Learning goals
1. To learn the organization of the brain and the brain anatomy of
the parietal and occipital lobes as well as the basal ganglia.
2. To understand how motion and perception are processed in the
brain.
3. To link lesions of the parietal and occipital lobes to motion and
perception disorders.
Source: Kolb Chapter 14
The Parietal Lobes
14.1 Parietal Lobe Anatomy
Subdivision of the Parietal Cortex
• The parietal region of the cerebral cortex lies between the frontal and
occipital lobes, underlying the parietal bone at the roof of the skull.
• Roughly demarcated anteriorly by the central fissure, ventrally by the
lateral (Sylvian) fissure, dorsally by the cingulate gyrus, and
posteriorly by the parieto-occipital sulcus.
• The principal regions of the parietal lobe, mapped in Figure 14.1A
and B, include the postcentral gyrus (Brodmann’s areas 3-1-2),
superior parietal lobule (areas 5 and 7), parietal operculum (area 43),
supramarginal gyrus (area 40), and angular gyrus (area 39).
• Together, the supramarginal gyrus and angular gyrus are often
referred to as the inferior parietal lobe.
• The parietal lobe can be divided into two functional zones: an anterior
zone including areas 3-1-2 and 43 and a posterior zone that includes
the remaining areas.
• The anterior zone is the somatosensory cortex; the posterior zone is
called the posterior parietal cortex.
• Over the course of human evolution, the parietal lobes have
undergone a major expansion, largely in the inferior region. à
difficult to compare with monkey brain as some areas don’t exist in
their brain.
• Another anatomist: parietal areas are called PA (parietal area A), PB,
and so forth, are three posterior parietal areas (PE, PF, PG) that von
Economo described in both humans and monkeys. à area PF is
equivalent to Brodmann’s areas 43 and 40 plus part of area 7 and PE
to area 5 and the remainder of area 7. Similarly, area PG is roughly
equivalent to Brodmann’s areas 39 and 40.
• An area significantly expanded in the human brain appears to consist
of the polymodal parts of area PG and adjoining polymodal cortex in the superior
temporal sulcus (STS). Polymodal cells receive inputs from more than one sensory
modality. Those in PG respond to both somatosensory and visual inputs, whereas those
, in the STS respond to various combinations of auditory, visual, and somatosensory
inputs.
The increased size of area PG and the STS is especially interesting, because this region is
anatomically asymmetrical in the human brain.
The asymmetry may be due to a much larger area PG (and possibly STS) on the right than on
the left. If PG has a visual function and is larger in humans, especially in the right hemisphere,
then we might expect unique visual symptoms after right parietal lesions. This indeed is the
case. à Note, however, that PG is also larger on the left in the human than in the monkey.
This leads us to expect unique deficits in humans after left-hemisphere lesions. This outcome,
too, is the case.
The regions in the intraparietal sulcus contribute to controlling saccadic eye movements (area
LIP) and visual control of object-directed grasping (AIP). The PRR has a role in visually guided
grasping movements. (A saccade is a series of involuntary, abrupt, and rapid small movements
or jerks made by both eyes simultaneously in changing the point of fixation.)
Connections of the parietal cortex
The anterior parietal cortex makes rather straightforward connections. Projections from the
primary somatosensory cortex (area 3-1-2 in Figure 14.1B) extend to secondary somatosensory
area PE (area 5), which has a tactile recognition function, as well as to motor areas including
the primary motor cortex (area 4) and the supplementary motor and premotor regions (area 6)
in the frontal lobes. The motor connections must be important for providing sensory
information about limb
position in movement control.
1. Area PE (Brodmann’s area 5 plus part of area 7) is basically somatosensory,
receiving most of its connections from the primary somatosensory cortex (areas 3-
1-2). PE’s cortical outputs are to the primary motor cortex (area 4) and to the
, supplementary motor (SMA) and premotor (6 and 8) regions, as well as to PF.
Area PE therefore plays some role in guiding movement by providing information
about limb position.
2. Area PF (part of area 7) has heavy input from the primary somatosensory cortex
(areas 3-1-2) through area PE. PF also receives inputs from the motor and
premotor cortex and a small visual input through area PG. PF’s efferent
connections are similar to those of area PE, and these connections presumably elaborate
similar information for the motor systems.
3. Area PG (part of area 7 and visual areas) receives more-complex connections
including visual, somesthetic (skin sensations), proprioceptive (internal stimuli),
auditory, vestibular (balance), oculomotor (eye movement), and cingulate
(motivational?). MacDonald Critchley (1953) first described area PG as the “parieto-
temporo-occipital crossroads,” which is apparent from the connectivity. Its function
likely corresponds to this intermodal mixing. Area PG is part of the dorsal stream that
controls spatially guided behavior with respect to visual and tactile information.
4. The close relation between the posterior parietal connections and the prefrontal
cortex (especially area 46) are apparent in the connections between the posterior
parietal cortex (PG and PF) and the dorsolateral prefrontal region. Additionally,
the prefrontal and the posterior parietal regions project to the same areas of the
paralimbic cortex and the temporal cortex as well as to the hippocampus and various
subcortical regions. These connections emphasize a close functional relation between
the prefrontal and parietal cortices. This relation probably has an important role in
controlling spatially guided behavior.
Anatomy of the Dorsal Stream
Kravitz and his colleagues identify three functional pathways leaving the posterior parietal
region and traveling to the premotor, prefrontal, and medial temporal regions.
• The parieto–premotor pathway is proposed as the principal “how” pathway.
