Reversing direction of the eye movement, or the retinal image motion, reverses the perceived depth, but a reversal of both (such as with a reversal in observer head movement) maintains depth constancy. An extra-retinal signal derived from the pursuit component of these compensatory eye movements is used to disambiguate the motion-from-depth. Regardless of whether the observer or stimulus translates, eye movements are generated to maintain foveation of a certain point in the scene. Using these same computer-generated, random-dot MP stimuli, recent psychophysical studies have shown that the underlying neural mechanisms rely on an extra-retinal signal from the pursuit eye-movement system for the disambiguation of depth from MP ( Nawrot, 2003a Nawrot, 2003b Naji & Freeman, 2004 Nawrot & Joyce, 2006). This random-dot methodology has been very important in the study of depth perception by allowing the isolation of particular stimulus variables, especially those required for the unambiguous perception of relative depth from MP.
#Motion parallax psychology definition free
However, with random-dot MP stimuli, individual stimulus dots can be shifted on the display screen to recreate the appropriate retinal image motion to simulate a three-dimensional shape during observer head translation or during display translation ( Rogers & Graham, 1979), free from many other potential depth cues. These other depth cues could include relative size, familiar size, interposition, relative brightness, and perspective, and each has the potential to confound an experimental study of MP. Following the success of computer-generated random-dot stereograms in the study of binocular stereopsis ( Julesz, 1960 1963 1971), the development of random-dot MP stimuli helped show “…that motion parallax can be a sufficient cue to the shape and depth of three-dimensional surfaces, in the absence of all other depth cues” ( Rogers & Graham, 1979). Various studies used illuminated points or needles ( Bourdon, 1898 1902 Graham et al., 1948), wires in frames ( Tschermak, 1939), points of light ( Gogel & Tietz, 1973), shadow projections of opaque objects ( Gibson et al, 1959) and paint or powder splattered panels ( Smith & Smith, 1963). This early research used a broad variety of visual stimuli. While early research revealed that MP was capable of producing the perception of depth, this research suggested that determining which points or portions of objects were nearest and farthest was often unreliable ( Gibson, Gibson, Smith & Flock, 1959). The study presented here investigates whether perspective information, generated by translation conditions, could have confounded results in previous studies of MP.Ī fundamental issue in understanding MP is determining the source of information for the unambiguous perception of relative depth. Observer translation, in addition to creating conditions of MP, can also create perspective information, a change in the relative retinal image size of various parts of an object as an observer moves past the object.
Another important source of depth information is provided by perspective, the optical geometry that produces the apparent convergence of parallel lines as they extend away from the observer. One important monocular depth cue, motion parallax (MP), is provided by relative movement of objects in the visual scene, relative to a stable point of fixation, during observer translation. To perform this task accurately, the visual system often relies on multiple visual depth cues. An important task for the human visual system is to interpret the three-dimensional world using information from a two-dimensional retinal image.
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