Tactile morphagnosia secondary to spatial deficits

A 73-year old man showed visual and tactile agnosia following bilateral hae morrhagic stroke. Tactile agnosia was present in both hands, as shown by hi s impaired recognition of objects, geometrical shapes, letters and nonsense shapes. Basic somatosensory functions and the appreciation of substance qu alities (hylognosis) were preserved. The patient's inability to identify th e stimulus shape (morphagnosia) was associated with a striking impairment i n detecting the orientation of a line or a rod in two- and three-dimensiona l space. This spatial deficit was thought to underlie morphagnosia, since i n the tactile modality form recognition is built upon the integration of th e successive changes of orientation in space made by the hand as it explore s the stimulus. Indirect support for this hypothesis was provided by the lo cation of the lesions, which could not account for the severe impairment of both hands. Only those located in the right hemisphere encroached upon the posterior parietal cortex, which is the region assumed to be specialised i n shape recognition. The left hemisphere damage spared the corresponding ar ea and could not, therefore, be held responsible for the right hand tactile agnosia. We submit that tactile agnosia can result from the disruption of two discre te mechanisms and has different features. It may arise from a parietal lesi on damaging the high level processing of somatosensory information that cul minates in the structured description of the object. In this case, tactile recognition is impaired in the hand contralateral to the side of the lesion . Alternatively, it may be caused by a profound derangement of spatial skil ls, particularly those involved in detecting the orientation in space of li nes, segments and complex patterns. This deficit results in morphagnosia, w hich affects both hands to the same degree. (C) 1999 Elsevier Science Ltd. All rights reserved.