Troubles with bubbles

The bubbles method is a recently developed variant of reverse correlation methods that have been used in psychophysics and physiology. We show mathematically that for the broad and important class of noisy linear observers, the bubbles method recovers much less information about how observers process stimuli than reverse correlation does. We also show experimentally that the unusual type of noise used in the bubbles method can drastically change human observers' strategies in psychophysical tasks, which reduces the value of the information that is obtained from a bubbles experiment. We conclude that reverse correlation is generally preferable to the bubbles method in its present form, but we also give suggestions as to how the bubbles method could be modified to avoid the problems we discuss.