Abstract
Purpose: Currently, it is not well known how people with central field loss (CFL) pursue moving targets, a behavior that has been attributed to the fovea (Krauzlis 2003). Optimizing pursuit behavior may be especially important for this population, as it can afford additional time to visualize a moving object of interest. As the position of the target changes during the task, it is important to assess pursuit characteristics in the context of the target’s location relative to a subject’s scotoma and preferred retinal loci (PRL). Here, we present results describing monocular smooth pursuit in CFL patients using a novel approach employing a scanning laser ophthalmoscope (SLO) that allows for a direct visualization of the target on the damaged retina. Methods: Six (5 AMD, 1 JMD) individuals with differing degrees of CFL were recruited. Monocular scotomas were assessed using standard microperimetry in an SLO. Subsequent to mapping each scotoma, observers pursued in a step ramp (Rashbass 1961) paradigm. They first fixated a central target (1°) for 1 second. The target then disappeared, and reappeared 6° away, moving at a constant speed (~5°/s), in one of 8 possible directions (in 45° increments), selected at random. Participants were asked to follow the target with their eyes. Results: CFL patients demonstrated reasonable pursuit of moving targets, with pursuit velocity gain (0.16-1.79) dependent on whether the target moved into or out of the scotoma. Gains were lowest for targets moving toward the scotoma and highest for those moving out of and away from it. Pursuit latencies mirrored gains. Fig 1 shows pursuit of a target moving out of the scotoma for one observer. A computed gain modulation index (MI) that characterized the difference between maximum and minimum gains across target trajectories correlated with scotoma size (Fig 2). Conclusions: Despite absence of the fovea, CFL patients could smoothly pursue moving targets during the majority of trials. Pursuit gains were highest for targets moving out of the scotoma, suggesting subjects were trying to catch up with the target. For targets starting opposite the scotoma and moving towards it, our results are contrary to the expectation that patients would pursue the target to avoid having it disappear into the scotoma. One potential explanation is that individuals have some knowledge of the scotoma and are unwilling to pursue a target if it means obscuring the center of the visual field.