Laurel J. Buxbaum

Knowledge of object manipulation and object function: dissociations in apraxic and nonapraxic subjects.

An influential account of selective semantic deficits posits that visual features are heavily weighted in the representations of animals, whereas information about function is central in the representations of tools (e.g., Warrington & Shallice, 1984 ). An alternative account proposes that information about all types of objects-animate and inanimate alike-is represented in a distributed semantic architecture by verbal-propositional, tactile, visual, and proprioceptive-motor nodes, reflecting the degree to which these systems were activated when the knowledge was acquired (e.g., Allport, 1985 ). We studied a group of left hemisphere chronic stroke patients, some of whom were apraxic, with measures of declarative tool and animal knowledge, body part knowledge, and function and manipulation knowledge of artifacts. Apraxic (n=7) and nonapraxic (n=6) subjects demonstrated a double dissociation of performance on tests of tool and animal knowledge, suggesting that the apraxic group was not simply more severely impaired overall. Apraxics were relatively impaired in manipulation knowledge, whereas nonapraxics tended to be relatively impaired in function knowledge. Apraxics were also more impaired with body parts than nonapraxics. The association of gestural praxis, tool knowledge, body part knowledge, and manipulation knowledge suggests a coherent basis for the organization of semantic artifact knowledge in frontoparietal cortical regions specialized for sensorimotor functions, and thus provides support for the distributed architecture account of the semantic system.

Ideomotor Apraxia: a Call to Action

Although ideomotor apraxia (IM) has been a topic of investigation since the early 20th century, progress in studying the models of various investigative groups. As a result, it is difficult to draw conclusions about the expected performance of IM patients on various tests of praxis (e.g. pantomime, single object use, gesture imitation, and naturalistic action); the relationship of IM to ideational apraxia (IA); the degree to which specific error types are diagnostic of one or the other disorder; the relationship of semantic knowledge to gesture representations; and the role of spatiomotor systems external to the stored gesture representation system. Here an updated model of IM is presented, informed by recent physiological and functional neuroimaging literature, as a step towards resolving some of these concerns. The model is intended to lay the groundwork for future investigations of specific performance patterns in different subtypes of IM.

Left inferior parietal representations for skilled hand-object interactions: evidence from stroke and corticobasal degeneration.

Patients with ideomotor apraxia (IM) are frequently more impaired in the production and imitation of object-related (transitive) than non-object-related, symbolic (intransitive) gestures, but reasons for this dissociation, and its anatomical underpinnings, remain unclear. Our theoretical model of praxis (Buxbaum, 2001) postulates that left inferior parietal lobe (IPL) gesture representations store information about postures and movements of the body and hand for skillful manipulation of familiar objects; in contrast, bilateral fronto-parietal dynamic calculations provide constantly-updated information about the current position and movement of the body and hand for both familiar and novel, transitive and intransitive movements. This account predicts distinct patterns of IM in patients with left IPL damage versus bilateral fronto-parietal involvement. Consistent with predictions, 16 stroke patients with left IPL damage were more impaired with transitive than intransitive gestures, whereas 4 patients with bilateral fronto-parietal damage due to corticobasal degeneration (CBD) were not [F (1, 18) = 8.5 p < .01]. Additionally, the hand posture component of transitive gestures was the most impaired aspect of gesture in CVA, but tended to be the least impaired aspect of gesture in CBD [F (3, 54) = 5.1, p < .005]. Finally, CVA patients were more impaired with transitive hand postures than meaningless or intransitive hand postures, whereas CBD patients showed the opposite pattern. These data indicate that the left IPL mediates representations of skilled hand-object interactions, as distinct from dynamic coding of the body in space, and suggest that the IPL maps between representations of object identity in the ventral stream and spatial body representations mediated by the dorsal system.

The Role of Semantic Memory in Object Use

Does semantic knowledge of objects mediate object selection and use? We present data from two patients that speak to this question. The first, DM, is a semantic dementia patient previously reported by Breedin, Saffran, and Coslett (1994) who, despite moderate to severe loss of functional and associative object knowledge, was nevertheless able to perform almost normally on single-object use and on more complex tests of naturalistic action. The second, HB, is a dementia patient who exhibited an executive disorder but performed as well as controls on a detailed battery of semantic memory and single-object use tests. Unlike DM, he made numerous errors on the naturalistic action tests, among which were errors of object selection and usage. Taken together, these data suggestthatintactsemantic memory forobjects is neithernecessary norsufficient to ensure good object utilisation in naturalistic action. The data cannot be accommodated by accounts postulating that action with objects is performed exclusively via no...

Deficient internal models for planning hand-object interactions in apraxia.

