Effect of tactile interference stimulation of the ear in human primary somatosensory cortex: a magnetoencephalographic study

The Homuncular Jigsaw: Investigations of Phantom Limb and Body Awareness Following Brachial Plexus Block or Avulsion

Many neuropsychological theories agree that the brain maintains a relatively persistent representation of one’s own body, as indicated by vivid “phantom” experiences. It remains unclear how the loss of sensory and motor information contributes to the presence of this representation. Here, we focus on new empirical and theoretical evidence of phantom sensations following damage to or an anesthetic block of the brachial plexus. We suggest a crucial role of this structure in understanding the interaction between peripheral and central mechanisms in health and in pathology. Studies of brachial plexus function have shed new light on how neuroplasticity enables “somatotopic interferences”, including pain and body awareness. Understanding the relations among clinical disorders, their neural substrate, and behavioral outcomes may enhance methods of sensory rehabilitation for phantom limbs.

Phantom limb sensations in the ear of a patient with a brachial plexus lesion

Referred phantom sensations are frequently reported following a peripheral injury. However, very few cases describe such sensations of the ear, and it remains unclear how the aural nerve territory can be remapped to one specific peripheral nerve region. We report on a patient with brachial plexus avulsion who underwent sensory testing and was asked to report the location of the stimulated site and any other sensations experienced. The patient spontaneously described the sensation of his arm being separate from his body. Despite visual input, he felt that his fist was closed, with his thumb pointing inward. Importantly, he felt clear and reproducible sensations from the affected arm when the ipsilateral ear was touched. These referred sensations were noted just 15 days after sustaining the injury. The arm nerve territory was systematically remapped to a specific aural nerve territory by applying both manual and electrical stimulation. Stimulation of the external ear, which is innervated by the vagus nerve, showed high spatial specificity for the dorsal and volar skin surfaces of the limb, and clearly delineated digits. Somatosensory-evoked potentials indicated that cortical adaptation in the somatosensory stream transferred a spatially organized map of the limb to the skin of the outer ear. This referral of sensations to the ear, as distinct from the face, provides evidence of highly specific topographical reorganization of the central nervous system following peripheral injury. Rapid map changes in the phantom sensation to the ear as a function of stimulation of vagus nerve suggest that the reorganization process can occur in cortex rather than in the brainstem.

Reappraisal of the somatosensory homunculus and its discontinuities

Neuroscience folklore has it that somatotopy in human primary somatosensory cortex (SI) has two significant discontinuities: the hands and face map onto adjacent regions in SI, as do the feet and genitalia. It has been proposed that these conjunctions in SI result from coincident sources of stimulation in the fetal position, where the hands frequently touch the face, and the feet the genitalia. Computer modeling using a Hebbian variant of the self-organizing Kohonen net is consistent with this proposal. However, recent work reveals that the genital representation in SI for cutaneous sensations (as opposed to tumescence) is continuous with that of the lower trunk and thigh. This result, in conjunction with reports of separate face innervation and its earlier onset of sensory function, compared to that of the rest of the body, allows a reappraisal of homuncular organization. It is proposed that the somatosensory homunculus comprises two distinct somatotopic regions: the face representation and that of the rest of the body. Principles of self-organization do not account satisfactorily for the overall homuncular map. These results may serve to alert computational modelers that intrinsic developmental factors can override simple rules of plasticity.

Functional connectivity between forearm and digits representations in human somatosensory area 3b

OBJECTIVE

We compared the effects of tactile interference to the forearm on magnetic responses evoked by electric stimulation of the little finger (D5) and the thumb (D1).

METHODS

Electric stimulation was delivered to D5 or D1 individually. In each stimulus session, magnetic recordings were conducted with or without concurrent tactile interference to the radial side of the anterior forearm.

RESULTS

With forearm interference, the amplitude of the primary response (N20m) following D5 stimulation was reduced to 90.7% of the control value without interference, while that following D1 stimulation was not affected (100.7%).

CONCLUSIONS

In human somatosensory area 3b, the representation of the forearm is immediately adjacent to that of the D5, and distant from that of the D1. Thus, the result suggests that the tactile interference effect on N20m depends on the cortical distance between electrically and mechanically activated 3b areas.

SIGNIFICANCE

Intrinsic synaptic connections between the 3b hand representation and its surroundings have been hypothesized as a neural basis for plastic changes of the human brain, such as a phantom hand phenomenon. The present finding implies that these connections may play some physiological roles even in normal adult humans.