Commentary/Feldman and Levin: Motor control tion is computed during the movement (sect. 11 of the target article by Feldman & Levin [F&L]). A similar conclusion follows independently from a hemispheric motor model (HMM) of inten- tional hand movements. The probability of obtaining the same conclusion independently at random is small. This observation increases the probability that both models are correct, including details that are part of only one model, but do not contradict the other one. The HMM is presented in Gilad & Fidelman (1990) and Fidelman & Gilad (1992). In these studies, the performance times for manual micromovements by right-handed males were corre- lated with their scores on tests for the right hemisphere. The subjects removed 12 objects, differing in shape and size, in a fixed sequence from one task board and replaced them in the appropri- ate recesses on the other task board. The hand movements were divided into the following four components (micromovements): Reach, Grasp, Move, and Position. Reach and Move are defined as spatial motions, whereas Grasp and Position are defined as exact motions. The experiment was filmed by a movie camera and the performance time of each component was determined by count- ing the frames. Two tests were applied for the right hemisphere. The first was the enumeration of dots presented simultaneously during 64 milliseconds (subitizing). The second test was a similar enumeration of different figures. The performance terms of the micromovements and the scores on the right hemispheric enumeration tests were correlated. Most of the correlation coefficients were positive, that is, the more efficient the right hemisphere, the slower the hand movements. This effect is larger for the left hand (controlled by the right hemisphere) than for the right hand (controlled by the left hemisphere) with a 2-tailed significance of p = 0.0212. That is, this effect is not related to a nonright hemispheric factor. According to Guiard et al. (1983), movements are performed with a smaller constant error when using the left hand than when using the right. We explained this finding and our own by the existence of two alternative strategies for operating the hands. One strategy is a slower but more accurate one, related to the right hemisphere (the left hand). The second is a fast and inaccurate movement related to the left hemisphere (the right hand). Because the right hand (left hemispheric strategy) is less accurate, it requires more corrections, which are performed by the left hemisphere. The exact motions (Grasp, Position) are more compound and require more adjustments. Conversely, the spatial motions, Reach and Move, require fewer adjustments while the hand is moving. That is, we may expect the positive correlations between the scores on the right hemispheric tests and the performance times of the spatial motions to be larger than the correlations related to exact motions. This hypothesis was confirmed experimentally with a 2-tailed significance of p = 0.004 for the right hand and p = 0.0212 for the left hand. These observations are explained by the HMM as indicating that the right hemispheric strategy is to compute a representation of the spatial goal state; then the hand is sent to its spatial target with minimum corrections. The left hemispheric strategy is to begin the motion before the right hemisphere completes the computation of the spatial goal state; the left hemisphere performs corrections quickly by feedback monitoring (Gilad & Fidelman 1990, p. 160). The longer performance time of the right hemi- spheric strategy is caused by the later initiation of the movements. The logical reason for this later initiation is that although the right hemispheric goal state of a subject serves both hands, the right hand is less accurate and faster. That is, the right hand does not fully apply the goal state and may begin its motion before the goal state is fully imprinted in the right hemisphere. It therefore applies the correction mechanism more extensively to correct the deviations. It should be noted that there is no clear distinction between the two strategies, and they may change continuously. We may accordingly expect that most subjects (except, perhaps, most extreme right hemispheric subjects) begin moving their hands before the computation of the destination is complete. According to the conservative control strategy of the Lambda model, a movement may be initiated without determining its distance. The distance may be determined during the movement by computing the time needed to come as close as possible to the target. That is, the central control of the movement comprises two components: the determination of the direction and of the dura- tion of the movement. This model is in line with the HMM. The spatial direction may be determined by the right hemisphere, whereas the temporal analysis may be performed by the left hemisphere. The spatial direction may be identical to the spatial goal state of the HMM; the arresting of the movement using temporal coding may be related to the left hemispheric correc- tions of the HMM. This arresting of movement is extended by F&L to double-joint and triple-joint movements. This extension may be identical to the left hemispheric series of corrections existing according to the HMM. According to the HMM, the more efficient the right hemi- sphere is, the more time that is required to compute the direction of the movement. This is explained by the necessity to inhibit the former goal state of the movement imprinted in the right hemi- sphere. This inhibition is more difficult if the right hemisphere is efficient (Gilad & Fidelman 1990, p. 159). An additional factor causing the longer time for the right hemispheric strategy may be related to a negative correlation between the efficiency of the left and right hemispheres caused by the influence of sex hormones (see references in Fidelman & Gilad 1992). A right hemispheric subject, therefore, has an inefficient left hemisphere that requires more time to compute the duration of the movement. This movement is hence slower, to allow more time for computing its duration. That is, even extreme right hemispheric subjects may begin their hand movements before the computation of their duration is complete. We observe that the Lambda model and the HMM can be integrated. Each of the models contributes details to the unified model. Moving models of motion forward: Explication and a new concept Thomas G. Fikes a and James T. Townsend b department of Psychology, University of Puget Sound, Tacoma, WA 98416. tfikes@ups.edu; b Department of Psychology, Indiana University, Bloomington, IN 47405 Abstract: We affirm the dynamical systems approach taken by Feldman and Levin, but argue that a more mathematically rigorous and standard exposition of the model according to dynamical systems theory would greatly increase readability and testability. Such an explication would also have heuristic value, suggesting new variations of the model. We present one such variant, a new solution to the redundancy problem. We find ourselves in an interesting predicament: we would like to believe that the theory put forth in the target article is essentially correct, but we are still not quite sure we understand it. Appar- ently we are not alone - Feldman and Levin's (F&L's) complaint is that others have consistently misrepresented the model as well. Unfortunately, the present paper does not appear to remedy the problem. Our commentary takes the form of a suggestion and an illustration. First, we suggest that the proponents of this and similar models (e.g., Bizzi et al. 1992) make better use of the mathematical conventions of dynamical systems theory. This would substantially improve clarity of the model and issues sur- rounding it. Perhaps Feldman and his colleagues believe that the use of quantitative notation will obstruct communication. In this instance, we suspect exactly the opposite is the case. Second, we illustrate the heuristic utility of this suggestion by taking another look at the redundancy problem (sect. 11.3 of the target article), briefly sketching an apparent misprediction from his theory. Then, BEHAVIORAL AND BRAIN SCIENCES (1995) 18:4 751