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Abstract

In a number of models of depth cue combination the depth percept is constructed via a weighted average combination of independent depth estimations. The inuence of each cue in such average
depends on the reliability of the source of information [1,5]. In particular, Ernst and Banks
(2002) formulate such combination as that of the minimum variance unbiased estimator that
can be constructed from the available cues. We have observed systematic differences in slant
discrimination performance of human observers when different types of textures were used as
cue to slant [4]. If the depth percept behaves as described above, our measurements of the
slopes of the psychometric functions provide the predicted weights for the texture cue for the
ranked texture types. However, the results for slant discrimination obtained when combining
these texture types with object motion results are difcult to reconcile with the minimum variance
unbiased estimator model [3]. This apparent failure of such model might be explained by
the existence of a coupling of texture and motion, violating the assumption of independence of
cues. Hillis, Ernst, Banks, and Landy (2002) [2] have shown that while for between-modality
combination the human visual system has access to the single-cue information, for withinmodality
combination (visual cues) the single-cue information is lost. This suggests a coupling
between visual cues and independence between visual and haptic cues. Then, in the present
study we combined the different texture types with haptic information in a slant discrimination
task, to test whether in the between-modality condition these cues are combined as predicted
by an unbiased, minimum variance estimator model. The measured weights for the cues were
consistent with a combination rule sensitive to the reliability of the sources of information, but
did not match the predictions of a statistically optimal combination.