Correlation between motor and phosphene thresholds: A transcranial magnetic stimulation study

Authors

  • Choi Deblieck,

    1. Ahmanson-Lovelace Brain Mapping Center, UCLA, Los Angeles, California
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  • Benjamin Thompson,

    1. Department of Psychology, UCLA, Los Angeles, California
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  • Marco Iacoboni,

    1. Ahmanson-Lovelace Brain Mapping Center, UCLA, Los Angeles, California
    2. Department of Psychiatry and Biobehavioral Sciences, David Geffen School of Medicine at UCLA, Los Angeles, California
    3. Brain Research Institute, David Geffen School of Medicine at UCLA, Los Angeles, California
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  • Allan D. Wu

    Corresponding author
    1. Ahmanson-Lovelace Brain Mapping Center, UCLA, Los Angeles, California
    2. Department of Neurology, David Geffen School of Medicine at UCLA, Los Angeles, California
    • Department of Neurology, David Geffen School of Medicine at UCLA, 710 Westwood Plaza, Reed Bldg A-153, Los Angeles, CA 90095-1769
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Abstract

Transcranial magnetic stimulation (TMS) has become a common tool for the brain mapping of a wide variety of cognitive functions. Because TMS over cortical regions of interest other than motor cortex often does not produce easily observable effects, the ability to calibrate TMS intensity for stimulation over nonmotor regions can be problematic. Previous studies reported no correlation between motor thresholds (MT) over the motor cortex and phosphene thresholds (PT) over the visual cortex. However, different thresholding methods, lighting, and eye-closure conditions were used to determine MT and PT. We investigated the correlation between resting MT (rMT), active MT (aMT), and PT in 27 dark-adapted healthy volunteers. All thresholds were measured with eyes-open in the dark and determined by gradually reducing stimulation intensity downward. All subjects had aMT and rMT; 21 subjects had measurable PT. rMT was 70.4% ± 9.8% (mean ± SD of maximum stimulator output); aMT was 61.1% ± 7.9%; PT was 82.2% ± 10.1%. A significant positive correlation was found between aMT and PT (r = 0.53; P = 0.014) with a trend toward correlation between rMT and PT (r = 0.43; P = 0.052). Our results suggest that sensitivity to TMS over visual and motor cortices may be correlated under similar thresholding procedures. They also provide a rationale for the use of easily obtained aMT to calibrate TMS intensities in brain mapping studies that employ TMS in cortical regions besides motor cortex. Hum Brain Mapp, 2008. © 2007 Wiley-Liss, Inc.

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