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Biomedical subjects

H Tiitinen

Publications and source records attributed to H Tiitinen.

At least 37 records · Page 2Linked to original sources
PubMed1999

Separation of contamination caused by coil clicks from responses elicited by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is accompanied with loud clicks that evoke auditory responses in the brain, confounding several types of TMS studies. We investigated the effects of these clicks with high-resolution EEG by applying TMS pulses at 3 magnitudes, with the coil placed either at 10 or 50 mm over the subjects' vertex and recording event-related potentials (ERPs). The clicks were found to elicit a positively displaced response at 150-250 ms post-TMS. Furthermore, clicks were found to interact with simultaneously presented auditory sinewave stimuli, resulting in an amplitude decrease in the auditory N1 response.

Adult↗
PubMed1999

A method for generating natural-sounding speech stimuli for cognitive brain research.

OBJECTIVE: In response to the rapidly increasing interest in using human voice in cognitive brain research, a new method, semisynthetic speech generation (SSG), is presented for generation of speech stimuli. METHODS: The method synthesizes speech stimuli as a combination of purely artificial processes and processes that originate from the natural human speech production mechanism. SSG first estimates the source of speech, the glottal flow, from a natural utterance using an inverse filtering technique. The glottal flow obtained is then used as an excitation to an artificial digital filter that models the formant structure of speech. RESULTS: SSG is superior to commercial voice synthesizers because it yields speech stimuli of a highly natural quality due to the contribution of the man-originating glottal excitation. CONCLUSION: The artificial modelling of the vocal tract enables one to adjust the formant frequencies of the stimuli as desired, thus making SSG suitable for cognitive experiments using speech sounds as stimuli.

Humans↗
PubMed1999

Brain responses reveal the learning of foreign language phonemes.

Learning to speak a new language requires the formation of recognition patterns for the speech sounds specific to the newly acquired language. The present study demonstrates the dynamic nature of cortical memory representations for phonemes in adults by using the mismatch negativity (MMN) event-related potential. We studied Hungarian and Finnish subjects, dividing the Hungarians into a naive (no knowledge of Finnish) and a fluent (in Finnish) group. We found that the MMN for a contrast between two Finnish phonemes was elicited in the fluent Hungarians but not in the naive Hungarians. This result indicates that the fluent Hungarians developed cortical memory representations for the Finnish phoneme system that enabled them to preattentively categorize phonemes specific to this language.

Adult↗
PubMed1997

Binaural interaction in the human brain can be non-invasively accessed with long-latency event-related potentials.

Invasive microelectrode measurements have demonstrated binaural interaction effects of summation, occlusion, and suppression. Here we demonstrate these phenomena in humans using non-invasive long-latency cortical event-related potential (ERP) components N1 and mismatch negativity (MMN). Subjects were presented with monaural and binaural stimulus trains consisting of frequent standard stimuli and deviant stimuli deviating from the standard either in frequency, intensity, or duration. The binaural N1 was smaller than the monaural N1, MMN for the intensity change was larger with binaural than monaural stimulation, whereas for the frequency and duration change, the MMN amplitude remained unchanged. Thus, cortical binaural interactions reflected suppression in the N1, summation for the intensity MMN, and occlusion for the frequency and duration MMNs.

Acoustic Stimulation↗
PubMed1997

The transient 40-Hz response, mismatch negativity, and attentional processes in humans.

1. Recent experimental studies on the neurophysiological basis of auditory selective attention and sensory memory forming the sensory-data basis for tuning the selective-attention system in humans are reviewed. 2. The results demonstrate that the transient 40-Hz response is enhanced by selective attention, attenuated in the course of long-term stimulation, but is not affected by changes in auditory stimuli. 3. Therefore, the 40-Hz response seems to be closely related to selective and sustained attention, whereas it does not seem to be associated with passive attention, as it does not reflect the detection of changes in auditory stimuli. 4. Changes in auditory stimulation are registered by pre-attentive sensory memory, indexed by the mismatch negativity (MMN), a change-specific component of the event-related potentials (ERPs). By this time, the transient 40-Hz response has already terminated. The magnitude of stimulus change is reflected in MMN latency. These latency changes predict changes in attentive reaction time (RT). 5. Thus, the pre-attentive memory mechanism seems to govern attentive detection of changes in the auditory environment. 6. It is concluded that the transient 40-Hz response is related to active attention and MMN is related to passive attention.

