Motor and phosphene thresholds: a transcranial magnetic stimulation correlation study.
OBJECTIVE
To investigate the stability of visual phosphene thresholds and to assess whether they correlate with motor thresholds.


BACKGROUND
Currently, motor threshold is used as an index of cortical sensitivity so that in transcranial magnetic stimulation (TMS) experiments, intensity can be set at a given percentage of this value. It is not known whether this is a reasonable index of cortical sensitivity in non-motor and hence whether it should be used in experiments where other cortical areas are targeted. Previous studies have indicated that phosphene threshold might be a suitable alternative in TMS studies of the visual system.


METHODS
Using single pulse TMS visual phosphene and motor thresholds were measured in 15 subjects. Both thresholds were retested in seven of these subjects a week later.


RESULTS
Visual phosphene thresholds, though stable within subjects across the two sessions, showed greater variability than motor thresholds. There was no correlation between the two measures.


CONCLUSIONS
TMS motor thresholds cannot be assumed to be a guide to visual cortex excitability and by extension are probably an inappropriate guide to the cortical excitability of other non-motor areas of the brain. Phosphene thresholds are proposed as a potential standard for inter-individual comparison in visual TMS experiments.
Stewart
L M
LM
Institute of Cognitive Neuroscience, 17 Queen Square, London WC1N 3AR, UK. l.stewart@ucl.ac.uk
Walsh
V
V
Rothwell
J C
JC
D000328:Adult; D002540:Cerebral Cortex / Q000502:physiology*; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D008297:Male; D009048:Motor Skills / Q000502:physiology*; D010719:Phosphenes / Q000502:physiology*; D015203:Reproducibility of Results; D012684:Sensory Thresholds; D050781:Transcranial Magnetic Stimulation