Biomedical subjects

John F Soechting

Publications and source records attributed to John F Soechting.

At least 19 recordsLinked to original sources

PubMed2006

Predicting curvilinear target motion through an occlusion.

When a tracked target is occluded transiently, extraretinal signals are known to maintain smooth pursuit, albeit with a reduced gain. The extent to which extraretinal signals incorporate predictions of time-varying behavior, such as gradual changes in target direction, is not known. Three experiments were conducted to examine this question. In the experiments, subjects tracked a target that initially moved along a straight path, then (briefly) followed the arc of a circle, before it disappeared behind a visible occlusion. In the first experiment, the target did not emerge from the occlusion and subjects were asked to point to the location where they thought the target would have emerged. Gaze and pointing behaviors demonstrated that most of the subjects predicted that the target would follow a linear path through the occlusion. The direction of this extrapolated path was the same as the final visible target direction. In the second set of experiments, the target did emerge after following a curvilinear path through the occlusion, and subjects were asked to track the target with their eyes. Gaze behaviors indicated that, in this experimental condition, the subjects predicted curvilinear target motion while the target was occluded. Saccades were directed to the unseen curvilinear path and pursuit continued to follow this same path at a reduced speed in the occlusion. Importantly, the direction of smooth pursuit continued to change throughout the occlusion. Smooth pursuit angular velocity was maintained for approximately 200 ms following target disappearance. The results of the experiments indicate that extraretinal signals indeed incorporate cognitive expectations about the time-varying behavior of target motion.

Cognition↗

PubMed2006

Factors influencing the radial-tangential illusion in haptic perception.

According to the radial-tangential illusion, in the horizontal plane, arm movements executed in directions radial to the trunk are sensed to be longer than movements of the same length in the orthogonal direction. It has been suggested that the illusion arises from the fact that radially directed movements are executed more slowly and require more effort. These suggestions were tested in a series of experiments, using a robotically controlled manipulandum. In all of the experiments subjects grasped the handle of the manipulandum, in some cases exploring the virtual boundary of a rectangle, while in others being guided along a rectangular contour by the robot. In a two-alternative forced choice design, subjects reported whether the rectangle was wide or narrow. In a control experiment, subjects manifested the radial-tangential illusion. Contrary to the hypothesis, the magnitude of this illusion was not altered when a resistive force was added in the tangential direction or when the ratio of movement times in the tangential and radial directions was changed. However, when the contour was explored in the counterclockwise direction, the illusion was much smaller than when it was explored in the clockwise direction. A second series of experiments, in which subjects only explored two sides (i.e., an L-shape), demonstrated that this effect arose from distortions induced by the serial ordering of the exploratory movements. The illusion was much smaller when the radial segment was explored first. We suggest that this distortion arises from the serial nature of haptic exploration, in which the length of the initial segment decreases as it is stored in working memory for subsequent comparison.

Attention↗

PubMed2006

Oculomotor responses to gradual changes in target direction.

Smooth pursuit tracking of targets moving linearly (in one dimension) is well characterized by a model where retinal image motion drives eye acceleration. However, previous findings suggest that this model cannot be simply extended to two-dimensional (2D) tracking. To examine 2D pursuit, in the present study, human subjects tracked a target that moved linearly and then followed the arc of a circle. The subjects' gaze angular velocity accurately matched target angular velocity, but the direction of smooth pursuit always lagged behind the current target direction. Pursuit speed slowly declined after the onset of the curve (for about 500 ms), even though the target speed was constant. In a second experiment, brief perturbations were presented immediately prior to the beginning of the change in direction. The subjects' responses to these perturbations consisted of two components: (1) a response specific to the parameters of the perturbation and (2) a nonspecific response that always consisted of a transient decrease in gaze velocity. With the exception of this nonspecific response, pursuit behavior in response to the gradual changes in direction and to the perturbations could be explained by using retinal slip (image velocity) as the input signal. The retinal slip was parallel and perpendicular to the instantaneous direction of pursuit ultimately resulted in changes in gaze velocity (via gaze acceleration). Perhaps due to the subjects' expectations that the target will curve, the sensitivity to the image motion in the direction of pursuit was not as strong as the sensitivity to image motion perpendicular to gaze velocity.

