Biomedical subjects

Hannu Tiitinen

Publications and source records attributed to Hannu Tiitinen.

17 recordsLinked to original sources

PubMed2006

From the repression of contents to the rules of the (narrative) self: a present-day cognitive view of the Freudian phenomenon of repressed contents.

In psychoanalysis, it is commonly thought that ideas (desires, fears, etc.) may be repressed, and that they can be made conscious. In this article, we shall apply cognitive viewpoints and assert that ideas do not exist in the unconscious as 'ready made', and thus repressed ideas cannot be 'brought' into consciousness. We suggest that the contents of consciousness are formed by processes on four levels: (1) unconscious brain processes, (2) the level of consciousness, (3) the level of self-consciousness, and (4) the level of narrative self-consciousness. From this point of view, the absence (or repression) of certain contents appears to be due to the missing of processes on Levels 1-4. Consequently, repressed contents appear in consciousness when appropriate processes take place. When studied in terms of our four-level model, repression may be treated as part of the study of the self. By applying the viewpoint of the self to the phenomenon of repression, the danger of the homunculus problem can be avoided. It also becomes apparent that certain fundamental problems met in the study of the self are the ones that Freud tried to solve in his meta-psychological writings.

Cognitive Science↗

PubMed2005

Neuromagnetic recordings reveal the temporal dynamics of auditory spatial processing in the human cortex.

In an attempt to delineate the assumed 'what' and 'where' processing streams, we studied the processing of spatial sound in the human cortex by using magnetoencephalography in the passive and active recording conditions and two kinds of spatial stimuli: individually constructed, highly realistic spatial (3D) stimuli and stimuli containing interaural time difference (ITD) cues only. The auditory P1m, N1m, and P2m responses of the event-related field were found to be sensitive to the direction of sound source in the azimuthal plane. In general, the right-hemispheric responses to spatial sounds were more prominent than the left-hemispheric ones. The right-hemispheric P1m and N1m responses peaked earlier for sound sources in the contralateral than for sources in the ipsilateral hemifield and the peak amplitudes of all responses reached their maxima for contralateral sound sources. The amplitude of the right-hemispheric P2m response reflected the degree of spatiality of sound, being twice as large for the 3D than ITD stimuli. The results indicate that the right hemisphere is specialized in the processing of spatial cues in the passive recording condition. Minimum current estimate (MCE) localization revealed that temporal areas were activated both in the active and passive condition. This initial activation, taking place at around 100 ms, was followed by parietal and frontal activity at 180 and 200 ms, respectively. The latter activations, however, were specific to attentional engagement and motor responding. This suggests that parietal activation reflects active responding to a spatial sound rather than auditory spatial processing as such.

Acoustic Stimulation↗

PubMed2005

Disentangling the effects of phonation and articulation: hemispheric asymmetries in the auditory N1m response of the human brain.

BACKGROUND: The cortical activity underlying the perception of vowel identity has typically been addressed by manipulating the first and second formant frequency (F1 & F2) of the speech stimuli. These two values, originating from articulation, are already sufficient for the phonetic characterization of vowel category. In the present study, we investigated how the spectral cues caused by articulation are reflected in cortical speech processing when combined with phonation, the other major part of speech production manifested as the fundamental frequency (F0) and its harmonic integer multiples. To study the combined effects of articulation and phonation we presented vowels with either high (/a/) or low (/u/) formant frequencies which were driven by three different types of excitation: a natural periodic pulseform reflecting the vibration of the vocal folds, an aperiodic noise excitation, or a tonal waveform. The auditory N1m response was recorded with whole-head magnetoencephalography (MEG) from ten human subjects in order to resolve whether brain events reflecting articulation and phonation are specific to the left or right hemisphere of the human brain. RESULTS: The N1m responses for the six stimulus types displayed a considerable dynamic range of 115-135 ms, and were elicited faster (approximately 10 ms) by the high-formant /a/ than by the low-formant /u/, indicating an effect of articulation. While excitation type had no effect on the latency of the right-hemispheric N1m, the left-hemispheric N1m elicited by the tonally excited /a/ was some 10 ms earlier than that elicited by the periodic and the aperiodic excitation. The amplitude of the N1m in both hemispheres was systematically stronger to stimulation with natural periodic excitation. Also, stimulus type had a marked (up to 7 mm) effect on the source location of the N1m, with periodic excitation resulting in more anterior sources than aperiodic and tonal excitation. CONCLUSION: The auditory brain areas of the two hemispheres exhibit differential tuning to natural speech signals, observable already in the passive recording condition. The variations in the latency and strength of the auditory N1m response can be traced back to the spectral structure of the stimuli. More specifically, the combined effects of the harmonic comb structure originating from the natural voice excitation caused by the fluctuating vocal folds and the location of the formant frequencies originating from the vocal tract leads to asymmetric behaviour of the left and right hemisphere.

