James Gordon

Accuracy of planar reaching movements

This study examined the variability in movement end points in a task in which human subjects reached to targets in different locations on a horizontal surface. The primary purpose was to determine whether patterns in the variable errors would reveal the nature and origin of the coordinate system in which the movements were planned. Six subjects moved a hand-held cursor on a digitizing tablet. Target and cursor positions were displayed on a computer screen, and vision of the hand and arm was blocked. The screen cursor was blanked during movement to prevent visual corrections. The paths of the movements were straight and thus directions were largely specified at the onset of movement. The velocity profiles were bell-shaped, and peak velocities and accelerations were scaled to target distance, implying that movement extent was also programmed in advance of the movement. The spatial distributions of movement end points were elliptical in shape. The major axes of these ellipses were systematically oriented in the direction of hand movement with respect to its initial position. This was true for both fast and slow movements, as well as for pointing movements involving rotations of the wrist joint. Using principal components analysis to compute the axes of these ellipses, we found that the eccentricity of the elliptical dispersions was uniformly greater for small than for large movements: variability along the axis of movement, representing extent variability, increased markedly but nonlinearly with distance. Variability perpendicular to the direction of movement, which results from directional errors, was generally smaller than extent variability, but it increased in proportion to the extent of the movement. Therefore, directional variability, in angular terms, was constant and independent of distance. Because the patterns of variability were similar for both slow and fast movements, as well as for movements involving different joints, we conclude that they result largely from errors in the planning process. We also argue that they cannot be simply explained as consequences of the inertial properties of the limb. Rather they provide evidence for an organizing mechanism that moves the limb along a straight path. We further conclude that reaching movements are planned in a hand-centered coordinate system, with direction and extent of hand movement as the planned parameters. Since the factors which influence directional variability are independent of those that influence extent errors, we propose that these two variables can be separately specified by the brain.

Trajectory control in targeted force impulses

SummaryIn the preceding study (Gordon and Ghez 1987), we showed that accurately targeted isometric force impulses produced by human subjects are governed by a pulse height control policy. Different peak forces were achieved by modulating the rate of rise of force while force rise time was maintained close to a constant value and independent of peak force. An early measure of the rate of rise of force, peak d2F/dt2, was scaled to the required force (target amplitude) and highly predictive of the peak force achieved. In six subjects examined, peak d2F/dt2 accounted for between 70% and 96% of the total variance in peak force. In the present study, we further examined these targeted responses to determine whether the residual variability not predicted by peak d2F/dt2 could be accounted for by adjustments to the force trajectories which compensated for initial errors in the scaling of the d2F/dt2. A statistical model of the determinants of peak force was tested. This model included two paths by which the target amplitude could independently influence the peak force achieved. The first path was preprogrammed pulse height control. In this path, target amplitude determined the initial rate of rise of force (peak d2F/dt2) which in turn determined the final peak force achieved. The second path was an independent influence of errors in the initial scaling of peak d2F/dt2 on peak force. Multiple regression analysis was performed on trajectory variables within the sets of responses by each subject in each condition to determine whether the second path contributed significantly to explaining the variance in peak force. In each subject and condition, there was a significant independent influence of error in d2F/dt2 on peak force, and the direction of this effect was to decrease the magnitudes of peak force errors. These compensatory adjustments accounted for between 1% and 14% of the total variance in peak force. Further multiple regression analyses revealed that inappropriate scaling of the initial phase of the trajectories was compensated for by shortening or prolonging the force rise time. These trajectory adjustments were in turn implemented by modulation of the timing and magnitude of the contractions in the agonist and antagonist muscles that produced the force trajectories. Because these compensatory adjustments were evident in the EMG pattern at latencies too short to be accounted for by peripheral feedback, we assume that they depend on internal monitoring of the unfolding neural commands. These internal feedback processes act in parallel with the programmed commands, both determining the force trajectory.

Accuracy of planar reaching movements - II. Systematic extent errors resulting from inertial anisotropy

