Pure linguistic interference during comprehension of competing speech signals

In certain situations, human listeners have more difficulty in understanding speech in a multi-talker environment than in the presence of non-intelligible noise. The costs of speech-in-speech masking have been attributed to informational masking, i.e. to the competing processing of the target and the distractor speech’s information. It remains unclear what kind of information is competing, as intelligible speech and unintelligible speech-like signals (e.g. reversed, noise-vocoded, and foreign speech) differ both in linguistic content and in acoustic information. Thus, intelligible speech could be a stronger distractor than unintelligible speech because it presents closer acoustic information to the target speech, or because it carries competing linguistic information. In this study, we intended to isolate the linguistic component of speech-in-speech masking and we tested its influence on the comprehension of target speech. To do so, 24 participants performed a dichotic listening task in which the interfering stimuli consisted of 4-band noise-vocoded sentences that could become intelligible through training. The experiment included three steps: first, the participants were instructed to report the clear target speech from a mixture of one clear speech channel and one unintelligible noise-vocoded speech channel; second, they were trained on the interfering noise-vocoded sentences so that they became intelligible; third, they performed the dichotic listening task again. Crucially, before and after training, the distractor speech had the same acoustic features but not the same linguistic information. We thus predicted that the distracting noise-vocoded signal would interfere more with target speech comprehension after training than before training. To control for practice/fatigue effects, we used additional 2-band noise-vocoded sentences, which participants were not trained on, as interfering signals in the dichotic listening tasks. We expected that performance on these trials would not change after training, or would change less than that on trials with trained 4-band noise-vocoded sentences. Performance was measured under three SNR conditions: 0, -3, and -6 dB. The behavioral results are consistent with our predictions. The 4-band noise-vocoded signal interfered more with the comprehension of target speech after training (i.e. when it was intelligible) compared to before training (i.e. when it was unintelligible), but only at SNR -3dB. Crucially, the comprehension of the target speech did not change after training when the interfering signals consisted of unintelligible 2-band noise-vocoded speech sounds, ruling out a fatigue effect. In line with previous studies, the present results show that intelligible distractors interfere more with the processing of target speech. These findings further suggest that speech-in-speech interference originates, to a certain extent, from the parallel processing of competing linguistic content. A magnetoencephalography study with the same design is currently being performed, to specifically investigate the neural origins of informational masking