Attention↗
PubMed1996

Processing of complex sounds in the human auditory cortex as revealed by magnetic brain responses.

Processing of simple and complex sounds in the human brain was compared by recording extracranial magnetic mismatch responses (MMNm; the magnetic counterpart of the mismatch negativity, or MMN) to frequency changes in these sounds. Generator sources, modeled as equivalent current dipoles (ECDs), of MMNm responses to a change in one frequency element of complex sounds (a chord and a serial tone pattern) were located in supratemporal auditory cortex, on average, 10 mm medially to the source of an MMNm elicited by an identical frequency change in a simple tone. These results suggest that at least partially different supratemporal neuron populations are involved in processing changes in simple and complex sounds and that sensory-memory representations for these sounds may be located in different fields of the auditory cortex.

Adult↗
PubMed1995

Sustained fields of tones and glides reflect tonotopy of the auditory cortex.

Cortical activation in response to two types of auditory stimuli, constant-frequency tones and frequency glides, was studied by measuring the magnetic field outside the head using a whole-head 122-channel magnetometer. Both the magnetic N1m and sustained responses were located in the supratemporal plane of the primary auditory cortex. The sustained responses both to constant-frequency tones and frequency glides reflect tonotopic organization of the auditory cortex both in depth and direction, thus revealing the underlying neuroanatomical structure of the auditory cortex.

Acoustic Stimulation↗
PubMed1995

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.

Brain↗
PubMed1994

The auditory transient 40-Hz response is insensitive to changes in stimulus features.

Ten subjects were presented with tone pips occasionally interspersed with deviant tone pips of a higher frequency. The transient 40-Hz response was insensitive to change in qualitative stimulus features. In contrast, stimulus changes elicited a later and slower event-related potential, the mismatch negativity (MMN). As a response to changes in stimulus features implies the existence of a memory system, and because changes in qualitative stimulus aspects do not activate the generator mechanisms underlying the 40-Hz response, the 40-Hz response can be dissociated from memory mechanisms. Furthermore, the analysis of phase-locked (synchronous) and non-phase-locked (asynchronous) responses revealed that the 40-Hz response might be caused by the synchronization of already active oscillators.

Acoustic Stimulation↗
PubMed1994

Attentive novelty detection in humans is governed by pre-attentive sensory memory.

Being able to detect unusual, possibly dangerous events in the environment is a fundamental ability that helps ensure the survival of biological organisms. Novelty detection requires a memory system that models (builds neural representations of) events in the environment, so that changes are detected because they violate the predictions of the model. The earliest physiologically measurable brain response to novel auditory stimuli is the mismatch negativity, MMN, a component of the event-related potential. It is elicited when a predictable series of unvarying stimuli is unexpectedly followed by a deviating stimulus. As the occurrence of MMN is not usually affected by the direction of attention, MMN reflects the operation of automatic sensory (echoic) memory, the earliest memory system that builds traces of the acoustic environment against which new stimuli can be compared. The dependence of attentive novelty detection on earlier, pre-attentive processes, however, has remained elusive. Previous, related studies seem to suggest a relationship between MMN and attentive processes, although no conclusive evidence has so far been shown. Here we address novelty detection in humans both on a physiological and behavioural level, and show how attentive novelty detection is governed by a pre-attentive sensory memory mechanism.

Acoustic Stimulation↗
PubMed1994

Long-term stimulation attenuates the transient 40-Hz response.

