maps in auditory cortex. The network architecture was constrained by properties observed
in auditory cortex and which include the modulating influences of the basal forebrain. The
simulations allowed the authors to formulate hypotheses on neuromodulatory effects in
auditory cortical plasticity and to suggest future simulations including other neural regions
(e.g. hippocampus) in order to model effects on learning and memory. Recently, we used
the SOM algorithm to simulate an example of cognitive plasticity, notably the cognitive
capacity to extract underlying regularities and to become sensitive to musical structures
and regularities via implicit and unsupervised learning processes.^31
An overview of regularities underlying Western tonal music
Western tonal music is an example of a highly structured system in our environment. It con-
tains a set of regularities (i.e. regularities of cooccurrence, frequency of occurrence, and psycho-
acoustic regularities) based on a limited number of elements. The Western tonal system that
contains a three-level organization is based on a set of 12 tones (C-C#-D-D#-E-F-F#-
G-G#-A-A#-B). The 12 tones are combined into subsets of seven tones, the scales. For each
tone of a scale, chords (e.g. major or minor) are constructed by adding two notes—creating
a second order of musical units. The tones in a scale and their associated chords define a
third level of organization in the tonal system: the keys. Keys sharing numerous tones and
chords are harmonically related, and the strength of harmonic relationship depends on the
number of shared events. The seven tones that belong to the underlying scale are in-key
tones, the remaining five tones are out-of-key tones. Similarly for chords, in-key chords are
differentiated from out-of-key chords. Hierarchies of functional importance exist among
tones and chords belonging to a given key: For example, chords built on the first, fifth, and
fourth scale degrees (referred to as tonic, dominant, and subdominant, respectively) have
more important functions than chords built on other degrees. From a psychological point
of view, the hierarchically important events of a key act as stable cognitive reference points^32
to which others are anchored.^33 As the tonal system is based on a restricted set of events, the
same event can define an in-key or an out-of-key event or can take different levels of func-
tional importance (i.e. tonal stability)—always depending on the established key.
The three levels of musical units occur with strong regularities of cooccurrence. Tones
and chords belonging to the same key are more likely to cooccur in a musical piece than
tones and chords belonging to different keys. Changes between keys are more likely to
occur between closely related keys than between less related ones. Also within-key hierarch-
ies are strongly correlated with the frequency of occurrence of tones and chords in Western
musical pieces. Tones and chords that have more important music-theoretic functions in a
given key are used with higher frequency (and longer duration).34–36
Despite the complexity of the system, Western tonal listeners become sensitive to the musical
structures by mere exposure to musical pieces obeying this system of regularities.35–37As
the number of opportunities to listen to musical pieces is so great in everyday life, this
implicit knowledge is plausibly learned by the same implicit processes as those investigated in
the laboratory.38–41Behavioural and neurophysiological research has provided evidence that
even nonmusician listeners are sensitive to the context dependency of musical events. The
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