Impaired access to manipulation features in Apraxia: evidence from eyetracking and semantic judgment tasks.

Apraxic patients are known for deficits in producing and comprehending skilled movements. Two experiments tested their implicit and explicit knowledge about manipulable objects in order to examine whether such deficits accompany impairment in the conceptual representation of manipulation features. An eyetracking method was used to test implicit knowledge (Experiment 1): participants viewed a visual display on a computer screen and touched the corresponding object in response to an auditory input. Manipulation relationship among objects was not task-relevant, and thus the assessment of manipulation knowledge was implicit. Like the non-apraxic control patients, apraxic patients fixated on an object picture (e.g., "typewriter") that was manipulation-related to a target word (e.g., 'piano') significantly more often than an unrelated object picture (e.g., "bucket") as well as a visual control (e.g., "couch"). However, this effect emerged later than in the non-apraxic control group, suggesting impaired access to manipulation features in the apraxic group. In the semantic judgment task (Experiment 2), participants were asked to make an explicit judgment about the relationship of picture triplets of manipulable objects by choosing the pair with similar manipulation features. Apraxic patients performed significantly worse on this task than the non-apraxic control group. Both implicit and explicit measures of manipulation knowledge show that apraxia is not merely a perceptuomotor deficit of skilled movements, but results in a concomitant impairment in representing manipulation features and accessing them for cognitive processing.