Acceleration↗

PubMed2005

Haptic feature extraction.

This study examined the process by which the shape of a haptically explored object is synthesized from the geometric characteristics of simpler constituent elements, such as arcs and ellipses. Subjects traced the outlines of virtual objects by means of whole arm movements. Each object consisted of the union of a large central ellipse and two smaller circles, extending upward and outward from the top left and right sides of the base. The sizes of the two circles and the eccentricity of the elliptical base were varied. After exploring the object's contour in the absence of vision, subjects reproduced the sensed shape by means of freehand drawing. Speed and force were modulated during the exploratory phase in a manner that suggested that subjects reacted to rather than predicted changes in curvature. Also, subjects typically devoted more time to exploring the part of the contour encompassing the two smaller circles. During drawing, individual features of the explored shape were reproduced with varying degrees of fidelity. Aspects related to the size and location of the smaller circles were reproduced better than was the eccentricity of the ellipse forming the base. Since subjects spent proportionally less time exploring the base, these results suggest that subjects selectively focused attention to regions of high spatial contrast and that the exploratory strategy introduced distortions in the haptically sensed shapes.

Form Perception↗

PubMed2005

The use of motion cues in the haptic sense of circularity.

We tested the hypothesis that speed cues are used to haptically identify changes in the curvature of the hand's trajectory. Subjects grasped the handle of a robotically-controlled manipulandum that was moved in the horizontal plane along various elliptical arcs following one of three different speed profiles. In one profile, a circular arc was traced at a constant speed whereas in the other two speed was constant for ellipses whose aspect ratios differed from unity. A two-alternative forced choice procedure was used to identify the ellipse that was sensed to be circular in each of the three experimental conditions. In unconstrained movements, speed varies with the radius of curvature. If speed cues are used to identify curvature during passive movements, one would expect that subjects' responses should be biased towards the ellipse traced at a constant speed. The results did not support this hypothesis, indicating that speed cues are not a major contributor in the haptic sensing of shape.

Algorithms↗

PubMed2005

Approaches to the study of haptic sensing.

This review surveys results from a new approach to the problem of haptic sensing, in which subjects use primarily proximal arm movements to explore the shapes of virtual objects. These shapes are generated using a robotically controlled manipulandum. We begin by summarizing distortions of simple geometric properties (such as the length and orientation of lines) in the haptic perception of space. We then consider the extent to which the sense of more complex shapes (such as quadrilaterals) can be explained by these geometric distortions, i.e., the extent to which the shape of a complex object is synthesized from simpler constituent elements, and some of the sensory cues that may be important in this process. Haptic and visual processing of shapes appear to lead to some similar illusions. However, we argue that the processing of haptic information differs fundamentally from visual processing in that the former requires the integration of information that evolves in time as well as in space.

Animals↗

PubMed2004

Distortions in the visual perception of shape.

It is known that visual illusions lead to a distorted perception of the length and orientation of lines, but it is not clear how these illusions affect the appreciation of the shape of closed forms. In this study two experiments were performed to characterize distortions in the visual perception of the shape of quadrilaterals and the extent to which these distortions were similar to the distortions of haptically sensed shapes. In the first experiment human subjects were presented with two quadrilaterals side by side on a computer monitor. One was a reference shape; the other was rotated and distorted relative to the first. The subjects used the computer mouse to adjust the corners of the distorted quadrilateral to match the shape of the target quadrilateral. They made consistent errors on this task: the adjusted quadrilateral was about 2% wider and about 2% shorter than the veridical shape. Furthermore, subjects adjusted the inner angles of the quadrilateral to make them closer to 90 degrees . The first type of error was also present in a second experiment in which, in a two-alternative forced-choice paradigm, subjects viewed a reference shape and were asked to indicate which of two transiently presented quadrilaterals was closest to the target shape. The width/height errors and the inner angle errors were comparable to those described previously when subjects felt the outline of a quadrilateral and then drew its reproduction in the absence of vision, suggesting that the distortion occurs in the process of remembering the shape.

