Automatic postural response systems in individuals with congenital total blindness

This study examined the effects of the absence of vision from birth on auto matic postural responses to platform displacements during stance. Postural responses were induced by producing randomly four types of perturbations wh ich consisted of forward and backward translations, and toe up and down rot ations. Nine congenitally totally blind and nine sighted adults served as s ubjects. EMG signals were recorded from four muscles in the right leg, and reaction time to somatosensory stimuli generated by platform displacements was measured by pushing a hand-held button. To assess the ability to contro l postural balance, the root mean square (RMS) values for lateral and anter o-posterior sway before, during, and after perturbations were calculated. T he EMG amplitude in the gastrocnemius muscle of a blind subject was smaller than that of a sighted subject with eyes closed. No significant difference s were found between blind and sighted subjects in EMG latencies of the low er extremity muscles in response to perturbations. The blind subjects had s ignificantly faster reaction times to somatosensory stimuli triggered by pl atform displacements, but in toe down rotations no significant difference w as found between blind and sighted subjects. The difference in the EMG late ncies and reaction times between the two groups suggests that blindness fro m birth may not affect the spinal stretch reflex, but may affect a volition al act mediated through the motor cortex. There were also no significant di fferences in the RMS values for postural sway between blind and sighted sub jects with eyes open or closed, although blind subjects swayed more after b ackward translations than did sighted subjects with eyes open. Results sugg est that the ability to control postural balance during perturbations was n ot affected by vision loss from birth. Our findings suggest that the automa tic postural response systems of humans are unaffected by the absence of vi sion from birth and are rather hard wired. (C) 2001 Elsevier Science I3.V. All rights reserved.