Acoustic Stimulation↗

PubMed2005

The use of stationarity and nonstationarity in the detection and analysis of neural oscillations.

Using available signal (i.e., spectral and time-frequency) analysis methods, it can be difficult to detect neural oscillations because of their continuously changing properties (i.e., nonstationarities) and the noise in which they are embedded. Here, we introduce fractally scaled envelope modulation (FSEM) estimation which is sensitive specifically to the changing properties of oscillatory activity. FSEM utilizes the fractal characteristic of wavelet transforms to produce a compact, two-dimensional representation of time series data where signal components at each frequency are made directly comparable according to the spectral distribution of their envelope modulations. This allows the straightforward identification of neural oscillations and other signal components with an envelope structure different from noise. For stable oscillations, we demonstrate how partition-referenced spectral estimation (PRSE) removes the noise slope from spectral estimates, yielding a level estimate where only peaks signifying the presence of oscillatory activity remain. The functionality of these methods is demonstrated with simulations and by analyzing MEG data from human auditory brain areas. FSEM uncovered oscillations in the 9- to 12-Hz and 15- to 18-Hz ranges whereas traditional spectral estimates were able to detect oscillations only in the former range. FSEM further showed that the oscillations exhibited envelope modulations spanning 3-7 s. Thus, FSEM effectively reveals oscillations undetectable with spectral estimates and allows the use of EEG and MEG for studying cognitive processes when the common approach of stimulus time-locked averaging of the measured signal is unfeasible.

Acoustic Stimulation↗

PubMed2005

Averaged and single-trial brain responses in the assessment of human sound detection.

We investigated sound detection in humans with magnetoencephalography and behavioural measurements. Sounds with intensity increasing smoothly over 125-1000 ms elicited a transient response in auditory cortex with a peak latency in the 200-600 ms range. Importantly, peak latency accurately predicted behavioural reaction time and was unaffected by attentional engagement. Peak amplitude was augmented when the study participants attended to the stimuli and when stimulus duration was decreased. For investigating the cause of these amplitude variations in the averaged response we designed a wavelet-based method for analysing single-trial responses. We found that attention affects the amplitude of the single-trial responses whereas the intensity slope of the stimulus modifies their latency distribution. The transient response reported here holds promise for rapid, objective hearing assessment not requiring a behavioural task.

Attention↗

PubMed2005

Left-hemispheric brain activity reflects formant transitions in speech sounds.

Connected speech is characterized by formant transitions whereby formant frequencies change over time. Here, using magneto-encephalography, we investigated the cortical activity in 10 participants in response to constant-formant vowels and diphthongs with formant transitions. All the stimuli elicited prominent auditory N100m responses, but the formant transitions resulted in latency modulations specific to the left hemisphere. Following the elicitation of the N100m, cortical activity shifted some 10 mm towards anterior brain areas. This late activity resembled the N400m, typically obtained with more complex utterances such as words and/or sentences. Thus, the present study demonstrates how magnetoencephalography can be used to investigate the spatiotemporal evolution in cortical activity related to the various stages of the processing of speech.

Acoustic Stimulation↗

PubMed2005

Spatial processing in human auditory cortex: the effects of 3D, ITD, and ILD stimulation techniques.

