Brain lateralization and Interpreting Intentions

M.A. Goodale and A.D. Milner discuss the possibility of separate neurological processing streams for visual sensory information in A neurological dissociation between perceiving objects and grasping them. They present the case of a woman, D.F., who has brain damage ventrally in the lateral occipital region and parasagittal occipitoparietal region. They tested separate processing streams by using D.F. as a subject.
D.F. was asked to perform a series of trials that showed the remarkable difference in her ability to perceive object orientation and her ability to direct accurate movements with objects in differing orientations. Her results were compared to control subjects. In one instance, D.F. was asked to report the orientation of lines corresponding with orientation slots, as well as the orientation of blocks on a table. She had huge errors in verbally reporting these orientations, even confusing horizontal for vertical. In sharp contrast, when asked to physically place cards in slots, she performed with excellence, and showed behavioral signs of comprehending the object’s orientation and correct placement from the moment her hand began moving. In addition to simply orientation, object size and form were tested with D.F. In these tests, she had difficulty reporting whether white plaques of differing dimensions were different or alike (52% accuracy), but when asked to pick up the plaques, she showed strong behavioral evidence of knowing the object’s dimension, and did not differ from control subjects.
These results suggest “separate processing systems not for different subsets of visual information, but for the different uses to which vision can be put.” This means that there must be an area that processes conscious perceptual judgments separate from one that operates our automatic visuomotor processes that mediate skilled movements. This necessitates serious study not just of the visual/sensory inputs but also the patterns of output connections, processed internally within the brain and determined independent of visual information type. In other words, we need to look at what people mean to do with what they see, and not just study what they are seeing.
This undercuts those who believe visual information is interpreted and responded to only by the visual cortex and then sent to other brain functions as either spatial localization or object identification. There is clear evidence in this research of multiple, distinct brain areas that correspond with specific systems for processing different intentions associated with the visual information. This may even suggest that our brain has evolved to process our intentions as carefully as it processes our senses.
Further research since 1991 has been performed by a number of scientists. As one example, Clavagnier et al. (2000) assert further evidence for “functional subdivisions”. Their research focused primarily on the posterior parietal cortex. The numerous studies in this realm all point to the great degree of lateralization in the human brain. Gazzaniga (2008) discusses the lateralization of the human brain’s language centers: Broca’s and Wernicke’s areas, and suggests that this lateralization may be what makes humans truly unique from our primate ancestors. We not only need to process the phonemes of speech and how to recreate those sounds, but also their function and meaning. It is the toil of processing the semantics and pragmatics of language that has led to some of the unique lateralizations in the human brain.
Works Cited:
Clavignier, S., et al. (2000).Two systems of spatial representation: evidence from parietal lesions in humans. European Journal of Neuroscience. 12, 2.
Gazzaniga, M.S. (2008). Human. New York, New York: HarperCollins.
Goodale, M.A., & Milner, A.D. (1991). A neurological dissociation between perceiving objects and grasping them. Nature. 349, 154-156.