Eye position effects in saccadic adaptation in macaque monkeys.

The saccadic amplitude of humans and monkeys can be adapted using intrasaccadic target steps in the McLaughlin paradigm. It is generally believed that, as a result of a purely retinal reference frame, after adaptation of a saccade of a certain amplitude and direction, saccades of the same amplitude and direction are all adapted to the same extent, independently from the initial eye position. However, recent studies in humans have put the pure retinal coding in doubt by revealing that the initial eye position has an effect on the transfer of adaptation to saccades of different starting points. Since humans and monkeys show some species differences in adaptation, we tested the eye position dependence in monkeys. Two trained Macaca fascicularis performed reactive rightward saccades from five equally horizontally distributed starting positions. All saccades were made to targets with the same retinotopic motor vector. In each session, the saccades that started at one particular initial eye position, the adaptation position, were adapted to shorter amplitude, and the adaptation of the saccades starting at the other four positions was measured. The results show that saccades that started at the other positions were less adapted than saccades that started at the adaptation position. With increasing distance between the starting position of the test saccade and the adaptation position, the amplitude change of the test saccades decreased with a Gaussian profile. We conclude that gain-decreasing saccadic adaptation in macaques is specific to the initial eye position at which the adaptation has been induced.

Fix your eyes in the space you could reach: neurons in the macaque medial parietal cortex prefer gaze positions in peripersonal space.

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.