Cite as:
Waugh S J, Badcock D R, 1998, "A common pathway for spatial localisation of luminance-defined and contrast-defined blobs" Perception 27 ECVP Abstract Supplement

A common pathway for spatial localisation of luminance-defined and contrast-defined blobs

S J Waugh, D R Badcock

Luminance-defined and contrast-defined stimuli are thought to be processed independently in the motion system (Derrington and Badcock, 1985 Vision Research 25 1869 - 1878), with the mechanism supporting motion of contrast-defined stimuli being more sluggish (Derrington, Badcock, and Henning, 1993 Vision Research 33 1785 - 1794). We examined these two suggestions with respect to the visual encoding of relative spatial position.

Thresholds for a three-blob alignment task were measured with stimuli which were 10 dB above detection threshold for a range of exposure durations (27 to 400 ms). Blob stimuli were constructed from dynamic random-dot noise and circularly symmetric Gaussian envelopes (sd=0.5 deg). The outer reference blobs were separated vertically by 6 deg, and judgments were made about the relative position of a central test blob. Alignment thresholds were obtained from psychometric functions generated by a self-paced method of constant stimuli. To address the issue of independence, a `mixed' condition where test and reference stimuli were of different type--luminance-defined reference and contrast-defined test blobs, or vice versa--was tested. Alignment thresholds were on average similar for all stimuli--luminance-defined, contrast-defined, and mixed. The effect of exposure duration on alignment thresholds, which was slight (power function with exponent of about -0.15), was similar for all stimuli, as was the effect of increasing signal strength (from 1.5 to 12 dB above detection threshold).

These findings do not support the need to propose separate first-order and second-order visual pathways in order to encode relative spatial localisation information for separated stimuli. [Supported by the Australian Research Council.]

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