Minna Huotilainen

Processing of novel sounds and frequency changes in the human auditory cortex: Magnetoencephalographic recordings

Whole-head magnetoencephalographic (MEG) responses to repeating standard tones and to infrequent slightly higher deviant tones and complex novel sounds were recorded together with event-related brain potentials (ERPs). Deviant tones and novel sounds elicited the mismatch negativity (MMN) component of the ERP and its MEG counterpart (MMNm) both when the auditory stimuli were attended to and when they were ignored. MMNm generators were located bilateral to the superior planes of the temporal lobes where preattentive auditory discrimination appears to occur. A subsequent positive P3a component was elicited by deviant tones and with a larger amplitude by novel sounds even when the sounds were to be ignored. Source localization for the MEG counterpart of P3a (P3am) suggested that the auditory cortex in the superior temporal plane is involved in the neural network of involuntary attention switching to changes in the acoustic environment.

Signal-space projections of MEG data characterize both distributed and well-localized neuronal sources

We describe the use of signal-space projection (SSP) for the detection and characterization of simultaneous and/or sequential activation of neuronal source distributions. In this analysis, a common signal space is used to represent both the signals measured by an array of detectors and the underlying brain sources. This presents distinct advantages for the analysis of EEG and MEG data. Both highly localized and distributed sources are characterized by the components of the field patterns which are measured by the detectors. As a result, a unified description of arbitrary source configurations is obtained which permits the consistent implementation of a variety of analysis techniques. The method is illustrated by the application of SSP to auditory, visual and somatosensory evoked-response MEG data. Single-trace evoked responses obtained by SSP of spontaneous activity demonstrate that a considerable discrimination against both system noise and uncorrelated brain activity may be achieved. Application of signal-space projections determined in the frequency domain to spontaneous activity illustrates the possibility of including temporal relationships into the analysis. Finally, we demonstrate that SSP is particularly useful for the description of multiple sources of distributed activity and for the comparison of the strengths of specific neuronal sources under a variety of different paradigms or subject conditions.

The discrimination of and orienting to speech and non-speech sounds in children with autism.

The present study aimed to find out how different stages of cortical auditory processing (sound encoding, discrimination, and orienting) are affected in children with autism. To this end, auditory event-related potentials (ERP) were studied in 15 children with autism and their controls. Their responses were recorded for pitch, duration, and vowel changes in speech stimuli, and for corresponding changes in the non-speech counterparts of the stimuli, while the children watched silent videos and ignored the stimuli. The responses to sound repetition were diminished in amplitude in the children with autism, reflecting impaired sound encoding. The mismatch negativity (MMN), an ERP indexing sound discrimination, was enhanced in the children with autism as far as pitch changes were concerned. This is consistent with earlier studies reporting auditory hypersensitivity and good pitch-processing abilities, as well as with theories proposing enhanced perception of local stimulus features in individuals with autism. The discrimination of duration changes was impaired in these children, however. Finally, involuntary orienting to sound changes, as reflected by the P3a ERP, was more impaired for speech than non-speech sounds in the children with autism, suggesting deficits particularly in social orienting. This has been proposed to be one of the earliest symptoms to emerge, with pervasive effects on later development.

Maturation of the auditory event-related potentials during the first year of life

This study examined the maturation of cortical auditory event-related potentials (ERPs) from birth until 12 months of age. In the 15 infants studied, all ERP peaks observable at 12 months of age, the P150, N250, P350, and N450 were identifiable already at birth. As in previous studies, the amplitudes of the ERP peaks increased and latencies shortened with increasing age. In addition, the time courses of the amplitude growth of these peaks differed from each other. It was concluded, that the generators of all the infantile ERP peaks are functional already at birth, and that the maturational changes in the waveform morphology can mostly be accounted for by the changing relative strengths of the different generators.

