Warrington and Taylor's 1978 Paper

Complementary deficits in perceptual classification in pure alexia and acquired prosopagnosia - New insights from two classic cases

Pure alexia and prosopagnosia traditionally have been seen as prime examples of dissociated, category-specific agnosias affecting reading and face recognition, respectively. More recent accounts have moved towards domain-independent explanations that postulate potential cross-links between different types of visual agnosia. According to one proposal, abnormal crowding, i.e. the impairment of recognition when features of adjacent objects are positioned too closely to each other, might provide a unified account for the perceptual deficits experienced by an agnosic patient. An alternative approach is based on the notion of complementary visual subsystems favouring the processing of abstract categories and specific exemplars, respectively. To test predictions of these two approaches with regard to pure alexia and prosopagnosia, we present previously unpublished data on digit recognition and visual crowding from two in the neuropsychological literature extensively studied patients, KD and MT (e.g., Campbell et al., 1986; Landis and Regard, 1988; Rentschler et al., 1994). Patient MT, diagnosed with pure alexia, showed pronounced abnormal foveal crowding, whereas KD, diagnosed with prosopagnosia, did not. These results form a distinct double dissociation with the performance of the two patients in other perceptual classification tasks involving Gabor micropatterns and textures, as well as Glass patterns, which revealed a significantly greater impairment in KD relative to MT. Based on an analysis of the specific task demands we argue that prosopagnosia and pure alexia may involve complementary deficits in instantiation and abstraction, respectively, during perceptual classification, beyond any category specificity. Such an explanation appears in line with previous distinctions between a predominantly left-hemispheric, abstract-category and a predominantly right-hemispheric, specific-exemplar subsystem underlying object recognition.

Critical brain regions for action recognition: lesion symptom mapping in left hemisphere stroke

A number of conflicting claims have been advanced regarding the role of the left inferior frontal gyrus, inferior parietal lobe and posterior middle temporal gyrus in action recognition, driven in part by an ongoing debate about the capacities of putative mirror systems that match observed and planned actions. We report data from 43 left hemisphere stroke patients in two action recognition tasks in which they heard and saw an action word ('hammering') and selected from two videoclips the one corresponding to the word. In the spatial recognition task, foils contained errors of body posture or movement amplitude/timing. In the semantic recognition task, foils were semantically related (sawing). Participants also performed a comprehension control task requiring matching of the same verbs to objects (hammer). Using regression analyses controlling for both the comprehension control task and lesion volume, we demonstrated that performance in the semantic gesture recognition task was predicted by per cent damage to the posterior temporal lobe, whereas the spatial gesture recognition task was predicted by per cent damage to the inferior parietal lobule. A whole-brain voxel-based lesion symptom-mapping analysis suggested that the semantic and spatial gesture recognition tasks were associated with lesioned voxels in the posterior middle temporal gyrus and inferior parietal lobule, respectively. The posterior middle temporal gyrus appears to serve as a central node in the association of actions and meanings. The inferior parietal lobule, held to be a homologue of the monkey parietal mirror neuron system, is critical for encoding object-related postures and movements, a relatively circumscribed aspect of gesture recognition. The inferior frontal gyrus, on the other hand, was not predictive of performance in any task, suggesting that previous claims regarding its role in action recognition may require refinement.

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.