Search PubMed⌕ Search

Biomedical subjects

H Carnahan

Publications and source records attributed to H Carnahan.

At least 19 recordsLinked to original sources
PubMed2001

Control strategies when intercepting slowly moving targets.

In 3 experiments, the authors investigated and described how individuals control manual interceptive movements to slowly moving targets. Participants (N = 8 in each experiment) used a computer mouse and a graphics tablet assembly to manually intercept targets moving across a computer screen toward a marked target zone. They moved the cursor so that it would arrive in the target zone simultaneously with the target. In Experiment 1, there was a range of target velocities, including some very slow targets. In Experiment 2, there were 2 movement distance conditions. Participants moved the cursor either the same distance as the target or twice as far. For both experiments, hand speed was found to be related to target speed, even for the very slowly moving targets and when the target-to-cursor distance ratios were altered, suggesting that participants may have used a strategy similar to tracking. To test that notion, in Experiment 3, the authors added a tracking task in which the participants tracked the target cursor into the target zone. Longer time was spent planning the interception movements; however, there was a longer time in deceleration for the tracking movements, suggesting that more visually guided trajectory updates were made in that condition. Thus, although participants scaled their interception movements to the cursor speed, they were using a different strategy than they used in tracking. It is proposed that during target interception, anticipatory mechanisms are used rather than the visual feedback mechanism used when tracking and when pointing to stationary targets.

Adult↗
PubMed2001

What causes specificity of practice in a manual aiming movement: vision dominance or transformation errors?

The withdrawal of vision of the arm during a manual aiming task has been found to result in a large increase in aiming error, regardless of the amount of practice in normal vision before its withdrawal. In the present study, the authors investigated whether the increase in error reflects the domination of visual afferent information over the movement representation developed during practice to the detriment of other sources of afferent information or whether it reflects only transformation errors of the location of the target from an allocentric to an egocentric frame of reference. Participants (N = 40) performed aiming movements with their dominant or nondominant arm in a full-vision or target- only condition. The results of the present experiment supported both of those hypotheses. The data indicated that practice does not eliminate the need for visual information for optimizing movement accuracy and that learning is specific to the source or sources of afferent information more likely to ensure optimal accuracy during practice. In addition, the results indicated that movement planning in an allocentric frame of reference might require simultaneous vision of the arm and the target. Finally, practice in a target-only condition, with knowledge of results, was found to improve recoding of the target in an egocentric frame of reference.

Adult↗
PubMed2000

The effect of illusory size on force production when grasping objects.

Milner and Goodale (1995) have proposed that visuomotor and perceptual processes are mediated by discrete visual systems that reflect the functional independence of action and perception. The visuomotor system is proposed to be insensitive to pictorial illusions of object size, whereas the perceptual system is reliably "tricked" by such figures. Brenner and Smeets (1996) and Jackson and Shaw (2000) demonstrated that grasp preshaping, but not grasping force, is immune to the Ponzo visual illusion, suggesting that not all visuomotor processes operate independently of the perceptual system. The present study investigated the effect of illusory object size on prehension kinematics and grasping dynamics (i.e., grip force and load force) as well as perceptual judgements of object size. Unlike previous investigations, object mass was held constant independent of changes in size. The Ponzo figure reliably affected perceptual estimates of object size, but this effect was restricted to one form of the illusion. Some aspects of the prehension movement were sensitive to veridical but not illusory object size (peak grip aperture, peak grip force, peak vertical wrist acceleration), whereas other movement parameters demonstrated illusory size effects (movement time, peak wrist velocity). Still other movement parameters were not sensitive to veridical or illusory object size (peak load force). Together the data suggest that certain prehension components are immune to pictorial illusions of object size, whereas others are not. Complex interactions between the perceptual and visuomotor systems appear to underlie the anticipatory scaling of grasping forces in prehension.

Acceleration↗
PubMed2000

Target velocity effects on manual interception kinematics.

Participants generated manual interception movements toward a target cursor that moved across a computer screen. The target reached its peak velocity either during the first third, at the midpoint, or during the last third of the movement. In Experiment 1 the view of the target was available for either the first 316, 633, 950, or 1267 ms, after which it disappeared. Results showed that for all viewing conditions, the timing of the interception velocity was related to the temporal properties of the target's trajectory. In Experiment 2, when the portion of the target trajectory that was viewed was reversed (such that participants did not see the first 316, 633, 950, or 1267 ms of the trajectory, but instead saw only the later portions of the trajectory), there was no clear relationship between the target trajectory and the timing of the aiming trajectory. These results suggest that participants use visual information early in the target's trajectory to form a representation of the target motion that is used to facilitate manual interception.