Here, the perception of auditory spatial information as indexed by behavioral measures is linked to brain dynamics as reflected by the N1m response recorded with whole-head magnetoencephalography (MEG). Broadband noise stimuli with realistic spatial cues corresponding to eight direction angles in the horizontal plane were constructed via custom-made, individualized binaural recordings (BAR) and generic head-related transfer functions (HRTF). For comparison purposes, stimuli with impoverished acoustical cues were created via interaural time and level differences (ITDs and ILDs) and their combinations. MEG recordings in ten subjects revealed that the amplitude and the latency of the N1m exhibits directional tuning to sound location, with the amplitude of the right-hemispheric N1m being particularly sensitive to the amount of spatial cues in the stimuli. The BAR, HRTF, and combined ITD + ILD stimuli resulted both in a larger dynamic range and in a more systematic distribution of the N1m amplitude across stimulus angle than did the ITD or ILD stimuli alone. Further, the right-hemispheric source loci of the N1m responses for the BAR and HRTF stimuli were anterior to those for the ITD and ILD stimuli. In behavioral tests, we measured the ability of the subjects to localize BAR and HRTF stimuli in terms of azimuthal error and front-back confusions. We found that behavioral performance correlated positively with the amplitude of the N1m. Thus, the activity taking place already in the auditory cortex predicts behavioral sound detection of spatial stimuli, and the amount of spatial cues embedded in the signal are reflected in the activity of this brain area.

Acoustic Stimulation↗

PubMed2004

Auditory event-related responses are generated independently of ongoing brain activity.

For researchers and clinical practitioners alike, evoked and event-related responses measured with MEG and EEG provide the means for studying human brain function and dysfunction. However, the generation mechanism of event-related responses remains unclear, hindering our ability to formulate viable theories of neural information processing. Event-related responses are assumed to be generated either (1) separately of ongoing, oscillatory brain activity or (2) through stimulus-induced reorganization of ongoing activity. Here, we approached this issue through examining single-trial auditory MEG data in humans. We demonstrate that phase coherence over trials observed with commonly used signal decomposition methods (e.g., wavelets) can result from both a phase-coherent state of ongoing oscillations and from the presence of a phase-coherent event-related response which is additive to ongoing oscillations. To avoid this problem, we introduce a method based on amplitude variance to establish the relationship between ongoing oscillations and event-related responses. We found that auditory stimuli do not give rise to phase reorganization of ongoing activity. Further, increases in spectral power accompany the emergence of event-related responses, and the relationship between spectral power and the amplitude of these responses can be accounted for by a linear summation of the event-related response and ongoing oscillation with a stochastically distributed phase. Thus, on the basis of our observations, auditory event-related responses are unique descriptors of neural information processing in humans, generated by processes separate from and additive to ongoing brain activity.

Acoustic Stimulation↗

PubMed2004

Human auditory event-related processes in the time-frequency plane.

Sensory stimuli produce phase-locked and non-phase-locked changes in brain activity as indexed by EEG and MEG. Time-frequency methods such as wavelets, when carefully applied, allow simultaneous analysis of both types of activity. Here we used wavelets of different time and frequency resolutions in combination with spatial mapping to identify these processes. We found that auditory stimulation leads to a pattern of large-magnitude power increases and small-magnitude power decreases. The power increases, ranging from the theta to the beta frequency band, were accounted for by the transient auditory responses P50m, N100m and P200m. Following these responses, we observed a power reduction of non-phase-locked activity which occurred 250-500 ms after stimulus onset in the 14-24 Hz frequency range and could be localized to the temporal and parietal brain areas. These results indicate that auditory event-related processes consist not only of the well-established transient responses but also of power reductions of ongoing, non-phase-locked brain processes.

Acoustic Stimulation↗

PubMed2004

Glides in speech fundamental frequency are reflected in the auditory N1m response.

