Electrical stimulation of this area yields sensations of tingling, numbness, and warmth in specific regions on the opposite side of the body. The information transmitted to S1 is tactile and proprioceptive, derived mainly from the dorsal column–medial lemniscus system and concerned mainly with sensory discrimination. The second somatosensory area (S2) lies on the upper bank of the sylvian fissure, adjacent to the insula. Localization of function is less discrete in S2 than in S1, but S2 is also organized somatotopically, with the face rostrally and the leg caudally. The sensations evoked by electrical stimulation of S2 are much the same as those of S1 but, in distinction to the latter, may be felt bilaterally.
Undoubtedly, the perception of sensory stimuli involves more of the cerebral cortex than the two discrete areas described above. Some sensory fibers probably project to the precentral gyrus and others to the superior parietal lobule. Moreover, S1 and S2 are not purely sensory in function; motor effects can be obtained by stimulating them electrically. It has been shown that sensory neurons in VPL, cuneate and gracile nuclei, and sensory neurons in the dorsal horns of the spinal cord all receive descending as well as ascending cortical projections. This reciprocal arrangement probably influences movement and the transmission and interpretation of some sensations as discussed in Chap. 7.
Because they are situated as way stations in a serial system, if the thalamus and subcortical projections are damaged, sensations such as pain, touch, pressure, and extremes of temperature do not reach consciousness. Damage to the sensory cortex allows some appreciation of contralateral sensory experience but degrades accurate localization, as well as the patient’s ability to make other fine sensory discriminations. This clinical application of these features is elaborated in the discussion of the sensory syndromes further on.
From this brief account of the various channels of sensory information, one must conclude that at every level there is the possibility of feedback control from higher levels. Most external and some internal stimuli are highly complex and induce activity in more than one sensory system. In every system there is sufficient redundancy to allow lesser-used systems to compensate partially for the deficits incurred by disease.
Terminology of Sensory Signs and Symptoms
A few terms require definition, as they are commonly encountered in discussions of sensation. Some of these, relating to pain, were mentioned in Chap. 7. Experimental data support the view that partially damaged touch, pressure, thermal, and pain fibers become hyperexcitable and generate ectopic impulses along their course, either spontaneously or in response to stimuli (Ochoa and Torebjork). These abnormal sensations are experienced as paresthesias, or dysesthesias if they are severe and distressing, as noted in Table 8-2. Another positive sensory symptom is allodynia, referring to a phenomenon in which a non-painful stimulus such as touch evokes pain.