This study examines the source of directiondependent errors in movement extent made by human subjects in a reaching task. As in the preceding study, subjects were to move a cursor on a digitizing tablet to targets displayed on a computer monitor. Movements were made without concurrent visual feedback of cursor position, but movement paths were displayed on the monitor after the completion of each movement. We first examined horizontal hand movements made at waist level with the upper arm in a vertical orientation. Targets were located at five distances and two directions (30° and 150°) from one of two initial positions. Trajectory shapes were stereotyped, and movements to more distant targets had larger accelerations and velocities. Comparison of movements in the two directions showed that in the 30° direction responses were hypermetric, accelerations and velocities were larger, and movement times were shorter. Since movements in the 30° direction required less motion of the upper arm than movements in the 150° direction, we hypothesized that the differences in accuracy and acceleration reflected a failure to take into account the difference in total limb inertia in the two directions. To test this hypothesis we simulated the initial accelerations of a two-segment limb moving in the horizontal plane with the hand at shoulder level when a constant force was applied at the hand in each of 24 directions. We compared these simulated accelerations to ones produced by our subjects with their arms in the same position when they aimed movements to targets in the 24 directions and at equal distances from an initial position. The magnitudes of both simulated and actual accelerations were greatest in the two directions perpendicular to the forearm, where inertial resistance is least, and lowest for movements directed along the axis of the forearm. In all subjects, the directional variation in peak acceleration was similar to that predicted by the model and shifted in the same way when the initial position of the hand was displaced. The pattern of direction-dependent variations in initial acceleration did not depend on the speed of movement. It was also unchanged when subjects aimed their movements toward targets presented within the workspace on the tablet instead of on the computer monitor. These findings indicate that, in programming the magnitude of the initial force that will accelerate the hand, subjects do not fully compensate for direction-dependent differences in inertial resistance. The direction-dependent differences in peak acceleration were associated with systematic variations in movement extent in all subjects, but the variations in extent were proportionately smaller than those in acceleration. This compensation for inertial anisotropy, which differed in degree among subjects, was associated with changes in movement duration. The possible contributions of elastic properties of the musculoskeletal system and proprioceptive feed-back to the compensatory variations in movement time are discussed. The finding that the magnitude of the initial force that accelerates the hand is planned without regard to movement direction adds support for the hypothesis that extent and direction of an intended movement are planned independently. Furthermore, the lack of compensation for inertia in the acceleration of the simple reaching movements studied here suggests that they are planned by the central nervous system without explicit inverse kinematic and dynamic computations.

Color vision in the peripheral retina. II. Hue and saturation.

Hue and saturation of spectral lights were measured (direct scaling) in the fovea and at 45 degrees in the periphery; all lights were of equal photopic retinal illuminance (1200 trolands). At each retinal location both large and small targets were used. As shown by previous studies, small peripheral targets appear desaturated and of uncertain hue, except long wavelengths which appear red. However, if target size is increased, saturation increases and a full range of hues is seen; the hue functions for large peripheral targets are comparable to foveal ones for very small targets. From a modified form of color matching, it was concluded that the color deficiency in the periphery is more tritanlike than deutanlike; this is strengthened by the observation, that, for small peripheral targets, hues are generally apportioned between two hue categories and the change from one to the other is at about 580 nm.

Bringing good teaching cases "to life": a simulator-based medical education service.

Realistic medical simulation has expanded worldwide over the last decade. Such technology is playing an increasing role in medical education not merely because simulator sessions are enjoyable, but because they can provide an enhanced environment for experiential learning and reflective thought. High-fidelity patient simulators allow students of all levels to “practice” medicine without risk, providing a natural framework for the integration of basic and clinical science in a safe environment. Often described as “flight simulation for doctors,” the rationale, utility, and range of medical simulations have been described elsewhere, yet the challenges of integrating this technology into the medical school curriculum have received little attention. The authors report how Harvard Medical School established an on-campus simulator program for students in 2001, building on the work of the Center for Medical Simulation in Boston. As an overarching structure for the process, faculty and residents developed a simulator-based “medical education service”—like any other medical teaching service, but designed exclusively to help students learn on the simulator alongside a clinician-mentor, on demand. Initial evaluations among both preclinical and clinical students suggest that simulation is highly accepted and increasingly demanded. For some learners, simulation may allow complex information to be understood and retained more efficiently than can occur with traditional methods. Moreover, the process outlined here suggests that simulation can be integrated into existing curricula of almost any medical school or teaching hospital in an efficient and cost-effective manner.

EMG patterns in antagonist muscles during isometric contraction in man: Relations to response dynamics

SummaryWe studied the EMG activity of biceps and triceps in human subjects during isometric force adjustments at the elbow. Rapid targeted force pulses exhibited stereotyped trajectories in which peak force was a linear function of the derivatives of force and the time to peak force was largely independent of its amplitude. These responses were associated with an alternating triphasic pattern of EMG bursts in agonist and antagonist muscles similar to that previously described for rapid limb movements. When the instructions demanded rapid force pulses, initial agonist bursts were of constant duration, and their magnitude was strongly related to peak force achieved. The timing of EMG bursts in antagonist pairs was closely coupled to the dynamics of the force trajectory, and the rising phase of the force was determined by both agonist and antagonist bursts. When peak force was kept constant and rise time systematically varied, the presence and magnitude of antagonist and late agonist bursts were dependent on the rate of rise of force, appearing at a threshold value and then increasing in proportion to this parameter. It is proposed that antagonist activity compensates for nonlinearity in muscle properties to enable the linear scaling of targeted forces which characterizes performance in this task.

