Language processing without motor processing is bilaterally symmetric
Abstract
Objective
To determine the degree to which brain activation lateralizes to the left hemisphere (LH) during passive language use that involves no motor activity or overt decision making.
Background
Noninvasive imaging tasks used as an index of laterality associated with language function in normal subjects and preoperative measures of laterality (e.g., the Wada test) typically require a verbal response, or decision-making accompanied by a motor response. LH laterality resulting from these measures may be more strongly representative of these processes than the ability to decode the speech signal. We hypothesized that the brain activation associated with the passive perception of auditory (A) and auditory+visual (AV) discourse is bilateral and that the linear addition of visual (V) information to A alone can not account for AV. A non-linear interaction of this type raises questions about (task subtraction) methods used to derive indexes of laterality and localize language function.
Design/Methods
Six female and five male right handed native English speakers participated. Functional MRI was performed at 1.5 T (GE Signa) with a head coil. Twenty-four spiral gradient echo T2* axial functional images were collected every 3 seconds. Subjects were asked to remain motionless and attentive as they viewed and/or heard stimuli. No overt motor response was required. Stimuli were audio discourse (A), audio and video discourse (AV) and video alone (V). Stimuli were interesting self-contained stories of approximately 24 seconds duration. A total of 224 whole brain images during each condition were collected. A general linear model was used to reveal brain activation related to each condition as well as to the comparison of AV and V (AV/V).
Results
Raw volumes of activation in the right and left hemispheres were tabulated and analyzed by means of a 2-way (condition X hemisphere) repeated measures ANOVA. There was a main effect of condition [F (3) = 8.408, p = 0.003] but no main effect of hemisphere. The interaction between condition and hemisphere was significant [F (3, 30) = 3.525, p = 0.0267] with a preferential activation of the LH being seen in the AV/V comparison and to a lesser extent in the AV condition. Further, the lateralization of AV could not be accounted for by the simple linear addition of A and V.
Conclusions
Our results suggest that the brain processes human discourse bilaterally. Strong LH lateralization of language function may be the result of motor and/or decision-making processes associated with tasks and task subtractions. Regional analysis of statistical images suggests that relative lateralization on perceptual tasks may relate to perception of motor processes initially associated with production. These results have implications for preoperative measures of laterality and for treatment of aphasia by suggesting an inherent bilaterality of language circuits that may be shifted by motor and/or decision making processes that accompany ecological language use. Further, the use of subtraction methods in imaging paradigms is questionable as activations associated with AV appear emergent in the LH and cannot be accounted for by A and V.
Study supported by: NIH-NIDCD DC-03378