| Humans and animals constantly interact with a dynamic environment. These interactions can include an observer moving towards a stationary object, an object moving towards a stationary observer or a combination of both thereof. Over the past few decades, numerous studies have tried to elucidate which sources of information are used to mediate these types of interactions. Given that the time required for performing an action is biologically constrained and often fixed (i.e. resulting from a sequence of muscle movements), it is possible that humans and animals use predictive timing information specified by visual information to guide their actions. One potential source of information often focused on in the literature is time-to-collision (alternatively time-to-contact; TTC). TTC is defined as the time remaining before contact between the observer and object, and can be derived in several ways.Among the different ways of perceiving TTC, an optic variable "tau" (Lee,1976) was particularly focused on in the literature. Tau is often regarded as an invariant; and as such, it is veridical and independent of other variables, such as the distance and velocity of the incoming object. Tau theory has argued that a tau strategy is necessary and sufficient to perceive TTC. Compared to the time-consuming and less accurate computation of distance to collision (DTC) and speed, tau provides a more direct and accurate estimate of TTC. The capability to perceive tau, as such, would be especially useful in situations which require immediate and accurate responses.Some laboratory studies have suggested that tau is used to perform a variety of TTC estimation tasks. Others, however, have argued that other variables can also impact these estimations.To evaluate the contribution of individual sources to TTC perception, it is necessary to first dissociate different co-varying variables that may have influenced TTC judgments. Among the studies that investigated the use of tau, two groups of studies deserve special considerations for their methods used in controlling visual information. The first group of studies devised a novel and systematic approach to isolate tau from other related optical variables. For instance, in an attempt to dissociate tau and rate of expansion at the moment of object presentation (see Equation 2), A two-dimensional matrix was created in which the two variables were systematically varied, one along each dimension, at the start of object trajectory. Cells of the matrix then formed trials using the values of these two variables as parameters. By examining responses to relative judgments of TTC, results showed that the observer was consistent with the use of a tau strategy, and that his judgment was independent of rate of expansion. Additionally, it was also found that the observer was able to specifically judge rate of expansion, which was done independently of tau. Thus, a conclusion was reached that separate and independent systems exist for estimating TTC and rate of expansion.Using systematic variations and well-controlled dissociation of movement parameters, along with the incorporation of cue conflict between the two sources of information that specify TTC; the current study aimed to quantify the relative contributions of tau and other variables such as DTC, speed and physical size of the incoming object during a TTC judgment task. In Experiment 1, the availability of distance information through the presence of ground was manipulated in order to better investigate the effects of DTC and speed. When distance information was presented, the target approached in a direction parallel to the ground surface, and projected a shadow directly underneath. In this situation, both target and its shadow provided potential depth information. However, as we varied target size between trials, DTC and speed information were most saliently provided by the contrast of the moving object and shadow along the ground, which could then be used to estimate TTCd. Responses in these conditions were then compared to when ground and shadow information were unavailable. In Experiments 2 and 3, we continued to provide ground depth information, but further manipulated the physical size of the object during some of the approaches. This TTCt manipulation led to inconsistencies between TTC specified by tau and TTC specified by the distance/speed ratio. Changes in responses due to these manipulations then allowed us to quantify the extent observers used tau. In Experiment 1. we investigated the contribution of four variables-TTC, DTC, target speed and physical size-during a relative TTC judgment task. Our results showed that participants could accurately discriminate trial-to-trial TTC differences based on TTC information. Discrimination thresholds revealed that TTC information was the most effective information for making TTC judgments compared to the other three variables, and that these results were consistent among all observers.Moreover, there was no noticeable difference between responses in with-ground and without-ground conditions. Altogether, our results suggested that depth information provided by the ground surface and target shadow, for the most part, did not influence TTC estimations. Thus, we concluded that DTC and speed information derived from ground presence affects little, if at all, the perception of TTC.Similar to Experiment 1, Experiment 2a showed that participants were most sensitive to trial-to-trial differences in TTC during a TTC estimation task, and not sensitive to variations of DTC, speed and target size. Weber fractions of curves based on TTC information were less than those produced by other variables. Additionally, we dissociated the effects of TTC by tau (TTCt) from TTC by DTC-speed (TTCd). If observers relied on tau (TTCt) to guide their judgments, responses would have differed between the three TTCt manipulation conditions. Responses, on the other hand, would have remained unchanged if observers relied instead on TTC specified by distance (TTCd) or other non-tau sources. Our results showed that responses were mostly affected by tau, and that manipulating TTCt reliably influenced TTC judgments. Specifically, enlarging the physical size of the target during approach caused observers to perceive a sooner arriving object. In contrast, decreasing the physical size of the target during approach caused participants to view the object as arriving later. The extent that individuals utilized tau, however, remained uncertain. While the present experiment only examined four observers, it was demonstrated that at least in the TTC-speed-size array, participants did not fully shift their responses to the extent we would expect had observers only used tau.Results in Experiment 3 indicated that participants could in fact discriminate trial-to-trial differences in DTC and speed. Therefore, the lack of sensitivity to DTC and speed observed in Experiment 2 was not due to inability to perceive the two variables. We also demonstrated that TTCt manipulations did not noticeably influence DTC and speed perception in the present study, thus confirming the validity of our manipulations.Experiment 4 demonstrated that even for a larger sample of naive participants, reducing the number of manipulations did not greatly change the pattern of results. Results consistently revealed that tau was the most effective and utilized source of information for judging TTC. When tau conflicted with ground-based depth information, these new participants continued to base their judgment to a large extent on tau. Tau by itself could not account for the total difference in response following manipulation (approximately 80%). Statistical analyses showed that this observed shift was different, although only marginally, to the expected 100% shift we would expect if observers had based their judgments entirely on tau. This suggested that observers relied on tau primarily for TTC estimations, but also used other variables, albeit to a smaller extent.In summary, the present study used an orthogonal design and cue-conflict paradigm to investigate how participants made relative TTC judgments when non-time variables and different sources of TTC were available. Results from Experiment 1 showed that when judging TTC of an approaching target, participants were most sensitive to TTC and much less sensitive to variations of other non-time variables such as the approaching target’s DTC, speed and physical size. Similar performances in with-ground and without-ground conditions further confirmed that participants relied mostly on TTC information during these estimations. Given that TTC information is used, we were also interested in what source of time information drove this sensitivity. Our results in Experiments 2 and 4 demonstrated that when different sources provided conflicting TTC information, observers were largely influenced by tau, and biased their judgments more towards the extent tau was manipulated. Altogether, we conclude that tau variable was the most useful cue when making relative TTC judgments, and others were less effective. |
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