Initial evidence of vowel space reduction in a subset of individuals with schizophrenia

Abstract

Objective: Acoustic phonetic measures have been found to correlate with negative symptoms of schizophrenia, thus offering a path toward quantitative measurement of such symptoms. These acoustic properties include F1 and F2 measurements (affected by tongue height and tongue forward/back position, respectively), which determine a general "vowel space." Among patients and controls, we consider two phonetic measures of vowel space: average Euclidean distance from a participant's mean F1 and mean F2, and density of vowels around one standard deviation of mean F1 and of F2.

Methods: Structured and spontaneous speech of 148 participants (70 patients and 78 controls) was recorded and measured acoustically. We examined correlations between the phonetic measures of vowel space and ratings of aprosody obtained using two clinical research measures, the Scale for the Assessment of Negative Symptoms (SANS) and the Clinical Assessment Interview for Negative Symptoms (CAINS).

Results: Vowel space measurements were significantly associated with patient/control status, attributed to a cluster of 13 patients whose phonetic values correspond to reduced vowel space as assessed by both phoenetic measures. No correlation was found between phonetic measures and relevant items and averages of ratings on the SANS and CAINS. Reduced vowel space appears to affect only a subset of patients with schizophrenia, potentially those on higher antipsychotic dosages.

Conclusions: Acoustic phonetic measures may be more sensitive measures of constricted vowel space than clinical research rating scales of aprosody or monotone speech. Replications are needed before further interpretation of this novel finding, including potential medication effects.

Keywords: Aprosody; Linguistics; Negative symptoms; Phonetics; Schizophrenia; Vowel space.

Figure 1.

Visual representation of theoretical canonical vowel space: four sample vowels with the pronunciations both in English spelling (in quotations) and the International Phonetic Alphabet, which is used to represent sounds unambiguously (in square brackets). The dots represent a speaker with more centralized articulations of the four vowels (a reduced vowel space).

Figure 2.

F1 and F2 scatterplots from two participants, the first with ED=298 and density= 44.5%, and second with ED=297, density=67.4%. While the EDs are basically the same, about two-thirds of the second participant’s vowel points are within one SD around the mean of either F1 or of F2 (or of both), less than half of the first participant’s vowel points are (showing that density is a different measurement from ED).

Figure 3.

Euclidean distance and density among both patients and controls