Color appearance in the peripheral retina: effects of stimulus size

Hue and saturation scaling were used to measure the appearance of spectral lights as a function of stimulus size for nine loci across the horizontal retinal meridian. At a given locus, each hue (R, Y, G, and B) grew as a function of stimulus size up to some asymptotic value. The parameter values of Michaelis-Menten growth functions fitted to the hue data were used to derive the sizes of the so-called perceptive fields of the hue mechanisms. The fields for all mechanisms increased with eccentricity, and this increase was greater on the temporal than on the nasal retina. By increasing stimulus size it was possible to achieve fovealike color vision to eccentricities of 20 deg. However, even the largest stimuli failed to produce fully saturated hues at 40 deg. The retinal size scales of the four hue mechanisms were not the same; those for R and B were similar, and these mechanisms had the smallest perceptive fields everywhere. The perceptive fields of the hue mechanisms at all loci were larger than anatomical estimates of the sizes of retinal receptive fields.

Organization of voluntary movement

There have recently been a number of advances in our knowledge of the organization of complex, multi-joint movements. Promising starts have been made in our understanding of how the motor system translates information about the location of external targets into motor commands encoded in a body-based coordinate system. Two simplifying strategies for trajectory control that are discussed are parallel specification of response features and the programming of equilibrium trajectories. New insights have also been gained into how neural systems process sensory information to plan and assist with task performance. A number of recent papers emphasize the feedforward use of sensory input, which is mediated through models of the external world, the bodys physical plant, and the task structure. These models exert their influence at both reflex and higher levels and permit the preparation of predictive default parameters of trajectories as well as strategies for resolving task demands.

National Growth in Simulation Training within Emergency Medicine Residency Programs, 2003-2008

OBJECTIVES The use of medical simulation has grown dramatically over the past decade, yet national data on the prevalence and growth of use among individual specialty training programs are lacking. The objectives of this study were to describe the current role of simulation training in emergency medicine (EM) residency programs and to quantify growth in use of the technology over the past 5 years. METHODS In follow-up of a 2006 study (2003 data), the authors distributed an updated survey to program directors (PDs) of all 179 EM residency programs operating in early 2008 (140 Accreditation Council on Graduate Medical Education [ACGME]-approved allopathic programs and 39 American Osteopathic Association [AOA]-accredited osteopathic programs). The brief survey borrowed from the prior instrument, was edited and revised, and then distributed at a national PDs meeting. Subsequent follow-up was conducted by e-mail and telephone. The survey concentrated on technology-enhanced simulation modalities beyond routine static trainers or standardized patient-actors (high-fidelity mannequin simulation, part-task/procedural simulation, and dynamic screen-based simulation). RESULTS A total of 134 EM residency programs completed the updated survey, yielding an overall response rate of 75%. A total of 122 (91%) use some form of simulation in their residency training. One-hundred fourteen (85%) specifically use mannequin-simulators, compared to 33 (29%) in 2003 (p < 0.001). Mannequin-simulators are now owned by 58 (43%) of the programs, whereas only 9 (8%) had primary responsibility for such equipment in 2003 (p < 0.001). Fifty-eight (43%) of the programs reported that annual resident simulation use now averages more than 10 hours per year. CONCLUSIONS Use of medical simulation has grown significantly in EM residency programs in the past 5 years and is now widespread among training programs across the country.

Entrainment to video displays in primary visual cortex of macaque and humans

Cathode ray tubes (CRTs) display images refreshed at high frequency, and the temporal waveform of each pixel is a luminance impulse only a few milliseconds long. Although humans are perceptually oblivious to this flicker, we show in V1 in macaque monkeys and in humans that extracellularly recorded action potentials (spikes) and visual-evoked potentials (VEPs) align with the video impulses, particularly when high-contrast stimuli are viewed. Of 91 single units analyzed in macaque with a 60 Hz video refresh, 29 cells (32%) significantly locked their firing to a uniform luminance display, but their number increased to 75 (82%) when high-contrast stimuli were shown. Of 92 cells exposed to a 100 Hz refresh, 21 (23%) significantly phase locked to high-contrast stimuli. Phase locking occurred in both input and output layers of V1 for simple and complex cells, regardless of preferred temporal frequency. VEPs recorded in humans showed significant phase locking to the video refresh in all seven observers. Like the monkey neurons, human VEPs more typically phase locked to stimuli containing spatial contrast than to spatially uniform stimuli. Phase locking decreased when the refresh rate was increased. Thus in humans and macaques phase locking to the high strobe frequency of a CRT is enhanced by a salient spatial pattern, although the perceptual impact is uncertain. We note that a billion people worldwide manage to watch TV without obvious distortion of their visual perception despite extraordinary phase locking of their V1s to a 50 or 60 Hz signal.