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I .2 Pattern Similarity and PR Tasks 9

Based on these elements we can describe a spike as any sequence consisting of a
subsequence of U primitives followed by a subsequence of D primitives or vice-
versa, with at least one U and one D and no other primitives between. Figures 1.7a
and 1.7b show examples of spikes and non-spikes according to this rule.

wave I
n I

other accel. decel.
n
wander shift I

Figure 1.7 Wave primitives for FHR signal: (a) Spikes; (b) Non-spikes; (c) Wave
hierarchy.

The non-spikes could afterwards be classified as accelerations, decelerations or
other wave types. The rule for acceleration could be: any up wave sequence
starting with at least one u primitive with no d's in between, terminating with at
least one d primitive with no M'S inibetween. An example is shown at the bottom of
Figure 1.7b. With these rules we could therefore establish a hierarchy of wave
descriptions as shown in Figure 1.7~.
In this description task the similarity of the objects (spikes, accelerations,
decelerations, etc., in this example) is assessed by means of a s~ruc~ural rule. Two
objects are similar if they obey the same rule. Therefore all spikes are similar, all
accelerations are similar, and so on. Note in particular that the bottom spike of
Figure 1.7a is, in this sense, more similar to the top spike than the top wave of
Figure 1.7b, although applying a distance measure to the values of the signal
amplitudes, using the first peak as time alignment reference, would certainly lead
to a different result!
The structural rule is applied here to the encoded sequence of the primitives, in
the form of a string ofprimitives, in order to see if the rule applies. For instance, a
machine designed to describe foetal heart rate tracings would encode the segment
shown in Figure 1.6b as "uduDUuud", thereby recognizing the presence of a spike.

1.3 Classes, Patterns and Features

In the pattern recognition examples presented so far a quite straightforward
correspondence existed between patterns and classes. Often the situation is not that
simple. Let us consider a cardiologist intending to diagnose a heart condition based
on the interpretation of electrocardiographic signals (ECG). These are electric