Janne Sinkkonen

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.

Test–retest reliability of mismatch negativity for duration, frequency and intensity changes

OBJECTIVESnThe test-retest reliabilities of the mismatch negativity (MMN) elicited by deviances in sound duration, frequency, and intensity were compared.nnnMETHODSnThe MMN was recorded with a 64-channel electroencephalograph (EEG) from 15 healthy adult subjects in two sessions over intervals of 1-27 days. During the recordings, subjects watched a silent movie while they were presented with one long stimulus sequence consisting of 6 types of tones. The standard tone (P = 0.8) of 75 ms in duration consisted of 3 lowest harmonic partials with 500 Hz as the fundamental frequency. Each of the 5 different deviant tones was presented with P = 0.04: frequency deviants (+/-5%, +/-10%), duration deviants (-66%, -33%), and intensity deviants (- 15 dB).nnnRESULTSnThe 66% duration decrement elicited MMN with the most replicable amplitude (r = 0.78) and latency (r = 0.76) among the deviances tested.nnnCONCLUSIONSnSince these reliabilities considerably exceed those reported previously, these data support the use of the duration decrement deviance presented with spectrally rich tones while investigating the integrity of the patients cognitive brain functions using the MMN.

Analysis of speech sounds is left-hemisphere predominant at 100-150 ms after sound onset

Hemispheric specialization of human speech processing has been found in brain imaging studies using fMRI and PET. Due to the restricted time resolution, these methods cannot, however, determine the stage of auditory processing at which this specialization first emerges. We used a dense electrode array covering the whole scalp to record the mismatch negativity (MMN), an event-related brain potential (ERP) automatically elicited by occasional changes in sounds, which ranged from non-phonetic (tones) to phonetic (vowels). MMN can be used to probe auditory central processing on a millisecond scale with no attention-dependent task requirements. Our results indicate that speech processing occurs predominantly in the left hemisphere at the early, pre-attentive level of auditory analysis.

Gabor filters: an informative way for analysing event-related brain activity

Frequency-specific, i.e., narrow-band brain, activity is traditionally analyzed on the basis of either a time- or frequency-domain representation of the signal. Here we demonstrate an alternative method based on Gabor functions which are well known for their optimal concentration in time and frequency. Using Gabor filtering, amplitude and frequency information can be separated clearly from one another and certain novel approaches to averaging become possible.

RAPID COMMUNICATION Scalp-Recorded Optical Signals Make Sound Processing in the Auditory Cortex Visible

1982) have demonstratedtonotopic organization in the auditory cortex. Thus,these methods, especially fMRI, are able to map brainareas involved in auditory processing on a millimeterscale. However, fMRI is not able to track the fine-grained temporal dynamics of the central auditorysystem in encoding and analyzing sound informationbecause its time resolution is limited by the propertiesof the hemodynamic changes, lagging neural activity atleast by several hundred milliseconds. In contrast,MEG provides temporally detailed information on cen-tral auditory processing (Lu¨tkenho¨ner and Stein-stra¨ter, 1998) but has severe problems in separatingsimultaneously activated adjacent sources from eachother and cannot indicate the extent and pattern of theactivated area. The present paper reports the firstevent-related optical signals (EROS; Gratton and Fabi-ani, 1998) from the functioning human auditory cortex.Usingthisnovelnoninvasivemeasureofcorticalactiva-tion, we were able to locate, in both time and space,neuronal generators for two of the most importantauditory functions, sound detection and change detec-tion (Na¨a¨ta¨nen, 1992).The EROS method combines both high spatial andhigh temporal resolution in a single measure (Grattonand Fabiani, 1998). In an EROS recording, a source ofintensity-modulated near-infrared light and a detectorare placed on the scalp a few centimeters apart fromeach other. The low-intensity light emitted by thesource diffuses through the skin, bone, and brain, andsome photons exit the head, reaching the detector. Aspatially high-resolution signal is achieved by selectingthe photons on the basis of their time of flight; thosephotons that take similar amounts of time to migratethrough the medium are likely to follow relativelysimilar paths. EROS is a measure of phase-shifts in themodulationenvelopeofthelightasthephotonsmigratethrough the brain tissue, which is optically modified byneural activation. Some of the optical changes in thebrain tissue are related to various hemodynamic phe-nomena lasting relatively long times compared withneural electric activity (Cannestra

