Asymmetric integration of allocentric and egocentric coordinates within
hemisphere and between two hemispheres
Yang Zhou, Yi-Ning Liu, Ming-Sha Zhang*
Institute of Neuroscience, Shanghai Institute of biological sciences, China Academy
of Sciences, Shanghai, PR China, 200031
*Corresponding author: Ming-Sha Zhang
E-mail: mingsha@ion.ac.cn
Abstract: Objective Spatial locations of objects in the surrounding environment are not
only perceived based on observer-centered (egocentric) coordinate, but also on
object-centered (allocentric) coordinate in which the referenced center is an object
instead of being the observer. For example, when a basketball player is doing
defense rebound he needs to know where the ball is related to his hands (egocentric
location) as well as the ball’s position related to the basketry (allocentric location).
In addition to spatial perception, allocentric coordinate has also been used in other
cognitive functions, such as spatial long-term memory, in which the locations of
objects are mapped in allocentric coordinate. In this case, the memorized spatial
relationships among objects never change regardless of the alteration of observer’s
location in space. However, when interacting with the surrounding environment,
one need first to know where the objects are related to his body, then the navigation
system will utilize the egocentric spatial information to guide one’s behavior. In
other worlds, the perceived or memorized allocentric spatial information needs to be
transformed to egocentric coordinate prior to the formation of motor command. It
has been known for long time that information could be transformed between
coordinates. Byrne and Becker proposed a model to explain the transformation from
egocentric coordinate to allocentric coordinate based on studies of single neuron
recording. Andersen and his colleagues found that the visual, memory and saccadic
activities in monkeys’ posterior parietal cortex (PPC) were strongly modulated by the
orbital position of the eyes during fixation. Such phenomenon has been named as
gain field modulation and linked to information transformation from retinotopic
coordinate to body-centered coordinate. However, these studies did not solve the
problem of how spatial information from allocentric coordinate converse to egocentric
coordinate. Methods In the present study, we measured the manual reaction time
(RT) while six human subjects were performing a position discrimination task in
which the behavior response purely relay on the allocentric information of a target
related to a referenced object (allocentric left or right), but regardless of the egocentric
positions of the target (egocentric left or right). Results Our results indicate that the
conversion of allocentric coordinate to egocentric coordinate has the following
characteristics: 1). The conversion is significantly faster when target’s allocentric
position is compatible with egocentric position (SAME) comparing with incompatible
condition (OPPO); 2). The conversion difference between SAME and OPPO is
increased following the increase of the visual angle between target’s location and the
observer’s sagittal middle line in egocentric coordinate; 3). The conversion difference
between SAME and OPPO is larger, in most right hand subjects (5 out of 6), when
target is in egocentric left than in egocentric right. Conclusion We conclude that
each hemisphere prefers to integrate the contralateral allocentric information with
contralateral egocentric information.
The greater RT in OPPO condition
accompanies with greater visual angle indicates that each hemisphere preferentially
receives and progresses egocentric contralateral information and has week input from
ipsilateral side. Moreover, the greater difference between SAME and OPPO when
target appears in egocentric left than in egocentric right, is consistent with findings in
previous studies, i.e. right hemisphere is preferentially dealing with spatial
information.
Keywords: Spatial perception, allocentric coordinate, egocentric coordinate,
coordinate integration, human psychophysics, manual reaction time