The parietal lobe is best known for its importance in attention. PET scans
show high activity in the parietal lobe during tasks that require concentrated
attention to one or more aspects of a stimulus. For example, if subjects are
required to specifically attend to the
color, velocity, or shape of an object in their view, blood flow to the parietal
lobe increases, indicating that they are using that area of the brain to carry
out the task. Damage to the parietal lobe results in a syndrome called neglect,
in which patients treat parts of their body or objects in part of their visual
field as though they did not exist. This effect is contralateral: if the right
parietal lobe is damaged, people will neglect their left side; if the left
parietal lobe is damaged, they will neglect their right side. For example, a
woman with damage to the right parietal lobe might put makeup on only the right
side of her face and eat the food on only the right side of her plate. The
visual system remains intact; they can see the neglected side just fine. But
because the parietal lobe is damaged, they cannot direct their attention to it,
so they fail to notice that it is there. Neglect syndrome is more likely to
occur as a result of damage to the right side of the parietal lobe than the
left. This is because the right parietal lobe mediates attention to both the
left and right fields, while the left controls attention to the right only.
Thus, when the left parietal lobe is damaged, the right can compensate, but the
converse is not true.
The occipital lobe comprises the most posterior section of the brain,
situated posterior to the parieto-occipital sulcus and directly above the
cerebellum (see
figure). The most
important feature of the occipital lobe is the primary visual cortex,
also called the striate cortex. The primary visual cortex receives and
interprets information from the retina of the eye. Signals from receptors in
the eye travel through optic nerves to the lateral geniculate nucleus, a small
structure near the center of the brain, where the information is organized and
relayed to the primary visual cortex. Like the
primary motor
cortex and the
somatic sensory
cortex, the primary visual
cortex is organized into a topographic map, with the size of receptor fields in
the cortex corresponding to the density of receptors at each location on the
retina. Also like the primary motor and somatic sensory areas, the primary
visual cortex is contralateral; the signals from the left side of each retina
are received by the right side of the visual cortex, and vice versa. Damage to
the visual cortex results in loss of vision in the contralateral visual field
(the visual field is only the left or right side of each eye, not the entire
contralateral eye). In addition to the primary visual cortex, overlapping areas
in the occipital and parietal lobes work together to control eye movements
following a moving target.
