Auditory Enhancement in Speech Perception

Summerfield et al. [22] described a perceptual effect of temporal spectral contrast which they called the ‘Flat-spectrum Vowels’ effect. They summed the first 50 harmonics of 100 Hz with equal amplitudes, creating a signal with a flat spectrum. Three sets of adjacent harmonics were then omitted at frequencies centred on the first three formants of a vowel, creating the ‘spectral complement’ of the vowel. Stimuli were constructed in which 500-ms segments of a vowel complement preceded and followed a 500-ms segment of the flat spectrum. Listeners identified the flat spectrum as the vowel whose complement surrounded it, despite the absence of formant peaks. The effect reveals the existence of processes that enhance newly-arriving acoustical energy in relation to pre-existing energy. These processes could improve the effective signal-to-noise ratio of intermittent signals in background noises. The aims of this paper are to discuss factors that may contribute to this effect and to illustrate roles it may play in the perception of speech.