unit thus activates itself in proportion to its own current activation and the
strength of the link. The strength of the link will be referred to as its weight. If
the weight iswi, then:
sn¼wt:This scheme is functionally identical to decay, but is easier to implement.
An alternative postulate to decay isinterference. Interference is the displace-
ment (or reduction in retrievability) of items in memory by more recently per-
ceived items. Interference seems to occur in both short-term memory (Waugh &
Norman, 1965) and long-term memory (Bjork, 1989), but in the present context
we are concerned primarily with short-term memory. Interference in short-term
memory is typically attributed to a capacity limitation in attention or activation
(J. R. Anderson, 1983; Shiffrin, 1975), although specific interactive effects have
been noted. In the context of network models, interference is the reduction in
activation of some units due to an increase in activation of others. Interference
assumes that the total amount of activation among a given set of units is lim-
ited, so that activation caused by the currently perceived event comes at the
expense of activation caused by earlier events. Interference is usually imple-
mented by introducing inhibition. Gjerdingen (1990) has used what amounts to
an interference mechanism, in which the activation of perceived events persists
until inhibited by more recent events.
Whether decay or interference accounts for forgetting in short-term memory
is a debate that goes back to the very beginnings of cognitive psychology (see
Neisser, 1967), and there is evidence for both (Reitman, 1974; Waugh & Nor-
man, 1965). The notion of persistence in a temporal composite, as outlined ear-
lier, is agnostic as to the mechanism and its implementation.
A temporal composite has also been adopted unwittingly by Parncutt and
Huron (1993)—although not in the form of a neural net—to account for key
tracking data. Parncutt and Huron refer to their representation asechoic mem-
ory. Echoic memory is an auditory sensory memory that persists for several
seconds, after which it is lost unless attended to (Darwin, Turvey, & Crowder,
1972; Neisser, 1967). Echoic memory enables us to relate what we are hearing at
this very moment to what we have just heard. It permits us to maintain a tem-
poral window wide enough to recognize a dynamic sound or parse a phrase.
The persistencesnmay also be influenced by segmentation cues—factors that
cue the listener to chord changes or to boundaries between groups, motifs,
phrases, or other segments. Segmentation cues could include phasic signals for
chord changes (see earlier) or any number of pitch, timing, and timbral cues in
either the composition or the performance (Bregman, 1990; Lerdahl & Jackend-
off, 1983; Palmer, 1989). A segmentation cue would causesnto be small, so that
a fresh temporal composite can be started for the next segment.
G. Tonal and Modal Composites
A temporal composite of a pitch-class representation may be called atonal
composite, and a temporal composite of an invariant pitch class representation
may be called amodalcomposite. Tonal composites that integrate information
between chord changes represent the chords that have been either played or
implied, and can account for aspects of the implication of harmony by melody.
Neural Nets, Temporal Composites, and Tonality 463