Stroke: “Neuropsychological evidence for a strategic control of multiple routes in imitation”

Imitation capacity and brain function of stroke patients

The paper I discuss today relates to imitation and investigates what routes stroke patients used when executing an imitation task that they were asked to perform.

When we watch something and imitate it, there are two routes.

One is the meaning route (indirect route), by which we understand the meaning of an action before imitating it, and the other is the direct route, by which we set aside understanding the meaning and start by imitating the form as-is. The direct route ultimately joins up with the indirect route.

This could be represented schematically like this:

→→→→Meaning route (direct route)→→→→→→→→→→→→→→

Action input→Visual analysis→Long-term memory→Working memory→Action output
↓　　　　　　　　　　　　　　　　　　　　　　　↑
Direct route→→→→→→→→→→→→→→→→→→

The experiment results suggest that damage to the route via understanding of meaning is involved when meaningful gestures are difficult to imitate, while conversely damage to the direct route is involved when meaningless gestures are difficult to imitate.

The results also suggest that the basal ganglia play an important role in the execution of imitation.

“Previous studies have suggested that imitators can reproduce known gestures shown by a model using a semantic, indirect route, and novel gestures using a sublexical, direct route. In the present study we aimed at testing the validity of such a dual-route model of action imitation. Patients with either left-brain damage (LBD) or right-brain damage (RBD) were tested on an action imitation task. Actions were either meaningful (n = 20) or meaningless (n = 20), and were presented in an intermingled list and, on a different day, in separate lists. We predicted that, in the mixed condition, patients would use a direct route to imitate meaningful and meaningless actions, as it allows the imitation of both action types. In the blocked condition, patients were expected to select the semantic route for meaningful actions and the direct route for meaningless actions. As hypothesized, none of the 32 patients showed dissociations between imitation of meaningful and meaningless actions in the mixed presentation. In contrast, eight patients showed a dissociation between imitation of meaningful actions and imitation of meaningless actions in the blocked presentation. Moreover, two of these patients showed a classical double dissociation between the imitation of the two action types. Results were interpreted in support of the validity of a dual-route model for explaining action imitation. We argue that the decrease in imitation of meaningful actions, relative to meaningless actions, is caused by a damage of the semantic route, and that the decline in imitation of meaningless actions, relative to meaningful actions, is produced by a breakdown of the direct route. The brain areas that were lesioned in all six LBD patients who showed a dissociation were in the superior temporal gyrus and the angular gyrus, whereas the two RBD subjects had common lesions of the pallidum and of the putamen. The brain structures affected in our patients with selective apraxia are consistent with those reported before in other neuropsychological reports.”

Reference URL: Neuropsychological evidence for a strategic control of multiple routes in imitation.

Comments

When imitating a meaningless action, the pathway is:
See→Imitate movement
but when imitating a meaningful action, the pathway is:
See→Understand meaning→Imitate movement

So, it seems that the difference between the two types of imitation is whether the route passes through meaning processing.

In addition, it is reported that the putamen and the hippocampus, which are deep within the brain, are heavily involved in imitation, suggesting that it cannot simply be marked down to left-brain damage and so on.