Jonathan Vaughan

Inhibition of return: Neural basis and function

Abstract A goal of neuropsychology is to connect cognitive functions with underlying neural systems. Posner (1984; in press) has proposed a framework for doing so in which elementary mental operations in cognitive models are expressed in terms of component facilitations and inhibitions in the performance of normal persons. Studies of brain-injured patients are used to link these components to underlying neural systems. In the area of spatial attention one such component is the tendency to inhibit orienting towards visual locations which have been previously attended (inhibition of return). Here we report studies in patients and normals which demonstrate the relationship of this component to neural systems which generate saccades. The first experiment showed that midbrain lesions impairing saccade generation produced a concurrent loss of the inhibition of return, whereas cortical components shown to impair facilitatory components did not. The second and third experiments show that the inhibition of return ...

Posture-based motion planning: applications to grasping.

This article describes a model of motion planning instantiated for grasping. According to the model, one of the most important aspects of motion planning is establishing a constraint hierarchy--a set of prioritized requirements defining the task to be performed. For grasping, constraints include avoiding collisions with to-be-grasped objects and minimizing movement-related effort. These and other constraints are combined with instance retrieval (recall of stored postures) and instance generation (generation of new postures and movements to them) to simulate flexible prehension. Dynamic deadline setting is used to regulate termination of instance generation, and performance of more than one movement at a time with a single effector is used to permit obstacle avoidance. Old and new data are accounted for with the model.

Time course of movement planning: selection of handgrips for object manipulation.

A goal of research on the cognitive control of movement is to determine how movements are chosen when many movements are possible. We addressed this issue by studying how subjects reached for a bar to be moved as quickly as possible from a home location to a target location. Ss generally grabbed the bar in a way that afforded a comfortable posture at the target location (the end-state comfort effect) and with the thumb toward the end of the bar that would be aligned with the target (the thumb-toward bias). The data suggested that subjects chose handgrips by retrieving instances of previous reaches, not by carrying out computations that treated candidate reaches as new behavioral events.

Optimal Movement Selection

Most physical tasks can be performed with an infinite number of movement patterns. How then are particular patterns selected? We propose that the contributions of individual limb segments depend on their own independently assessed fits to task demands. An advantage of this system is that coordination among limb segments can be achieved without explicit control of limb-segment interactions. In addition, the system allows segments that are still functioning to compensate for segments that are disabled. To test the model, we first asked subjects to oscillate the fingertip over varying distances at varying rates, using only the finger, hand, or forearm. Based on their performance, we identified the optimal amplitude and frequency of movement for each limb segment. Then we allowed the subjects to use the finger, hand, and forearm however they wished. We demonstrate that the relative contribution of each limb segment to fingertip displacement is predicted by the similarity of the optimal amplitude and frequency of that segment to the required amplitude and frequency of fingertip displacement. Because our model is similar to models proposed for learning and perception, common computational approaches appear viable for motor control and other more widely studied activities underlying information processing and behavior.

Plans for Grasping Objects

Through the lens of prehension research, we consider how motor planning is influenced by people’s perception of, and their intentions for how to act in, the environment. We review some noteworthy prehension phenomena, including a number of studies from our own labs which demonstrate the “end-state comfort effect,” the discovery of sequential effects in motor planning, and the finding that postural end states are known before movements begin. The existence of these phenomena highlights the role that mental representation plays in motor control. We review a recent model of motor control which can account for both perception-related and intention-related features of motor planning.

Coordination of reaching and grasping by capitalizing on obstacle avoidance and other constraints.

