Visibility of elliptical Gaussian blobs

Contrast detection thresholds for horizontally and vertically oriented elliptical stimuli with a 2D-Gaussian luminance profile in space and a Gaussian envelope in time were measured as a function of their spatial scale parameters for two different presentation times. For slowly varying stimuli, threshold is independent of stimulus size whenever the minor axis of the stimuli exceeds 10-20 min of arc. If the presentation time is shorter, the occurrence of size invariance also depends on the area of the stimulus. In general no significant dependence of the threshold on stimulus orientation is found. Where size invariance occurs it holds that: (1) the threshold is determined by the ratio between the major and the minor axis of the stimulus; (2) the threshold is highest for circular blobs; (3) for greatly elongated blobs the threshold varies as a power of the ratio. For slowly varying stimuli, this power equals -1/2; if the stimulus varies rapidly in time, thresholds depend much less on the ratio, unless the minor axis is small. The results are predicted by a multi-layer receptive field model in which the detection units resemble circular symmetric X-type cells, and in which the total response is obtained after Pythagorean summation of the contributions of the individual units. Probability summation cannot explain our results.