Greg
Detre
Wednesday,
17 May, 2000
Brain
& Behaviour � Prof Rolls
The parietal cortex is where the dorsal (or �where�)
visual stream�s information about motion and location is integrated with
somatosensory input and the other senses to develop representations of our
position relative to the contents of the outside world.
This coordinating role requires the parietal cortex
to be well-connected. The principle thalamocortical projections to the parietal
cortex are from the LP-pulvinar complex (which receives from the superior
colliculus and the pretectum � see below), and also the thalamic intra-laminar
nuclei. The posterior parietal cortex, especially MT and MST, are where the
dorsal visual stream terminates, traceable back to the magno-cellular layers of
the LGN and the larger ganglial m-cells in the retina. In the other direction,
the parietal cortex�s connections can be mapped with horse-radish peroxidase
and radioactive amino acids, demonstrate a hierarchical organisation gathering
at common destinations, e.g. prefrontal cortex and the limbic system
The posterior parietal cortex can be approximately
subdivided by function into: (Sakata et al., 1997)
lateral intraparietal area
(LIP) � saccades
posterior parietal region
(PRR) � planning reaching movements
anterior intraparietal
area (AIP) � grasping
As can be seen, the parietal cortex plays quite a
high-level role. One view is Andersen�s, that the different sensory modalities
are originally represented in different coordinate frames, but brought together
in the areas of the posterior parietal cortex. There, the LIP and PRR encode
the spatial location of visual and auditory signals, using the eye as a common
reference frame. These neurons code a goal for movement in multiple coordinate
frames so that different cordinate transformations can be accomplished with the
same population of neurons depending on how these cells are read by other brain
areas.
The coordinate transformation for determining
spatial location and forming plans operates on three abstract representations,
combining information from the different modalities of sensory input before
being projected as efferent signals:
head-centred � combines information
about eye position and the location of a visual stimulus on the retina
body-centred � combines information
about head, eye and retinal position
world-centred � combines vestibular
signals with eye poistion and retinal position
This view of the parietal cortex as integral to the
planning and execution of limb movements is supported by human and monkey
studies. Investigation of the neural processes subserving the production of
movement requires various approaches: behavioural, physiological and brain
imaging. Various studies (Mountcastle et al. 1975, Andersen et al. 1992) have
demonstrated that the posterior parietal cortex has a role in programming
actions and in transforming sensory signals into plans for motor behaviours.
The particular responses of neurons in the PRR vary,
though they are largely during sensory input and movement. Stein (1992) claimed
that these are two characteristics of all posterior parietal neurons:
1.
combinations of sensory, motivational and motor information are received
2.
their response is greatest when the animal attends to, or moves towards, a
target
All of them respond to movements of the eyes and to
the position of the eye in its socket (some are most responsive to
behaviourally relevant stimulus, e.g. a reward). Some are barely activated by
stationary visual stimuli but respond strongly when attention is directed or
eye/arm movement made towards stimulus. Some respond to manipulation of the
object, or its structural features.
Given all of this, we have reason then to expect
posterior parietal neurons to be transforming sensory information into commands
for directing attention and guiding motor outputs. We can look to evidence from
human posterior parietal lesions to corroborate this:
impaired distinguishing
left from right
impaired mental
manipulations of objects
spatial deficits � perhaps
due to damage to temporal-parietal polysensory regions (Goodale & Milner,
1993), rather than to the dorsal stream�s role in visuomotor guidance - right
hemisphere lesions (greater polysensory growth in the right hemisphere) give
rise greater deficits on complex spatial tasks
The AIP and the premotor areas are connected
reciprocally (Metelli et al., 1994), and many AIP neurons are selective for
object shape and size (Sakata et al., 1997). Others are active during object
fixation and visually-guided grasping movements (which we know because they do
not discharge in the dark, for instance), whereas the prefrontal neurons are
active when grasping is performed, even with different effectors (e.g.
left/right hand, mouth etc.), with others selective for specific grip type.
There, specific knowledge about actions and their implementations is stored, as
well as simplifying the association between a sensory stimulus (e.g. visually
presented object) and appropriate motor response. Rizzolatti, Fogassi and Gallese
suggest that the �mirror neurons� form a basic system of action recognition,
and the early stages of an internal model, since they are active both when the
monkey performs and observes an action. This could explain the curious
phenomenon of what is left unaffected by lesions in the ventral stream �
subjects remain able to grip objects correctly, even though their impairments
to shape and form recognition are such that they are unable to verbalise what
the object actually is.
The parietal cortex is integral in the planning and
execution of eye movements. There are many other regions where neuronal
activity also correlates with saccades: the frontal cortex, basal ganglia,
cerebellum and brain stem. The superior colliculus is the key structure, since
it coordinates the various inputs from the forebrain, and provides the location
for the transformation into outputs for the control of eye movements. The fact
that a comparison of neuronal activity in the superior colliculus with its
cortical inputs identifies the same type of activity supports the idea of
distributed processing (i.e. a given transformation does not occur in a
particular area, but progresses across a series of regions).
Saccadic eye movements are intimately related to
visual attention � the content
of a subject�s visual attention = traceable from their saccades. In the
posterior parietal cortex, neurons responding to visual stimuli fire more
vigorously when the stimuli are the targets of saccades. Saccades are
attentional signals, which don�t depend on visual stimuli or eye movements (but
are relevant to both). This can be contrasted with the superior colliculus,
where enhanced activity is associated only with saccades, not with saccade-free
behaviour.
It is the posterior parietal cortex that identifies
the location, local orientation and motion of an object relative to the viewer
� a �viewer-centred system�. There are many visual areas in the posterior
parietal region, with multiple projections to motor systems for the eyes and
limbs, necessary for its roles in visual attention and grasping, for example,
as evidenced by monkey neurons, whose activity is dependent on concurrent
behaviour of the animal with respect to visual stimulation.. There are also
connections to the prefrontal cortex, which plays a role in the STM of location
of events in space.
Human lesions provide
further evidence of the function of the parietal cortex. Damage to the parietal
lobe gives rise to diverse range of physical symptoms, especially in terms of
non-verbal cognitive functions.
Human parietal lesions to the right side initially
cause dramatic attentional deficits (such as dorsal and ventral simultagnosias
(from Farah, 1990)). Subjects act as if the objects in the neglected field do
not exist, and have difficulty making eye movements into that field. Balint�s
syndrome usually arises from bilateral lesions of posterior parietal and
prestriate cortex, where patients tend to see and describe only one object at a
time. Subjects make few saccades and seem unable to shift the focus of their
attention from the fovea (�stickiness�). Even after recovery, contralateral
saccades are inaccurate and take longer to initiate.
Lesioning monkeys can be instructive, because it
allows us to more precisely localise damage and isolate effects than the often
patchy or diffuse non-experimental lesions that we encounter in human subjects.
Lesions to the monkey�s posterior parietal cortex give raise to increased
latency of saccades, targetting inaccuracy and selective neglect. Unilateral
lesions give rise to preferentially attending to stimuli in the contralateral
hemi-field.
I have shown that the parietal cortex incorporates
and moves on from the processing in the �where� visual system, by incorporating
information from other modalities, particularly the somatosensory. However,
this should not be seen necessarily as evidence for a single, unified spatial
map, but rather different representations of space for different behavioural
needs and levels of complexity (such as simple movements as opposed to
topographical knowledge). This leads to the formation of an even more abstract,
and useful, representation of our extra-personal space, and influences motor
input to our eyes and limbs.