Following blindly––smooth pursuit of tactile and proprioceptive stimuli

Abstract

Abstract: : Purpose:Smooth pursuit is generally thought to depend upon retinal slip for its generation, but tactile, acoustic and proprioceptive stimuli have also been effective. Illusory motion may also be tracked (eg, sigma pursuit). We examined smooth pursuit made in darkness to proprioceptive stimuli and to an object moving across areas of skin with high (glabrous surface of the index finger) and low (forearm) tactile resolution. Our hypothesis was that stimulating the more sensitive surface would provide a better motion signal and hence better pursuit. We further hypothesised that both would yield better tracking than the purely proprioceptive input generated by motion of the forearm and hand. Methods: 18 young to middle–aged subjects took part. Eye movements were recorded in total darkness with an IR limbus tracker. Tactile stimuli moved lightly against the skin between ±10° at 10°/s. Proprioceptive input came from subjects moving a slider held between thumb and index finger horizontally at as close as possible to the same distance and speed as the tactile stimuli. Pursuit gain and percentage time spent tracking were computed. Two trials of each task were run. Results: As there were no significant differences between trials, averaged results are presented. Average gains were: proprioceptive, 0.75; index finger, 0.57; forearm, 0.53. ANOVA found that proprioceptive tracking was significantly better than pursuit of tactile stimuli at either location. There was no significant difference between the two tactile task pursuit gains. Comparing percentage time spent tracking for the two tactile tasks yielded 11.9 and 12.4% for finger and forearm; these again did not differ. Conclusions: The 10x difference in tactile spatial resolution between finger and forearm did not influence pursuit quality. This suggests that the tactile sensory motion signal per se does not drive the eye movements directly but that it instead elicits an internal (and fragile—note the percent tracking time) motion percept, which, in turn, drives pursuit. Proprioception—or efference copy—was a significantly more stable and continuously effective stimulus. The percentage tracking time was lower than in previous reports, but this may relate to the very light pressure applied to the skin.

Keywords: eye movements: saccades and pursuits • eye movements • perception