Abstract Reaching and grasping an object can be viewed as the solution of a multiple-constraint satisfaction problem. The constraints include contact with the object with the appropriate effectors in the correct positions as well as generation of a collision-free trajectory. We have developed a computational model that simulates reaching and grasping based on these notions. The model, rendered as an animation program, reproduces many basic features of the kinematics of human reaching and grasping behavior. The core assumptions of the model are: (1) tasks are defined by flexibly organized constraint hierarchies; (2) manual positioning acts, including prehension acts, are first specified with respect to goal postures and then are specified with respect to movements towards those goal postures; (3) goal postures are found by identifying the stored posture that is most promising for the task, as determined by the constraint hierarchy, and then by generating postures that are more and more dissimilar to the most-promising stored posture until a deadline is reached, at which time the best posture that was found during the search is defined as the goal posture; (4) depending on when the best posture was encountered in the search, the deadline for the search in the next trial is either increased or decreased; (5) specification of a movement to the goal posture begins with straight-line interpolation in joint space between the starting posture and goal posture; (6) if an internal simulation of this default movement suggests that it will result in collision with an obstacle, the movement can be reshaped until an acceptable movement is found or until time runs out; (7) movement reshaping occurs by identifying a via posture that serves as a body position to which the actor moves from the starting posture and then back to the starting posture, while simultaneously making the main movement from the starting posture to the goal posture; (8) the via posture is identified using the same posture-generating algorithm as used to identify the goal posture. These processes are used both for arm positioning and, with some elaboration, for prehension. The model solves a number of problems with an earlier model, although it leaves some other problems unresolved.

Speed and sequential effects in reaching.

To investigate the impact of future task demands on reaching, participants performed repetitive sagittal-plane reaches at low and high speeds. In a control condition, they reached from a start location to a target and back. In the experimental conditions, they reached from the start to the target, then to a second target (the location of which varied between trials), then back to the first target, and finally back to the start. Contributions of the hip, shoulder, and elbow to reaches made to the first target depended on the second targets location, on movement speed, and on repetition. Participants combined sustained and transient postural adjustments to minimize effort. The results support the knowledge model of movement selection (D. A. Rosenbaum, L. D. Loukopoulos, R. G. M. Meulenbroek, J. Vaughan, & S. E. Engelbrecht, 1995) but also call for its elaboration. Variants of the model are explored through simulations of the above study.

Adaptation of a reaching model to handwriting: How different effectors can produce the same written output, and other results

This report shows how a model initially developed for the control of reaching can be adapted for the control of handwriting. The main problem addressed by the model is how people can produce essentially the same written output with different effectors (e.g., the preferred or nonpreferred hand, the foot, or even the mouth). The model is based on the assumption that writers strive for invariant graphic outputs when they write with different effectors, when they write on surfaces with different orientations, or when they write large or small script; such output invariance is an essential requirement for later recognition of the written result. Given this assumption, the question is how the motor system enables the relevant effectors to generate the necessary pen strokes. The adapted model provides one possible answer to this question. It is the first fully working model of multijoint activity underlying writing and related graphic tasks. We describe how the model differs from other models developed in the past, and we review the models strengths and weaknesses.

Remembered positions: stored locations or stored postures?

Abstract Many recent studies indicate that memory for final position is superior to memory for movement. There is ambiguity about what is meant by the term final position, however. Is it final spatial location or final posture? According to a recently proposed theory by Rosenbaum et al., which maintains that stored postures form the basis for movement planning, when people try to return to recently reached positions, they should try to adopt the postures they just occupied. An alternative view, which holds that movements are primarily planned with respect to spatial locations, predicts that subjects should tend to return to places in external space. We describe an experiment that tested these opposing predictions. The experiment relied on the notion that if people store and use postures, they should ”copy” the posture adopted with one arm to the other arm when possible. The results support this hypothesis.

Saccades Directed at Previously Attended Locations in Space

Having previously fixated a location increases the latency of subsequent saccadic eye movements towards that location. In a saccadic pursult task, the last saccade of each trial was directed directly at, or up to three deg away from the location of a previous fixation on that trial. Saccades to a previously fixated location were longer by 8 to 15 msec than those directed 3 deg away from any previously fixated location, even 1700 msec after the prior fixation. The elevation in saccade latency is consistent with an attentional bias in favor of fresh sources of information.