Biol. Cybern. 46, 111-118 (1983) Biological Cybernetics 9 Springer-Verlag 1983 The Information Transmitted at Final Position in Visually Triggered Forearm Movements Barbara Sakitt, Francis Lestienne, and Thomas A. Zeffiro Department of Psychology, Massachusetts Institute of Technology, Cambridge, USA Abstract. Visually triggered forearm movements were analyzed by an Information Theory approach. Human subjects made smooth movements which were charac- terized by moderate speeds, ranging about 100 degrees per second, by continuity in the position and velocity traces, and attainment of final average EMG levels before completion of the movement. We calculated the information transmitted by final position, biceps EMG, triceps EMG, and the ratio of the EMGs. The results were: (1) The information transmitted by final joint angle increased with number of targets but gradually levelled off. The maximum value was slightly over 3 bits, corresponding to an equivalent number of less than nine independent arm positions for a single movement. (2)The information transmitted by the ratio of the EMGs exceeds that transmitted by the biceps or triceps alone. (3)A previous theoretical prediction based on a spring model (Sakitt, 1980a) gives a moderately good fit to the experimental EMG ratio as a function of final position over a large range of angles. Our results lend consistency to two ideas about the nature of visually triggered forearm move- ments. First, our finding about the EMG ratio suggests that the basic motor program for final position is probably in terms of relative allocation of inner- vations, rather than looking up individual values. Second, single movements of this kind transmit sur- prisingly little information. If this is the case, it sug- gests that very fine accuracy is not achieved by a single program but requires feedback in order to program and execute additional movement. In this paper we have used an application of Information Theory to investigate the amount of * Present address: Laboratoire de Physiologie Neurosensorielle, CNRS, Paris, France information transmitted by an arm movement. Shannon (1948) formulated Information Theory in such a way as to form a quantitative, precise measure that corresponded to our intuitive notion of infor- mation. By so doing, he and many others afterwards were able to develop powerful theorems about com- munications systems. The basic components of a com- munication system include a source that is sending out messages ; a channel with or without noise, over.which the message is sent; and a receiver or destination for the message. Although information theory was initially used by physical scientists, N. Weiner and others who founded the field of cybernetics were interested in applying these ideas to living systems. The essential connection is that: (1)a stimulus array is an infor- mation source because a particular stimulus can con- vey a message; (2) that the human or animal is a noisy channel over which information must be recoded and transrnitted; and (3)the output is a response, which could be an arm movement, the number of impulses in a single neuron, etc. A well-known application of information in move- ment control was introduced by Fitts (1954). This approach is related to the tradeoff between speed and accuracy in movement and employs a measure related to information per unit time. In our study we do not use this application because we are interested in the amount of information transmitted per movement. This is related to applications in sensory absolute judgement where subjects are required to estimate, for instance, the intensity of a visual stimulus. From such data, the amount of information transmitted is calcu- lated. We have adapted this approach to the motor system although some changes are needed since the responses we are concerned with are physical measures like final position, and electromyographic activities, not verbal reports. But the basic quantity we are interested in is the amount of information per arm movement, not the rate of information per unit time.