Perception of action boundaries in patients with frontal lobe damage.

The prefrontal cortex is known to be involved in action planning and in controlling behaviour. Neuropsychological evidence also supports the idea that the prefrontal cortex is generally involved in processing complex events, such as action knowledge. Actions are represented in memory as sequences of goal-directed discrete events. Complex events can be divided into discrete smaller units and organised in a hierarchical manner. The aim of the present study was to investigate the ability of 9 patients with prefrontal lesion and 12 healthy controls to parse action sequences into meaningful events. Subjects were requested to detect the transitions between events under three different orientation instructions: (1) spontaneous; (2) small events; and (3) large event. Both normal subjects and patients identified significantly more events under the small-oriented condition. However, contrary to normal controls (NCs), patients with prefrontal damage showed considerable difficulties in detecting large event units than small event units. These results strongly suggest that prefrontal cortex is specifically involved in parsing and recognising clusters of action sequences. These findings also show that the way in which instructions orient subjects' attention, has an effect on how action information is encoded and represented in memory.