For many professional musicians, mental rehearsal is an integral part of their performance preparation. In neurobiological terms, mental rehearsal and preparation can be seen to encompass several distinct elements. First, there is the interpretation of the score in terms of an internal representation of sound (i.e. mentally hearing the music when reading the score). Second, there is the committing of the score to memory, which includes not only the notation on the stave, but also the marks of expression. Playing from memory reduces the cognitive load of performance and allows greater attention to be given to assessment of the sound being produced. Only when memorization has been achieved does the final stage of using the score to support a virtual rehearsal of the movements required to perform it become possible. In its most advanced form, it may even be used to explore different options for expressive interpretation. For this to be possible, the brain must create an internal image not only of the movements but also of the precise effect they will have on the sound. Although they are combined holistically, each of these tasks requires a different set of mental skills; we will concentrate primarily on the mental rehearsal of playing movements. This assumes a complete familiarity with and mastery of the instrument, as the imagined movements must remain accurate in the absence of auditory feedback, and is therefore an option open only to advanced players (Lotze et al. 2003). This form of physical imagery is not unique to musicians; it is widely used in sports that involve complex stereotyped movements. High-jumpers, for example, can often be seen mentally practising using minimal or reduced movements before a crucial jump. Perhaps surprisingly, the effectiveness of mental practice in improving aspects of the dynamics of movement, such as the accuracy of a movement trajectory, has been verified experimentally (Yaguez et al. 1998). In one study, the effect of mental rehearsal was compared with physical practice of the same simple musical passage on the keyboard (Pascual-Leone, 2001). The subjects practised mentally (i.e. rehearsing the activity in the absence of any movement or muscle activity) or physically for 2 h daily over 5 days. The effect of this on the representation of the hand in the motor cortex was assessed by comparing the efficacy of transcranial magnetic stimulation in eliciting movement of the trained and untrained hand. Although the section of the primary motor cortex that drives the muscles of the hand remained silent in the group carrying out only mental rehearsal, the size of the areas that were able to activate the muscles moving the fingers increased, while their threshold for activation was reduced. This was accompanied by a demonstrable improvement in the accuracy of motor performance, although it was not as great as that achieved through physical practice. However, at the end of the study, a single session of physical practice in the mental rehearsal group was sufficient for them to reach parity of performance with the physical rehearsal group. This result is in general agreement with those of other less physiologically rigorous studies, which suggest that while mental practice is better than no practice, it is not as good as actual practice (Gabrielsson, 1999). Professional musicians who play a great deal can be prone to overuse injury, so one advantage of mental practice is that it can be used to refine or improve performance while reducing this risk.
Clearly one source of information missing in mental rehearsal is exteroceptive and proprioceptive feedback. The less experienced the player, the more important this feedback will be, but in experienced players in whom the movements of the fingers have become fully automatic, its importance may be reduced. There is also a lack of auditory feedback as there is no tangible output from the virtual activity. The effect of its absence on performance accuracy has been investigated in experienced pianists playing on a silent keyboard (Finney & Palmer, 2003). For substantial excerpts of previously learned music, there was no significant difference in error rate between playing with or without sound. In simple sight reading tests, however, although the absence of auditory feedback had no effect on performance from the score, it did have a deleterious effect on the accuracy of the music when it was subsequently repeated from memory, indicating that it remains important for the initial stages of learning even in experienced players.
With the exception of the primary motor and sensory areas, many of the other cortical regions that are normally active during playing are also active during virtual practice (Langheim et al. 2002; Lotze et al. 2003; Meister et al. 2004). Functional imaging studies have revealed activity in premotor and supplementary motor areas (Fig. 3C–F). The premotor area was active when the physical performance of a melody was silently re-created in the mind, though not when the melody was simply recalled. Perhaps surprisingly, there was no activity in the primary auditory area during these experiments, despite the fact that virtual rehearsal generally requires a vivid mental realization of the sound associated with the virtual ‘movements’. This is in contrast to imagining a scene, which does produce activity in visual cortical areas. However, when the rehearsal involved real hand movements, activity was present in the right primary auditory cortex and left auditory association cortex even if the hand was not in contact with the instrument (Lotze et al. 2003). This suggests that some link exists between the primary motor and auditory areas (Bangert & Altenmuller, 2003). The right primary auditory cortex is the main region involved in the perception of pitch, harmony and timbre, and its level of activity during silent practice with actual finger movements is greater in professional musicians than in amateurs. A link in the opposite direction between the primary auditory and primary motor cortices has also been demonstrated. A study of advanced piano students demonstrated that listening to a piece of keyboard music with which they were already familiar caused involuntary activity in the primary motor cortex even though no contraction of muscles moving the fingers took place (Haueisen & Knosche, 2001). Activity in the motor cortex occurred in the region controlling a finger just before the note it would have played was sounded and so it mirrored the activity that would have been required for playing. No such response was seen in a control group of similarly experienced singers who were not pianists. This type of connection would undoubtedly support the ability to play music by ear.