Empirical and theoretical issues in the perception of time to contact.

Four questions concerning the perceptual source of information about time to contact (tc) are addressed: (a) What conditions are required for the optic variable tau to play a role in the perception of tc? (b) When these conditions are met, does tau alone provide sufficient information for accurate timing of interceptive actions? (c) Does a distance divided by velocity account of tc perception provide a convincing alternative to an account that is based on tau? (d) Is there any empirical evidence that distinguishes the two accounts? A "global" type of tau variable and a "local" type of tau variable are distinguished, each with different limitations. The discussion is largely concerned with local tau variables, 2 versions of which are identified. It is concluded that tau alone cannot provide sufficient information for skilled timing. An extended tau-based account presented in an earlier article (Tresilan, 1990) is discussed. It is argued that no extant empirical data can distinguish the extended account from the distance divided by velocity account.     

Perceptual and cognitive processes in time-to-contact estimation: Analysis of prediction-motion and relative judgment tasks

Three classes of task appear to involve time-to-contact (TTC) information: coincidence anticipation (CA) tasks, relative judgment (RJ) tasks, and interceptive actions (IAs). An important type of CA task used to study the perception of TTC is the prediction-motion (PM) task. The question of whether it is possible to study the perceptual processes involved in the timing of IAs using PM and RJ tasks is considered. A revised version of the tau hypothesis is proposed as an account of the perceptual information processing involved in the control of fast IAs. This draws on the distinction between “motor” and “cognitive” visual systems. It is argued that task variables affect whether “cognitive” information processing is involved in performance and can determine whether TTC information is used at all. Evidence is reviewed that suggests that PM and RJ tasks involve cognitive processing. It is argued that target viewing time, TTC at response initiation, amount of practice, and whether there is a period between target disappearance and response are task variables that determine whether cognitive processing will influence responding.     

Time-to-Topple: Haptic Angular Tau

The temporal pattern of the force needed to rotate an object around a supporting edge at constant angular velocity is analyzed. We show that haptic angular tau, the momentary value of the force divided by its rate of change, specifies the time remaining until the object reaches its topple point, that is, its unstable equilibrium position. Previous analyses of ratios of this general form, the inverse of the relative rate of change of a variable, have shown the availability of acoustic and visual information for the time-to-contact between a moving object and a station point, and acoustic information for vessel-filling. The present results document the availability of haptic information for a noncontact event and suggest that the tau analysis of physical variables may be a fruitful tactic in perceptual studies of many types of events.     

Prospective information for pass decisional behavior in rugby union

Decision-making requires the perception of relevant information variables that emerge from the player–environment interaction. The purpose of the present article is to empirically assess whether players’ decisional behavior about which type of pass to make is influenced by the spatio-temporal variable tau. Time series positional data of rugby players were analyzed from video footage taken in real match scenarios. The tau of the distance motion gap between attacker and defender was calculated, along with the duration of the next pass. Results revealed that the initial tau value predicted 64% of the variance found in pass duration. A qualitative distinction of tau dynamics between two periods of the approach between the attacker and the defender was also observed. We argue that the time-to-contact between the attacker and the defender may yield information about future pass possibilities. Additionally, the informational fields constraining attacker–defender interaction may be viewed as a convergent channeling of possibilities towards a single pass solution.     

On the nature of perceptual information during letter perception

Letter perception has been traditionally viewed as a process in which individual features are accumulated over time. In order to test this notion, a special stimulus set was created having little or no featural redundancy. Using a masking paradigm, confusion matrices were generated at each of eight interstimulus intervals. Few, if any, of the predictions made by the feature accumulation models were upheld. Instead, it is suggested that letter perception is better thought of as a global-to-local process. When a letter is presented, an observer initially perceives a large array of perceptual data. Over time, a clearer view of the stimulus emerges as the perceptual system brings the letter into focus. Thus, global information about the letter is available quite early in processing, while the letter’s more local aspects become available only after relatively extensive perceptual processing.     