Adult↗

PubMed2004

Modulation of grasping forces during object transport.

Subjects held an instrumented object in a tripod grasp and moved it in the horizontal plane in various directions. The contact forces at the digits were measured and the grip force was decomposed into 2 components: a manipulating force responsible for accelerating the object and a grasping force responsible for holding the object steady. The grasping forces increased during the movement, reaching a peak near the time of peak velocity. The grasping forces also exhibited directional tuning, but this tuning was idiosyncratic for each subject. Although the overall grip forces should be modulated with acceleration, the load force did not vary during the task. Therefore the increase in the grasping force is not required to prevent slip. Rather, it is suggested that grasping force increases during translational motion to stabilize the orientation of grasped objects.

Acceleration↗

PubMed2004

Smooth pursuit tracking of an abrupt change in target direction: vector superposition of discrete responses.

The directional control of smooth pursuit eye movements was studied by presenting human subjects with targets that moved in a straight line at a constant speed and then changed direction abruptly and unpredictably. To minimize the probability of saccadic responses in the interval following the target's change in direction, target position was offset so as to eliminate position error after the reaction time. Smooth pursuit speed declined at a latency of 90 ms, whereas the direction of smooth pursuit began to change later (130 ms). The amplitude of the offset in target position did not affect the subsequent smooth pursuit response. In other experiments, the target's speed or acceleration was changed abruptly at the time of the change in direction. Step changes in speed elicited short-latency responses in smooth pursuit tracking but step changes in acceleration did not. In all instances, the earliest component of the response did not depend on the parameters of the stimulus. The data were fit with a model in which smooth pursuit resulted from the vector addition of two components, one representing a response to the arrest of the initial target motion and the other the response to the onset of target motion in the new direction. This model gave an excellent fit but further analysis revealed nonlinear interactions between the two vector components. These interactions represented directional anisotropies both in terms of the initial tracking direction (which was either vertical or 45 degrees ) and in terms of the cardinal directions (vertical and horizontal).

Brain↗

PubMed2004

Interception of targets using brief directional cues.

There are time delays in visuomanual and oculomotor pathways, and some of these time delays may be due to the finite time required to process visual motion signals and to extract accurate information about the speed and direction of the motion. The present experiments were designed to ascertain the time required to obtain a reliable estimate of the direction of target motion. Subjects were asked to indicate the final direction of a moving target, which abruptly changed direction and shortly thereafter disappeared, by pointing to its expected emergence at the boundary of an occlusion. Subjects made small but consistent errors that overestimated the target's change in direction. These errors depended little on the amount of time the target was visible (ranging from 50 to 400 ms) after it changed direction. Pointing direction was strongly correlated with gaze, which was dominated by a saccade initiated shortly after the target changed direction. The pointing errors were explained by the fact that the saccade always intercepted the (occluded) target, but then continued in the same direction toward the boundary of the occlusion. The analysis reveals that target direction was estimated accurately even at the shortest viewing time.

Cues↗

PubMed2004

Deficits in ocular and manual tracking due to episodic ataxia type 2.

Four patients with a novel mutation leading to episodic ataxia type 2 were studied in a task that required them to track target motion either with the eyes or with the index finger of the right hand. The target initially moved in a straight line and then changed direction at an unpredictable time by an unpredictable amount. On the day of testing, 3 of the patients were evaluated as normal on a neurological exam, whereas the fourth was severely ataxic. Nevertheless, all 4 showed deficits in tracking behavior with common features. Ocular tracking tended to result in hypermetric saccades at longer than normal latencies. Smooth pursuit tracking was absent in 1 patient and had lower than normal gain in the others. Deficits in manual tracking showed similarities to the deficits in ocular tracking, with hypermetric compensations for changes in target direction. The similarities in the deficits in manual and ocular tracking suggest that they are subject to similar control by the cerebellar structures.

