Minimal spectral contrast of formant peaks for vowel recognition as a function of spectral slope

Perceptual sensitivity to spectral properties of earlier sounds during speech categorization

Speech perception is heavily influenced by surrounding sounds. When spectral properties differ between earlier (context) and later (target) sounds, this can produce spectral contrast effects (SCEs) that bias perception of later sounds. For example, when context sounds have more energy in low-F₁ frequency regions, listeners report more high-F₁ responses to a target vowel, and vice versa. SCEs have been reported using various approaches for a wide range of stimuli, but most often, large spectral peaks were added to the context to bias speech categorization. This obscures the lower limit of perceptual sensitivity to spectral properties of earlier sounds, i.e., when SCEs begin to bias speech categorization. Listeners categorized vowels (/ɪ/-/ɛ/, Experiment 1) or consonants (/d/-/g/, Experiment 2) following a context sentence with little spectral amplification (+1 to +4 dB) in frequency regions known to produce SCEs. In both experiments, +3 and +4 dB amplification in key frequency regions of the context produced SCEs, but lesser amplification was insufficient to bias performance. This establishes a lower limit of perceptual sensitivity where spectral differences across sounds can bias subsequent speech categorization. These results are consistent with proposed adaptation-based mechanisms that potentially underlie SCEs in auditory perception.

SIGNIFICANCE STATEMENT

Recent sounds can change what speech sounds we hear later. This can occur when the average frequency composition of earlier sounds differs from that of later sounds, biasing how they are perceived. These "spectral contrast effects" are widely observed when sounds' frequency compositions differ substantially. We reveal the lower limit of these effects, as +3 dB amplification of key frequency regions in earlier sounds was enough to bias categorization of the following vowel or consonant sound. Speech categorization being biased by very small spectral differences across sounds suggests that spectral contrast effects occur frequently in everyday speech perception.

Modest, reliable spectral peaks in preceding sounds influence vowel perception

When a spectral property is reliable across an acoustic context and subsequent vowel target, perception deemphasizes this cue and shifts toward less predictable, more informative cues. This phenomenon (auditory perceptual calibration) has been demonstrated for reliable spectral peaks +20 dB or larger, but psychoacoustic findings predict sensitivity to more modest spectral peaks. Listeners identified vowel targets following a sentence with a reliable +2 to +15 dB spectral peak centered at F2 of the vowel. Vowel identifications weighted F2 significantly less when reliable peaks were at least +5 dB. Results demonstrate high sensitivity to reliable acoustic properties in the sensory environment.

The role of formant amplitude in the perception of /i/ and /u/

Although recent evidence reconfirmed the importance of spectral peak frequencies in vowel identification [Kiefte and Kluender (2005). J. Acoust. Soc. Am. 117, 1395-1404], the role of formant amplitude in perception remains somewhat controversial. Although several studies have demonstrated a relationship between vowel perception and formant amplitude, this effect may be a result of basic auditory phenomena such as decreased local spectral contrast and simultaneous masking. This study examines the roles that local spectral contrast and simultaneous masking play in the relationship between the amplitude of spectral peaks and the perception of vowel stimuli. Both full- and incomplete-spectrum stimuli were used in an attempt to separate the effects of local spectral contrast and simultaneous masking. A second experiment was conducted to measure the detectability of the presence/absence of a formant peak to determine to what extent identification data could be predicted from spectral peak audibility alone. Results from both experiments indicate that, while both masking and spectral contrast likely play important roles in vowel perception, additional factors must be considered in order to account for vowel identification data. Systematic differences between the audibility of spectral peaks and predictions of perceived vowel identity were observed.

On the effectiveness of whole spectral shape for vowel perception

The formant hypothesis of vowel perception, where the lowest two or three formant frequencies are essential cues for vowel quality perception, is widely accepted. There has, however, been some controversy suggesting that formant frequencies are not sufficient and that the whole spectral shape is necessary for perception. Three psychophysical experiments were performed to study this question. In the first experiment, the first or second formant peak of stimuli was suppressed as much as possible while still maintaining the original spectral shape. The responses to these stimuli were not radically different from the ones for the unsuppressed control. In the second experiment, F2-suppressed stimuli, whose amplitude ratios of high- to low-frequency components were systemically changed, were used. The results indicate that the ratio changes can affect perceived vowel quality, especially its place of articulation. In the third experiment, the full-formant stimuli, whose amplitude ratios were changed from the original and whose F2's were kept constant, were used. The results suggest that the amplitude ratio is equal to or more effective than F2 as a cue for place of articulation. We conclude that formant frequencies are not exclusive cues and that the whole spectral shape can be crucial for vowel perception.