Adult↗
PubMed2000

The visuomotor system resists the horizontal-vertical illusion.

The effect of the horizontal-vertical illusion on the visual and visuomotor systems was investigated. Participants (N = 8) viewed horizontal and vertical lines in an inverted-T stimulus and judged whether the two line segments were the same or different lengths. Participants also reached out and grasped either the vertical or the horizontal line segment of the stimulus. Perceptually, participants succumbed to the illusion; that is, they judged Ts of equal horizontal and vertical line lengths to be different and Ts of unequal line lengths to be the same. When reaching toward the same stimuli, however, the size of their grip aperture was scaled appropriately for the various line lengths. Thus, whereas the perceptual system succumbed to the illusion, the visuomotor system did not. Those results support a model proposed by M. A. Goodale and A. D. Milner (1992), who posited separate cortical pathways for visual perception and visually guided action.

Adult↗
PubMed1999

Target viewing time and velocity effects on prehension.

The goal of the present study was to understand which characteristics (movement time or velocity) of target motion are important in the control and coordination of the transport and grasp-preshape components of prehensile movements during an interception task. Subjects were required to reach toward, grasp and lift an object as it entered a target area. Targets approached along a track at four velocities (500, 750, 1000 and 1250 mm/s) which were presented in two conditions. In the distance-controlled condition, targets moving at all velocities traveled the same distance. In the viewing-time-controlled condition, combinations of velocity and starting distances were performed such that the moving target was visible for 1000 ms for all trials. Analyses of kinematic data revealed that when, target distance was controlled, velocity affected all transport-dependent measures; however, when viewing time was controlled, these dependent measures were no longer affected by target velocity. Thus, the use of velocity information was limited in the viewing-time-controlled condition, and subjects used other information, such as target movement time, when generating the transport component of the prehensile movement. For the grasp-preshape component, both peak aperture and peak-aperture velocity increased as target velocity increased, regardless of condition, indicating that target velocity was used to control the spatial aspects of aperture formation. However, the timing of peak aperture was affected by target velocity in the distance-controlled condition, but not in the viewing-time-controlled condition. These results provide evidence for the autonomous generation of the spatial and temporal aspects of grasp preshape. Thus, an independence between the transport and grasp-preshape phases was found, whereby the use of target velocity as a source of information for generating the transport component was limited; however, target velocity was an important source of information in the grasp-preshape phase.

Adolescent↗
PubMed1998

Manual asymmetries in response to rapid target movement.

Two studies are reported that examine reaching responses of the two hands to unexpectedly displaced targets, located along the midline and to the left and right. In the first study, subjects used the index finger to point to the targets. The left hand movement time and time to peak velocity were longer than for the right hand. For both hands, performance was superior when reaching into ipsilateral space. No evidence was found for early (before peak velocity) trajectory corrections in response to perturbations. In the second study, subjects reached and grasped dowels which appeared to move unexpectedly. Again, movement times were smaller when reaching with the right hand when compared to the left, but only on trials when the dowel was located on the subject's right. When subjects reached to the left, the left hand had shorter movement times. Early trajectory corrections were observed in the velocity profiles; the pattern was similar for both hands. These data are discussed in terms of visual processing abilities of the left and right cerebral hemispheres, and visual processing abilities when pointing and grasping.

Hand↗
PubMed1997

An investigation into movement planning and execution deficits in individuals with schizophrenia.

The purpose of this study was to examine whether individuals with schizophrenia demonstrate slowing in movement planning (reaction time), or in movement execution (movement time and velocity), in comparison to normals. Twelve schizophrenic and 12 control right-handed males performed aiming movements with a mouse (controlled by either the left or right hand) on a graphics tablet towards targets of differing sizes and distances appearing on a computer screen. Results showed that, for reaction time, the controls were faster than the schizophrenics and the latter had a left hand advantage for movement preparation while the controls showed no such differences. For movement time, no group differences were found. However, as expected, movement times were shorter (i.e., movements were faster) to the large, and near targets. These results support Fitts' law (Fitts, 1954) which describes a relationship where aiming movement time decreases as target distance decreases, and target size increases. We provide evidence here for a movement planning deficit in schizophrenic patients with no decrement in movement execution compared to controls.

Adult↗
PubMed1997

Anticipatory locomotor adjustments for accommodating versus avoiding level changes in humans.

