Commissurotomy subjects show lateralized difference between manual and oral responding

Area of Science:

• Neuroscience
• Cognitive Psychology
• Speech and Hearing Science

Background:

• The right ear advantage (REA) is a well-documented phenomenon in verbal dichotic testing, suggesting left hemisphere dominance for language processing.
• The influence of response modality on the REA, particularly in individuals with cerebral commissurotomy, remains incompletely understood.

Purpose of the Study:

• To investigate how different response modes (oral vs. manual) affect the right ear advantage in verbal dichotic listening.
• To determine if response modality influences the engagement of specific cerebral hemispheres or the suppression of ipsilateral pathways.

Main Methods:

• Four subjects with complete cerebral commissurotomy were tested using an ascending digits tape.
• Stimuli were presented dichotically with varied volume levels (equal and raised) in each ear.
• Responses were recorded orally and manually (pointing/writing) using the dominant hand.

Main Results:

• Oral responses consistently yielded a right ear advantage across all conditions and difficulty levels.
• Manual responses (pointing, writing) elicited a left ear report under specific conditions (raised left ear volume, low digit load).
• Increasing task difficulty (more digits) enhanced the right ear advantage and reduced left ear performance, irrespective of response mode.

Conclusions:

• Response modality significantly modulates the observed right ear advantage in verbal dichotic testing.
• Oral responses may mask the true extent of left hemisphere lateralization by engaging or suppressing specific neural pathways.
• Manual response modes may provide a more sensitive measure of cognitive lateralization in certain populations.