Efficiency of graphical perception

The term graphical perception refers to the part played by visual perception in analyzing graphs. Computer graphics have stimulated interest in the perceptual pros and cons of different formats for displaying data. One way of evaluating the effectiveness of a display is to measure the efficiency (as defined by signal-detection theory) with which an observer extracts information from the graph. We measured observers' efficiencies in detecting differences in the means or variances of pairs of data sets sampled from Gaussian distributions. Sample size ranged from 1 to 20 for viewing times of 0.3 or 1 sec. The samples were displayed in three formats: numerical tables, scatterplots, and luminance-coded displays. Efficiency was highest for the scatterplots (approximately equal to 60% for both means and variances) and was only weakly dependent on sample size and exposure time. The pattern of results suggests parallel perceptual computation in which a constant proportion of the available information is used. Efficiency was lowest for the numerical tables and depended more strongly on sample size and viewing time. The results suggest serial processing in which a fixed amount of the available information is processed in a given time.     

Functional stabilization of unstable fixed points: human pole balancing using time-to-balance information

Humans are often faced with tasks that require stabilizing inherently unstable situations. The authors explored the dynamics of human functional stabilization by having participants continually balance a pole until a minimum time criterion was reached. Conditions were manipulated with respect to geometry, mass, and characteristic "fall time" of the pole. Distributions of timing between pole and hand velocities showed strong action-perception coupling. When actions demonstrated a potential for catastrophic failure, the period of hand oscillation correlated well with the perceptual quantity "time to balance" (tau(bal) = theta/theta), but not other quantities such as theta and theta alone. This suggests that participants were using available tau(bal) information during critical conditions, although they may not have been attending to this type of perceptual information during typical, noncritical motions of successful performance. In a model analysis and simulation, the authors showed how discrete tau(bal) information may be used to adjust the parameters of a controller to perform this task.     

An indirect method of measuring perceived distance from familiar size

Two methods of measuring perceived distance as a function of familiar size were compared in five experiments. The method which uses the perception of motion concomitant with a motion of the head, unlike the method of verbal report, is considered to provide a measure of perceived distance that is unaffected by factors of cognitive distance. The results of the experiments indicate that although the perceived egocentric distance of an object can vary somewhat as a function of the cue of familiar size, the larger variation often found with verbal reports of distance is based upon cognitive, not perceptual, information. The cognitive information is interpreted as resulting from the perception of the object as off-sized and the observer’s assumption that the perceived size of an object will vary inversely with its physical distance.     

Aging and the detection of observer and moving object collisions

The authors examined age-related differences in the detection of collision events. Older and younger observers were presented with displays simulating approaching objects that would either collide or pass by the observer. In 4 experiments, the authors found that older observers, as compared with younger observers, had less sensitivity in detecting collisions with an increase in speed, at shorter display durations, and with longer time-to-contact conditions. Older observers also had greater difficulty when the scenario simulated observer motion, suggesting that older observers have difficulty discriminating object motion expansion from background expansion from observer motion. The results of these studies support the expansion sensitivity hypothesis-that age-related decrements in detecting collision events involving moving objects are the result of a decreased sensitivity to recover expansion information.     

Unity of perception

Perceptual scientists have recently enjoyed success in constructing mathematical theories for specific perceptual capacities, capacities such as stereovision, auditory localization, and color perception. Analysis of these theories suggests that they all share a common mathematical structure. If this is true, the elucidation of this structure, the study of its properties, the derivation of its consequences, and the empirical testing of its predictions are promising directions for perceptual research. We consider a candidate for the common structure, a candidate called an "observer". Observers, in essence, perform inferences; each observer has a characteristic class of perceptual premises, a characteristic class of perceptual conclusions, and its own functional relationship between these premises and conclusions. If observers indeed capture the structure common to perceptual capacities, then each capacity, regardless of its modality or manner of instantiation, can be described as some observer. In this paper we develop the definition of an observer. We first consider two examples of perceptual capacities: the measurement of visual motion, and the perception of depth from visual motion. In each case, we review a formal theory of the capacity and abstract its structural essence. From this essence we construct the definition of observer. We then exercise the definition in discussions of transduction, perceptual illusions, perceptual uncertainty, regularization theory, the cognitive penetrability of perception, and the theory neutrality of observation.     

