Although stereoscopic depth perception has been a common subject of research since 1838, it is only recently that a fundamental division between two types of stereoscopic processing -- first-order and second-order -- has been identified. The precise functioning of the neural mechanisms that underlie these processing streams, however, is not yet understood. It was first confirmed that first-order stereopsis mechanisms are best investigated with vertically oriented stimuli at small disparities , whereas second-order stereopsis mechanisms prefer stimuli at large disparities. It has been postulated that the second-order stereopsis mechanisms are insensitive to stimulus spatial frequency content, but this was found not to be the case. Four different explanations were investigated to account for this result: nonlinearities of the early visual system and screen, absolute and relative orientation of the stimuli, and the possibility of the second-order mechanism being multiple. The last of these possibilities was found to be the most plausible, based on the data collected. It has long been thought that vergence may play an important role in stereopsis because both systems use disparity cues. This relationship was investigated using briefly presented "second-order" stimuli and both experienced and naive subjects. A disparity sign judgement (front/back) to disparate stimuli was made while vergence eye movements were recorded. Stimuli were presented over a range of orientations, presentation times, disparities and contrasts. The vergence responses were found to be uncorrelated with the disparity sign judgements and it was concluded that vergence does not influence stereopsis under these conditions. Finally, the role that first- and second-order stereopsis mechanisms play within tilt percepts induced by orientation disparities were investigated. Subjects were presented with two different types of stimuli with either carrier, envelope, or both carrier and envelope orientation disparities. The stimulus with no "first-order reference" could not be perceived in depth with the envelope orientation disparity alone. When this stimulus had both carrier and envelope orientation disparities, depth tilt was still perceived. It was concluded that the perception of stereoscopic tilt is a first-order process.

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