| Humans can perceive objects' position and orientation based on visual and auditory signals, and can perceive distance between objects and oneself, which called perception of egocentric distance. Besides perceiving objects in 3D-space, perceiving objects in space-time is more important. Constantly experience and react to movements in their environments, humans need to know and forecast when the object will reach which position. Moving towards an observer, or called approaching or looming is one critical type of movement. To implement intercepting and avoiding accurately, observers need to estimate the time remaining before impending collision actually occurs (time-to-collision, TTC). Perception of distance and time-to-collision are critically important sensorimotor skills. There are many behavioural studies found evidence that the sensory information specifying distance and time-to-collision can come from both visual and auditory sources via several cues in each sensory modality.Two contrasting theoretical approaches to TTC perception are currently predominant: one is structuralism approach and the other is Gibson's ecological-direct approach. Structuralist insisted that TTC perception is based on information about the instantaneous distance and perceived speed of the approaching object. While in ecological framework, observers are supposed to direct use information about the time-to-collision (TTC). TTC of the object can be specified by an optical variable called tau, which is equal to the inverse of the relative rate of expansion of the object's retinal image (TTC =θ/(dθ/dt)). Based on auditory information, TTC can be specified by the inverse of relative rate of increase of the object's sound intensity (TTC = 2I/(dI/dt)). Whether tau is indeed used to judge TTC is still being discussed.To make sure which kind of information is been used to judge distance and TTC, a 3D stimulus created using Virtools 3.0 is employed, authoring software used to develop interactive 3D content. Through this software participants were provided with realistic simulations of visual and auditory information associated with a real, approaching object. Method of constant stimulus was used to measure participants' psychometric function of distance perception and TTC perception. Two approaching car moving at constant speed was visible/audible for a period of time then disappeared (and/or became silent) well before the point of collision. When the second car disappeared, the participants were asked to judge which car is more near to their position (distance judgment task) or which car will reach their position first (TTC judgment task). All these four experiments were done in the Virtual Reality Lab, the visual stimulus were presented by LCD or stereo projection, and the auditory information was presented through stereo earphone. Twenty-eight participants with normal vision and hearing were tested individually.Exp. 1 and Exp. 2 has the same design: 7(TTC) X 5(speed) X 3(modality). All the variables were tested within subjects and were presented together in a fully randomized design. The reference stimuli were held constant at 2.5s (TTC) and 20m/s (speed) respectively, the comparison stimuli was varied in 35 levels. The task is to judge which car would reach their position first, that is to judge which TTC is shorter. The result is, in visual condition, participants were most sensitive to TTC information; in auditory condition, participants were most sensitive to distance information; in audiovisual condition, participants were most sensitive to TTC information, as the visual condition. Change of moving speed had no effect on visual and audiovisual TTC perception, but affected PSE of auditory TTC significantly. Observer tended to underestimate TTC of approaching object moving fast in auditory condition. In visual condition, participants consider both distance and speed factors to make TTC judgments, not as in auditory condition, they judge TTC only considered distance.Two independent variables were added in Exp. 3 and Exp. 4, which were Tasks and Starting Size. The design is 7 (TTC)×5(speed)×5(starting size)×3(modality)×2 (tasks). Tasks means in each certain block, participants were instructed to judge distance or judge TTC. Starting size means the size of object as comparison stimuli was controlled, both in visual angle and sound intensity. The result is, in all of the three modality conditions, discrimination threshold of distance perception is lower than which of TTC perception. That is to say, to judge distance is an easier task than to TTC judgment task. In distance task, participants were most sensitive to distance information, but in TTC task, participants used different information to judge TTC in different modality. In visual condition, participants were still most sensitive to TTC information; in auditory condition, participants were still most sensitive to distance information; in audiovisual condition, participants were sensitive to both TTC and distance information, while a little more sensitive to TTC information. Factor of moving speed did not affect PSE and JND of visual TTC perception, but affected PSE of auditory TTC perception significantly. Starting size of object affected distance perception significantly, the bigger the object was, the nearer the visual distance judged, while the farther the auditory distance judged. But starting size of object had no effects on both visual and auditory TTC perception.Relative distance and relative TTC judgment is applied as experimental paradigm, to measure the discrimination threshold of distance perception and TTC perception. PSE and JND of six kinds of information was analyzed individually, which were visual angle (0), the time-dependent derivative of the rate of the image's expansion (dθ/dt), sound intensity (I) and the time-dependent derivative of the change in sound intensity over time (dI/dt). Observers may use the information which he most sensitive to make distance and TTC judgments. The result is, in visual condition, participants use distance information to judge distance, use TTC information to judge TTC; in auditory condition, participants use distance information to make both distance and TTC judgments; in audiovisual condition, participants use distance information judge distance, use both TTC and distance information to judge TTC, but a little more sensitive to TTC information. In visual condition, TTC perception wasn't affected by moving speed and starting size of visual angle. It suggested that participants judged TTC based on tau variable. In auditory condition, TTC perception didn't affect by starting size of intensity, but moving speed. It suggested that auditory TTC perception is not accounted to acoustic tau, but to instantaneous distance information. |
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