The control of locomotion has been studied from various perspectives related to the tasks of pattern generation, equilibrium control or adaptation to the environment. The last of these locomotor components has received comparably less attention, specifically pertaining to anticipatory adjustments. Continuing the work which has been conducted on both humans and cats, the present paper explores the nature of the differences in anticipatory locomotor adjustments for obstacle avoidance versus the accommodation to level changes. Six subjects walked in six different environments including no obstructions, a simple obstacle, two different level changes (a platform and stairs), and a combination of an obstacle with each respective level change. Full dynamic analyses allowed comparison of muscle torques as well as muscle power generated and absorbed at the lower limb joints across conditions. It was found that the previously shown robust lower limb reorganization characterized by a knee flexor generation strategy was upheld in all conditions when the obstacle was present. Pure level changes involved an augmentation of the ongoing hip strategy inherent in normal level walking. In the compound environment of obstructed level changes, subjects chose to combine an augmentation of hip flexor power with a reorganization to active knee flexion. The results are discussed from the point of view of general principles of mechanical coordination and the exploitation of intersegmental dynamics for foot transport.

Accommodation, Ocular↗
PubMed1996

Visuomotor control when reaching toward and grasping moving targets.

Two experiments were conducted to examine the influence of target motion on the control and coordination of reaching and grasping movements. In Experiment 1 all the measures which reflected the transport or reaching phase of prehension (movement time, peak velocity, time to peak velocity, and transport tau margin) were sensitive to the velocity of the target, with subjects moving even slower than during the stationary trials, when the target was moving very slowly. Measures which reflected grasp formation such as the size of maximum aperture and the grasping tau margin were not influenced by target speed. To further investigate this dissociation between grasp and transport, different object sizes and even slower object speeds were introduced in Experiment 2. Transport results similar to those found in the first study were observed. However, in Experiment 2, both size of peak aperture and the grasping tau margin were sensitive to target movement. The results of these two studies are discussed in terms of the limitations of optic variables in the visual regulation of movement, and the independence of control of the reach and grasp phase of prehension.

Acceleration↗
PubMed1996

Influence of object size on prehension in leukotomized and unleukotomized individuals with schizophrenia.

The aim of this study was to investigate the influence of schizophrenia and frontal leukotomy on the control and perceptual-motor coordination of reaching and grasping movements. Reaches to various-sized drinking glasses were videotaped for unleukotomized and leukotomized adults with schizophrenia, and for healthy age-matched controls. Kinematic analyses showed that while the two patient groups moved more slowly than the controls, the proportion of overall movement time spent accelerating to reach peak velocity and the proportion of overall movement time spent decelerating before contact with the glasses was similar for all three groups. For grasp formation, the two patient groups, which did not differ, opened their hands wider than did the controls; however, all three groups opened their hands to the appropriate size in response to the various glass sizes. The frontal lesions of the leukotomized patients were not related to motor control deficits when compared to the control schizophrenic patients. As well, the perceptual abilities of the patients were adequate as evidenced by their ability to scale their grasp to glass size.

Aged↗
PubMed1996

The influence of summary knowledge of results and aging on motor learning.

This study examined whether older adults (mean age = 75.0 years) use summary knowledge of results (KR) to facilitate learning in a manner similar to that of young adults (mean age = 22.5 years). All subjects were required to learn a computer-key-pressing task in a specified goal time. During acquisition, subjects received either KR after every trial, or summary KR. All subjects then performed no-KR retention trials. In acquisition, KR after every trial facilitated timing accuracy for both the younger and older groups in comparison to the summary KR groups. The young subjects were equally variable in both KR practice groups. For older subjects summary KR facilitated more consistent performance. In retention, the summary groups were more accurate than those subjects who received KR after every trial. There were no accuracy or variability differences between the two age groups during retention. These results suggest that older adults are able to use summary KR to facilitate learning in a manner similar to that of young adults.

Adult↗
PubMed1996

Delayed visual feedback while learning to track a moving target.

Two studies investigated the effects of delayed visual feedback on manual tracking. In Experiment 1, individuals practiced with visual feedback provided either immediately (0 delay) or with a 333-ms delay. During acquisition, the 0 delay group performed with less error than the 333-ms delay group. A retention test with 0 delay feedback was performed with the least error by the 0 delay group. A transfer test using a different 0 delay tracking pattern, was performed with the least error by the 333-ms delay group. In Experiment 2, individuals practiced at six different delays. Error increased as training feedback delay increased. For retention there were no differences between the delay groups during the 0 delay retention. At a 417-ms retention, test error decreased as training feedback delay increased. Results indicate that error during acquisition does not necessarily impair learning and that feedback delays can be beneficial for learning.

