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Biomedical subjects

A P Georgopoulos

Publications and source records attributed to A P Georgopoulos.

At least 19 recordsLinked to original sources
PubMed1992

The motor cortex and the coding of force.

The relation of cellular activity in the motor cortex to the direction of two-dimensional isometric force was investigated under dynamic conditions in monkeys. A task was designed so that three force variables were dissociated: the force exerted by the subject, the net force, and the change in force. Recordings of neuronal activity in the motor cortex revealed that the activity of single cells was directionally tuned and that this tuning was invariant across different directions of a bias force. Cell activity was not related to the direction of force exerted by the subject, which changed drastically as the bias force changed. In contrast, the direction of net force, the direction of force change, and the visually instructed direction all remained quite invariant and congruent and could be the directional variables, alone or in combination, to which cell activity might relate.

Animals↗
PubMed1992

Three-dimensional drawings in isometric conditions: planar segmentation of force trajectory.

Normal human subjects grasped an isometric handle with an unrestrained, pronated hand. They were asked to exert forces continuously to draw lemniscates (figure eights) in specified or self-chosen planes and in the presence or absence of a three-dimensional visual feedback cursor and a visual template. In every condition, the mean plane orientation in the force space differed appreciably between the two loops of the figure, as described previously by Soechting and Terzuolo (1987a) for free drawing arm movements. These findings suggest that the planar segmentation of the motor trajectory is not a consequence of joint motion but arises from central constraints related to the production of motor trajectory in space.

Feedback↗
PubMed1992

Three-dimensional drawings in isometric conditions: relation between geometry and kinematics.

Normal human subjects grasped a 3-D isometric handle with an otherwise unrestrained, pronated hand and exerted forces continuously to draw circles, ellipses and lemniscates (figure-eights) in specified planes in the presence or absence of a 3-D visual force-feedback cursor and a visual template. Under any of these conditions and in all subjects, a significant positive correlation was observed between the instantaneous curvature and angular velocity, and between the instantaneous radius of curvature and tangential velocity; that is, when the force trajectory was most curved, the tangential velocity was lowest. This finding is similar to that obtained by Viviani and Terzuolo (1982) for 2-D drawing arm movements and supports the notion that central constraints give rise to the relation between geometric and kinematic parameters of the trajectory.

Female↗
PubMed1992

Motor cortical activity in a memorized delay task.

Two rhesus monkeys were trained to move a handle on a two-dimensional (2D) working surface in directions specified by a light at the plane. They first captured with the handle a light on the center of the plane and then moved the handle in the direction indicated by a peripheral light (cue signal). The signal to move (go signal) was given by turning off the center light. The following tasks were used: (a) In the non-delay task the peripheral light was turned on at the same time as the center light went off. (b) In the memorized delay task the peripheral light stayed on for 300 ms and the center light was turned off 450-750 ms later. Finally, (c) in the non-memorized delay task the peripheral light stayed on continuously whereas the center light went off 750-1050 ms after the peripheral light came on. Recordings in the arm area of the motor cortex (N = 171 cells) showed changes in single cell activity in all tasks. In both delay tasks, the neuronal population vector calculated every 20 ms after the onset of the peripheral light pointed in the direction of the upcoming movement, which was instructed by the cue light. Moreover, the strength of the population signal showed an initial peak shortly after the cue onset in both the memorized and non-memorized delay tasks but it maintained a higher level during the memorized delay period, as compared to the non-memorized task.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Cognitive spatial-motor processes. 7. The making of movements at an angle from a stimulus direction: studies of motor cortical activity at the single cell and population levels.

