Spatial extent and eccentricity effects for detection of luminance-defined and contrast-defined blob stimuli

Evidence from masking studies suggests that the detection of luminance-defined and contrast-defined spatial stimuli under foveal viewing conditions are detected by independent mechanisms (Schofield and Georgeson, 1999). Results also indicated similarly sized spatial integration regions for these foveally-viewed stimuli. The aim of this study is to further characterise spatial summation regions for the detection of luminance-defined and contrast-defined stimuli at the fovea and in the periphery. Luminance-defined stimuli were constructed by adding random dot noise to a Gaussian profile, whereas contrast-defined stimuli were constructed by multiplying random dot noise to a Gaussian profile. Random dot noise was dynamic and care was taken to eliminate unwanted artefacts. Blobs varied in size from a sigma of 0.06 deg to 2 deg and were presented for 400 msec. Detection thresholds for all sizes were measured at the fovea and at 2.5, 5 and 10 deg in the inferior visual field. A self-paced, temporal 2AFC paradigm was used to obtain performance estimates. Spatial summation areas for contrast-defined and luminance-defined stimuli obtained under foveal viewing were similar in extent, confirming the previous result. However for all measured eccentricities, spatial integration areas for contrast-defined stimuli were larger than those found for luminance-defined stimuli. Detection thresholds obtained for single-sized stimuli at the different eccentricities also reveal a steeper rate of fall-off for contrast-defined stimuli than for luminance-defined stimuli. These results suggest that in peripheral vision, different underlying processes limit the detection of luminance-defined and contrast-defined spatial targets.