Abstract
Retinal image motion, absent other visual depth cues, is perceptually depth-sign ambiguous. For motion parallax (MP), pursuit eye movement signals disambiguate the depth-sign of the opposing directions of retinal motion. Disambiguation is achieved with extraordinarily brief stimulus presentations: 30 msec with unimpeded viewing and 70 msec with processing disrupted with masking. Transcranial magnetic stimulation (TMS) of the Slow-Eye Movement region of the Frontal Eye Fields (FEFsem) disrupts both the generation of pursuit eye movement signals and perception of depth from MP. Considering proximity of FEF regions for pursuit and saccadic eye movements, involvement of saccadic eye movements in the disruption of MP must be explored. To investigate the role of saccadic eye movements in the perception of depth-sign from MP, we varied stimulus presentation duration (PD) in an adaptive staircase. PD started at 167 msec, and moved between a floor of 17 msec and a ceiling of 333 msec. The random-dot stimulus depicted a vertically oriented sinusoid with a peak local dot velocity of 4.7 deg/s. To elicit pursuit, stimulus window translation was 9.4 deg/s. To elicit saccades, the window stepped laterally 1 deg. Five conditions compared: stationary stimulus window, lateral window translation (pursuit), stepped window movement (saccade). Eye position was monitored. Absent stimulus window translation, PD tracked towards the ceiling, meaning these stimuli were perceptually ambiguous. Conditions with pursuit found PD tracking towards the floor, meaning these stimuli were depth-sign unambiguous. The saccade condition found PD tracking towards the ceiling, meaning that saccades do not disambiguate the depth-sign of MP. Saccadic eye movements do not disambiguate depth from MP as do pursuit eye movements. This means that any disruption of MP produced by TMS of FEFsem is linked to disruption of pursuit, and not linked to a possible disruption of the saccadic system.