Ear-EEG allows extraction of neural responses in challenging listening scenarios - A future technology for hearing aids?

Advances in brain-computer interface research have recently empowered the development of wearable sensors to record mobile electroencephalography (EEG) as an unobtrusive and easy-to-use alternative to conventional scalp EEG. One such mobile solution is to record EEG from the ear canal, which has been validated for auditory steady state responses and discrete event related potentials (ERPs). However, it is still under discussion where to place recording and reference electrodes to capture best responses to auditory stimuli. Furthermore, the technology has not yet been tested and validated for ecologically relevant auditory stimuli such as speech. In this study, Ear-EEG and conventional scalp EEG were recorded simultaneously in a discrete-tone as well as a continuous-speech design. The discrete stimuli were applied in a dichotic oddball paradigm, while continuous stimuli were presented diotically as two simultaneous talkers. Cross-correlation of stimulus envelope and Ear-EEG was assessed as a measure of ongoing neural tracking. The extracted ERPs from Ear-EEG revealed typical auditory components yet depended critically on the reference electrode chosen. Reliable neural-tracking responses were extracted from the Ear-EEG for both paradigms, albeit weaker in amplitude than from scalp EEG. In conclusion, this study shows the feasibility of extracting relevant neural features from ear-canal-recorded "Ear-EEG", which might augment future hearing technology.

Attention modulates the use of spectral attributes in vowel discrimination: behavioral and event-related potential evidence.

Speech contains a variety of acoustic cues to auditory and phonetic contrasts that are exploited by the listener in decoding the acoustic signal. In three experiments, we tried to elucidate whether listeners rely on formant peak frequencies or whole spectrum attributes in vowel discrimination. We created two vowel continua in which the acoustic distance in formant frequencies was constant but the continua differed in spectral moments (i.e., the whole spectrum modeled as a probability density function). In Experiment 1, we measured reaction times and response accuracy while listeners performed a go/no-go discrimination task. The results indicated that the performance of the listeners was based on the spectral moments (especially the first and second moments), and not on formant peaks. Behavioral results in Experiment 2 showed that, when the stimuli were presented in noise eliminating differences in spectral moments between the two continua, listeners employed formant peak frequencies. In Experiment 3, using the same listeners and stimuli as in Experiment 1, we measured an automatic brain potential, the mismatch negativity (MMN), when listeners did not attend to the auditory stimuli. Results showed that the MMN reflects sensitivity only to the formant structure of the vowels. We suggest that the auditory cortex automatically and pre-attentively encodes formant peak frequencies, whereas attention can be deployed for processing additional spectral information, such as spectral moments, to enhance vowel discrimination.

Left prefrontal cortex activation during sentence comprehension covaries with grammatical knowledge in children.

Children's language skills develop rapidly with increasing age, and several studies indicate that they use language- and age-specific strategies to understand complex sentences. In the present experiment, functional magnetic resonance imaging (fMRI) and behavioral measures were used to investigate the acquisition of case-marking cues for sentence interpretation in the developing brain of German preschool children with a mean age of 6 years. Short sentences were presented auditorily, consisting of a transitive verb and two case-marked arguments with canonical subject-initial or non canonical object-initial word order. Overall group results revealed mainly left hemispheric activation in the perisylvian cortex with increased activation in the inferior parietal cortex (IPC), and the anterior cingulate cortex (ACC) for object-initial compared to subject-initial sentences. However, single-subject analysis suggested two distinct activation patterns within the group which allowed a classification into two subgroups. One subgroup showed the predicted activation increase in the left inferior frontal gyrus (IFG) for the more difficult object-initial compared to subject-initial sentences, while the other group showed the reverse effect. This activation in the left IFG can be taken to reflect the degree to which adult-like sentence processing strategies, necessary to integrate case-marking information, are applied. Additional behavioral data on language development tests show that these two subgroups differ in their grammatical knowledge. Together with these behavioral findings, the results indicate that the use of a particular processing strategy is not dependent on age as such, but rather on the child's individual grammatical knowledge and the ability to use specific language cues for successful sentence comprehension.

Top-down knowledge supports the retrieval of lexical information from degraded speech.

How is it that the human brain is capable of making sense from speech under many acoustically compromised conditions? The support through top-down knowledge is inevitable but can we identify brain measures of this matching process between degraded auditory input and possible meaning? To answer these questions, the present study investigated the modulation of the induced gamma-band activity (GBA) in the auditory domain in response to degraded speech. During an EEG experiment subjects first listened to digitally degraded unintelligible speech signals (derived from German nouns). In an exposure sequence, half of the nouns were presented in a non-degraded intelligible format and memorized, while in the crucial test sequence subjects listened to all degraded speech signals again and were asked to identify the words. The induced GBA (40-Hz range) showed an increase at left temporal electrode sites around 350 ms only for words correctly identified in the test sequence. No differences in induced GBA were evident in the baseline sequence; neither did the evoked brain potentials yield any comparable effect. We conclude that the observed enhancement in induced gamma-band activity reflects a matching process of top-down lexical memory traces with degraded sensory input to form a comprehendible speech percept. The findings are highly corroborant to analogous studies in the visual system. They lend further plausibility to a left-lateralized fronto-temporal network enabling lexically guided speech perception, and they demonstrate the complementary role of time-sensitive brain analyses in discerning the functional neuroanatomy of speech.

Recovery from aphasia as a function of language therapy in an early bilingual patient demonstrated by fMRI.

Knowledge about the recovery of language functions in bilingual aphasic patients who suffer from left-hemispheric stroke is scarce. Here, we present the case of an early bilingual patient (German/French) with chronic aphasia. Functional magnetic resonance imaging (fMRI) was used to investigate neural correlates of language performance during an overt picture naming task in German and French (a) 32 months after stroke to assess differential recovery of both languages as a function of the preceding language therapy that was provided exclusively in German and (b) after additional short-term intensive (German) language training. At the first investigation behavioral performance confirmed selective recovery of German naming ability which was associated with increased functional brain activation compared to the French naming condition. Changes in behavioral performance and brain activation pattern as disclosed by fMRI after an additional experimental treatment were confined to the trained (German) language and indicate bilateral neuroplastic reorganization. No generalization to the untrained (French) language was observed. The present case results demonstrate use and/or training-dependent differential recovery of expressive language functions and an enhanced pattern of brain activation as a function of the rehabilitation efforts that were focussed exclusively on the patient's German language abilities.

Gender differences in functional hemispheric asymmetry during processing of vowels as reflected by the human brain magnetic response.

A number of findings indicate gender differences in language-related functional hemispheric brain asymmetry. To test if such gender-specific laterality is already present at the level of vowel-processing, the auditory evoked magnetic field was recorded in healthy right-handed male and female participants in response to the German synthetic vowels [a], [e] and [i]. Female participants exhibited stronger N100m responses than male participants over the left hemisphere. This observation was highly reliable across repeated experimental sessions. The present lateralization shows that previous findings suggesting a stronger left-hemispheric dominance for verbal material in males than in females can not be generalized to basic speech elements. Furthermore, the present results support the importance of controlling for gender ratio in studies of phonetic processing.