Читать книгу The Handbook of Speech Perception - Группа авторов - Страница 78

Asking participants to categorize a continuum of speech stimuli varying in equal acoustic steps from one phonetic category to another results in a categorical‐like function, for example, [d] to [t] varying in 10 millisecond (ms) voice‐onset time steps. There is a range of stimuli consistently categorized as [d] and a range of stimuli consistently categorized as [t], with one or two stimuli at the edges of or on the boundary between the two categories less consistently categorized. Nonetheless, it turns out that not all members within a phonetic category are perceived equally.

Using more sensitive measures than phonetic categorization including reaction time (Pisoni & Tash, 1974) and judgments of category goodness (Miller, 1997; Iverson & Kuhl, 1996), speech‐perception studies have shown that listener reaction times are slower in identifying a stimulus on a continuum, and their judgments of category goodness are reduced as stimuli approach the phonetic category boundary. Variations in task requirements support the robustness of these gradient effects (e.g. Carney, Widin, & Viemeister, 1977). Thus, phonetic categories are graded and have an internal structure to them (see Miller, 1997). In this sense, categories are not truly binary representations that are either present or absent, but rather some exemplars of a category are better representations of the category than others.

Such findings support a functional architecture in which the degree of activation of a representation is itself graded and influences, as well, the degree of activation of potential competitors. As a stimulus approaches the phonetic category boundary, its activation decreases and there is a concomitant increase in the activation, and hence the extent of competition with the contrasting phonetic category representation. For example, assume a [da]–[ta] continuum ranging in 10 ms steps from 0–40 ms voice onset time (VOT) with a category boundary of 20 ms. As described earlier, there is competition between stimuli that share acoustic properties. Thus, presentation of a 40 ms stimulus (perceived as a [d]) would compete with the representation of the contrasting voiced phonetic category [t]. However, a stimulus with a VOT of 30 ms is a poorer exemplar of the voiceless phonetic category, and thus not only does it activate the phonetic representation of [t] more weakly, but there is an increase in the activation of the contrasting voiced phonetic category [d] (see Blumstein, Myers, & Rissman, 2005).

Neural evidence also supports the gradient nature of phonetic categories. Both temporal and frontal areas show graded responses as a function of the goodness of the phonetic category input, with the least activation for the best exemplar of the phonetic category and increased activation as stimuli on a continuum approach the phonetic category boundary (Blumstein, Myers, & Rissman, 2005; Frye et al., 2007; Guenther et al., 2004). Importantly, other neural areas (middle frontal gyrus, supramarginal gyrus) fail to show such graded activation, displaying sensitivity only to between‐phonetic‐category and not to within‐phonetic‐category differences (Joanisse, Zevin, & McCandliss, 2007; Myers et al., 2009). That there is both graded and categorical perception of phonetic categories reflects two critical aspects of speech perception: the need for sensitivity to fine acoustic differences on the one hand, and sensitivity to category membership on the other. We will return to this point in the Conclusion of this chapter.