Adult↗

PubMed2003

Haptic synthesis of shapes and sequences.

Haptic perception of shape is based on kinesthetic and tactile information synthesized across space and time. We studied this process by having subjects move along the edges of multisided shapes and then remember and reproduce the shapes. With eyes closed, subjects moved a robot manipulandum whose force field was programmed to simulate a quadrilateral boundary in a horizontal plane. When subjects then reproduced the quadrilateral using the same manipulandum, with eyes still closed but now with the force field set to zero, they made consistent errors, overestimating the lengths of short segments and underestimating long ones, as well as overestimating acute angles and underestimating obtuse ones. Consequently their reproductions were more regular than the shapes they had experienced. When subjects felt the same quadrilaterals with the same manipulandum but drew them on a vertical screen with visual feedback, they made similar errors, indicating that their distortions reflected mainly perceptual rather than motor processes. In a third experiment, subjects explored the 3 sides of an open shape in a fixed order. The results revealed a temporal pattern of interactions, where the lengths and angles of previously explored segments influenced the drawing of later segments. In all tasks, our subjects were as accurate as subjects in earlier studies who haptically explored only single lines or angles, suggesting that the mental processes that synthesize haptic data from multiple segments into complete shapes do not introduce any net error.

Adolescent↗

PubMed2003

Time constants in the perception of a change in the direction of motion in humans.

Motion signals are subject to spatio-temporal filtering at early stages of processing. In general, motion can be characterized by two parameters: speed and direction. This study sought to determine the time constants for the filtering of the directional component of the motion signal. In a forced-choice discrimination task, subjects were asked to choose the more abrupt change in direction of a target that moved through two 90 degrees corners. At each corner, direction of motion was low-pass filtered. Subjects were able to reliably perform this task if the filter time constants differed by >20 ms.

Choice Behavior↗

PubMed2003

Coarticulation in fluent fingerspelling.

In speech, the phenomenon of coarticulation (differentiation of phoneme production depending on the preceding or following phonemes) suggests an organization of movement sequences that is not strictly serial. In the skeletal motor system, however, evidence for comparable fluency has been lacking. Thus the present study was designed to quantify coarticulation in the hand movement sequences of sign language interpreters engaged in fingerspelling. Records of 17 measured joint angles were subjected to discriminant and correlation analyses to determine to what extent and in what manner the hand shape for a particular letter was influenced by the hand shapes for the preceding or the following letters. Substantial evidence of coarticulation was found, revealing both forward and reverse influences across letters. These influences could be further categorized as assimilation (tending to reduce the differences between sequential hand shapes) or dissimilation (tending to emphasize the differences between sequential hand shapes). The proximal interphalangeal (PIP) joints of the index and middle fingers tended to show dissimilation, whereas at the same time (i.e., during the spelling of the same letters) the joints of the wrist and thumb tended to show assimilation. The index and middle finger PIP joints have been shown previously to be among the most important joints for computer recognition of the 26 letter shapes, and therefore the dissimilation may have served to enhance visual discrimination. The simultaneous occurrence of dissimilation in some joints and assimilation in others demonstrates an unprecedented level of parallel control of individual joint rotations in an essentially serial task.

Acoustic Stimulation↗

PubMed2003

Bias and sensitivity in the haptic perception of geometry.

