| Abstract |
The aim of the present study was to elucidate the cortical regions that are activated
during grasping execution in the presence or absence of visual information, as well
as the ones implicated in grasping observation. For this reason, we employed the
quantitative autoradiographic method of 2 [14C]-deoxyglucose (2DG). This method
uses radioactive deoxyglucose, an analog of glucose, to trace glucose consumption
and therefore local functional activity, since glucose is the main source of energy for
brain cells. Furthermore, it provides direct quantitative assessment of the brain
activity based on glucose consumption, has the highest spatial resolution (20μm) as
compared to other imaging methods and allows the identification of the affected
cortical areas by means of cytoarchitectonic criteria.
The 2DG method was applied in Macaca mulatta monkeys, trained at the
following tasks:
a) execution of grasping in light (EL),
b) execution of grasping in dark (ED),
c) observation of grasping performed by the experimenter (O),
d) observation of the experimenter’s hand reaching towards the behavioural
apparatus with hand/fingers extended (Cm), and
e) exposure to experimental conditions similar to the ED task, without grasping
execution (Cd).
In this study we focused on the cortical areas located in the medial parietal
convexity, the anterior bank of the parietooccipital cortex and the posterior part of the
medial intraparietal bank namely V6, V6A, 5IPp, PGm/7m, area 31 and retrosplenial
cortex (areas 29 and 30).
The net activations induced by the EL task, involve areas 5IPp, PGm/7m,
RSC and V6 at both hemispheres, and area V6Ad at the hemisphere contralateral to
the moving forelimb. The ED task influenced the metabolic activity in areas V6Ad and
60
PGm/7m contralaterally and 5IPp and V6 bilaterally. The O task increased the
metabolic activity of areas PGm/7m, 31, RSC and V6 at both hemispheres.
Common activations for the EL and the O tasks were observed in RSC and PGm/7m
at both hemispheres. Execution of grasping in light or in dark (EL and ED tasks),
induced common activations in areas V6Ad and PGm/7m at the contralateral
hemispheres and in area 5IPp and part of area V6 at both hemispheres.
As far as area V6 is concerned, it could be involved in the visual and
somatosensory quidance of grasping movements. In support with this view are its
connections with arm-related areas such as MIP and V6A which in turn project to
premotor area 6.
The contralateral activation of area V6A only in the grasping-execution tasks,
demonstrates that V6A is related with the execution of grasping only and not with the
observation of the same action. Presumably, the arm-movement related activity
observed in the dorsal part of V6A, corresponds to neurons that integrate motorsignals
related to arm movements with somatosensory signals evoked by these
movements, in the service of online quidance of the reaching-to-grasp behaviour.
Area PGm/7m which was found bilaterally activated during the EL and O
tasks and contralaterally activated during the ED tasks, may be involved in the
combination of visuomanual and oculomotor signals for the composition of motor
commands (based on kinesthetic and visual signals) for movement generation. The
higher metabolic activity observed in this area for the execution tasks as compared to
the observation task, may be an indication for the stronger involvement of this area in
execution than in recognition of actions.
Area 31, which was activated only during observation and not during
execution of grasping, may contribute to memory proccesses and in attribution of an
action (i.e. grasping) to another agent and not to the self. Similarly, area V6A which
was activated during execution and not during observation of the reaching-to-grasp
behaviour, could also be involved in the attribution of grasping to the performer (self)
and not to another agent.
The RSC which was bilaterally activated during the EL and the O tasks, may
process contextual associations during object recognition, encoding the current
salience of objects in the immediate (visual) scene for orienting and navigating
towards them. Also it may be involved in topographic spatial navigation, for situating
oneself within a larger environment, using the immediate scene as a cue.
Area 5IPp is an area which is reported for the first time in the literature and
only partially corresponds to area PIP. The strong bilateral involvement of area 5IPp
in the grasping execution tasks performed either in light or in darkness, suggests that
61
this area could participate in the visual and somatosensory quidance of the reachingto-
grasp behaviour. More specifically, it could be implicated in providing object
positional information, useful for reaching, and object feature/shape information by
cross modal (visual and tactile) matching, useful for grasping. The involvement of
area 5IPp only in execution and not in observation tasks, implies a possible role of
this area in the attribution of an action to the performer (i.e. the monkey) and not to
another agent (i.e. the experimenter).
|
Search
