Illusory contours and surfaces without amodal completion and depth stratification.

Cognitive and figural cues were studied in modified Ehrenstein figures made from letters of the alphabet instead of radial lines. Capital letters with and without terminators (L, J vs O, D) were used, oriented towards or away from the central gap. Three groups, of 14 subjects each, estimated the magnitude of either (i) the illusory contour, (ii) brightness enhancement, or (iii) apparent depth. Strong illusory contour formation and brightness enhancement, but no depth stratification, were perceived in figures devoid of apparent occlusion and amodal completion. These results demonstrate that the Ehrenstein illusion can arise from line ends--with no need for perceptual completion, showing that illusory boundaries and surfaces can be dissociated from apparent depth. Results support a bottom-up explanation in terms of end-stopped neurons in the visual cortex. Conversely, top-down processes appear to be responsible for depth stratification.