Abstract
We investigated whether the categorical perception (CP) of speech might also provide a mechanism that aids its perception in noise. We varied signal-to-noise ratio (SNR) [clear, 0 dB, −5 dB] while listeners classified an acoustic-phonetic continuum (/u/ to /a/). Noise-related changes in behavioral categorization were only observed at the lowest SNR. Event-related brain potentials (ERPs) differentiated category vs. category-ambiguous speech by the P2 wave (~180–320 ms). Paralleling behavior, neural responses to speech with clear phonetic status (i.e., continuum endpoints) were robust to noise down to −5 dB SNR, whereas responses to ambiguous tokens declined with decreasing SNR. Results demonstrate that phonetic speech representations are more resistant to degradation than corresponding acoustic representations. Findings suggest the mere process of binning speech sounds into categories provides a robust mechanism to aid figure-ground speech perception by fortifying abstract categories from the acoustic signal and making the speech code more resistant to external interferences.
Highlights
A basic tenet of perceptual organization is that sensory phenomena are subject to invariance: similar features are mapped to common identities by assigning similar objects to the same membership (Goldstone and Hendrickson, 2010), a process known as categorical perception (CP)
More robust neural activity predicted steeper psychometric functions at the individual level. These findings suggest the neural processing of speech sounds carrying clear phonetic labels predicts more dichotomous categorical decisions at the behavioral level; whereas neural responses to ambiguous speech tokens do not predict perceptual categorization
By measuring neuroelectric brain activity during rapid classification of SIN, our results reveal three main findings: (1) speech identification is robust to acoustic interference, degrading only at very severe noise levels; (2) the neural encoding of speech is enhanced for sounds carrying a clear phonetic identity compared to phonetically ambiguous tokens; and (3) categorical neural representations are more resistant to external noise than their categorically ambiguous counterparts
Summary
A basic tenet of perceptual organization is that sensory phenomena are subject to invariance: similar features are mapped to common identities (equivalence classes) by assigning similar objects to the same membership (Goldstone and Hendrickson, 2010), a process known as categorical perception (CP). In the context of speech, CP is demonstrated when gradually morphed sounds along an equidistant acoustic continuum are heard as only a few discrete classes (Liberman et al, 1967; Pisoni, 1973; Harnad, 1987; Pisoni and Luce, 1987; Bidelman et al, 2013). Equal physical steps along a signal dimension do not produce equivalent changes in percept (Holt and Lotto, 2006). Listeners treat sounds within a given category as perceptually similar despite their otherwise dissimilar acoustics. To arrive at categorical decisions, acoustic cues are presumably weighted and compared against internalized “templates”
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