Internally generated and externally triggered actions are physically distinct and independently controlled.

Area of Science:

• Neuroscience
• Motor Control
• Human Movement Science

Background:

• Everyday actions involve balancing internally generated intentions with externally triggered reactions.
• Previous research often studied these action types separately, despite their crucial interaction.
• Understanding the relationship between internally generated and externally triggered movements is vital for motor control.

Purpose of the Study:

• To investigate the relationship between internally generated and externally triggered actions.
• To examine how the motor system processes and switches between these two action types.
• To provide peripheral physiological evidence for distinct neural preparation levels.

Main Methods:

• Electromyography (EMG) of the first dorsal interosseous muscle was measured.
• Participants performed index finger key presses in internally generated and externally triggered conditions.
• A 'truncation' condition forced a switch from intentional to externally triggered action to assess processing.

Main Results:

• Internally generated actions exhibited significantly greater muscle activation than externally triggered actions.
• Response times were delayed by approximately 54.7 ms in the truncation condition compared to externally triggered responses.
• Distinct EMG patterns for internally generated and externally triggered actions were maintained during truncation, suggesting separate processing.

Conclusions:

• The motor system may process internally generated and externally triggered actions separately, even when the motor output is identical.
• Switching between action types does not leverage pre-existing preparation, leading to response delays.
• Findings support neuroimaging studies indicating greater preparation for internally generated actions.