Two rhesus monkeys were trained to move a handle on a two-dimensional (2-D) working surface either towards a visual stimulus ("direct" task) or in a direction orthogonal and counterclockwise (CCW) from the stimulus ("transformation" task), depending on whether the stimulus appeared dim or bright, respectively. Thus the direction of the stimulus (S, in polar coordinates) and the direction of the movement (M) were the same in the direct task but differed in the transformation task, such that M = S + 90 degrees CCW. The task (i.e. brightness) condition (k = 2, i.e. direct or transformation) and the direction of the stimulus (m = 8, i.e. 8 equally spaced directions on a circle) resulted in 16 combinations (k x m = 16 "classes") that were varied from trial to trial in a randomized block design. In 8 of these combinations the direction of the stimulus was the same for both tasks, whereas the direction of the movement was the same in the remaining 8 cases. The electrical signs of cell activity (N = 394 cells) in the arm area of the motor cortex (contralateral to the performing arm) were recorded extracellularly. The neural activity was analyzed at the single cell and neuronal population levels, and a modeling of the time course of single activity during the transformation task was carried out. We found the following. (a) Individual cells were active in both tasks; no cells were found that were active exclusively in only one of the two tasks. The patterns of single cell activity in the transformation task frequently differed from those observed in the direct task when the stimulus or the movement were the same. More specifically, cells could not be consistently classified as "movement"-or "stimulus"-related for frequently the activity of a particular cell would seem "movement-related" for a particular stimulus-movement combination, "stimulus-related" for another combination, or unrelated to either movement or stimulus for still another combination. Thus no real insight could be gained from such an analysis of single cell activity. (e) In a different analysis, we explored the idea that a changing directional signal could be detected in the time course of single cell activity during the reaction time. For that purpose we modeled the time course of single activity observed in the transformation task as a linear, weighted combination of influences from the direct task, taking the time patterns of cell activity during the stimulus, intermediate and movement directions in the direct task as estimates of the postulated directional influences.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Variance↗
PubMed1991

Cognitive spatial-motor processes. 4. Specification of the direction of visually guided isometric forces in two-dimensional space: information transmitted and effects of visual force-feedback.

The information transmitted (Ti) by the direction of two-dimensional (2-D) isometric forces at different stereoscopic depths was studied in 50 naive human subjects using an isometric manipulandum and random dot stereograms generated in a color display (Massey et al. 1988). Subjects viewed the display through appropriate color filters and perceived the image of a disk rotated about a horizontal axis on the frontal plane; the top of the disk was rotated around that axis by 15, 45, 60 and 80 degrees away from the subject. Each of these disks involved a different amount of stereoscopic depth perception which was lowest for the 15 degrees and highest for the 80 degrees tilt. Subjects were instructed to exert force in the direction of a visual target presented on the disk in a reaction time task. The instantaneous force exerted by the subjects on the manipulandum was shown on the disk in the form of a feedback cursor. Information transmitted, reaction time (RT) and systematic directional deviations were calculated. We found the following. (a) Ti increased with input information but at a lower rate; at the highest level of input information studied (5.91 bits), Ti was 4.1 bits at the 15 degrees tilt. This high value of Ti suggests that directional information for isometric force is processed very efficiently. However, this Ti was consistently lower than that transmitted by the direction of movement (Georgopoulos and Massay, 1988). (b) Ti did not differ significantly among the 15-60 degrees tilt but was 0.19 bits less for the 80 degrees tilt. RT did not differ among the 15-80 degrees tilts.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Cognitive spatial-motor processes. 5. Specification of the direction of visually guided isometric forces in two-dimensional space: time course of information transmitted and effect of constant force bias.

The effects of an external constant force bias on the information transmitted (Ti) by the direction of isometric force exerted in 2-dimensional (2-D) space by human subjects were studied using an isometric manipulandum and random dot stereograms generated in a color display (Massey et al. 1988, Massey et al. 1990). Subjects exerted force on the manipulandum such that a visual force-feedback cursor would move in the direction of a visually defined stimulus in the stereo display. The time course of force development and the gain of directional information during increasing force intensity were also studied. We found the following. (a) When no bias force was applied, the force exerted by the subject increased from near zero to greater than 200 gram-force at the end of a trial and was close to the visually defined direction. When a constant bias force of 110 gram-force was applied in various directions in blocks of trials, the force exerted by the subject increased in time, as above; however, its direction also changed in time so that the instantaneous vector sum of the bias force and the force exerted by the subject pointed close to the visually defined direction. The Ti and the reaction time (RT) did not differ significantly in the two experimental conditions. These results suggest that the directional control of isometric forces is very efficient, especially in relation to visuomotor coordination. (b) The Ti was calculated at various levels of force intensity, as the latter increased from approximately 50 gram-force to 200 gram-force.(ABSTRACT TRUNCATED AT 250 WORDS)

Cognition↗
PubMed1991

Cognitive spatial-motor processes. 6. Visuomotor memory scanning.

