Perceptual Doping: An Audiovisual Facilitation Effect on Auditory Speech Processing, From Phonetic Feature Extraction to Sentence Identification in Noise
Authors: Moradi Shahram , Lidestam Björn , Ning Ng Elaine Hoi , Danielsson Henrik , Rönnberg Jerker 
Published: June 2018

Objective:
We have previously shown that the gain provided by prior audiovisual (AV) speech exposure for subsequent auditory (A) sentence identification in noise is relatively larger than that provided by prior A speech exposure. We have called this effect “perceptual doping.” Specifically, prior AV speech processing dopes (recalibrates) the phonological and lexical maps in the mental lexicon, which facilitates subsequent phonological and lexical access in the A modality, separately from other learning and priming effects. In this article, we use data from the n200 study and aim to replicate and extend the perceptual doping effect using two different A and two different AV speech tasks and a larger sample than in our previous studies.

Design:
The participants were 200 hearing aid users with bilateral, symmetrical, mild-to-severe sensorineural hearing loss. There were four speech tasks in the n200 study that were presented in both A and AV modalities (gated consonants, gated vowels, vowel duration discrimination, and sentence identification in noise tasks). The modality order of speech presentation was counterbalanced across participants: half of the participants completed the A modality first and the AV modality second (A1–AV2), and the other half completed the AV modality and then the A modality (AV1–A2). Based on the perceptual doping hypothesis, which assumes that the gain of prior AV exposure will be relatively larger relative to that of prior A exposure for subsequent processing of speech stimuli, we predicted that the mean A scores in the AV1–A2 modality order would be better than the mean A scores in the A1–AV2 modality order. We therefore expected a significant difference in terms of the identification of A speech stimuli between the two modality orders (A1 versus A2). As prior A exposure provides a smaller gain than AV exposure, we also predicted that the difference in AV speech scores between the two modality orders (AV1 versus AV2) may not be statistically significantly different.

Results:
In the gated consonant and vowel tasks and the vowel duration discrimination task, there were significant differences in A performance of speech stimuli between the two modality orders. The participants’ mean A performance was better in the AV1–A2 than in the A1–AV2 modality order (i.e., after AV processing). In terms of mean AV performance, no significant difference was observed between the two orders. In the sentence identification in noise task, a significant difference in the A identification of speech stimuli between the two orders was observed (A1 versus A2). In addition, a significant difference in the AV identification of speech stimuli between the two orders was also observed (AV1 versus AV2). This finding was most likely because of a procedural learning effect due to the greater complexity of the sentence materials or a combination of procedural learning and perceptual learning due to the presentation of sentential materials in noisy conditions.

Conclusions:
The findings of the present study support the perceptual doping hypothesis, as prior AV relative to A speech exposure resulted in a larger gain for the subsequent processing of speech stimuli. For complex speech stimuli that were presented in degraded listening conditions, a procedural learning effect (or a combination of procedural learning and perceptual learning effects) also facilitated the identification of speech stimuli, irrespective of whether the prior modality was A or AV.