Abnormal reliance on object structure in apraxics' learning of novel object-related actions.

We assessed the prediction that object structural cues could benefit the learning of object-action relationships in ideomotor apraxia (IMA). A total of 15 patients with left-hemisphere stroke, 11 of whom exhibited IMA, and 10 healthy subjects were trained to match novel gestures to novel tool pictures that were either High- or Low-Afforded by their associated tools. Learning was assessed with recognition and production tests. Only IMA patients demonstrated better recognition of High- than Low-Afforded gestures, and their recognition of High-Afforded gestures was statistically comparable to the other groups. This finding suggests that apraxics may rely abnormally on object structure when learning to associate novel gestures and tools. Finally, the "affordance benefit" was associated with relative sparing of structures in the dorsal visual processing stream. These data are consistent with the proposal that two routes may mediate skilled action, one specialized for stored information, and the other responsive to object structure, and that deficient gesture learning may be compensated by "bootstrapping" intact dorsal stream coding of action.