An experiment examined 2-choice reaction under fixed or variable mapping rules. Each display contained a signal, M, and a symbol representing a rule, I, that transformed it. In some conditions I and M came on in sequence. RT was always shorter under fixed than under variable I. When I was variable, presenting I or M in advance led to shorter RT, the reduction being greater for I. There were independent transition effects for I and M from one trial to the next, being greater for I than for M: RT was shorter with repetition than with alternation under variable I and conversely under fixed I. RT was longer with contra-than with homolateral mapping under fixed I or if I was variable and given in advance, otherwise it was invariant with mapping.

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A case is made that typing, speaking, and playing music are members of a family of skills that can be studied within a common theory. The theory is first developed in the context of typing data: Alternative models for latency data on typing arc examined, and it is argued that hierarchic rather than finite-stale models are required to explain the data. Then it is shown that a hierarchic model can readily account for errors and error detection in typing and also that similar models can account for phenomena in speech and piano playing. The basic theory is that the intention of response output is translated in two successive stages and is represented differently in memory buffers in each stage. The second representation is a string of commands governing response output, while the first representation, from which it derives, controls the timing and expressive aspects of output.

Perhaps some of the most refined forms of timing arise in musical performance, particularly in the coordination between musicians playing together. Studies of timing in solo and duet piano performances are described, in which the musicians gave repeat performances of the music. In both solo and duet performances there was expressive use of timing, modulating the tempo of the music and the phase relationship between the voices, and the expressive forms were similar in successive performances of the piece. There was also evidence of separate timing control of the metre and of the production of notes and rests. Thus timing in musical performance is best modelled by assuming two levels of timekeeper, one pacing the metre and the other contained in the movement trajectories of note production, computed by motor procedure in relation to the metre. It is argued that expressive forms are derived from an interpretation of the music rather than memorized; and that coordination between voices in the music is achieved at the level of the metre.

The concept of rhythm points to diverse phenomena in skilled performance. A theoretical frame sufficiently general to deal with these is presented, together with data from a variety of skills. It is argued that a motor system, responsible for movement production, can produce movements that realize given time-scales and hence can act as a timekeeper. In contexts requiring temporal coordination, a more abstract timekeeper, or clock, can provide temporal references for a motor system or several motor subsystems; this enables complex timing such as is found in musical performance. Skilled movement is teleological in the sense that it aims at targets in space and time. Its timing is based on an internal schedule of motor events-the targets of movement-not movement onsets: The motor system arranges the latter on considerations of movement economy or expressiveness. Tapping, handwriting, typing, playing music, and speech are examined as instances of skills having a near-miss periodicity."Skilled performance is rhythmical." Like many apparent truisms this one is weakened by ambiguity, in this case in the meaning of rhythm. If we ask what is common between the rhythms of speech, playing tennis, and playing music, we find that it refers to different properties of time series, relevant in varying degrees to the different skills. Among these are properties of temporal pattern, periodicity, stress, expression, and quality of movement. The purpose here is to develop a theoretical frame for rhythm that can encompass this variety of phenomena.We can illustrate the different senses of rhythm by taking music as a paradigm of serial-structured forms. Excluding some of folk music and a handful of modern freeform compositions, the timing in a piece of