James R. Tresilian

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.

Visually timed action: time-out for ‘tau’?

Bringing about desirable collisions (making interceptions) and avoiding unwanted collisions are critically important sensorimotor skills, which appear to require us to estimate the time remaining before collision occurs (time-to-collision). Until recently the theoretical approach to understanding time-to-collision estimation has been dominated by the tau-hypothesis, which has its origins in J.J. Gibsons ecological approach to perception. The hypothesis proposes that a quantity (tau), present in the visual stimulus, provides the necessary time-to-collision information. Empirical results and formal analyses have now accumulated to demonstrate conclusively that the tau-hypothesis is false. This article describes an alternative approach that is based on recent data showing that the information used in judging time-to-collision is task- and situation-dependent, is of many different origins (of which tau is just one) and is influenced by the information-processing constraints of the nervous system.

Coupling of grip force and load force during arm movements with grasped objects

Numerous studies have investigated the kinematics of arm movements; others have examined grip forces during static holding of objects. However, the coordination of grip force and arm movement when moving grasped objects has not been documented. We show that grip force is finely modulated in phase with load force during movements with grasped objects in which load force varies with acceleration. A tight coupling between grip and load force is seen in point-to-point and cyclic movements of varying rate and direction. We conclude that in transporting an object, the programming of grip force is part and parcel of the process of planning the arm movement.

Improving vision: neural compensation for optical defocus

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur, or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for thedefocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid–range (5–25) cycles deg−1 spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two–stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid–range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter–ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Perceptual Information for the Timing of Interceptive Action

Time-to-contact is an important quantity for controlling activities which involve the timing of interactions with objects and surfaces in motion relative to an observer. Two alternative means for obtaining perceptual information that might be used to obtain the time-to-contact required to correctly time an interaction have been contrasted: a method based on the perception of distance and velocity, and a method due to Lee involving a perceptual variable called tau. A monocular version of the first method is presented and shown to place a highly unrealistic and arbitrary limitation on the capabilities of the visual system. The second method is reviewed and its limitations discussed. Several means by which these limitations can be overcome are presented. Recently reported results from experiments which involved catching self-luminous balls in the dark are interpreted in terms of timing information available to the subject, and the notions of intermodal and multimodal timing information are introduced. Finally, the possibility that timing information is available to an observer which does not involve the variable tau is considered. It is concluded that many questions regarding the perception of time-to-contact remain unresolved and that much empirical research remains to be done.

Attention in action or obstruction of movement? A kinematic analysis of avoidance behavior in prehension

Abstract Obstacle avoidance strategies are of two basic but interrelated types: moving around an obstacle to that body parts do not come too close, and slowing down. In reaching-to-grasp, avoidance may involve the transport component, the grasp formation component, or both. There has been little research that has directly examined obstacle avoidance strategies during reaches-to-grasp. Several recent reports describe experiments in which reaches-to-grasp were made when nontarget objects were present in the workspace. The effects of these nontargets were interpreted as being due to their distracting effects rather than their obstructing effects. The results of these studies are reinterpreted as being due to the non-target’s obstructing effects. The obstacle interpretation is more parsimonious and better predicts the pattern of results than the distractor interpretation. Predictions of the obstacle interpretation were examined in an experiment in which participants were required to reach to grasp a target in the presence of another object in various locations. The results were exactly in line with the interpretation of the object as an obstacle and the data show how grasp and transport movements are subtly adjusted so as to avoid potential obstacles. It is proposed that people move so as not to bring body parts within a minimum preferred distance from nontarget objects within the workspace. What constitutes the preferred distance in a particular context appears to depend upon the speed of movement and a variety of psychological factors related to the cost that a person attaches to a collision.