Visual contribution to the multistable perception of speech

The multistable perception of speech, or verbal transformation effect, refers to perceptual changes experienced while listening to a speech form that is repeated rapidly and continuously. In order to test whether visual information from the speaker's articulatory gestures may modify the emergence and stability of verbal auditory percepts, subjects were instructed to report any perceptual changes during unimodal, audiovisual, and incongruent audiovisual presentations of distinct repeated syllables. In a first experiment, the perceptual stability of reported auditory percepts was significantly modulated by the modality of presentation. In a second experiment, when audiovisual stimuli consisting of a stable audio track dubbed with a video track that alternated between congruent and incongruent stimuli were presented, a strong correlation between the timing of perceptual transitions and the timing of video switches was found. Finally, a third experiment showed that the vocal tract opening onset event provided by the visual input could play the role of a bootstrap mechanism in the search for transformations. Altogether, these results demonstrate the capacity of visual information to control the multistable perception of speech in its phonetic content and temporal course. The verbal transformation effect thus provides a useful experimental paradigm to explore audiovisual interactions in speech perception.     

Visual information about rigid and nonrigid motion: a geometric analysis

A mathematical analysis is presented that attempts to describe the available visual information about rigid and nonrigid motion and the three-dimensional structure of rigidly moving objects. Unlike other approaches, the analysis is based on the geometric relations among a set of trajectories defined over an extended region of space-time. Two experiments are reported in which observers viewed computer simulations of moving objects and were required to judge whether the observed motion appeared to be rigid or nonrigid. The results suggest that the mathematical limitations of a trajectory-based analysis of visual information are consistent with the perceptual limitations of actual human observers.     

Development of perceptual expertise in emotion recognition

How do children’s early social experiences influence their perception of emotion-specific information communicated by the face? To examine this question, we tested a group of abused children who had been exposed to extremely high levels of parental anger expression and physical threat. Children were presented with arrays of stimuli that depicted the unfolding of facial expressions, from neutrality to peak emotions. The abused children accurately recognized anger early in the formation of the facial expression, when few physiological cues were available. The speed of children’s recognition was associated with the degree of anger/hostility reported by the child’s parent. These data highlight the ways in which perceptual learning can shape the timing of emotion perception.     

Stability and skill in driving

Two experiments addressed the relation between postural stability, perceptual sensitivity, and stability of driving performance. A vehicle was fitted with differential GPS for measuring position and speed, position sensors for measuring brake and accelerator depression, force transducers for measuring door, console and footrest bracing forces, and an accelerometer for measuring the 3D accelerations of the vehicle. In Experiment 1, we investigated whether the initiation of deceleration and the control of braking might be due to sensitivity to the perceptual variable tau, which specifies time-to-contact (TTC), and in particular, whether its first derivative, tau-dot, is used to maintain a constant deceleration profile. Using both untrained experienced drivers (EDs) and trained driving instructors from the Holden Performance Driving Centre (HPDC), results confirmed that, regardless of skill level, tau-dot was maintained at a value close to 0.5 and, as predicted by Lee [Perception 5 (1976) 437], braking was initiated when TTC approximately 5 s. In Experiment 2, we wished to quantify the purported differences in driving behaviour between EDs and HPDC instructors during a variety of everyday manoeuvres. Results indicated that instructors utilised a different cornering trajectory, a different emergency braking strategy, and were able to perform a high-speed swerve and recovery task more effectively than the EDs. In general, the instructors applied greater bracing forces using the door and console compared with EDs. The instructors also applied greater footrest forces during emergency braking than did the EDs. The greater use of bracing by instructor drivers to resist g-forces represents a strategy of active stabilisation that enhances both postural stability, as well as overall stability and consistency of driving performance. Results are discussed with regard to the dynamics of perceptual-motor coordination, and how increased stability might improve sensitivity to relevant perceptual information. We conclude that driver-training programmes that focus on increasing driver stability (as a pre-requisite for increased control) show great promise as a means to improving one's attention during driving, and hence have the potential to dramatically improve road safety in general.     

