Isolating Neural Signatures of Conscious Speech Perception with a No-Report Sine-Wave Speech Paradigm.

Identifying neural correlates of conscious perception is a fundamental endeavor of cognitive neuroscience. Most studies so far have focused on visual awareness along with trial-by-trial reports of task-relevant stimuli, which can confound neural measures of perceptual awareness with postperceptual processing. Here, we used a three-phase sine-wave speech paradigm that dissociated between conscious speech perception and task relevance while recording EEG in humans of both sexes. Compared with tokens perceived as noise, physically identical sine-wave speech tokens that were perceived as speech elicited a left-lateralized, near-vertex negativity, which we interpret as a phonological version of a perceptual awareness negativity. This response appeared between 200 and 300 ms after token onset and was not present for frequency-flipped control tokens that were never perceived as speech. In contrast, the P3b elicited by task-irrelevant tokens did not significantly differ when the tokens were perceived as speech versus noise and was only enhanced for tokens that were both perceived as speech and relevant to the task. Our results extend the findings from previous studies on visual awareness and speech perception and suggest that correlates of conscious perception, across types of conscious content, are most likely to be found in midlatency negative-going brain responses in content-specific sensory areas.

Hearing Shapes: Event-related Potentials Reveal the Time Course of Auditory-Visual Sensory Substitution.

In auditory-visual sensory substitution, visual information (e.g., shape) can be extracted through strictly auditory input (e.g., soundscapes). Previous studies have shown that image-to-sound conversions that follow simple rules [such as the Meijer algorithm; Meijer, P. B. L. An experimental system for auditory image representation. Transactions on Biomedical Engineering, 39, 111-121, 1992] are highly intuitive and rapidly learned by both blind and sighted individuals. A number of recent fMRI studies have begun to explore the neuroplastic changes that result from sensory substitution training. However, the time course of cross-sensory information transfer in sensory substitution is largely unexplored and may offer insights into the underlying neural mechanisms. In this study, we recorded ERPs to soundscapes before and after sighted participants were trained with the Meijer algorithm. We compared these posttraining versus pretraining ERP differences with those of a control group who received the same set of 80 auditory/visual stimuli but with arbitrary pairings during training. Our behavioral results confirmed the rapid acquisition of cross-sensory mappings, and the group trained with the Meijer algorithm was able to generalize their learning to novel soundscapes at impressive levels of accuracy. The ERP results revealed an early cross-sensory learning effect (150-210 msec) that was significantly enhanced in the algorithm-trained group compared with the control group as well as a later difference (420-480 msec) that was unique to the algorithm-trained group. These ERP modulations are consistent with previous fMRI results and provide additional insight into the time course of cross-sensory information transfer in sensory substitution.

ERP signatures of conscious and unconscious word and letter perception in an inattentional blindness paradigm.

A three-phase inattentional blindness paradigm was combined with ERPs. While participants performed a distracter task, line segments in the background formed words or consonant-strings. Nearly half of the participants failed to notice these word-forms and were deemed inattentionally blind. All participants noticed the word-forms in phase 2 of the experiment while they performed the same distracter task. In the final phase, participants performed a task on the word-forms. In all phases, including during inattentional blindness, word-forms elicited distinct ERPs during early latencies (∼200-280ms) suggesting unconscious orthographic processing. A subsequent ERP (∼320-380ms) similar to the visual awareness negativity appeared only when subjects were aware of the word-forms, regardless of the task. Finally, word-forms elicited a P3b (∼400-550ms) only when these stimuli were task-relevant. These results are consistent with previous inattentional blindness studies and help distinguish brain activity associated with pre- and post-perceptual processing from correlates of conscious perception.

Electrophysiological assessment of the time course of bilingual visual word recognition: Early access to language membership.

Previous research examining the time course of lexical access during word recognition suggests that phonological processing precedes access to semantic information, which in turn precedes access to syntactic information. Bilingual word recognition likely requires an additional level: knowledge of which language a specific word belongs to. Using the recording of event-related potentials, we investigated the time course of access to language membership information relative to semantic (Experiment 1) and syntactic (Experiment 2) encoding during visual word recognition. In Experiment 1, Spanish-English bilinguals viewed a series of printed words while making dual-choice go/nogo and left/right hand decisions based on semantic (whether the word referred to an animal or an object) and language membership information (whether the word was in English or in Spanish). Experiment 2 used a similar paradigm but with syntactic information (whether the word was a noun or a verb) as one of the response contingencies. The onset and peak latency of the N200, a component related to response inhibition, indicated that language information is accessed earlier than semantic information. Similarly, language information was also accessed earlier than syntactic information (but only based on peak latency). We discuss these findings with respect to models of bilingual word recognition and language comprehension in general.

Who do you love, your mother or your horse? An event-related brain potential analysis of tone processing in Mandarin Chinese.

In Mandarin Chinese, word meaning is partially determined by lexical tone (Wang, 1973). Previous studies suggest that lexical tone is processed as linguistic information and not as pure tonal information (Gandour, 1998; Van Lanker & Fromkin, 1973). The current study explored the online processing of lexical tones. Event-related potentials were obtained from 25 Mandarin speakers while they listened to normal and anomalous sentences containing one of three types of semantic anomalies created by manipulating the tone, the syllable, or both tone and syllable (double-anomaly) of sentence-final words. We hypothesized N400 effects elicited by all three types of anomalies and the largest by the double-anomaly. As expected, all three elicited N400 effects starting approximately 150 ms poststimulus and continuing until 1000 ms in some areas. Surprisingly, onset of the double-anomaly effect was approximately 50 ms later than the rest. Delayed detection of errors in this condition may be responsible for the apparent delay. Slight differences between syllable and tone conditions may be due to the relative timing of these acoustic cues.