The cortical dynamics underlying the perception of constant and gliding speech fundamental frequency (F0) was investigated in 10 subjects using magnetoencephalography (MEG). The stimuli comprised vowels having either constant, ascending or descending F0s and tones of corresponding frequencies, matched with the vowels in intensity or loudness. The amplitude of the N1m response was highly sensitive to F0 variation embedded in vowels and insensitive to corresponding variation in tones. The latency of the N1m elicited by the tones with respect to vowels was significantly delayed. Thus, the speech-specific behavior of the N1m arises out of cortical sensitivity to the acoustic structure of voiced speech, that is to the F0 and its harmonics, which underlie the perception of pitch and intonation in speech.

Acoustic Stimulation↗

PubMed2004

Human posterior auditory cortex gates novel sounds to consciousness.

Life or death in hostile environments depends crucially on one's ability to detect and gate novel sounds to awareness, such as that of a twig cracking under the paw of a stalking predator in a noisy jungle. Two distinct auditory cortex processes have been thought to underlie this phenomenon: (i) attenuation of the so-called N1 response with repeated stimulation and (ii) elicitation of a mismatch negativity response (MMN) by changes in repetitive aspects of auditory stimulation. This division has been based on previous studies suggesting that, unlike for the N1, repetitive "standard" stimuli preceding a physically different "novel" stimulus constitute a prerequisite to MMN elicitation, and that the source loci of MMN and N1 are different. Contradicting these findings, our combined electromagnetic, hemodynamic, and psychophysical data indicate that the MMN is generated as a result of differential adaptation of anterior and posterior auditory cortex N1 sources by preceding auditory stimulation. Early ( approximately 85 ms) neural activity within posterior auditory cortex is adapted as sound novelty decreases. This alters the center of gravity of electromagnetic N1 source activity, creating an illusory difference between N1 and MMN source loci when estimated by using equivalent current dipole fits. Further, our electroencephalography data show a robust MMN after a single standard event when the interval between two consecutive novel sounds is kept invariant. Our converging findings suggest that transient adaptation of feature-specific neurons within human posterior auditory cortex filters superfluous sounds from entering one's awareness.

Adult↗

PubMed2004

Transient brain responses predict the temporal dynamics of sound detection in humans.

The neural events leading up to the conscious experience of stimulus events have remained elusive. Here we describe stimulation conditions under which activation in human auditory cortex can be used to predict the temporal dynamics of behavioral sound detection. Subjects were presented with auditory stimuli whose energy smoothly increased from a silent to a clearly audible level over either 1, 1.5, or 2 s. Magnetoencephalographic (MEG) recordings were carried out in the passive and active recording conditions. In the active condition, the subjects were instructed to attend to the auditory stimuli and to press a response key when these became audible. In both conditions, the stimuli elicited a prominent transient response whose emergence is unexplainable by changes in stimulus intensity alone. This transient response was larger in amplitude over the right hemisphere and in the active condition. Importantly, behavioral sound detection followed this brain activation with a constant delay of 180 ms, and further the latency variations of the brain response were directly carried over to behavioral reaction times. Thus, noninvasively measured transient events in the human auditory cortex can be used to predict accurately the temporal course of sound detection and may therefore turn out to be useful in clinical settings.

Adult↗

PubMed2003

The auditory N1m reveals the left-hemispheric representation of vowel identity in humans.

The cortical correlates of the perception of the sustained vowels /a/, /o/ and /u/ were studied by using whole-head magnetoencephalography (MEG). The three vowels which were located on a line in the space spanned by the first (F1) and second (F2) formants and having equal F2-F1 differences evoked equally strong auditory N1m responses at 120 ms after stimulus onset. The left-hemispheric distribution of the source locations, estimated by equivalent current dipoles, reflected the acoustic similarity of the vowels: the growing distance of the vowels in the F2,F1-space was accompanied by a growing distance between the centres of gravity of activation elicited by each vowel. Thus, direct evidence for the orderly left-hemispheric representation of phonemes in human auditory cortex was found.

Adult↗

PubMed2003

Visual short-term memory load affects sensory processing of irrelevant sounds in human auditory cortex.

