Figure 1.

(a) A frame from the drifting stimulus used to drive OKN, (b) typical “sawtooth” pattern of eye positions (grey line) elicited by a rightward then a leftward drifting stimulus. The red/green shaded trace is the first derivative (velocity) coded for sign (green = left, red = right). Periods of tracking in the stimulus-direction interspersed by saccades in the opposite direction is characteristic of OKN. We allow for the latency in onset of OKN (here around 300 ms) by analysing a 2 second window of eye movements offset by a latency fixed per observer across all trials (Δ). (c) In order to categorise each instantaneous estimate of eye-velocity as either “saccade” or “tracking” we compare it to a threshold (τ), using its sign to categorise the direction (as θ or θ + π).