Proceedings Fonetik 2009 - Institutionen för lingvistik

Proceedings, FONETIK 2009, Dept. of Linguistics, Stockholm Universitylow amplitude oscillations are lost and the sequencesare interpreted as plateaus that arelonger (and therefore fewer within the analysiswindow) than those measured in speech segmentsproduced with higher F0. Such momentarychanges in the structure of the plateaus areinterpreted by the program's arbitrary code asindicating "deception". Under typical circumstances,flagging "lie" in association with loweringof F0 will give the illusion that the programis doing something sensible because F0tends to be lower when a speaker produces fillersduring hesitations than when the speaker'sspeech flows normally. Since the "lie-detector"is probably calibrated with responses to questionsabout obvious things the speaker will tendto answer using a typical F0 range that willgenerally be higher than when the speaker hasto answer to questions under split-attentionloads. Of course, when asked about events thatdemand recalling information, the speaker willtend to produce fillers or speak at a lowerspeech rate, thereby increasing the probabilityof being flagged by the system as attempting to"lie", although in fact hesitations or lowering ofF0 are known to be no reliable signs of deception.Intentionally or by accident, the illusion ofseriousness is further enhanced by the randomcharacter of the LVA outputs. This is a directconsequence of the technology's responses toboth the speech signal and all sorts of spuriousacoustic and digitalization accidents. The instabilityis likely to confuse both the speaker andthe "certified examiner", conveying the impressionthat the system really is detecting somebrain activity that the speaker cannot control 3and may not even be aware of! It may even givethe illusion of robustness as the performance isequally bad in all environments.Figure 2. The figures above show synthetic vowelsconstructed by algebraic addition of delayedversions of a natural glottal pulse. Thesewaveforms lead generate different “emotionaloutputs” depending on the relationship betweenthe F0 of the waveform being tested and the F0of the “calibration” waveform.low, the final portions of the vocal tract'sdamped responses to more sparse glottal pulseswill tend to achieve lower amplitudes in betweenconsecutive pulses. Given the technology'svery crude amplitude quantization, theseThe UK's DWP's evaluation of LVAThe UK's Department of Work and Pensionshas recently published statistics on the resultsof a large and systematic evaluation of theLVA-technology iii assessing 2785 subjects andcosting £2.4 million iv . The results indicate thatthe areas under the ROC curves for seven districtsvary from 0.51 to 0.73. The best of these3 Ironically this is true because the output is determinedby random factors associated with room acoustics, backgroundnoise, digitalization problems, distortion, etc.224