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Daniela Balslev, Chris Miall; Degraded eye proprioception after 1Hz rTMS over the anterior parietal cortex. Journal of Vision 2008;8(6):484. doi: 10.1167/8.6.484.
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The extraocular muscles have proprioceptive receptors that project to the primary somatosensory cortex (Wang et al., Nat Neurosci, 2007). The role of eye proprioception in oculomotor behavior is unknown. A step in elucidating this role would be to observe this behavior in the absence of proprioception. To the best of our knowledge, no method for acute proprioceptive deafferentation of the eye muscles in healthy humans is currently available.
We aimed to reduce proprioception in the right eye with 1Hz rTMS over the left somatosensory cortex, a method previously shown to reduce hand proprioception (Balslev et al., J. Neurosci 2004). We measured the perceived straight ahead in monocular vision and in darkness, a task sensitive to changes in eye proprioception (Gauthier et. al, Science, 1990). Healthy subjects sat with the head fixed in front of an array of 48 LEDs at 2 degree separation. A random LED was first lit, and the subjects verbally instructed left or right movement until they perceived it to be directly in front of their nose.
rTMS over the somatosensory cortex but not over motor cortex shifted the perceived straight ahead a mean of 3 degrees to the left . This effect can be explained by the underestimation of the angle of gaze of the right eye when fixating the target.
To test whether this effect was related to eye proprioception we applied a passive deviation by pushing the right eye through the closed eyelid before the task. The shift to the left in the perceived straight ahead induced by somatosensory rTMS was significantly reduced by the passive eye displacement. This displacement did not affect the perceived straight ahead at baseline or after motor cortex rTMS.
We suggest that depression of somatosensory cortex using rTMS interferes with the processing of eye proprioception.
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