The left-brain interpreter is a neuropsychological concept developed by the psychologist Michael S. Gazzaniga and the neuroscientist Joseph E. LeDoux. It refers to the construction of explanations by the left brain hemisphere in order to make sense of the world by reconciling new information with what was known before. The left-brain interpreter attempts to rationalize, reason and generalize new information it receives in order to relate the past to the present. Left-brain interpretation is a case of the lateralization of brain function that applies to "explanation generation" rather than other lateralized activities. Although the concept of the left-brain interpreter was initially based on experiments on patients with split-brains, it has since been shown to apply to the everyday behavior of people at large.
Discovery The concept was first introduced by Michael Gazzaniga while he performed research on split-brain patients during the early 1970s with Roger Sperry at the California Institute of Technology. Sperry eventually received the 1981 Nobel Prize in Medicine for his contributions to split-brain research. In performing the initial experiments, Gazzaniga and his colleagues observed what happened when the left and right hemispheres in the split brains of patients were unable to communicate with each other. In these experiments when patients were shown an image within the right visual field (which maps to the left brain hemisphere), an explanation of what was seen could be provided. However, when the image was only presented to the left visual field (which maps to the right brain hemisphere) the patients stated that they didn't see anything. However, when asked to point to objects similar to the image, the patients succeeded. Gazzaniga interpreted this by postulating that although the right brain could see the image it could not generate a verbal response to describe it.
Experiments Since the initial discovery, a number of more detailed experiments have been performed to further clarify how the left brain "interprets" new information to assimilate and justify it. These experiments have included the projection of specific images, ranging from facial expressions to carefully constructed word combinations, and functional magnetic resonance (fMRI) tests. Many of the studies and experiments build on the initial approach of Gazzaniga in which the right hemisphere is instructed to do things that the left hemisphere is unaware of, e.g. by providing the instructions within the visual field that is only accessible to the right brain. The left-brain interpreter will nonetheless construct a contrived explanation for the action, unaware of the instruction the right brain had received. The typical fMRI experiments have a "block mode", in which specific behavioral tasks are arranged into blocks and are performed over a period of time. The fMRI responses from the blocks are then compared. In fMRI studies by Koutstaal the level of sensitivity of the right visual cortex with respect to the single exposure of an object (e.g. a table) on two occasions was measured against the display of two distinct tables at once. This contrasted with the left hemisphere's lower level of sensitivity to variations. Although the concept of the left-brain interpreter was initially based on experiments on patients with split brains, it has since been shown to apply to the everyday behavior of people at large. A hierarchical organization of the lateral prefrontal cortex has been developed in which different regions are categorized according to different "levels" of explanation. The left lateral orbitofrontal cortex and ventrolateral prefrontal cortex generate causal inferences and explanations of events, which are then evaluated by the dorsolateral prefrontal cortex. The subjective evaluation of different internally generated explanations is then performed by the anterolateral prefrontal cortex.
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