We used whole-head magnetoencephalography (MEG) to investigate neural activity in human auditory cortex elicited by irrelevant tones while the subjects were engaged in a short-term memory task presented in the visual modality. As compared to a no-memory-task condition, memory load enhanced the amplitude of the auditory N1m response. In addition, the N1m amplitude depended on the phase of the memory task, with larger response amplitudes observed during encoding than retention. Further, these amplitude modulations were accompanied by anterior-posterior shifts in N1m source locations. The results show that a memory task for visually presented stimuli alters sensory processing in human auditory cortex, even when subjects are explicitly instructed to ignore any auditory stimuli. Thus, it appears that task demands requiring attentional allocation and short-term memory result in interaction across visual and auditory brain areas carrying out the processing of stimulus features.

Acoustic Stimulation↗

PubMed2002

Aging and cholinergic modulation of the transient magnetic 40-Hz auditory response.

Cholinergic blockade by scopolamine, a central muscarinic receptor antagonist, may produce transient memory impairment in healthy subjects, and it has been used as a neurochemical model of cognitive degeneration in aged individuals. To observe the muscarinic modulation of memory and cortical auditory processing, nine cognitively intact elderly subjects (59-80 years) were studied using neuropsychological tests and 122-channel magnetoencephalography (MEG) after an administration of scopolamine hydrobromide (0.3 mg, i.v.) or glycopyrrolate (0.2 mg, i.v.), a peripheral muscarinic antagonist. A double-blind randomized crossover design was used in two sessions separated by at least 1 week. Scopolamine, but not glycopyrrolate, produced a transient impairment of verbal memory performance in the elderly subjects. MEG indicated that the auditory-evoked 40-Hz magnetic response was significantly larger after scopolamine than after glycopyrrolate administration. Furthermore, reanalysis of our earlier results in younger subjects (20-31 years), basically supporting the present MEG findings, tentatively suggests that the scopolamine effects on the 40-Hz response may be slightly pronounced with aging. In sum, the transient magnetic 40-Hz auditory response may be useful in studies on brain cholinergic deficits in elderly subjects.

Aged↗

PubMed2002

Human cortical dynamics determined by speech fundamental frequency.

Evidence for speech-specific brain processes has been searched for through the manipulation of formant frequencies which mediate phonetic content and which are, in evolutionary terms, relatively "new" aspects of speech. Here we used whole-head magnetoencephalography and advanced stimulus reproduction methodology to examine the contribution of the fundamental frequency F0 and its harmonic integer multiples in cortical processing. The subjects were presented with a vowel, a frequency-matched counterpart of the vowel lacking in phonetic contents, and a pure tone. The F0 of the stimuli was set at that of a typical male (i.e., 100 Hz), female (200 Hz), or infant (270 Hz) speaker. We found that speech sounds, both with and without phonetic content, elicited the N1m response in human auditory cortex at a constant latency of 120 ms, whereas pure tones matching the speech sounds in frequency, intensity, and duration gave rise to N1m responses whose latency varied between 120 and 160 ms. Thus, it seems that the fundamental frequency F0 and its harmonics determine the temporal dynamics of speech processing in human auditory cortex and that speech specificity arises out of cortical sensitivity to the complex acoustic structure determined by the human sound production apparatus.

Adult↗

PubMed2002

Cortical processing of speech sounds and their analogues in a spatial auditory environment.

We used magnetoencephalographic (MEG) measurements to study how speech sounds presented in a realistic spatial sound environment are processed in human cortex. A spatial sound environment was created by utilizing head-related transfer functions (HRTFs), and using a vowel, a pseudo-vowel, and a wide-band noise burst as stimuli. The behaviour of the most prominent auditory response, the cortically generated N1m, was investigated above the left and right hemisphere. We found that the N1m responses elicited by the vowel and by the pseudo-vowel were much larger in amplitude than those evoked by the noise burst. Corroborating previous observations, we also found that cortical activity reflecting the processing of spatial sound was more pronounced in the right than in the left hemisphere for all of the stimulus types and that both hemispheres exhibited contralateral tuning to sound direction.

Acoustic Stimulation↗