Motor imagery (MI) has been associated with planning stages of motor production, and in particular, with internal models that predict the sensory consequences of motor commands and specify the motor commands required to achieve a given outcome. In this study we investigated several predictions derived from the hypothesis that ideomotor apraxia (IM), a deficit in pantomime and imitation of skilled actions, may be attributable in part to deficits in internal models for planning object-related actions, in the face of relatively intact on-line, feedback-driven control of action. This hypothesis predicts that in IM, motor imagery should be (a) strongly correlated with other motor tasks not providing strong visual, tactile, and proprioceptive feedback from objects, i.e., object-related pantomime and imitation; (b) poorly correlated with performance tasks providing strong environmental feedback about the locations of effectors and targets, i.e., actual interaction with objects; and (c) particularly deficient in conditions that are computationally difficult for the motor planning system. Eight left fronto-parietal stroke patients with IM, five stroke patients without IM, and six healthy matched controls imagined grasping dowels and widgets presented at varying orientations, and actually grasped the same objects. The experimental predictions were confirmed. In addition, patients with IM and motor imagery deficits were significantly more likely than the non-apraxic group to have lesions in the intraparietal sulcus, a region previously implicated in imagery for hand-object interactions. The findings suggest a principled explanation for the deficits of IM patients in object-related gesture pantomime, imitation, and learning of new object-related gestures.

Naturalistic action impairment in closed head injury.

The authors sought to determine whether errors of action committed by patients with closed head injury (CHI) would conform to predictions derived from frontal lobe theories. In Study 1, 30 CHI patients and 18 normal controls performed routine activities, such as wrapping a present, under conditions of graded complexity. CHI patients committed more errors even on the simplest condition; but, except for a higher proportion of omitted actions, their error profile was very similar to that of controls. Study 2 involved a subset of patients whose performance in Study 1 was within normal limits. When these high functioning patients were asked to perform the routine tasks under still more taxing conditions, they, too, committed errors in excess of the control group. Accounts based on frontal mechanisms have a difficult time explaining the overall pattern of findings. An alternative based on limited-capacity resources is suggested.

Action knowledge, visuomotor activation, and embodiment in the two action systems

Scientific interest in the relationship between cognition and action has increased markedly in the past several years, fueled by the discovery of mirror neurons in monkey prefrontal and parietal cortex and by the emergence of a movement in cognitive psychology, termed the embodied cognition framework, which emphasizes the role of simulation in cognitive representations. Guided by a functional neuroanatomic model called the Two Action Systems account, which posits numerous points of differentiation between structure‐ and function‐based actions, we focus on two of the major issues under recent scrutiny: the relationship between representations for action production and recognition, and the role of action in object representations. We suggest that mirror neurons in humans are not critical for full action understanding, and that only function‐based (and not structure‐based) action is a component of embodied object concepts.

The Naturalistic Action Test: A standardised assessment for everyday action impairment

The Naturalistic Action Test (NAT) measures everyday action impairment associated with damage to higher cortical functions. The tasks, procedures and scoring system were developed through extensive research. An earlier research instrument (the Multi-Level Action Test) was streamlined to create the NAT, which is scored for steps accomplished and for commission of a small set of recognised errors. This paper describes the NATs psychometric properties, based on a study of 100 patients undergoing rehabilitation for TBI, left CVA, or right CVA (Part 1) and a follow-up study with approximately half of the original cohort (Part 2). Psychometric properties with respect to scoring reliability, internal consistency, and concurrent criterion validity were measured in Part 1 and found to be acceptable. Construct validity was assessed against a battery of attention tests; significant correlations were obtained between NAT scores and measures of arousal, visuospatial attention, and working memory. Part 2 showed that NAT scores succeeded in predicting functional outcome above and beyond motor disability and age, and that it outperformed other tests in this regard, including those that measure specific attentional capacities. We discuss the potential uses of the NAT in the clinic and as a screening tool for research.

Function and manipulation tool knowledge in apraxia: Knowing ‘what for’ but not ‘how’

Abstract Several accounts of the semantic system posit that function information plays a critical role in the representations of man-made objects. Alternative possibilities are that man-made objects such as tools are organized according to manner of manipulation, or that both function and manipulation information figure importantly and distinctly in man-made object representations. An agnosic patient, FB, reported by Sirigu et al. (Brain 1991; 114: 727-41) provides support for the latter view. FB was able to demonstrate how to manipulate objects whose function he did not recognize. We now report two severely apraxic patients whose performance, together with that of FBI indicates that function and manipulation information may doubly dissociate. On ‘declarative’ semantic tests not requiring gesture production, our subjects demonstrate severely impaired manipulation knowledge in the context of relatively intact (and, in one subject, perfectly unimpaired) function knowledge. The double dissociation provides further support for the notion that function and manipulation knowledge are critical and distinct features of the representations of manipulable man-made objects.

Naturalistic action production following right hemisphere stroke

An unselected group of right hemisphere, semi-acute stroke patients (n = 30) was run on a laboratory test of naturalistic action production and was found to commit errors of action at a higher rate than what was previously reported for recovering head injury patients [Schwartz et al., Naturalistic action impairment in closed head injury. Neuropsychology, 1997, 8, 59-72]. There were strong similarities in how these two patient groups responded to variations in task demands and in the pattern of errors they produced. Hemispatial biases were evident in the errors of right hemisphere patients with neglect but not those without neglect; and neglect patients also many errors that were unrelated to the spatial layout. We argue that a non-specific resource limitation--which might translate as reduced arousal or effort--is central to the breakdown of naturalistic action production after brain damage, and right hemisphere patients are especially vulnerable to this resource limitation and its behavioral consequences.