June 2004
Volume 4, Issue 8
Vision Sciences Society Annual Meeting Abstract  |   August 2004
Eccentricity effects on spatial alignment for luminance-defined and contrast-defined blob stimuli
Author Affiliations
  • Subash Sukumar
    Anglia Polytechnic University, Cambridge, UK
Journal of Vision August 2004, Vol.4, 786. doi:10.1167/4.8.786
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      Subash Sukumar, Sarah J. Waugh; Eccentricity effects on spatial alignment for luminance-defined and contrast-defined blob stimuli. Journal of Vision 2004;4(8):786. doi: 10.1167/4.8.786.

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      © ARVO (1962-2015); The Authors (2016-present)

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Contrast-defined blob stimuli are detected by mechanisms that are about 3 times larger than those that detect luminance-defined blob stimuli for eccentricities up to 10 deg (Sukumar and Waugh, 2002). This study aims to examine whether such differences lead to differences in the precision of relative position processing. Alignment stimuli consisted of a test blob in between two reference blobs placed horizontally and presented for 400 msec. Luminance-defined stimuli were constructed by adding random dot noise to a Gaussian, whereas contrast-defined stimuli were constructed by multiplying the noise by a Gaussian. Noise was dynamic and care was taken to eliminate unwanted artefacts. Alignment thresholds were measured for blobs at a constant multiple above their detection thresholds, where reference blobs were separated by 12× their spread at each eccentricity (0, 2.5, 5, 10 deg). Thresholds were also measured for reference blob separations of 3 to 96× spread, where the test blob was fixated. A self-paced, single-interval forced-choice paradigm was used to obtain performance estimates. For blobs of a fixed small spread and separation, alignment thresholds and their rates of increase with increasing eccentricity were similar for luminance-defined, contrast-defined and “mixed” stimuli. For blobs scaled in size with eccentricity, thresholds increased similarly with scale and eccentricity. For luminance-defined stimuli, the threshold-separation dependence function was constant for separations up to about 25x spread, then increased (Toet et al, 1987). For contrast-defined stimuli, the function tended to show threshold increases at smaller separations. Our results indicate that despite differences found in the mechanisms detecting these stimuli, information appears to feed into common or very similar localisation mechanisms across a substantial part of the visual field.

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Sukumar, S., Waugh, S. J.(2004). Eccentricity effects on spatial alignment for luminance-defined and contrast-defined blob stimuli [Abstract]. Journal of Vision, 4( 8): 786, 786a, http://journalofvision.org/4/8/786/, doi:10.1167/4.8.786. [CrossRef]

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