Does eccentric fixation alter head movement strategy for smooth pursuit?



Age-related macular degeneration (AMD) can often lead to the loss of the fovea and the surrounding central visual field. This type of vision loss is extremely common (affecting nearly 7% of individuals over 40 in the United states alone, Klein et al. 2011) and can present particular challenges for oculomotor tasks that rely on the high-acuity foveal retina. For certain tasks, individuals develop a new, eccentric fixational area - the preferred retinal locus (PRL). We have previously shown that smooth pursuit is impaired in individuals with macular degeneration, and the degree of impairment depends on the direction of target motion relative to the location of the damaged retina (scotoma) and the fixational PRL (Shanidze et al. 2016). One might consider that it is the lack of the fovea that causes deficit in smooth pursuit performance. However, we have also shown that scotoma size and PRL location do not directly contribute to variations in smooth pursuit gain (Shanidze et al. 2017). Another consideration could be that individuals' eccentric fixation leads to an eccentric eye position that might impede smooth pursuit in the standard head-fixed experiment, especially for target directions along fovea-PRL axis (Stahl 2001). Thus, head movements could become particularly important for successful pursuit in this population. In this experiment we examined head movement strategies in 7 individuals with AMD (75-95, 4M) and compared them to 4 age-matched controls (72-76, 1M). Participants pursued a ramp target that stepped from the center by 6° in one of 6 possible directions (4 cardinal, 2 oblique) and moved at 10°/s in the opposite direction, through the center. We found that there was variability in total head excursion in smooth pursuit in both groups. In the control group, 1 participant made large pursuit head movements, whereas the others relied almost entirely on eye movements. In the AMD group, 3 participants consistently relied on head movements across all target directions. However, for 2 others, the range of head movements was variable across directions, and 2 others relied solely on eye movements for pursuit. Overall, there was no significant difference between total smooth pursuit head movement across groups (μAMD = 2.57, μCont = 1.37, p>0.05, 2-Way r.m. ANOVA). For both groups, there was more head movement in the horizontal than vertical direction (μH = 2.74, μV = 1.06, p=0.03). When we examined the relationship between PRL eccentricity and total head movement for all individuals who did exhibit smooth pursuit head movements, we found a slight negative relationship for the horizontal direction, however, these data were limited by the reduced sample size. Overall we find that individuals with AMD have similar head movement range during smooth pursuit as age-matched controls, despite known reduction in smooth pursuit eye velocity gain and often-eccentric eye position. 

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