Orienting the finger opposition space during prehension movements

Two experiments are reported that examined the act of prehension when subjects were asked to grasp with their thumb and index finger pads an elongated object resting horizontally on a surface and placed at different orientations with respect to the subject. In Experiment 1, the pad opposition preferences were determined for the six angles of orientation examined. For angles of 90 degrees (object parallel to frontal plane) or less, no rotation of the wrist (pronation) was used; for angles 110 degrees or greater, pronation was systematically employed to reorient the finger opposition space. Only one angle, 100 degrees , produced any evidence of ambiguity in how to grasp the object: Approximately 60% of these grasps involved pronation and 40% did not. Using the foregoing grasp preference data, in Experiment 2 we examined the kinematics of the wrist and elbow trajectories during prehension movements directed at an object in different orientations. Movement time, time to peak acceleration, velocity, and deceleration were measured. No kinematic differences were observed when the object orientation either required (110 degrees ) or did not require (80 degrees ) a pronation. By contrast, if the orientation was changed at the onset of the movement, such that an unpredicted pronation had to be introduced to achieve the grasp, kinematics were affected: Movement time was increased, and the time devoted to deceleration was lengthened. These data are interpreted as evidence that when natural prehension occurs, pronation can be included in the motor plan without affecting the movement kinematics. When constraints are imposed on the movement execution as a consequence of a perturbation, however, the introduction of a pronation component requires kinematic rearrangement.     

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the rnistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interresponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This "peripheral" hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.     

Graphemic analysis underlying literacy

Three experiments are described that studied the role of detailed graphemic analysis upon the ability to read text. College students named letters in various approximations to English, with frequency of individual letters constant. Findings were that later skill at reading varied with the order of approximation to English of the letters that were named, that the spacing of the letter sequences was important to this result, and, finally, that the influence of specific visual practice extended to the typeface on which the naming and reading were carried out. Hence, rather than a letter-by-letter process or its opposite, a wholly semantic analysis, reading is shown to be intimately dependent upon details of visual analysis of patterns or letter sequences.     

Control theories in motor behavior

The goal of contemporary motor control theorist is the delineation of the “language” of movements. That is, in what unambiguous code are the parameters of movement specified, given the composition of the human body? In this pursuit not only are the elements of the language of movement sought, but the rules of combination or syntax of movement are also to be derived. This paper compares a number of motor control theories according to the form of control they exhibit and according to their ability to address issues in the area. Recurring theoretical trends in motor control are examined and the evidence for each is reviewed, emphasizing their explanatory power in the classical problems of control: motor equivalence (Hebb 1949), complexity (Bernstein 1967), and variability (Glencross 1980; Schmidt 1975, 1976).     

Movement substructures change as a function of practice in children and adults

An experiment is reported in which participants at 6 (n = 20), 9 (n = 20), and 24 years (n = 20) of age either received or did not receive practice on a rapid aiming task using the arm and hand. The purpose of the experiment was to document the changes in movement substructures (in addition to movement time) as a function of practice. After receiving 10 baseline trials, subjects in the practice groups received 30 practice trials followed by 10 retention trials on each of 5 days, while subjects in the no-practice group had only baseline and retention trials. Retention-only trials were divided into primary (reflecting the ballistic controlled part of the movement) and secondary (reflecting corrective movement adjustments) submovements. In addition, jerk (the 3rd derivative of movement displacement) was calculated as an estimate of the smoothness of the movement. Participants increased the primary submovement as a function of practice; however, the increases were substantially larger in the children (25-30%) than in the adults (10%). Participants also decreased jerk as a function of practice and the decreases were greater in children than in adults. The results suggest that with practice the primary submovement is lengthened so that it ends nearer the target, especially in children. Associated with the primary submovement covering a larger percentage of the movement length and time, movements became smoother.     

Effect of orienting the finger opposition space in the control of reach-to-grasp movements

Participants (N = 13) made reach-to-grasp movements to an elongated object with or without a forearm pronation movement. Grasp and transport components of movements performed without forearm pronation differed from those performed when participants preplanned forearm pronation. The transport distance traveled after peak aperture (aperture closure distance) was unchanged, however, suggesting that participants initiated aperture closure on the basis of the distance of the hand from the target. When they suddenly pronated the forearm in response to a perturbation, aperture kinematics were altered from a monophasic to a biphasic profile and aperture closure distance was shortened. Conversely, a sudden reorientation to a nonpronated position minimized those changes. Thus, the relationship between transport and aperture components is differentially altered depending on online reorientation of the forearm.     

Control' of reflexive and voluntary saccades in the gap effect.

In two experiments, we examined whether voluntary and reflexive saccades shared a common fixation disengagement mechanism. Participants were required to perform a variety of tasks, each requiring a different level of information processing of the display prior to execution of the saccade. In Experiment 1, participants executed either a prosaccade or an antisaccade upon detecting a stimulus array. In Experiment 2, participants executed a prosaccade to a stimulus array only if the array contained a target item. The target could be a line (easy search) or a digit (difficult search). The critical manipulation in both experiments was the relative timing between the removal of the fixation stimulus and the onset of the stimulus array. In both experiments, it was found that saccadic latencies were shortest when the fixation stimulus was removed before the onset of the stimulus array--a gap effect. It was concluded that reflexive and voluntary saccades share a common fixation disengagement mechanism that is largely independent of higher level cognitive processes.     

Control of reflexive and voluntary saccades in the gap effect

In two experiments, we examined whether voluntary and reflexive saccades shared a common fixation disengagement mechanism, Participants were required to perform a variety of tasks, each requiring a different level of information processing of the display prior to execution of the saccade. In Experiment 1, participants executed either a prosaccade or an antisaccade upon detecting a stimulus array. In Experiment 2, participants executed a prosaccade to a stimulus array only if the array contained a target item. The target could be a line (easy search) or a digit (difficult search). The critical manipulation in both experiments was the relative timing between the removal of the fixation stimulus and the onset of the stimulus array. In both experiments, it was found that saccadic latencies were shortest when the fixation stimulus was removed before the onset of the stimulus array—a gap effect. It was concluded that reflexive and voluntary saccades share a common fixation disengagement mechanism that is largely independent of higher level cognitive processes.