Analysis of Variance↗
PubMed1996

Proprioception during manual aiming in individuals with shoulder instability and controls.

Recurrent glenohumeral joint instability is a common orthopaedic problem. One possible cause of this repeated instability is a lack of neuromuscular control and kinaesthetic sense of the glenohumeral joint. The purpose of this study was to determine whether there is a deficit in joint proprioception in subjects with recurrent anterior glenohumeral instability as compared with individuals with no previous shoulder pathology. Subjects were asked to generate pointing movements with their uninjured limb and to match this limb position with the injured limb. Movements of the pointing limb were measured with an optoelectric three-dimensional movement analysis system. These movements were performed in three conditions: 1) with full vision, 2) without vision, and 3) without vision with vibration to the posterior deltoid muscle. For the temporal and spatial measures, there were no significant differences between the control and shoulder instability groups. However, the kinematic data describing arm trajectory formation showed a performance decrement in the no vision with vibration condition for the subjects in the shoulder instability group, suggesting that they suffer from a proprioceptive deficit.

Adult↗
PubMed1995

Manual performance in leukotomized and unleukotomized individuals with schizophrenia.

Seven leukotomized adults with schizophrenia (LS), eight unleukotomized adults with schizophrenia (ULS), and eight healthy control (C) individuals were required to reach toward and grasp a small object that was either stationary or moving. Reflective markers were placed on the subject's index finger, thumb and wrist, and movements were videotaped. As expected the LS and ULS groups moved slower than the C group when the target was stationary. However, when the target was moving, all three groups moved faster, with the LS and C groups having the same movement times, and the ULS group having the fastest movement time. When the timing of the reaching trajectory was assessed, the LS group spent less time decelerating and closing their hands around the object, indicating their movements were controlled with less precision. When grasp formation was analyzed, for the stationary condition, the maximum apertures of the LS and ULS groups were not different, and both were larger than those of the C group. For the moving target condition, aperture increased for all groups but was smallest for the C group, intermediate for the LS group and largest for the S group. There was actually less within subject variability in peak aperture and maximum aperture closing speed for the LS and ULS groups in comparison to the C group, perhaps indicating a limited repertoire of potential motor responses for the patient groups. These results suggest that individuals with schizophrenia are able to use redundant information as well as controls, and that leukotomized individuals with schizophrenia have greater motor control deficits than unleukotomized schizophrenics.

Adult↗
PubMed1995

A preliminary analysis of the coordination of reaching, grasping, and walking.

The purpose of this investigation was to describe the strategies used by individuals when motor systems controlling locomotion and prehension must be used simultaneously to perform a task. Subjects were required to perform five tasks: walk normally, walk and pick up a small object, walk and pick up a large object, pick up a small object from a stationary standing position, and pick up a large object while standing. The grasping and walking movements were videotaped and the frequencies of various behaviors were observed (type of grasp, location of contact with the object, support leg during contact with the object, timing of gaze toward object). Characteristics of the grasp were influenced by object size, not by whether the subject was walking. Gaze was shifted to the object before initiation of reach and this pattern was not influenced by size of the object or movement of the subject. Finally, subjects preferred using an ipsilateral support leg while generating the reach, which is a deviation from normal gait patterns. Findings indicate that the motor control system may be hierarchically organized in such a way that stability of the lower limbs supersedes the control of the movement patterns of an upper limb.

Adult↗
PubMed1995

Relationship between movement planning and psychopathology profiles in schizophrenia.

BACKGROUND: There has been evidence that psychopathology in schizophrenia consists of three separable syndromes: reality distortion, disorganisation, and psychomotor poverty. The objective of this study was to explore the relationship between planning and execution of movement and each of the syndromes in schizophrenia. METHOD: Twenty-one right handed DSM-III-R schizophrenic patients performed a total of 80 trials of a motor movement task, varying distance of movement x size of the target x hand. Times taken to plan the movement (RT) and to carry it out (MT) were examined for their relationship with contemporaneous as well as lifetime profiles of the three syndromes in schizophrenia. RESULTS: Significant correlations are reported between RT and current as well as lifetime measures of disorganisation syndrome. Somewhat weaker correlations are reported between RT and psychomotor poverty, but only for the right-handed tasks. Partial correlations suggest that the influence of neuroleptic medication explains all but one of the correlations between psychomotor poverty and RT, but does not account for the relationship between disorganisation and RT. No other relationship emerged between any of the movement and symptom measures. CONCLUSIONS: These findings indicate that dysfunction in movement planning is related primarily to concurrent disorganisation, as well as to the prominence of disorganisation over the patient's history.

Adult↗