Auditory Streaming versus Apparent Motion
We have been examining the phenomenon of auditory stream segregation as an
example of how phenomena of auditory organization can exhibit the same
complexities as are found in vision. This has led us to see interesting parallels
in the principles that govern auditory stream segregation and visual grouping.
But we have not yet discussed the most striking parallel, that between auditory
stream segregation and the phenomenon of apparent motion in vision. Appar-
ent motion is the perceptual effect that used to be very popular on the bill-
boards of theatres, where the switching on and off of a series of electric light
bulbs in sequence gave the experience of movement. In the laboratory it is
usually created in a much simpler form. Two electric lamps, often seen as small
white dots in an otherwise black room, are alternately switched on, each for a
brief instant, so that a movement is seen that dances back and forth between
the lights, always moving from the light that has just been flashed to the light
that is currently being flashed. If the lamps are close together, it may seem that
the light itself is moving back and forth. At greater distances the experience is
just an impression of movement.
In 1915, Ko ̈rte formulated a number of laws relating the duration, brightness,
and spatial separation of the lamps to the strength of the impression of move-
ment. Ko ̈rte’s third law stated that within certain ranges, if you want to increase
the spatial separation between the lamps and still have a strong impression of
motion, you had to slow down the alternation of flashes. It was almost as if the
movement would not be able to keep up with the alternation of flashes if they
were far separated in space unless the flashes were slowed down to compen-
sate for their separation.
A more elaborate form of the apparent motion effect strongly resembles the
streaming effect.^6 Instead of two lamps, there are six, arranged in a horizontal
row as shown in figure 9.12. They are arranged so that there is a wider gap
between the left triplet of lights and the right triplet than there is between the
lights within each triplet. If we label the lamps with the digits 1 to 6 from left to
right, the order in which the lights are to be flashed can be expressed as the
sequence 142536, repeated endlessly with no pause between repetitions. In this
sequence there is an alternation between left-triplet and right-triplet flashes. At
very low speeds, there is no apparent motion at all. The lights appear simply to
go on and off in sequence. At a somewhat higher speed, the true sequence
(142536) is seen as a form of irregular left-and-right motion between members
of the two triplets. Then, as the speed is increased, the motion appears to split
into two separate streams, one involving the leftmost three lamps and the other
the rightmost three. The leftmost path of motion is 1–2–3 and the rightmost one
Figure 9.12
A visual display used to demonstrate visual motion segregation. Two groups of three lamps are
arranged in a horizontal row.
The Auditory Scene 229