The mismatch negativity for duration decrement of auditory stimuli in healthy subjects

The amplitude and latency of the mismatch negativity (MMN) elicited by occasional shorter-duration tones (25 and 50 ms) in a sequence of 75 ms standard tones were studied in 40 healthy subjects (9-84 years). The replicability and age dependence of the MMN-responses were determined. The 25 ms deviant tone evoked a clear response in 39 of the subjects, while the 50 ms deviant tone evoked an observable MMN only in 32 of the subjects. The MMN peak amplitude for the 25 ms deviants was significantly larger than for the 50 ms deviants. There was no significant difference in the peak latencies (measured from stimulus offset). For the 25 ms deviant, the amplitude diminished with increasing age. The MMN curves for the 25 ms deviant, measured on separate days in 14 subjects, looked very replicable. As a result of noise and filtering effect, the product-moment correlations were poor. The results indicate that the signal-to-noise ratio for the MMN to 25 ms deviants, obtained even in a 25 min recording session, is large enough for clinical use and individual diagnostics when undetectable (or very low amplitude) MMN is used as a sign of pathology. However, judged from the low correlation coefficients, despite the good replicability in visual evaluation, better methods for MMN quantification have to be used for clinical follow-up.

Task effects on fatigue symptoms in overnight driving

This study examined the effects of task and time-on-task on fatigue symptoms in overnight driving. Four participants drove an instrumented car 1200 km overnight and completed the same trip as passengers on another night. Subjective ratings of drowsiness, eye blink frequency and duration, microsleeps, and steering-wheel inputs were analysed as a function of time-on-task, and for separate samples when meeting oncoming heavy vehicles. Four video cameras were used to monitor the road view and the face of both the driver and passenger. In terms of eye closure duration, the reported microsleeps were shorter while driving (mean = 0.7 s, SD = 0.2 s) than as a passenger (mean = 2.6 s, SD = 2.0 s). Blink frequency increased with time-on-task as expected, indicating tiredness, and decreased when approaching an oncoming heavy vehicle, indicating attentive response to a potential critical situation. No consistent effect of time-on-task on high-frequency steering-wheel inputs when meeting oncoming heavy vehicles was found. The results raise the important question of what makes a driver wake from a microsleep earlier than a passenger and, given proper monitoring of long eyelid closures, what the proper intervention should be.

Age-related functional differences between auditory cortices : a whole-head MEG study

Auditory evoked magnetic fields (AEFs) were recorded from 10 healthy younger and 10 older subjects using a whole-head magnetometer. Two blocks of tone pips were presented to the left ear with constant inter-stimulus intervals (ISIs) of 0.5 and 2.5 s. The amplitude of P50m, unlike that of N100m, was larger in the older subjects. In both groups, the peak latencies of P50m and N100m responses were significantly shorter over the contralateral than ipsilateral cortex. The interhemispheric latency difference of NIOOm was significantly increased with age. These findings suggest that ageing delays signal processing in the ipsilateral auditory cortex and that ageing affects consecutive AEFs in a different manner.

Harmonic partials facilitate pitch discrimination in humans: electrophysiological and behavioral evidence

The effect of the spectral tone structure on pre-attentive and attentive pitch discrimination was investigated. The mismatch negativity (MMN) component was recorded from reading subjects to pitch changes of identical magnitude in pure tones with only one sinusoidal frequency component and in spectrally rich tones with two additional harmonic partials. In a separate condition, subjects were asked to indicate detection of pitch change by a button press. The MMN was elicited with a larger amplitude and shorter latency by change in spectrally rich tones than by change in pure tones. Furthermore, the subjects behavioral responses were more accurate for spectrally rich tones than for sinusoidal tones. Together these data indicate that pre-attentive and attentive pitch discrimination is facilitated with spectrally rich sounds in comparison to pure sinusoidal tones.