Appreciation of texture depends mainly on cutaneous impressions, but recognition of the shapes and sizes of objects is based on sensory experience from deeper receptors as well. Inability to recognize shape and form is frequently a manifestation of cortical disease, but a similar clinical defect will occur if tracts that transmit proprioceptive and tactile sensation are interrupted by lesions of the spinal cord and brainstem (and, of course, of the peripheral nerves). This type of sensory defect is called stereoanesthesia (see further on, under “Posterior [Dorsal] Column Syndrome”) and is distinguished from astereognosis, which connotes an inability to identify an object by palpation, even though the primary sense data (touch, pain, temperature, and vibration) are intact. In practice, pure astereognosis is rarely encountered, and the term is employed when the impairment of superficial and vibratory sensation in the hands seems to be of insufficient severity to account for the defect in tactile object identification. Defined in this way, astereognosis is either rightor left-sided and, with the qualifications mentioned below, is the product of a lesion in the opposite hemisphere, involving the sensory cortex, particularly S2 or the thalamoparietal projections.
The traditional doctrine that somatic sensation is represented only in the contralateral parietal lobe is not absolute. Beginning with the report by Oppenheim in 1906, there have been patients who showed bilateral astereognosis or loss of tactile sensation as a result of an apparently unilateral cerebral lesion. These observations were corroborated by Semmes and colleagues, who tested a large series of patients with traumatic lesions involving either the right or left cerebral hemisphere. They found that the impairment of sensation (particularly discriminative sensation) following right-and left-sided lesions was not strictly comparable; the left hand as well as the right tended to be impaired by injury to the left sensorimotor region, whereas only the left hand tended to be affected by injury to the right sensorimotor region. These observations, with minor qualifications, were also confirmed by Carmon and by Corkin and associates (1965), who investigated the sensory effects of cortical excisions in patients with focal epilepsy. Thus it appears that certain somatosensory functions in some patients are mediated not only by the contralateral hemisphere but also by the ipsilateral one, although the contribution of the former is undoubtedly the more significant.
This concept of left hemispheric dominance in respect to tactile perception has also been questioned by Carmon and Benton, who found that the right hemisphere is particularly important in perceiving the direction of tactile stimuli. Also, Corkin and associates observed that patients with lesions of the right hemisphere show a consistently greater failure of tactile-maze learning than those with left-sided lesions, pointing to a relative dominance of the right hemisphere in the mediation of tactile performance involving a spatial component. Certainly the related phenomenon of sensory inattention or extinction is more prominent with lesions of the right as opposed to the left parietal lobe and is most informative if the primary and secondary sensory cortical areas are spared. These matters are considered further on in this chapter and in Chap. 21.