Our ability to recognize and manipulate objects relies on our haptic sense of the objects' geometry. But little is known about the acuity of haptic perception compared to other senses like sight and hearing. Here, we determined how accurately humans could sense various geometric features of objects across the workspace. Subjects gripped the handle of a robot arm which was programmed to keep the hand inside a planar region with straight or curved boundaries. With eyes closed, subjects moved the manipulandum along this virtual wall and judged its curvature or direction. We mapped their sensitivity in different parts of the workspace. We also tested subjects' ability to discriminate between boundaries with different degrees of curvature, to sense the rate of change of curvature, and to detect the elongation or flattening of ellipses. We found that subjects' estimates of the curvature of their hand path were close to veridical, and did not change across the workspace though they did vary somewhat with hand path direction. Subjects were less accurate at judging the direction of the hand path in an egocentric frame of reference, and were slightly poorer at discriminating between arcs of different curvature than at detecting absolute curvature. They also consistently mistook flattened ellipses and paths of decreasing curvature (inward spirals) for circles-and mistook arcs of true circles for arcs of tall ellipses or outward spirals. Nevertheless, the sensitivity of haptic perception compared well with that of spatial vision in other studies. Furthermore, subjects detected curvature and directional deviations much smaller than those that actually arise for most reaching movements. These findings suggest that our haptic sense is acute enough to guide and train motor systems and to form accurate representations of shapes.

Biofeedback, Psychology↗

PubMed2003

Interactions between ocular motor and manual responses during two-dimensional tracking.

Tracking of a moving target usually involves coordinated movements of the eye and the hand. To study the extent to which one behavior influences the other, eye and hand movements were recorded during three conditions (eye alone, hand alone, and eye and hand together) where subjects tracked a target that initially moved in a straight line and then made an abrupt and unpredictable change in direction. The response latencies of the eye and hand were influenced by the presence of the other tracking modality. More specifically, the latency for the hand was decreased during concomitant ocular tracking, whereas the latency for the eye was increased during combined hand-eye tracking. Moreover, the velocity profile of the smooth pursuit component of ocular tracking was different when the hand also tracked the target. Taken together, these observations support the hypothesis that at least part of the neural substrate underlying tracking is shared by the two modalities.

Hand↗

PubMed2003

Biological constraints simplify the recognition of hand shapes.

This study sought to identify constraints that might lead to a concise system of recognizing fingerspelling hand shapes. Previous studies of grasping suggested that hand shape is controlled using combinations of a small number of neuromuscular synergies, but fingerspelling shapes appear to be more highly individuated and, therefore, might require a larger number of degrees of freedom. Static hand postures of the American Sign Language manual alphabet were recorded by measuring 17 joint angles. Principal components (PCs) analysis was compared to the use of subsets of individual variables (i.e., joint angles) for reduction in degrees of freedom. The first four PCs were similar across subjects. Classification using weightings from these four components was 86.6% accurate, while classification using four individual variables was 88.5% accurate (thumb abduction, as well as flexion at the index and middle finger proximal interphalangeal joints and the ring finger metacarpalphalangeal joint). When chosen for each subject, particular four-variable subsets yielded correct rates above 95%. This superior performance of variable subsets over PC weighting vectors suggests that the reduction in degrees of freedom is due to biomechanical and neuromuscular constraints rather than synergistic control. Thus, in future application to dynamic fingerspelling, reasonable recognition accuracy might be achieved with a significant reduction in both computational and measured degrees of freedom.

Cluster Analysis↗

PubMed2003

Using arm configuration to learn the effects of gyroscopes and other devices.

Previous studies have perturbed the association between motor commands and arm movements by applying forces to the arm during two-dimensional movements. These studies have revealed that, when the normal hand path is perturbed, subjects gradually adapt their motor commands to return to this path. The present study used the spin of a gyroscope to create a complex perturbation, as subjects reached to targets presented in three dimensions. Hand path did not change, but the whole-arm geometry ("arm configuration" in four dimensions) was altered. Over a series of several hundred reaches to various targets, subjects gradually returned the arm movement to its normal configuration. Furthermore, during the course of this learning, subjects used a strategy that involved manipulating arm posture. A similar strategy was observed when subjects made reaching movements with a rod attached to the upper arm to change its inertial characteristics. In both cases, the gradual return to the normal arm movement was accomplished without an increase in kinetic energy, suggesting that arm postures and movements (kinematics) and muscular forces (kinetics) may be mutually optimized. In contrast to previous studies, the present results highlight the role of arm configuration (rather than hand path) in learning and control.

Adaptation, Physiological↗