Weighted linear cue combination with possibly correlated error

We test hypotheses concerning human cue combination in a slant estimation task. Observers repeatedly adjusted the slant of a plane to 75 degrees. Feedback was provided after each setting and the observers trained extensively until their setting error stabilized. The slant of the plane was defined by either linear perspective alone (a grid of lines) or texture gradient alone (diamond-shaped texture elements) or the two cues together. We chose a High and Low variance version of each cue type and measured setting variability in four single-cue conditions (Low, High for each cue) and in the four possible combined-cue conditions (Low-Low, Low-High, etc.). We compared performance in the combined-cue conditions to predictions based on single-cue performance. The results were consistent with a linear combination of estimates from cues. Six out of eight observers did better with combined cues than with either cue alone. For three observers, performance was consistent with optimal combination of uncorrelated cues. Three other observers' results were also consistent with optimal combination, but with the assumption that internal cue estimates were correlated. The remaining two observers were consistent with sub-optimal cue combination.