A perception-action strategy for hummingbirds

Many human and animal tasks are thought to be controlled with the tau informational variable. It is widely accepted that controlling the rate of change of tau (tau) during decelerative tasks, such as when braking or landing, is one common perceptual control strategy. However, many tasks require accelerating before decelerating to a goal, such as reaching. An advancement of tau theory shows how a single action formula may be used to control the full action unit from initiation to peak velocity, and to rest at the goal, with the same perceptual tau information as before and accounting for the same decelerative kinematics as before. Here, we test the theory against data from high-speed video of a hummingbird flying to its flower feeder. We find that the theory accounts for 97% of the variance in the data, and thus supports it.     

The effect of body posture on long-range time-to-contact estimation

On Earth, gravity accelerates freely moving objects downward, whereas upward-moving objects are being decelerated. Do humans take internalised knowledge of gravity into account when estimating time-to-contact (TTC, the time remaining before the moving object reaches the observer)? To answer this question, we created a motion-prediction task in which participants saw the initial part of an object's trajectory moving on a collision course prior to an occlusion. Observers had to judge when the object would make contact with them. The visual scene was presented with a head-mounted display. Participants lay either supine (looking up) or prone (looking down), suggestive of the ball either rising up or falling down toward them. Results showed that body posture had a significant effect on time-to-contact estimation, but only when occlusion times were long (2.5 s). The effect was also rather small. This lack of immediacy in the posture effect suggests that TTC estimation is initially robust toward the effect of gravity, which comes to bear only as more time is allowed for post-processing of the visual information.     

Stimulus information vs processing time in auditory pattern recognition

Previous studies have shown that pitch perception improves with increases in tone duration. An increase in tone duration increases the information available in the stimulus presentation. Other recent experiments have shown that sufficient perceptual processing time is also necessary for accurate pitch perception. The present study determines the relative contribution of stimulus information and processing time in an absolute pitch indentification task. The results indicate that processing time is more critical than stimulus information. The results are described accurately by a model that describes the perceptual process in terms of the information in the stimulus and the time the information is available for perceptual processing.     

Perceptual organization of apparent motion in the Ternus display

A typical Ternus display has three sequentially presented frames, in which frame 1 consists of three motion tokens, frame 2 (blank) defines the interstimulus interval, and frame 3 has similar motion tokens with their relative positions shifted to the right. Interestingly, what appears to be a seemingly simple arrangement of stimuli can induce one of two distinct apparent-motion percepts in the observer. The first is an element-motion perception where the left-end token is seen to jump over its two neighboring tokens (inner tokens) to the right end of the display. The second is a group-motion perception where the entire display of the three tokens is seen to move to the right. How does the visual system choose between these two apparent-motion perceptions? It is hypothesized that the choice of motion perception is determined in part by the perceptual organization of the motion tokens. Specifically, a group-motion perception is experienced when a strong grouping tendency exists among the motion tokens belonging to the same frame. Conversely, an element-motion perception is experienced when a strong grouping tendency exists between the inner motion tokens in frames 1 and 3 (i.e. the two tokens that overlap in space between frames). We tested this hypothesis by varying the perceptual organization of the motion tokens. Both spatial (form similarity, 3-D proximity, common surface/common region, and occlusion) and temporal (motion priming) factors of perceptual organization were tested. We found that the apparent-motion perception of the Ternus display can be predictably affected, in a manner consistent with the perceptual organization hypothesis.