Parietal Lobe

Space, numbers and time, object manipulation, movement, navigation, sensory integration, body image, sense of self

The parietal lobes lie between the frontal and occipital lobes, integrating sensory signaling to provide a sense of embodiment and location in space. Afferent (incoming) sensory fibers are mapped to the primary somatosensory cortex in an upside- down fashion. Penfield helped to map the so-called sensory homunculus. As with the motor cortex, he used an electrode to stimulate the surface of the brain, carefully noting responses.[1] Body parts are represented in accordance with evolutionary developed needs. For example, more cortex is devoted to hands than to calves. The somatosensory cortex is cross-wired. Our motor, sensory, and perceptual systems are all cross-wired, and it is unclear why.

The parietal lobe is also involved in vision. The dorsal stream of visual processing, running forward from the primary visual cortex in the occipital lobe and up through the parietal lobe, is for locating things in space—the “where” stream. There is also a ventral stream, running from the primary visual cortex forward through the lower part of the temporal lobe, for discerning what we see—the “what” stream. These complementary streams help us to locate and recognize objects. The streams may be able to function independently if either one of them sustains damage. So, for example, a person with damage in the ventral stream might be able to interact spatially with things but might be unable to describe them.

Let us consider an evolutionary perspective on the visual streams of processing to understand them better. In chapter “Consciousness as a Scientific Problem”, we examined the visual perception account by Marr. This account represents the common view that visual perception is a matter of building an internal representation of the environment, which is then used for interacting in the world. Neuroscientists Melvyn Goodale and David Milner, however, maintain that vision did not evolve first for perception of the world, but evolved first for action. In their view, the dorsal stream of visual processing allows action in the world without us being consciously aware of what we see (Goodale and Milner 1992).

The parietal lobe grounds us environmentally at a fundamental level prior to conscious perception. It allows us to reach out, grasp, and interact with things in a prerepresentational way. Goodale and Milner report from an experiment in which a patient with visual impairment could put a card through a slot while being unable to say how the slot was oriented (Goodale and Milner 1992, pp. 22-24). Goodale and Milner explain this with the idea that the dorsal stream evolved to guide our actions in the world without higher awareness. We will revisit the dorsal stream and visually guided action as we examine the broader question of why consciousness evolved.

  • [1] Penfield could do this mapping in conjunction with his Montreal procedure for operating on epileptic patients, during which he stimulated the brain diagnostically to learn what not to cut into.
 
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