Spatial updating in amnesia using an eye movement analogue of a path integration task

Path integration (PI) allows organisms to navigate home by updating their location in reference to the route's starting point. We previously demonstrated a PI-like process in eye movements using an eyetracking version of commonly used PI tasks. As the hippocampus/medial temporal lobes (MTL) have been implicated in updating self-position via whole-body PI, we investigated whether the hippocampus/MTL is necessary for the spatial updating of gaze position. Using our eyetracking PI-analog task, we tested two individuals with hippocampal lesions, DA and BL; BL's hippocampal damage is relatively circumscribed to his dentate gyrus, but he has additional volume loss in the right precuneus and left superior-posterior parietal cortex. Participants followed routes with their eyes guided by visual onsets and, when subsequently cued, returned to the starting point or mid-route location. Surprisingly, despite DA's extensive MTL damage, his accuracy was comparable to that of control participants, but unlike the control participants, he showed increased saccade latency and little to no gaze revisits to enroute locations when returning to the start location. BL's accuracy was reduced relative to that of the control participants. Additionally, in contrast to DA, BL demonstrated an increased reliance on overt, enroute revisits. The behavior of the two amnesic cases, who each differ from the control participants and show distinct patterns from one another, suggests that spatial updating of gaze position reflects interactive processes supported by the hippocampus/MTL and posterior parietal cortex.