Summers R J, Meese T S, 2005, "Summation of pictorial depth cues with motion and disparity gradients" Perception 34 ECVP Abstract Supplement
Summation of pictorial depth cues with motion and disparity gradients
R J Summers, T S Meese
Our unified experience of the world derives from a detailed analysis and subsequent recombination of its component parts. A good investigative tool for this process is the summation paradigm in which sensitivities (SENS) to each of a pair of independent signals (A and B) are measured. The two signals are then weighted in a compound stimulus to equate their detectability. A comparison of sensitivities to a signal alone (eg SENSA) and that measured in the compound (eg SENS'A) gives the summation ratio (SR = SENS'A/SENSA). If the signals are detected independently, then SR = 1, though probability summation can increase this to about 1.2. If quadratic summation occurs, then SR = SQRT(2), consistent with linear summation of the two signals and independent limiting noise on each of the signal channels. Crucially, the observer must disregard the noise associated with the irrelevant signal in the single-signal conditions. If this is not possible (eg the signals are accessed only through the summation process), then signals and noise are summed, and SR = 2. Here we use this paradigm to investigate the summation of pictorial (size and contrast) and non-pictorial (disparity) depth cue gradients. Stimuli were two-dimensional arrays (13 × 13 elements) of grating patches arranged evenly over an invisible square grid, each subject to a small level of random positional jitter, and viewed through a circular aperture (diameter = 17.95 deg). Stimulus duration was 200 ms and black screens ensured there was no extraneous visual stimulation. Summation between pictorial depth cues was quadratic, suggesting the signals could be addressed independently, but summation between pictorial and disparity gradients was less than this, suggesting possible subsystems for pictorial and non-pictorial depth cues. This hypothesis will be tested by measuring summation between each of the present three gradients and a motion gradient.
[Supported by the Wellcome Trust.]
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