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.

Method: 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.

Result: 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.

Conclusion: 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.