• The parieto–prefrontal pathway is proposed to have visuospatial functions,
especially related to visuospatial working memory.
• The parieto–medial temporal pathway, which flows directly to the hippocampus and
parahippocampal regions as well as indirectly via the posterior cingulate and
retrosplenial cortex, is proposed to have a role in spatial navigation.
• Thus, the posterior parietal cortex would contribute to the dorsal stream by
participating in nonconscious visuospatial behavior, that is, reaching for and
grasping objects.
Kravitz and coworkers emphasize these three pathways within the dorsal stream, but others
may well be found. The researchers note connections with V5 and the superior temporal sulcus,
regions involved in motion and form processing, as likely candidates.
The goal of all dorsal stream pathways is to guide visuospatial behavior through motor
output, so the parieto–prefrontal and parieto–mediotemporal pathways must eventually
influence motor output, though more indirectly than does the parieto–premotor pathway.
,14.2 A Theory of Parietal-Lobe Function
• If we consider the anterior (somatosensory) and posterior (spatial) parietal zones
functionally distinct, we can identify two independent parietal-lobe contributions. The
anterior zone processes somatic sensations and perceptions. The posterior zone
specializes primarily in integrating sensory input from the somatic and visual regions
and to a lesser extent from other sensory regions, mostly for controlling movements—
reaching and grasping as well as whole-body movements in space.
• The posterior parietal cortex also plays a significant role in mental imagery, especially
related to both object rotation and navigation through space.
• As we think about how the brain manages different tasks, an internal representation of
the location of different objects around us seems obvious—a sort of map in the brain
of where things are. Furthermore, we assume that the map must be common to all of
our senses, because we can move without apparent effort from visual to auditory to
tactile information. More than seven decades of clinical observations of patients with
parietal injury demonstrate that the parietal lobe plays a central role in creating this
brain map. But what precisely is the map?
• We take for granted that the world around us is as we perceive it and thus that the brain
employs a unified spatial map. That is, real space must be mapped topographically in
the brain because that is how it appears to us.
• Unfortunately, scant evidence supports the existence of such a map in the brain. More
likely is a series of neural representations of space that vary in two ways.
• First, different representations serve different behavioral needs. Second, spatial
representations vary, from simple ones applicable to controlling simple movements to
abstract ones that may represent information such as topographic knowledge.
Behavioral Uses of Spatial Information
• We need spatial information about the location of objects in the world, both to direct
actions at those objects and to assign meaning and significance to them.
• Just as form is coded in more than one way in visual processing, so too is spatial
information. The critical factor for both form and space lies in how the information is
to be used.
• Recall the two basic types of form recognition, one for recognizing objects and the
other for guiding movements to objects. We can think of spatial information in the same
way.
Object Recognition
• The spatial information needed to determine relations between objects, independent of
what the individual’s behavior might be, is very different from the spatial information
needed to guide eye, head, or limb movements to objects.
• In the latter case, visuomotor control must be viewer-centered; that is, the object’s
location and its local orientation and motion must be determined relative to the viewer.
• Furthermore, because the eyes, head, limbs, and body are constantly moving,
computations about orientation, motion, and location must take place every time we
wish to undertake an action.
, •Details of an object’s characteristics, such as color, are irrelevant to visuomotor
guidance of viewer-centered movements.
• The brain operates on a “need-to-know” basis. Having too much information may be
counterproductive for any given system.
• In contrast with the viewer-centered system, the object-centered system must be
concerned with such properties as the object’s size, shape, color, and relative location
so that the objects are recognized when they are encountered in different visual contexts
or from different vantage points. In this case, the details of the objects themselves
(color, shape) are important. (Knowing where the red cup is relative to the green one
requires identifying each cup.)
• The temporal lobe codes objects’ relational properties. Part of this coding probably
occurs in the polymodal region of the superior temporal sulcus and another part in the
hippocampal formation.
Movement Guidance
• To accommodate the many differing viewer-centered movements (eyes, head, limbs,
body, separately and in combinations) requires separate control systems.
• Eye control is based on the optical axis of the eye, for example, whereas limb control
is probably based on the positions of the shoulders and hips. These are vastly different
types of movements.
• We have considered many visual areas in the posterior parietal region and multiple
projections from the posterior parietal regions to the frontal lobe motor structures for
the eyes (frontal eye fields, area 8) and limbs (premotor and supplementary motor).
Connections to the prefrontal region (area 46) have a role in short-term memory for the
location of events in space.
• Results of single-cell studies in the posterior parietal lobes of monkeys confirm the
posterior parietal’s role in visuomotor guidance. The activity of these neurons depends
on the concurrent behavior of an animal with respect to visual stimulation.
• In fact, most neurons in the posterior parietal region are active both during sensory
input and during movement. For example, some cells show only weak responses to
stationary visual stimuli, but if the animal makes an active eye or arm movement toward
the stimulus or even if it just shifts its attention to the object, the discharge of these cells
is strongly enhanced.
• Some cells are active when a monkey manipulates an object: they respond to its
structural features, such as size and orientation. That is, these neurons are sensitive to
the features that determine the hand’s posture during object manipulation. Other cells
move the eye to allow the fine vision of the fovea to examine objects.
• John Stein (1992) emphasized that the responses of posterior parietal neurons have two
important characteristics in common. First, they receive combinations of sensory,
motivational, and related motor inputs. Second, their discharge is enhanced when an
animal attends to a target or moves toward it. These neurons are therefore well suited
to transforming requisite sensory information into commands for directing attention
and guiding motor output. We can predict, therefore, that posterior parietal lesions
