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Abstract:
How much time does our visual system need to perform stereopsis? Viewed pseudoscopically, an opaque square floating above a random-dot
pattern appears as a rectangular cut-out. When the pattern moves
vertically upwards, an illusory gap with undefined depth position is
perceived at the upper edge of the square. This phenomenon is called
Delayed Stereopsis Illusion (DSI).
The DSI-gap carries the pattern of the moving plane, its spatial
depth, however, is perceived somewhere between the moving pattern and
the cut-out. In analogy with Julesz's noman's-land we called this
DSI-gap trailing-edge no-man's-land. Its width indicates the 3-D
computation time needed to determine spatial depth of the pattern, which
virtually appears from nowhere.
Data were gathered psychophysically with a computer generated model
system. In three experimental series E1-E3 14 subjects marked the width
of the DSI-gap under various welldefined conditions with two different
methods. A total of 881 single measurements were
performed, 212 of them in E1, 384 in E2 and 285 in E3. The results
indicate interindividually different 3-D computation times between 50
and 80 ms.
Learning, and pattern parameters like spatial frequency did not
significantly influence the perceived width of the DSI-gap.
Regarding the perceived shift of moving patterns according to de Valois
and de Valois (1991), an adequate correction of the delays concluded
from the measured DSI gaps is discussed. In any case, the minimum
presentation time of 17 ms, at which Julesz´ dynamic
random-dot-stereograms are just recognizable, is much too short to
determine the position in depth in each single frame. The 3-D system
rather seems to check that the depth situation has not changed, and
maintains the percept of the floating square.
[Supported by the Deutsche Forschungsgemeinschaft.]
