Perceptual Coherence : Hearing and Seeing

212 Perceptual Coherence
pointed out that the split is obligatory; the notes form two streams even if
the listener is trying to hear the two tones together.
In making judgments about apparent motion and auditory segregation,
the auditory and visual perceptual systems metaphorically are asking
whether the two stimuli could have come from the same object. For both
systems, the expectation is that objects change slowly. Thus, if the interval
between the two stimuli is relatively short, and yet the physical separation
or frequency difference is large, the perceptual systems judge that the two
stimuli come from different objects. Two lights are seen flashing alternately
without connecting movement, and two tones are heard as coming from
different sources.
Auditory and Visual Hysteresis
Hysteresis refers to the tendency to stick with the initial percept even as the
stimulus changes greatly. The same effect found for apparent visual movement
occurs for the alternating tone sequences used by van Noorden
(1975). In some of the continuous tonal sequences, the frequencies of the
two tones gradually became more different. Listeners heard a coherent sequence
at frequency ratios that would normally result in two streams; conversely,
if the frequencies of tones were gradually made more similar,
listeners heard two streams at frequency ratios that would normally lead to
a coherent sequence.
Theories of Motion Perception
Apparent Motion of Rigid Arrays
Over the last 25 years, the consensus has been that there is no single motion
detection system. Originally, the notion was that there were two systems.
The first captured small quick movements of complicated patterns in one
eye based on global matches, while the second captured longer slower
movements of simple figures based on feature matches that could be integrated
across the two eyes. 2 Although such a dichotomy has proven inadequate,
the basic concept of two or more systems still seems sound. In what
follows, I begin by discussing the types of experimental results that lead to
the short/long motion distinction, and then discuss how this notion has
evolved in the past 10 years.
2. The term feature is used rather loosely. Features are defined by the context. Convex or
concave would be a feature in a mixed set of figures, but not if all the figures were convex or
concave.