Fourteen human subjects performed in a modified Sternberg memory-scanning task. First, they made a series of 2-6 movements in different directions from a central point towards peripheral lights on a planar working surface ("list trials"). Then, after a warning signal, one of the previous list stimuli, except the last, was presented again ("test trial"). Subjects were instructed to move in the direction of the stimulus which was presented next in sequence in the list. The mean reaction time (RT) in the test trials increased as a linear function of the number of movements, S, in the list: Mean RT (ms) = 105 + 205.8S (2 less than or equal to S less than or equal to 6). This finding suggests that the task involves memory scanning of visuomotor list items.

Cognition↗
PubMed1991

Cortical control of motor behavior at the cellular level.

The studies reviewed in this paper describe the relations of single-cell activity in central motor structures to complex visuomotor tasks and document the fact that various cortical areas process visuomotor information in parallel. Moreover, the studies provide clear evidence that the map in the motor cortex is modifiable and dynamically maintained.

Animals↗
PubMed1990

Parietal cortex neurons of the monkey related to the visual guidance of hand movement.

A class of neurons specifically related to hand movements was studied in the posterior parietal cortex while the monkeys manipulated different types of objects. We examined the neuronal activity during manipulation of objects by the hand in the light and in the dark. Fifty-five neurons were active during manipulation in the dark and were classified as "hand-movement-related" neurons. Of these, 38/55 (69%) cells were also influenced by the visual stimulus. Most of the hand-movement-related neurons were selective in the type of objects manipulated. Moreover, some of these cells were selective in the axis of orientation of the object. These results suggest that the hand-movement-related neurons of the parietal cortex are concerned with the visual guidance of the hand movement, especially in matching the pattern of movement with the spatial characteristics of the object to be manipulated.

Animals↗
PubMed1989

Visuomotor coordination in reaching and locomotion.

Locomotion and reaching have traditionally been regarded as separate motor activities. In fact, they may be closely connected both from an evolutionary and a neurophysiological viewpoint. Reaching seems to have evolved from the neural systems responsible for the active and precise positioning of the limb during locomotion; moreover, it seems to be organized in the spinal cord. The motor cortex and its corticospinal outflow are preferentially engaged when precise positioning of the limb is needed during locomotion and are also involved during reaching and active positioning of the hand near objects of interest. All of these motor activities require visuomotor coordination, and it is this coordination that could be achieved by the motor cortex and interconnected parietal and cerebellar areas.

Animals↗
PubMed1989

Mental rotation of the neuronal population vector.

A rhesus monkey was trained to move its arm in a direction that was perpendicular to and counterclockwise from the direction of a target light that changed in position from trial to trial. Solution of this problem was hypothesized to involve the creation and mental rotation of an imagined movement vector from the direction of the light to the direction of the movement. This hypothesis was tested directly by recording the activity of cells in the motor cortex during performance of the task and computing the neuronal population vector in successive time intervals during the reaction time. The population vector rotated gradually counterclockwise from the direction of the light to the direction of the movement at an average rate of 732 degrees per second. These results provide direct, neural evidence for the mental rotation hypothesis and indicate that the neuronal population vector is a useful tool for "reading out" and identifying cognitive operations of neuronal ensembles.

Animals↗
PubMed1989

Cognitive spatial-motor processes. 3. Motor cortical prediction of movement direction during an instructed delay period.

We studied the activity of 123 cells in the arm area of the motor cortex of three rhesus monkeys while the animals performed a 2-dimensional (2-D) step-tracking task with or without a delay interposed between a directional cue and a movement triggering signal. Movements of equal amplitude were made in eight directions on a planar working surface, from a central point to targets located equidistantly on a circle. The appearance of the target served as the cue, and its dimming, after a variable period of time (0.5-3.2 s), as the "go" stimulus to trigger the movement to the target; in a separate task, the target light appeared dim and the monkey moved its hand towards it without waiting. Population histograms were constructed for each direction after the spike trains of single trials were aligned to the onset of the cue. A significant increase (3-4x) in the population activity was observed 80-120 ms following the cue onset; since the minimum delay was 500 ms and the average reaction time approximately 300 ms, this increase in population activity occurred at least 680-720 ms before the onset of movement. A directional analysis (Georgopoulos et al. 1983, 1984) of the changes in population activity revealed that the population vector during the delay period pointed in the direction of movement that was to be made later.

