Effects of talker variability on spectral contrast effects.

Spectral contrast effects are context-dependent effects that influence the way we perceive certain sounds. Evidence of these effects can be seen in experiments where a precursor sound (e.g. a sentence) is followed by a target vowel sound (like /ɪ/ as in "bit" or /ɛ/ as in "bet"). If the precursor's frequency was emphasized in areas more consistent with the frequency of /ɛ/, listeners tend to perceive the target sound to be the opposite i.e. /ɪ/. A recent study shows using sentence precursors from 200 different talkers diminishes these effects questioning previous claims that talker variability has no influence on spectral contrast effects (Assgari & Stilp, 2015; Lain, Liu, Lotto, & Holt 2012). This study investigated the influence of one, four, eight, and sixteen talkers using conversational speech. Sentences were filtered with +5 dB filter gain to emphasize frequency regions consistent with either /ɛ/ or /ɪ/. One-talker, four-talker, and eighttalker conditions all produced contrast effects while the sixteen-talker condition failed to produce an effect. Results suggest talker variability has a greater influence on spectral contrast effects than previously thought. EFFECTS OF TALKER ON SPECTRAL CONTRAST EFFECTS 3 Effects of Talker Variability on Spectral Contrast Effects Speech perception is a complex task that involves deciphering words from a broad range of sounds. Whether we are listening to a high-pitched voice in an echoing lecture hall or a man on a noisy street, we are able to understand speech with relative ease. Not only do we extract speech from this general sound signal, but we also categorize the different speech sounds into vowels so we may understand the speech. This poses a question of what mechanisms we use in categorizing vowels within speech that lead to our perception. Is our auditory system equipped with the ability to categorize vowel sounds based on their absolute frequencies or is speech perceived based on relative frequencies of a talker’s sounds? The latter theory was proposed by Joos (1948) and was later used to explain experiments done by Ladefoged & Broadbent (1957) who showed that listeners identified the same word differently after listening to a sentence whose formant frequencies were modified. The current study seeks to explore the effects described by Ladefoged & Broadbent (1957), now known as spectral contrast effects, in relation to whether the number of talkers has an influence on our experience of these contrast effects. In 1957, Ladefoged and Broadbent published a seminal paper that showed categorization of vowels can be influenced by the context in which the vowel sounds are placed. They performed an experiment where they created variations of the sentence “Please say what this word is,” manipulating its formant frequencies. The formant frequency is the frequency at which a sound reaches a relative maximum or a spectral peak and allows us to differentiate between vowels. In their experiment, they modified the sentence to shift the formant frequency towards the low region (towards a vowel sound like /ɪ/ as in "bit") or the high region (towards a vowel sound like /ɛ/ as in "bet") of the first formant (F1). They then synthesized target words like “bit” and “bet”. Participants were asked to distinguish these target words after listening to the EFFECTS OF TALKER ON SPECTRAL CONTRAST EFFECTS 4 manipulated sentences. Following an unmodified sentence, almost all participants judged the word in question as intended. When participants heard the sentence modified to emphasize the frequency of /ɛ/ (high F1), almost all participants judged the test word to be “bit” regardless of whether “bit” or “bet” was played. Similarly, when “bit” or “bet” was preceded by a sentence with /ɪ/ peaks emphasized (low F1), participants almost always judged the test word to be “bet” (Ladefoged & Broadbent, 1957). The tendency for people to judge the test word as opposite of what was emphasized in the preceding sentence is what is now known as the spectral contrast effect (Stilp, Anderson, & Winn, 2015). The spectral contrast effect has been interpreted and explained in many ways. One explanation that seeks to reconcile different reports of this effect explains it in terms of sensory adaptation (Stilp et al., 2015). Our sensory system tends to become less sensitive to stable stimuli in a given context (Barlow, 1961; Stilp et al., 2015). This allows us to be able to quickly perceive any changes in stimuli. In other words, we tend to perceptually magnify the difference between stable properties of a stimulus and any changes in the stimulus (Stilp et al., 2015). In context of Ladefoged & Broadbent (1957), this means that when participants adapted to the stable spectral peaks emphasizing a certain vowel in a sentence, they magnified the difference between the test word and the preceding sentence. This shifted their perception to the opposite direction of what was emphasized in the preceding sentence. There have been many studies showing this effect is very robust and can be produced for a wide range of sounds. Watkins (1991) found that contrast effects hold for manipulations like presenting the test vowels and precursor sentence to different ears, using male and female sentences followed by male test vowels, and when sentences were reversed to produce unnatural sounds. This effect can also be observed in English-Spanish bilingual, Spanish-, English-, or EFFECTS OF TALKER ON SPECTRAL CONTRAST EFFECTS 5 Dutch-speaking listeners using different native and non-native languages (Sjerps & Smiljanić, 2013). There have also been a number of studies that show spectral contrast effects extend beyond speech sounds. For example, contrast effects hold for musical instruments where instead of test words, participants heard a tenor saxophone and French horn (Stilp, Alexander, Kiefte, & Kluender, 2010). Even simple compilations of sine tones (or “tone histories”) that vary in duration and number of tones can be used to produce the effect (Holt, 2005; Holt, 2006). Traditionally, sentences or precursor sounds that are used in spectral contrast effect experiments are modified by introducing relatively large peaks [e.g. +30 decibel (dB) peaks] in key frequency regions (see Stilp et al., 2015). Such large peaks may not be common in natural settings (Assgari & Stilp, 2015). In experiments done by Stilp et al. (2015), a significant contrast effect was observed with as small as +5 dB filter gain. However, using such a small filter gain resulted in a significantly smaller effect when compared to the effect observed with +20 dB filter gain (Stilp et al., 2015). A major question surrounding spectral contrast effects is whether it is purely an acoustic phenomenon or it can be subject to higher-level influences like talker information. In the light of the above studies, it is clear that spectral contrast effects can be produced in a variety of contexts. Laing et al. (2012) observed contrast effects when they adjusted formant frequencies to create sentence precursors that perceptually appear to be produced by different talkers. The authors claimed that talker information plays no role in spectral contrast effects (Laing et al., 2012). This way of inducing talker variability may not fully represent differences found in actual talkers (Pollack et al., 1954; Watkins, 1991). A more recent study showed that a change in talker can significantly diminish spectral contrast effects and thus challenges this notion that talker information plays no role in spectral contrast effects (Assgari & Stilp, 2015). These experiments EFFECTS OF TALKER ON SPECTRAL CONTRAST EFFECTS 6 added either a +5 dB or a +20 dB spectral peak to sentences from one talker versus 200 different talkers. No significant difference was observed between conditions when filter gain was +20 dB. However, when a +5 dB peak was added, there was a significant decrease in the size of the contrast effect when sentences were produced by 200 different talkers. Therefore, there seems to be an increased sensitivity to the number of talkers when modest peaks are added. The current study hopes to investigate the number of talkers needed to significantly reduce the contrast effect. Assgari & Stilp (2015) used 200 talkers and 200 sentences to introduce variation in speech. This large amount of variation may not be necessary to diminish spectral contrast effects and such a variation may not be of practical significance. Other experiments of talker variability look at word recognition. A classic study by Creelman (1957) showed that there was decreased performance in word recognition as the number of talkers increased from one, two, four, eight, and sixteen talkers. This study will likewise look at one talker, four talkers, eight talkers, and sixteen talkers speaking 160, 40, 20, and 10 sentences respectively. Using a modest, more natural, +5dB filter gain, it can be predicted that spectral contrast effects will be diminished with increasing talker variability.