Oscillations, Waves, and Interactions - GWDG

28 H. W. Strube
that overlap at most half, normalized cross-correlation coefficients are then formed
with shifts within ±0.3 ms. The largest of all these coefficients yields a meassure,
“Glottal-to-Noise Excitation ratio” (GNE) [20].
To obtain a graphical representation appropriate for clinical routine, a two-dimensional
plot was desired. For this purpose we considered 6 jitter and 6 shimmer
measures, the MWC, and 3 variants each of NNE, CHNR and GNE. The dependencies
between these were investigated using rank correlation and mutual information. It
turned out that the GNE was most independent of the irregularity measures. By
means of principal-component analysis it was shown that, for pathological voices, two
dimensions explain 95% of variance (for normal voices, four dimensions are required).
In this way, a diagram resulted with an abscissa that was an average of a jitter
measure, a shimmer measure and the MWC, and a (linearly transformed) GNE as
ordinate, the “Göttinger Hoarseness Diagram” (GHD).
Normal voices are located at the lower left, aphonic voices at the upper right
in the diagram. Different groups (e. g., persons, vowels, medical diagnoses) can be
represented by ellipses, where the principal axes indicate the standard deviations
with respect to abscissa and ordinate. Fig. 1 shows an example for various cancer
groups.
As in the regions of normal and of aphonic voices two dimensions are not really
sufficient, it was also tried to obtain a finer resolution using Kohonen feature maps [21]
(example see Fig. 2) and discriminant analysis [22]. Also, the relevance of several
breathiness measures for classifying voice pathologies was investigated [23], resulting
in a dimensionality of 7 for benign disorders and 3 for cancer groups.
Figure 2. Kohonen feature map, projected into the GHD plane. Note the folds (marked
by ellipses) near the regions of normal and aphonic voices, indicating importance of higher
dimensions.