Temporal window of integration of auditory information in the human brain

A deviation in the acoustic environment activates an automatic change-detection system based on a memory mechanism that builds a neural trace representing the preceding sounds. The present study revealed that the auditory-cortex mechanisms underlying this sensory memory integrate acoustic events over time, producing a perception of a unitary auditory event. We recorded magnetic responses (MMNm) to occasional stimulus omissions in trains of stimuli presented at a constant stimulus-onset asynchrony (SOA) that was, in different blocks, either shorter or longer in duration than the assumed length of the temporal window of integration. A definite MMNm was elicited by stimulus omission only with the three shortest SOAs used: 100, 125, and 150 ms, but not with 175 ms. Thus, 160-170 ms was estimated as the length of the temporal window used by the central auditory system in integrating successive auditory input into auditory event percepts.

Visual cortex activation in blind humans during sound discrimination

We used a whole-scalp magnetometer with 122 planar gradiometers to study the activity of the visual cortex of five blind humans deprived of visual input since early infancy. Magnetic responses were recorded to pitch changes in a sound sequence when the subjects were either counting these changes or ignoring the stimuli. In two of the blind subjects, magnetic resonance images were also obtained, showing normal visual cortex macroanatomy. In these subjects, the magnetic responses to counted pitch changes were located at visual and temporal cortices whereas ignored pitch changes activated the temporal cortices almost exclusively. Also in two of the other three blind, the visual-cortex activation was detectable in the auditory counting task. Our results suggest that the visual cortex of blind humans can participate in auditory discrimination.

Newborn infants can organize the auditory world

The perceptual world of neonates is usually regarded as not yet being fully organized in terms of objects in the same way as it is for adults. Using a recently developed method based on electric brain responses, we found that, similarly to adults, newborn infants segregate concurrent streams of sound, allowing them to organize the auditory input according to the existing sound source. The segregation of concurrent sound streams is a crucial step in the path leading to the identification of objects in the environment. Its presence in newborn infants shows that the basic abilities required for the development of conceptual objects are available already at the time of birth.

Statistical language learning in neonates revealed by event-related brain potentials.

BackgroundStatistical learning is a candidate for one of the basic prerequisites underlying the expeditious acquisition of spoken language. Infants from 8 months of age exhibit this form of learning to segment fluent speech into distinct words. To test the statistical learning skills at birth, we recorded event-related brain responses of sleeping neonates while they were listening to a stream of syllables containing statistical cues to word boundaries.ResultsWe found evidence that sleeping neonates are able to automatically extract statistical properties of the speech input and thus detect the word boundaries in a continuous stream of syllables containing no morphological cues. Syllable-specific event-related brain responses found in two separate studies demonstrated that the neonatal brain treated the syllables differently according to their position within pseudowords.ConclusionThese results demonstrate that neonates can efficiently learn transitional probabilities or frequencies of co-occurrence between different syllables, enabling them to detect word boundaries and in this way isolate single words out of fluent natural speech. The ability to adopt statistical structures from speech may play a fundamental role as one of the earliest prerequisites of language acquisition.

Top-down effects can modify the initially stimulus-driven auditory organization

We recorded event-related potentials (ERPs) and magnetic fields (ERFs) of the human brain to determine whether top-down control could modulate the initial organization of sound representations in the auditory cortex. We presented identical sound stimulation and manipulated top-down processes by instructing participants to either ignore the sounds (Ignore condition), to detect pitch changes (Attend-pitch condition), or to detect violations of a repeating tone pattern (Attend-pattern condition). The ERP results obtained in the Attend-pattern condition dramatically differed from those obtained with the other two task instructions. The magnetoencephalogram (MEG) findings were fully compatible, showing that the neural populations involved in detecting pattern violations differed from those involved in detecting pitch changes. The results demonstrate a top-down effect on the sound representation maintained in auditory cortex.

Combined mapping of human auditory EEG and MEG responses

Auditory electric and magnetic P50(m), N1(m) and MMN(m) responses to standard, deviant and novel sounds were studied by recording brain electrical activity with 25 EEG electrodes simultaneously with the corresponding magnetic signals measured with 122 MEG gradiometer coils. The sources of these responses were located on the basis of the MEG responses; all were found to be in the supratemporal plane. The goal of the present paper was to investigate to what degree the source locations and orientations determined from the magnetic data account for the measured EEG signals. It was found that the electric P50, N1 and MMN responses can to a considerable degree be explained by the sources of the corresponding magnetic responses. In addition, source-current components not detectable by MEG were shown to contribute to the measured EEG signals.