The left and right cerebral hemispheres are grossly similar in their anatomy, morphology, physiology, and chemistry. Yet, a striking number of functional asymmetries have been described, ranging from differences in the processing of basic sensory features to differences in emotion, language, and problem solving. This disparity between the general neural similarity of the two hemispheres and the distinctiveness of their functions highlights the limits of our current understanding of the mapping between neural and functional properties. Whereas there is now a better sense of which brain areas may be involved in various language functions, the computations that these brain areas are performing are not yet understood well enough to afford an explanation of why a left hemisphere (LH) area is crucially involved in a given function while the right hemisphere (RH) homologue – with the same basic cell types, neurochemistry, and inputs and outputs – is not. In the Federmeier Lab we are interested in forwarding studies to allow us to increase this understanding.

To examine hemispheric differences in meaning processing, we combine visual half-field (VF) presentation methods with ERP measures. Whereas behavioral studies of lateralization examine only the extent to which stimuli are preferentially processed by one hemisphere, studies that involve concurrent brain measures offer the possibility to decompose the lateralized contribution of the processes that underlie performance on that task. One can thus ask whether, for instance, processing in extrastriate visual areas (e.g., perceptual components) and/or semantic processing (N400) and/or explicit memory functioning is asymmetric, and, if so, how these asymmetries relate to one another and to hemispheric biases evident in overt behavioral responses.

Indeed, our studies have shown that some of the critical inferences made about hemispheric differences from behavioral data do not hold up when tested with ERPs. For example, whereas theories derived from behavioral data have asserted that the RH is incapable of constructing message-level meaning representations from, e.g., sentence contexts, our ERP studies have consistently shown that the RH’s semantic processing is as sensitive to message-level information as that in the LH (e.g., Federmeier and Kutas, 1999; Federmeier and Kutas, 2002; Coulson, Federmeier, et al., submitted; Federmeier et al., submitted). While both hemispheres seem to be able to employ world knowledge to comprehend sentences, however, they also seem to use context information differently (see figure). We have put forward a theoretical framework for understanding the pattern of ERP data in the context of previous neuropsychological and behavioral results (e.g., Federmeier and Kutas, 2002; Federmeier, Mai, and Kutas, submitted). Whereas left hemisphere-initiated processing seems oriented toward prediction and the use of top-down cues, right hemisphere-initiated processing seems biased toward the veridical maintenance of information and integration with working memory. Such a division of labor across the hemispheres may help the brain deal with the inherent tradeoff between efficiency and accuracy in information processing.

This theoretical framework, derived from observed differences in language comprehension, also has important implications for memory. With an expectancy strategy, incoming information is rapidly assimilated so that it can be used to generate predictions about likely upcoming items. With an integration strategy, in contrast, the focus is on maintaining context information so that its fit with new incoming information can be assessed. This suggests that, if the two hemispheres are using these different strategies during language comprehension, language-related asymmetries may co-occur with – or even be partially driven by – hemispheric differences in how memory resources are allocated. We recently tested this idea using a continuous recognition memory paradigm that varied both VF of presentation and the lag between study and test (Federmeier and Benjamin, submitted). Consistent with the framework’s predictions, I observed a striking cross-over effect in the RTs to hits, with faster responses to items studied initially by the LH after short lags but faster responses to items studied initially by the RH after long lags. These data lay the groundwork for a series of studies exploring hemispheric asymmetries for memory functions in conjunction with hemispheric differences in language processing.