Motor and phosphene thresholds: a transcranial magnetic stimulation correlation study

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Abstract

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.

Introduction

The use of transcranial magnetic stimulation (TMS) in investigations of the motor system has advanced further than in studies of the visual system. One of the main reasons for this is that a muscle response elicited by TMS and measured by electromyogram (EMG) provides a reasonably objective measure of the individual subject's sensitivity to stimulation. This measure can then be used in other motor experiments to calibrate the level of stimulation across subjects by applying TMS at a given percentage of each subject's motor threshold. A standardized method of calibrating stimulation intensity to an individual subject's sensitivity is important for at least two reasons. Firstly, motor thresholds are commonly used as a valuable guide in establishing the envelope of safety in TMS studies [16].. Secondly, a standardized method of setting stimulation level according to an individual's sensitivity allows findings from different TMS studies to be more directly comparable. Non-motor TMS experiments do not at present benefit from such a standardized procedure. Studies of visual cortex have variously used stimulator output levels ranging from 35 to 100% of stimulator output to achieve functional disruption [2], [6], [13] but rarely are these levels chosen on the basis of an individual's sensitivity to TMS.

Single pulse or repetitive TMS can be used to readily elicit phosphenes in healthy subjects [12]. Two studies have used TMS to elicit phosphenes in migraine sufferers in order to determine whether, as a group, the excitability of occipital cortex differs from that of non-migraine sufferers [1], [4]. The present study suggests that phosphene threshold be used as a measure of occipital cortex excitability in healthy subjects, in the same way that motor threshold is used as an index of cortical excitability of this area. In order to investigate the potential usefulness of phosphene thresholds as an analogue of motor thresholds, it is first necessary first to establish whether phosphene thresholds correlate with motor thresholds. If this were found to be the case, experimenters would have the option of using visual or motor measures to standardize the stimulation intensity to the individual subject's sensitivity. If they were not found to correlate, one might propose that TMS studies of vision or cognition should calibrate stimulation intensity as a function of phosphene rather than motor threshold since the former is more likely to reflect the sensitivity of the areas which will be stimulated. However, in order for phosphene thresholds to be truly useful, they must be demonstrated to be as robust as motor thresholds.

Section snippets

Methods

Fifteen subjects, aged between 19 and 37 and all right handed, took part in the experiment. All 15 had visual and motor thresholds measured and seven of them returned to have these measures repeated at least 1 week later. Subjects reported absence of epilepsy, migraine, or any other neurological condition in themselves and their known family history. Ethical committee approval was granted for all procedures.

The stimulator used was a Magstim TM model 200 (Magstim, Whitland, Dyfed) connected to a

Phosphene thresholds

Stimulation, applied over the occipital midline and between 2 and 4 cm above the inion, elicited phosphenes in all 15 subjects. Phosphene threshold ranged between 35 and 85% of stimulator output. Phosphene thresholds of the seven subjects who were retested at least 1 week later ranged between 35 and 65% stimulator output. There was a significant correlation between the phosphene thresholds at the two times of testing (Fig. 1, correlation coefficient=0.7; P<0.01).

Motor thresholds

Stimulation over the right motor

Discussion

The findings of this study were:

  • 1.

    Phosphene and motor thresholds were robust across testing sessions held at least a week apart.

  • 2.

    Phosphene and motor thresholds differed within subjects with phosphene threshold greater than motor threshold except in two cases.

  • 3.

    Phosphene thresholds were more variable across subjects than motor thresholds.

  • 4.

    Phosphene and motor thresholds did not correlate with each other.

The finding that visual phosphene thresholds were stable across different testing sessions held at

Acknowledgements

This work was supported by the MRC. V. Walsh's work is supported by a Royal Society University Research fellowship.

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