The effect of long-term stimulation on the transient 40-Hz response was studied in 5-h long experimental sessions. Ten human subjects were presented with series of 1000 Hz tone pips occasionally replaced by 1200 Hz tone pips. The transient 40-Hz response was compared with two event-related potential (ERP) components, the N1 and the mismatch negativity (MMN) during the different phases of the long-lasting sessions. The 40-Hz and N1 onset responses attenuated while the MMN did not. Thus, the 40-Hz response might indicate vigilance.

Acoustic Stimulation↗
PubMed1993

Interaction between representations of different features of auditory sensory memory.

The neurophysiological basis of sensory memory was studied by measuring the magnetic counterpart (MMNm) of the mismatch negativity (MMN) with a whole-head 122-channel magnetometer. The MMNm is a response to a difference in the presented stimulus and a neuronal memory trace formed by repeated standard stimuli. This trace must contain information about the feature differing in the deviant. Keeping one feature (frequency) constant, we studied how other stimulus features affect the strength of the MMNm. The MMNm to a frequency change was weaker when the other features varied than when they were constant. This suggests that the MMNm to a frequency change is not independent of other stimulus features.

Acoustic Stimulation↗
PubMed1993

Selective attention enhances the auditory 40-Hz transient response in humans.

Studies of human auditory and somatosensory modalities have shown that there is an oscillatory response in the gamma-band (at about 40 Hz) frequency which is elicited by either steady state or transient stimulation. The auditory 40-Hz response is generated at least partially in the auditory cortex as a result of thalamocortical interaction and may serve perceptual integration and conscious perception. A connection to selective attention has been implied in human and animal studies, although the evidence is inconclusive. Moreover, fundamental differences between the human and animal 40-Hz responses prohibit generalization. Furthermore, most experiments have used steady-state stimulation during which the brain does not regain its resting state between stimuli as it does when transient stimulation is used. Here we study the effect of selective attention on the auditory gamma-band (40-Hz) transient response using subjects listening to tone pips presented in one ear while ignoring a concurrent sequence of tone pips in the other ear. The 40-Hz response was larger when subjects paid attention to stimuli rather than ignored them. This attention effect was most pronounced over the frontal and central scalp areas. Our results demonstrate a physiological correlate of selective attention in the 40-Hz transient response in humans.

Acoustic Stimulation↗
PubMed1993

Mismatch negativity to slight pitch changes outside strong attentional focus.

The mismatch negativity (MMN) component of the auditory event-related potential (ERP) is elicited by infrequent, physically deviant stimuli in a sequence of frequent homogeneous stimuli ("standards"). It has been suggested that the MMN is generated by an automatic (attention-independent) neural mismatch process between the sensory input and a memory trace encoding the physical features of the standard stimulus. The MMN independence of attention was addressed in the present study. Standard stimuli and two types of deviant stimuli, differing from standards in frequency either "widely" (5%) or "slightly" (3%) were dichotically presented in random order at a very rapid rate. The subject attended either to left- or right-ear stimuli, counting the number of "slight deviants" in that ear. A reading condition with the same stimuli was also included. Even in the present attend conditions with very strong attentional focus, the MMN was elicited by the slight deviants in the unattended input stream. Furthermore, its amplitude was similar to that of the MMN elicited by the equivalent deviant stimuli during reading. The results suggest that auditory frequency is fully analyzed even in the absence of attention.

Adult↗
PubMed1993

Memory-related processing of complex sound patterns in human auditory cortex: a MEG study.

Responses of the human brain to a complex sound pattern were recorded with a 24 channel magnetometer. The sound pattern consisted of 9 successive 50 ms segments, each with a different frequency. An infrequent change in the frequency of one of the segments elicited a magnetic mismatch response (MMNm) which peaked at about 200 ms after the deviant segment onset and resembled the electrical mismatch negativity (MMN). The equivalent current dipole which best explained the MMNm was located in the supratemporal auditory cortex, suggesting that a memory trace for the sound pattern was stored in that region.

Adult↗