Phosphene thresholds evoked with single and double TMS pulses

Area of Science:

• Neuroscience
• Neurophysiology
• Sensory Perception

Background:

• Transcranial magnetic stimulation (TMS) is used to study visual perception.
• Phosphenes, the perception of light without external visual stimuli, can be evoked by TMS.
• Optimizing TMS parameters for phosphene generation is crucial for research.

Purpose of the Study:

• To quantitatively compare the effectiveness of double pulses versus single pulses in evoking TMS phosphenes.
• To determine if double pulses offer a significant advantage in phosphene threshold measurements.
• To investigate the effect of stimulus onset asynchrony (SOA) and pulse form on phosphene thresholds.

Main Methods:

• Ten healthy subjects participated in the study.
• Single pulse TMS thresholds were compared to double pulse thresholds at various SOAs (2-20ms).
• Both biphasic and monophasic pulse forms were tested; fusion time was also determined.

Main Results:

• Double pulses slightly lowered thresholds to approximately 90% of single pulse thresholds, independent of SOA.
• Biphasic pulses were more effective than monophasic pulses.
• Fusion time averaged 45ms but showed significant inter-individual variability.

Conclusions:

• Double pulses provide only a minor efficiency gain over single pulses for TMS phosphenes.
• Current recommendations for using double pulses in phosphene studies are not supported for SOAs up to 20ms.
• Phosphene threshold studies do not benefit from the application of double pulses.