Abstract
Interacting in the peripersonal space requires coordinated arm and eye movements to visual targets in depth. In primates, the medial posterior parietal cortex (PPC) represents a crucial node in the process of visual-to-motor signal transformations. The medial PPC area V6A is a key region engaged in the control of these processes because it jointly processes visual information, eye position and arm movement related signals. However, to date, there is no evidence in the medial PPC of spatial encoding in three dimensions. Here, using single neuron recordings in behaving macaques, we studied the neural signals related to binocular eye position in a task that required the monkeys to perform saccades and fixate targets at different locations in peripersonal and extrapersonal space. A significant proportion of neurons were modulated by both gaze direction and depth, i.e., by the location of the foveated target in 3D space. The population activity of these neurons displayed a strong preference for peripersonal space in a time interval around the saccade that preceded fixation and during fixation as well. This preference for targets within reaching distance during both target capturing and fixation suggests that binocular eye position signals are implemented functionally in V6A to support its role in reaching and grasping.
Highlights
Primate lifestyle requires frequent relocations in space and coordinated movements of the eyes and hands to interact with objects
We found that neurons in V6A a) respond during the shifts of gaze performed in 3D space, and b) are tonically modulated by 3D eye position
The animals were required to fixate on ten LEDs turned on (LED) targets located in different positions in the 3D space (Fig. 1A)
Summary
Primate lifestyle requires frequent relocations in space and coordinated movements of the eyes and hands to interact with objects. For this behavior, a reliable visual percept of threedimensional (3D) space needs to be constructed and integrated in a movement plan involving several effectors. Humans with lesions in PPC showed deficits in perceiving the spatial relationship between objects and their own body, and performed inaccurate reaching movements [1,2,3]. Damages to PPC were shown to affect the depth component of visually-guided reaching movements [4,5]. Psychophysical studies suggested that, in order to reach objects in depth, information about the vergence angle is critical [6,7]. Vergence has been tightly related to the fixation distance [8], which, together with the retinal disparity signal that specifies the distance of an object from the fixation plane, have been proposed to encode object location in 3D space [9]
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