The relationship between behavioral language laterality, face laterality and language performance in left-handers

Abstract

Left-handers provide unique information about the relationship between cognitive functions because of their larger variability in hemispheric dominance. This study presents the laterality distribution of, correlations between and test-retest reliability of behavioral lateralized language tasks (speech production, reading and speech perception), face recognition tasks, handedness measures and language performance tests based on data from 98 left-handers. The results show that a behavioral test battery leads to percentages of (a)typical dominance that are similar to those found in neuropsychological studies even though the incidence of clear atypical lateralization (about 20%) may be overestimated at the group level. Significant correlations were found between the language tasks for both reaction time and accuracy lateralization indices. The degree of language laterality could however not be linked to face laterality, handedness or language performance. Finally, individuals were classified less consistently than expected as being typical, bilateral or atypical across all tasks. This may be due to the often good (speech production and perception tasks) but sometimes weak (reading and face tasks) test-retest reliabilities. The lack of highly reliable and valid test protocols for functions unrelated to speech remains one of the largest impediments for individual analysis and cross-task investigations in laterality research.

Fig 1. Reaction time lateralization indices for the laterality tasks.

Reaction time lateralization index (LI) distributions for the picture visual half field (VHF), word VHF, Optimal Viewing Position (OVP), dichotic listening and face VHF tasks. LIs are sorted from most negative to most positive scores per task. Note that only the dichotic listening chart has a deviating scale on the y-axis to improve visibility because LIs were based on left/right ear matches and not reaction times.

Fig 2. Percentage participants categorized as showing typical, bilateral or atypical lateralization based on their reaction times.

Reaction time distributions expressed as percentages and divided in a typical, bilateral and atypical category for the picture visual half field (VHF), word VHF, Optimal Viewing Position (OVP), Dichotic Listening (DL) and face VHF tasks. Panel A classifies the participants based on raw differences between the left and right visual field or ear. The data in Panels B and C are based on lateralization indices (LIs) that represent a mean reaction time difference of 10 ms (same values as in Panel A for the DL task) and 0 ms respectively.

Fig 3. Scatter plots contrasting two tasks.

Data points represent individual lateralization indices (LIs) based on reaction times. VHF = Visual Half Field; OVP = Optimal Viewing Position; DL = Dichotic Listening. Note that only the dichotic listening chart has a deviating scale on the y-axis to improve visibility because LIs were based on left/right ear matches and not reaction times.