The effect of obstacle position on reach-to-grasp movements

Abstract. Numerous everyday tasks require the nervous system to program a prehensile movement towards a target object positioned in a cluttered environment. Adult humans are extremely proficient in avoiding contact with any non-target objects (obstacles) whilst carrying out such movements. A number of recent studies have highlighted the importance of considering the control of reach-to-grasp (prehension) movements in the presence of such obstacles. The current study was constructed with the aim of beginning the task of studying the relative impact on prehension as the position of obstacles is varied within the workspace. The experimental design ensured that the obstacles were positioned within the workspace in locations where they did not interfere physically with the path taken by the hand when no obstacle was present. In all positions, the presence of an obstacle caused the hand to slow down and the maximum grip aperture to decrease. Nonetheless, the effect of the obstacle varied according to its position within the workspace. In the situation where an obstacle was located a small distance to the right of a target object, the obstacle showed a large effect on maximum grip aperture but a relatively small effect on movement time. In contrast, an object positioned in front and to the right of a target object had a large effect on movement speed but a relatively small effect on maximum grip aperture. It was found that the presence of two obstacles caused the system to decrease further the movement speed and maximum grip aperture. The position of the two obstacles dictated the extent to which their presence affected the movement parameters. These results show that the anticipated likelihood of a collision with potential obstacles affects the planning of movement duration and maximum grip aperture in prehension.

Some recent studies on the extraretinal contribution to distance perception

Some recent studies on the extraretinal contribution to distance perception are reviewed. These experiments demonstrate that vergence can provide reliable information for judgments on the distance of proximal targets in the absence of all other cues. We argue that, although vergence is an unreliable cue at large fixation distances and is subject to a strong contraction bias when studied in isolation, these facts do not imply a minor role for vergence in near-space perception. When additional depth and distance cues are added, the contribution of vergence information becomes more complicated. We present results which indicate that the different cues to depth and distance are combined in a manner that can result in unexpected distortions of visual space. A simple heuristic model which can produce the observed distortions is outlined.

Hitting a moving target: Perception and action in the timing of rapid interceptions

Different interceptive tasks and modes of interception (hitting or capturing) do not necessarily involve similar control processes. Control based on preprogramming of movement parameters is possible for actions with brief movement times but is now widely rejected; continuous perceptuomotor control models are preferred for all types of interception. The rejection of preprogrammed control and acceptance of continuous control is evaluated for the timing of rapidly executed, manual hitting actions. It is shown that a preprogrammed control model is capable of providing a convincing account of observed behavior patterns that avoids many of the arguments that have been raised against it. Prominent continuous perceptual control models are analyzed within a common framework and are shown to be interpretable as feedback control strategies. Although these models can explain observations of on-line adjustments to movement, they offer only post hoc explanations for observed behavior patterns in hitting tasks and are not directly supported by data. It is proposed that rapid manual hitting tasks make up a class of interceptions for which a preprogrammed strategy is adopted—a strategy that minimizes the role of visual feedback. Such a strategy is effective when the task demands a high degree of temporal accuracy.

Four questions of time to contact: a critical examination of research on interceptive timing.

Four questions concerning the use and perception of time to contact, tc, are identified. (i) Is tc information used in the timing of interceptive actions? (ii) If so, what control strategies are used? (iii) What are the perceptual sources of tc information and which of them do people use? (iv) How is the information extracted by the perceptual systems? Research relevant to these questions is reviewed and analysed. In connection with question (i), theoretical work on the special case of catching a moving object is analysed. It is concluded that treatments of catching which involve the use of tc information provide the best account of timing. In connection with question (ii), two types of control strategy suggested in the literature are identified: an intermittent strategy and a continuous strategy. Evidence for a continuous strategy is reconsidered and shown to be at least as well if not better accounted for by an intermittent strategy. Other empirical evidence for intermittent control is also discussed. In connection with question (iii) a simple unifying method is outlined with which all tc information so far presented in the literature can be derived, and examples are given. The viability of various types of information as sources of tc is examined by considering the errors which would result from their use. Finally, in connection with question (iv) the role of ‘looming detectors’ in the extraction of tc information is considered. These are frequently proposed as mechanisms for extracting the tc information provided by Lees optic variable, tau. The analysis provided indicates that, despite the existence of a well-known and popular theory, due mainly to Lee, about how interceptive actions are timed, very little is actually known about perceptual timing. It is not yet certain whether tc information is used in interceptive timing tasks, what kinds of control strategies are involved, what sources of information people use to time their actions, or what perceptual processing is involved in the extraction of tc information.