Astereognosis (or tactile agnosia if only one hand is affected) is the inability to identify an object by active touch of the hands without other sensory input, such as visual or sensory information. An individual with astereognosis is unable to identify objects by handling them, despite intact elementary tactile, proprioceptive, and thermal sensation. Despite cross-talk between the dorsal and ventral cortices, fMRI results suggest that those with ventral cortex damage are less sensitive to object 3D structure than those with dorsal cortex damage. Unlike the ventral cortex, the dorsal cortex can compute object representations. Thus, those with object recognition impairments are more likely to have acquired damage to the dorsal cortex.
While astereognosis is characterized by the lack of tactile recognition in both hands, it seems to be closely related to tactile agnosia (impairment connected to one hand). Tactile agnosia observations are rare and case-specific. Josef Gerstmann recounts his experience with patient JH, a 34-year-old infantryman who suffered a lesion to the posterior parietal lobe due to a gunshot. Following the injury, JH was unable to recognize or identify everyday objects by their meaning, origin, purpose and use with his left hand using tactile sensation alone. His motility performance, elementary sensitivity, and speech were intact, and he lacked abnormalities in brain nerves.