Post unilateral lesion response biases modulate memory: Crossed double dissociation of hemispheric specialisations

Abstract

We propose that what appears to be hemispheric specialisation in the memory domain, as indexed by effects of unilateral brain lesions, is to a great extent explainable as response bias: left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern. To test this prediction a group of 40 non-confabulatory cases with a verbal and non-verbal retention deficit (hypomnesia), subsequent to a unilateral lesion, was assembled from the literature. A group of non-amnesic cases with confabulation, paramnesia, false memories or memory-laden hallucination (dysfunctional hypermnesia), due to a unilateral lesion, was also assembled from the literature (N=72). Most of the hypomnesic patients had left hemisphere lesions (73%, p<.005, two tailed) while most of the hypermnesic patients had right hemisphere lesions (78%, p<.0005, two tailed). This crossed double dissociation held good despite statistical control of the lesion's locus within the hemisphere, its size or its aetiology, presence of aphasic symptoms, psychiatric comorbidity, the patient's age, gender, or hand preference, and several other potentially confounding variables.

Keywords:

• Psychic tonus
• Memory
• Response bias
• Hemispheric specialisation
• Unilateral lesions

Acknowledgements

This research was made possible by a grant from the Fonds de Recherche en Santé du Québec (FRSQ) and the Fonds Québécois de Recherche sur la Société et Culture (FQRSC) to the first author and a scholarship from the Fonds Québécois de Recherché en Sciences et Technologie to the third author. We thank Isabelle Rouleau for her comments on the typescript.