A form of ideational apraxia as a delective deficit of contention scheduling

In this paper we studied three brain-damaged patients: the first two, DR and FG, had limb apraxia whilst the third was a control patient (WH2) with an executive function disorder but without limb apraxia. DR and FG were impaired in carrying out everyday actions, whilst they maintained the ability to sequence photographs representing those same activities. The failure in the action production task was not caused by visual agnosia for objects, as the patients could recognise them from sight. Nor was it produced by a loss of knowledge about their functions (De Renzi & Lucchelli, 1988), as DR and FG could identify objects from descriptions of their use. WH2's pattern of performance doubly dissociated from that of the apraxic patients, namely spared action production on the multiple object test, but faulty sequencing of photographs. WH2's difficulties in sequencing photographs were not due to a failure to understand the task, as she could sequence stimuli other than actions (e.g., shapes and numbers). Nor were the differences due to a loss of knowledge about the actions, since she could perform and identify them from photographs. These results show that the kind of apraxia observed in DR and FG is not produced by a degraded action sequence representation (Lehmkuhl & Poeck, 1981; Poeck & Lehmkuhl, 1980). We interpreted our results within a contention scheduling model (Cooper & Shallice, 2000; Norman & Shallice, 1986).