Internally Generated and Externally Triggered Motor Actions; Are they the same?

monkeys (Macca fascicularis, Macca mulatto) have shown that the supplementary motor area (SMA) directs actions on the basis of proprioceptive cues concerning the animal’s own state. Secondly, the arcuate premotor area seems to direct actions on the basis of visual cues about the outside environment (Passingham, 1987). Anatomically, the arcuate premotor area lies on the lateral surface of the cortex and the SMA on the medial surface. These findings support other work which suggests that the SMA is specifically involved in the retrieval of motor memories for the initiation of self-paced movements (Tanji, 1994). Further work with monkeys also suggests that the SMA and PMC are functionally specialized areas, with the former being more involved in movements that are internally generated and the latter in actions that are externally guided (Halsband, Matsuzaka and Tanji, 1994)

Whereas the evidence for the existence of two functionally specialized premotor systems in monkeys is quite strong, functional imaging work suggests that the dichotomy may be less sharp in humans. In one study, using human subjects

measurements of rCBF showed that several areas were involved in the production of internally generated actions as compared to a rest condition (Jahanshahi et al, 1995). Areas included the dorsolateral preffontal cortex (DLPFC), SMA, anterior cingulate, the lateral premotor cortex, parietal area 40, insular cortex, the thalamus and the putamen. No area was specifically activated for externally driven actions as compared to internally generated actions. The DLPFC was much more active in the case of internally generated movements than for externally triggered movements, but was still active in both classes of action. These results support the idea of a medial premotor system, including the SMA, that is involved in the production o f volitional, internally

Chapter 3.-Internally Generated and Externally Triggered Motor Actions: Are they the same?

generated actions. They do not however support the notion of a fiinctional dissociation between the SMA and lateral premotor area.

If such a functional specialisation exists, what might be the relationship between these two premotor systems in humans? That is, what is the effect of forcing a subject to change from their internal preparations to move, to an externally driven mode of response production? A number of types of interaction are plausible. One possibility is that the two premotor systems are independent, in which case, changing the context from internally generated to externally triggered would comprise a complete change in the premotor structures involved in movement production. Another possibility is that the two premotor systems are facilitatory. This would mean that, if one is preparing an internally generated movement and is suddenly forced to make an identical but externally triggered response, there should be a facilitatory effect on RT as compared to an SRT condition. It may also be expected that in this situation some physiological characteristics of the response such as the underlying muscle activation will be affected. A third possibility is that the two premotor systems are inhibitory. If this were true, one would expect higher RTs when a subject is forced to switch from their internal preparations to an external mode of response production, and perhaps a change in some of the physiological attributes of the response. Recent work by Haggard & Astor (unpublished study) has used RT type paradigms to investigate internally generated and externally triggered movements. These authors found that, during a “truncation” condition, when preparation for a voluntary (internally

generated) action was interrupted by an SRT stimulus, RT was delayed as compared to a straightforward SRT condition in which subjects simply responded to a response

Chapter 3.•Internally Generated and Externally Triggered Motor Actions: Are they the same?

stimulus. This result is interesting because the motor action (a simple key press) that subjects made was physically identical in both conditions.

Truncation provides a direct means of examining the effects of changing the context in which a movement is produced. Hence, in the above study it was shown that changing the context of an action from being mostly internally generated to being mostly externally triggered delays the action’s execution. These results generate at least two interesting questions. Firstly, why does this delay in action execution occur? Secondly, does truncation affect any other properties of the action other than its latency? One possible partial answer to the first question is that subjects are caught in a situation in which they have to switch from their internal preparations to an

externally driven mode. However, this in itself is an interesting phenomenon: that the motor system treats apparently identical motor outputs as two different tasks simply on the basis of the context in which they are performed. If this were the case, does this switch relate to a switch from the medial to the lateral premotor system? And if so, at what point in the movement production process does this switch occur? Assuming that there are indeed two specialized premotor systems, it would be extremely

interesting and important to determine the degree to which they are independent from one another, or the degree to which they can interact with each other. Figure 3a(i) - (iii) illustrates schematically the idea of internally generated action, externally triggered action and truncated action in which subjects must switch from an internal mode of response production to an external mode.