed more light into thequestion because important parameters did not deserve consideration (see *   Present adress: Joan López-Moliner. Department of Neuroscience, Erasmus MC, Dr.Mollewaterplein 50, NL-3015 GE Rotterdam, The Netherlands. e-mail:,   1   Usually in vision research, “velocity” refers to the motion vector, which is the speed in acertain direction.    J. López-Moliner at al.94 Regan, Kaufman, and Lincoln, 1986). The range of velocity, which wasfrequently too fast (e.g. Kaufman, Cyrulnick, Kaplowitz, Melnick, andStoff, 1971; Runeson, 1975) or the dependence on motor skill (Gottsdanker,1952), among others parameters, made difficult to lead to more generalconclusions.Except for the “never-replicated” results reported by Rosenbaum(1975), who concluded that constant acceleration and velocity are perceivedaccurately and directly, no other work provides data for supporting the ideaof direct computation of acceleration. Instead, more recent work showedconvincingly that acceleration is only perceived via changes in velocity(Werkhoven, Snippe, and Toet, 1992). Werkhoven et al. used the sameparadigm that allowed Nakayama and Tyler (1981) to demonstrate thathumans are able to directly sense visual motion and they do not perceive itfrom change of object’s position over time. Hogervorst and Eagle (2000)reported that acceleration plays an important role in recovering three-dimensional structure from motion. Brouwer, Brenner and Smeets (in press)showed that humans can detect changes of velocity even with shortpresentation times (300 ms), although they conclude that acceleration is notused to initiate locomotion in catching balls.However, most of the studies mentioned above have addressed theperception of acceleration within a 2-D space. As far as we know, situationswhere the observer has to face an accelerating object on a head on collisionpath have not been systematically studied. Motion in depth describes amotion pattern of the retinal image that is different from that generated bymotion in the fronto-parallel plane (see figure 1). Furthermore, there isstrong psychophysical evidence for independence of motion-in-depthchannels. For example, changing-size channels do respond when the targetmotion is along the line-of-sight only (Regan and Beverly, 1978). Adifference between motion in the frontal plane and motion in depth that isrelevant to us is related to the velocity-position debate. As mentioned above,there is empirical evidence that the visual system infers 2D motion viavelocity detectors (Nakayama and Tyler, 1981; Seiffert and Cavanagh,1998). Conversely, detection of different kinds of second-order motion,included motion in depth, seems to be achieved with a mechanism sensitiveto change in position. For example, motion in stereo-defined stimuli thatoscillated in depth was recovered by tracking position (near, far) instead of velocity (Seiffert and Cavanagh, 1998). Such a kind of mechanism would beattention-modulated (Cavanagh, 1992) and when attention is distractedaway from the moving target, motion in depth processing would decline.Some studies (e.g. Gray, 2000) support this hypothesis.As long as motion in depth involves a kind of feature (e.g. position)tracking system, the following question arises: could acceleration in motion  Perception of acceleration 95   in depth be detected by such system? If acceleration in the fronto-parallelplane is sensed, as evidence to date suggest, from comparing speeds atdifferent times, then it is likely that first-order derivatives of motion can beextracted. In other words, acceleration would be perceived indirectly fromtracking changes of velocity. However, to the extent that motion in depth isbased on changes of position, any sense of acceleration in depth would haveto be extracted from changes of position. It would be needed, therefore, akind of second-order derivative 2 of position. In this paper, we address thisissue in two experiments.In a first experiment, observers had to estimate the arrival time of approaching synthetic objects by using monocular information only, therebyremoving stereo-based distance cues that could feed the motion-in-depthmechanism. Changing optical size will be the only available cue for anaccurate temporal response to be the expansion pattern. This would feed thechanging-size channel of the motion-in-depth mechanism. Since optical sizeconfounds larger objects that are further away with nearer smaller objects,no information on relative position in depth is provided in Experiment 1.We, hence, expect a lack of sense of acceleration in this experiment. Figures1a and 1b illustrate the similarity of the available monocular visualinformation between constant velocity and accelerated situations. Figure 1. (a) Velocity information in 2D motion. (b) Rate of expansionfor an object that is approaching an observer. Note that speed profiles 2 Acceleration can be defined as the second order temporal derivative of the positionfunction.  J. López-Moliner at al.96  for constant velocity and acceleration are very different in 2D motion,while information on rate of expansion is hardly distinguishablebetween non-uniform and uniform approaching velocity in the case of motion in depth. In Experiment 2, we introduce stereo-motion by changing relativedisparity. There is strong evidence (Cumming and Parker, 1994) that stereo-motion is mainly detected by means of temporal changes in binoculardisparity instead of inter-ocular velocity differences. Some authors had thenecessity of postulating the existence of two distinct stereoscopic systems(see Regan, 1991 for a revision): a position-in-depth system that wouldrespond to static disparity and a motion-in-depth system, which woulddetect relative disparity. Therefore, following this hypothesis, stereo-motionwould not be detected by tracking changes in position. However, morerecent studies (e.g. Cumming and Parker, 1994; Seiffert and Cavanagh,1998) provide compelling arguments against that. Assuming that stereo-motion is perceived via changes in position-in-depth, any sense of acceleration would be based on the extraction of a second-order derivativeof position. Since information on relative position in depth is available inExperiment 2, we expect a more accurate estimation of the arrival time,thereby supporting second-order computations (see general discussion). EXPERIMENT 1 The geometric layout of the simulated situation is illustrated in figure2. In this experiment, only the monocular variables were considered.Monocular variables include the visual angle ( θ    ) subtended by the objectand its first temporal derivative ( θ  ’ ) : the rate of expansion. It is well knownthat the ratio θ   /  θ  ’ , known as τ (Lee, 1976), signals the time to contact (e.g.López-Moliner, Maiche and Estaún., 2000; Regan and Hamstra, 1993).However, its use has recently been questioned (Maiche, López-Moliner andEstaún, 2000; López-Moliner and Bonnet, 2002; Smith, Flach, Dittman andStanard, 2001). In order to perform the task, the knowledge resulting from θ    and θ  ’ or some combination of them, is the only available source of information in Experiment 1.However, τ signals time-to-contact accurately if approaching velocityis constant. If the movement is accelerated or decelerated, the computationof  τ would either overestimate or underestimate respectively the arrivaltime. Figure 1 shows the temporal course of monocular variables for bothuniform and non-uniform motion in the 2D and motion-in-depth situations.  Perception of acceleration 97    As can be noted, on the basis of rate of expansion (figure 1b), acceleratedobjects are almost indistinguishable from the constant velocity case.Conversely, note the difference between constant velocity and accelerationin the 2D situation (figure 1a).The aim of this experiment is to examine whether the observers’responses took into account future changes of velocity when onlymonocular cues of motion in depth are available. METHODSubjects . 8 subjects with normal (4) or corrected-to-normal (4) visionparticipated in experiment 1. Three subjects are authors of this paper andhad foreknowledge of the aim of the experiment. P d   Dt     I  θ    α L  screen
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