Evoked Potentials↗
PubMed1988

Callosal and association neurons in the cortical space: a spectral analysis approach.

The tangential distributions of callosal neurons of area 5 projecting homotopically to the contralateral hemisphere and of association neurons of areas 4 and 6 projecting to ipsilateral area 5 were determined in the macaque monkey by using neuroanatomical methods based on the retrograde transport of horseradish peroxidase. Both distributions were studied qualitatively through 2-dimensional reconstructions of the cortical areas of origin and quantitatively through a spectral analysis. This approach facilitated the characterization of the spatial periodicities contained in these distributions revealing that, in area 5, callosal neurons were organized in bands of various shapes and width; these bands were composed of more discrete clusters of cells. In the frontal lobe, association neurons projecting to ipsilateral area 5 were arranged similarly. This study suggests that a common principle underlies the tangential organization of both callosal and association projecting cells in different cortical areas and emphasizes a basic similarity of interhemispheric and intrahemispheric connections.

Animals↗
PubMed1988

Cognitive spatial-motor processes. 2. Information transmitted by the direction of two-dimensional arm movements and by neuronal populations in primate motor cortex and area 5.

We measured the amount of information transmitted by the direction of two-dimensional (2-D) arm movements of human subjects, and by neuronal populations in the motor cortex and area 5 of monkeys. We also compared the information transmitted by a motor cortical population when the predictability of the direction of movement was varied, i.e. when the target of the movement was the same, or different, in successive trials. The information transmitted by a neuronal population was measured using the "population vector" code (Georgopoulos et al. 1983, 1986) as the stimulated directional output of the population. We found the following. (a) The information transmitted per movement increased in all cases with input information but more slowly than the maximum possible. No asymptote had been reached at 6.64 bits of input information; the best human performance at that level was 4.5 bits. (b) The average information transmitted by a motor cortical population (N = 253 cells) was approximately 0.5 bits higher than that of best human performance at all levels of input information. These findings indicate that whereas information loss at the level of motor cortex, and during stages preceding it, increases with increasing input information, this loss remains constant at all levels of input information during processes intervening between motor cortex and movement. (c) The information transmitted by the neuronal population at a constant level of input information increased with the number of cells in the population and had not plateaued at the population size of 253 cells used in this study. (d) Consistently higher amounts of information were transmitted by motor cortical than by area 5 populations at all levels of input information (up to 7 bits) examined. The difference was approximately 0.16 bits. (e) Finally, the information transmitted by a motor cortical population was slightly higher (by 0.07 bits) when the movement was more than less predictable. Although this difference was small, it was consistent at all levels of input information (3-7 bits) and was statistically highly significant (p less than 0.005, paired t test). These results establish the information theoretical approach as a useful method for comparing (a) behavioral and neural data, (b) neural data between different brain areas, and (c) neural data obtained under different behavioral manipulations.

Adult↗
PubMed1988

A method for studying the control of three-dimensional isometric forces using dynamic stereogram.

A system has been developed for measuring the three-dimensional (3D) isometric forces produced by the arm in response to a visual stimulus. The output of 3 load cells is combined to determine the components of the 3D response force. The maximum force range of the load cells is 2000 g which can be read with a resolution of 4 g and with a measured accuracy of +/- 10 g. An overload stop protects the device to a load of 23 kg. The visual instruction to the subject concerning the direction and magnitude of the 3D force vector to be produced is presented using a vivid spherical stereogram. This stereogram is produced using the anaglyphic technique where the left and right images are separated by color filters. A dense aggregation of points defines the target cursor, which can be positioned anywhere inside or on the surface of the sphere. This cursor can represent a stationary or a moving target. A force feedback cursor can be provided to indicate to the subject his/her response. The color display on the cathode ray tube is produced by a display generator driven by a computer.

Amplifiers, Electronic↗