An investigation of the interactions between stereopsis, orientation perception, and higher-order visual processes.
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.