Tactile apraxia - Unimodal apractic disorder of tactile object explorationassociated with parietal lobe lesions

Tactile apraxia is characterized by an isolated disturbance of hand movemen ts for use of and interaction with an object (transitive movements) in the presence of preserved intransitive movements (movements without use of an o bject, for example repetitive movements or gestures). It is, however, still unclear whether motor and sensory abnormalities represent causal or associ ated features of tactile apraxia. To address this question, quantitative ki nematic recordings of exploratory finger movements (transitive movements) a nd rapid alternating finger movements (intransitive movements) were studied in 20 healthy volunteers and 22 patients with focal lesions of the parieta l, anterofrontal and motor cortex. The most severe deficits of manual objec t exploration were found in patients with parietal lesions, using the hand contralateral to the lesion. Patients with lesions of the anterior parietal lobe who exhibited prominent sensory deficits and astereognosia showed a d ecrease in frequency and regularity of exploratory finger movements and a m arked increase in exploration space, Patients with posterior parietal lesio ns exhibiting severe astereognosia, apraxia and deficits in dexterity had a greater decrease in frequency and regularity of manipulative movements, bu t a less pronounced increase of exploration space than the patients with an terior parietal lesions. Although the patients with parietal lobe lesions c ould generate rapid alternating finger movements, the regularity of these m ovements was also impaired, In comparison, patients with frontal lobe lesio ns exhibited impaired contralesional manipulatory and rapid alternating fin ger movements but no sensory abnormalities or astereognosia, We conclude th at tactile apraxia represents a deficit in the programming of exploratory f inger movements mediated by the parietal lobe. The comparison with lesions of other regions participating in the cortical network for tactile that apr axia of exploratory parietal lobe lesions represents a disturbance distinct from elementary motor or sensory abnormalities, but closely related to ste reognostic functioning.