Modulation of Human Velocity Storage Sampled During Intermittently-illuminated Optokinetic Stimulation

When a stationary human subject is suddenly exposed to constant-velocity full-field optokinetic stimulation, slow-phase eye velocity rapidly approaches stimulus velocity without the gradual build-up ("velocity storage") readily seen in other species. Subsequent to extinguishing illumination, the presence of velocity storage is suggested by the persistence of a gradually-declining component of optokinetic after-nystagmus (OKAN). In the past, characterizing modulation of velocity storage has been tedious. It is now shown that such modulation can be characterized by periodic "sampling" of OKAN using "intermittently-illuminated" optokinetic stimuli (light-on: 9.7 s; darkness: 2 s). Six subjects viewed an intermittently-illuminated optokinetic drum turning with a square wave angular velocity profile of 60 deg/s peak amplitude and 0.01 Hz frequency. The resulting modulation of velocity storage was approximately exponential with time constant, T = 5-11 s, and asymptote, A = 10-17 deg/s. A significant negative correlation was observed between T and A values. In a given subject, T and A values agreed (generally within +/- 20%) with values obtained employing previously-used methods, suggesting that velocity storage behaved linearly during periodic optokinetic stimuli of less than 60 deg/s. The new method of sampling OKAN permits the use of arbitrary stimulus profiles required to observe, or to confidently predict, velocity storage response during natural behavioural movements, which was not feasible with older methods. Sampling also increases the ease and speed (roughly three-fold) of data acquisition.