Fig. 2.

a Determination of phone-reading period: the beginning of each of the reading periods of 2 s was determined by finding the phone’s peak velocity (relative to the head) in the z axis (vertical direction) and adding 50 ms to the time at which peak velocity occurred. b Exemplar data from one participant showing the phone’s relative motion during a reading trial at customary speed. The black portion of line in each figure represents the 2 s ‘stable’ period, i.e., phone-reading period. The outcome variables (D_RES_Acc-SD, UD_GAZE_Acc-SD and RL_GAZE_Acc-SD) were determined for this ‘stable’ period only. The left-hand panels show the linear and angular displacement of the phone relative to the eyes; D_RES, indicates the resultant, relative linear displacement, i.e., viewing distance, and UD_GAZE and RL_GAZE, indicate the relative angular displacement of the phone in the Up-Down and Right-Left directions respectively. The middle and right-hand panels show the associated velocities and accelerations, i.e., the first and second derivatives of the displacements shown in the left-hand panels