Upper limb exteroceptive somatosensory and proprioceptive sensory afferent modulation of hemispatial neglect

In many paradigms of stimulation techniques aimed at reducing hemispatial neglect, somatosensory and proprioceptive stimulation are often interchanged, although the anatomical and functional pathways transmitting these signals are clearly different. Therefore, we have investigated the effects of one somatosensory stimulation technique, the cyclic pressure application (CPA), and have compared them with the effects of left transcutaneous electrical stimulation (TENS) on the expression of left hemispatial neglect in 13 stroke patients, as assessed by two visuospatial exploration tasks: the Star Cancellation task and Schenkenberg's Line Bisection task. In a first experiment, four treatment conditions were given: TENS, CPA, TENS + CPA, as well as a placebo condition. For each patient, the intensity of the TENS was determined, based on his/her conscious somatosensory threshold for the electrical impulses. In order to determine whether unconscious proprioceptive afferent information instead of exteroceptive somatosensory stimulation is a sufficient condition to improve hemispatial neglect, we carried out a second experiment, only with patients suffering from complete somatosensory loss. The effects of the different treatments were investigated, using the same sequence as for Experiment 1, but, this time, the applied intensity of TENS was manipulated over two conditions: (1) one in which the intensity of TENS stimulation was below the motor (proprioceptive) threshold and (2) one in which the intensity was determined following the degree of proprioceptive stimulation demonstrated by the point at which a visible muscle contraction during the stimulation could be elicited. The results demonstrated that proprioceptive-based TENS stimulation is a sufficient condition to reduce hemispatial neglect, even when a severe somatosensory loss was present.