Agnosia: Tactile

Everyday object recognition involves contributions from all of our senses. We recognize objects not only by their shape and color, but also by their sounds, textures, and temperatures. Our brains are organized to integrate information across senses, but each sense has its own separate neural object recognition system. To understand how the brain achieves multisensory object recognition, researchers examine how brain damage leads to the selective failure of recognition by an individual sensory system. This entry focuses on how impairments of tactile object recognition, or tactile agnosia, contribute to our understanding of normal object recognition. The entry reviews issues regarding different types of tactile agnosia, brain regions involved in tactile agnosia, sensory and motor contributions to tactile agnosia, and the existence of tactile agnosia separate from visuospatial disorders.

Agnosia is defined as the impaired recognition of familiar objects, faces, or sounds, despite adequate perceptual and intellectual capabilities. Tactile agnosia, or somatosensory agnosia, refers to a deficit in recognizing common objects by touch following brain damage, despite sufficiently intact tactile sensation, memory, and general intellectual function. Tactile agnosia is distinct from tactile aphasia, an inability to verbally label a felt object, or tactile inexperience in which the patient is not familiar with the object.

Tactile agnosia is modality specific: Patients who have difficulty recognizing objects by touch can still recognize objects by vision, audition, taste, or smell. Patients describe their condition as feeling like their hand is “numb” or “stiff,” that the feeling is not distinct or strong enough to identify the objects by touch. Patients with visual agnosia have similar complaints saying their glasses appear “dirty” or “don't work.”

Patients with tactile agnosia usually have unilateral lesions (i.e., brain damage on only one side of the brain). The hand that is contralateral to the lesion (on the opposite side of the lesion) is the hand that is “agnostic.” A patient with a left hemisphere lesion will have difficulty recognizing objects with his or her right hand. The unilateral nature of tactile agnosia provides an advantage to investigating tactile agnosia: patients provide their own control comparisons because patients' ability to recognize felt objects with their non-agnostic, ipsilateral hand (i.e., on the same side of the lesion) can be compared directly with their inability to recognize the same objects and perform the same tasks with their contralateral agnostic hand. Thus, difficulties recognizing objects with their agnostic hand cannot be attributed to labeling problems or a lack of knowledge about the objects.