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

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

Revised Tau Hypothesis: A Consideration of Wann's (1996) Analyses

It is shown that the results of J. P. Wann's (1996) analyses of empirical data concerning the role of the tau margin in timing interceptive actions are accommodated by a recent hypothesis of perceptual timing that proposes that task constraints and stimulus conditions are the basic determiners of what information is used in timing. These constraints and conditions are such that the use of multiple sources of information is the rule in timing, only a restricted class of fast interceptive actions is timed by tau margin information, and timing tasks will often involve sources of information that are not directly identifiable as sources of time-to-collision information. J. P. Wann's zeta ratio provides one potential source of timing information useful in certain conditions but does not provide an alternative to the tau margin. It is shown how J. P. Wann's criticisms of a preprogrammed model of timing in a ball-punching task can be met by developing a more sophisticated and biologically plausible model that better fits the data than the zeta ratio alternative

A simple empirical law for a class of visually timed interceptive actions

People are highly skilled at intercepting moving objects and capable of remarkably precise timing - in the order of a few milliseconds. The timing of interceptions is constrained by two situational factors: the time available to plan and complete the action and the temporal precision required by the task. Recent research on interceptive actions indicates that a highly replicable relationship is likely to exist between movement timing and task constraints. This is analogous to the classic Fitts' law relationship that relates movement time (MT) to task variables for movements aimed at stationary targets. We report two experiments that allowed a description of the relationship for an anticipatory interception task (without target pursuit). Two quantities that determine the required temporal precision were varied in experiment 1 - target speed and size. Available time (viewing time, VT) was kept constant and relatively large (1.6 s). MT was found to decrease with increases in target speed and decreases in target size. These results conformed to a specific empirical relationship between MT and the task variables. In experiment 2, VT and target speed were varied. MT was found to decrease monotonically with decreases in VT and the effect of target speed declined, becoming undetectable at the shortest VT (0.4s). These results can be interpreted as providing direct information about the 'rule' used by the nervous system to pre-determine MT in anticipatory interceptions

Measuring the development of motor-control processes in neurodevelopmental disorders

Montacute House; Pencil, 7 3/4 x 9 1/2" (sight); S. lower right Chas. R. Mackintosh; Inscr. lower right, Montecute House; Part of the Dr. Thomas Howarth collection, sold at Christies, Feb. 17, 1994. "Thomas Howarth, professor, architectural historian, collector (b in England 1914, d at Toronto 21 July 2000). Howarth reawakened interest in the great Scottish architect, Charles Rennie Mackintosh (1868-1928), through articles, a comprehensive monograph, exhibitions, and lifelong advocacy and collecting. A prescient collector of Mackintosh's work, Howarth amassed a huge and varied collection." Source: Canadian Encyclopedia; http://www.thecanadianencyclopedia.com/ (accessed 2/14/2008

The size-distance paradox is a cognitive phenomenon

The perceived size of a fixated object is known to be a function of the perceived fixation distance. The size-distance paradox has been posited as evidence that the perceived distance of a fixated object is, in turn, influenced by the object’s perceived size. If this is correct then it challenges a widely accepted account (modified weak fusion) of how the nervous system combines multiple sources of information. We hypothesised that the influence of perceived size on the perception of distance is likely to be restricted to conscious perceptual judgements. If our hypothesis is correct then the size-distance paradox should not be observed when observers make action-based distance judgements. In line with this expectation we observed the size-distance paradox when participants made verbal reports on target distance but found no paradoxical judgements in a group who were asked to point at the target. We therefore suggest that the size-distance paradox should not be taken as evidence that perceived size feeds back into distance perception

A framework for considering the role of afference and efference in the control and perception of ocular position

It has been well established that extra-retinal information is used in the perception of visual direction and distance. Furthermore, a number of studies have established that both efference copy and afferent discharge contribute to the extra-retinal signal. Despite this, no model currently exists to explain how the signals which arise through oculomotor control contribute to perception. This paper attempts to provide such a framework. The first part of the paper outlines the framework [the cyclopean equilibrium point (EP) model] and considers the binoculus or cyclopean eye from the perspective of a current account of motor control (the EP hypothesis). An existing model is used to describe how the nervous system could utilise available efference copy and afferent extra retinal signals when determining the direction and distance of cyclopean fixation. Although the cyclopean EP model is speculative, it allows for a parsimonious framework when considering the oculomotor contribution to perception. The model has the additional advantage of being consistent with current theories regarding the control and perception of limb movement. The second part of the paper shows that the model is biologically plausible, demonstrates the use of the proposed model in describing the central control of eye movements with regard to non-conjugate peripheral adaptation and reconciles seemingly disparate empirical findings

Getting the measure of vergence weight in nearness perception

Combining multiple sources of information allows the human nervous system to construct an approximately Euclidean representation of near (personal) space. Within this space, binocular vergence is an important source of egocentric distance information. We investigated how the nervous system determines the significance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy between vergence information and other cues and (2) reduced vergence demand. The results also provided evidence that the nervous system represents vergence related distance information in units of nearness (the reciprocal of distance)

A curious illusion suggests complex cue interactions in distance perception

Binocular perception of the distance to, and between, point light targets depends on vergence angle: Increasing vergence angle decreases apparent distance and vice versa. Placing a prism "base out" requires increased convergence for target fixation; "base in" requires decreased convergence: The triangulation account of distance perception predicts that apparent target distance should decrease and increase respectively. It was found that the results predicted from the triangulation account were not observed. Egocentric target distance was judged to be greater regardless of prism orientation or target distance. A heuristic model provided an explanation for this phenomenon and allowed for the prediction of modulations of the overestimate with simple manipulations of the viewing environment. Further experiments confirmed these predictions and demonstrated that the effects of the prism could be greatly attenuated by adding additional distance cues