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

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 medial temporal lobe (MTL), and specifically, the entorhinal cortex (ERC), is implicated in updating self-position via whole-body PI, we investigated its role in updating gaze position. Two individuals with hippocampal lesions, DA and BL, were tested on our eyetracking PI-analog task in which participants followed routes with their eyes guided by visual onsets and, when subsequently cued, returned to the starting point or mid-route location. As DA’s MTL damage was extensive, encompassing the ERC and hippocampus, we expected DA’s performance to be impaired compared to that of his controls. However, DA’s accuracy was comparable to that of control participants, and he showed little to no gaze revisits to enroute locations when returning to the start location. BL’s performance was expected to be impaired due to hippocampal dysfunction affecting mnemonic and perceptual discrimination (involving the dentate gyrus), with additional signs of visuospatial compromise, possibly in relation to volume loss in the parietal cortex that may otherwise support PI. Indeed, BL’s accuracy was reduced relative to that of the control participants, and he demonstrated an increased reliance on overt, enroute, revisits. These findings suggest that processes underlying spatial updating of gaze position overlap with those supporting whole-body movement that may depend on contributions from the MTL and/or parietal cortex. In the case of DA, residual executive function abilities and/or visuospatial strategies supported by frontal or parietal cortices may support eye movement-based spatial updating and compensate for MTL damage.