Alexander Gutschalk

Morphology of Heschl's gyrus reflects enhanced activation in the auditory cortex of musicians

Using magnetoencephalography (MEG), we compared the processing of sinusoidal tones in the auditory cortex of 12 non-musicians, 12 professional musicians and 13 amateur musicians. We found neurophysiological and anatomical differences between groups. In professional musicians as compared to non-musicians, the activity evoked in primary auditory cortex 19–30 ms after stimulus onset was 102% larger, and the gray matter volume of the anteromedial portion of Heschls gyrus was 130% larger. Both quantities were highly correlated with musical aptitude, as measured by psychometric evaluation. These results indicate that both the morphology and neurophysiology of Heschls gyrus have an essential impact on musical aptitude.

Deconvolution of 40 Hz steady-state fields reveals two overlapping source activities of the human auditory cortex

Steady-state auditory evoked fields were recorded from 15 subjects using a whole head MEG system. Stimuli were 800 ms trains of binaural clicks with constant stimulus onset asynchrony (SOA). Seven different SOA settings (19, 21, 23, 25, 27, 29 and 31 ms) were used to give click rates near 40 Hz. Transient responses to each click were reconstructed using a new algorithm that deconvoluted the averaged responses to the different trains. Spatio-temporal multiple dipole modelling in relation to 3D MRI scans revealed two overlapping source components in both the left and right auditory cortex. The primary sources in the medial part of Heschls gyrus exhibited a N19-P30-N40 m pattern. The secondary, weaker sources at more lateral sites on Heschls gyrus showed a N24-P36-N46 m pattern. When applied to transient middle latency auditory evoked fields (MAEFs) recorded at SOAs of 95-135 ms, the primary sources imaged activities similar to the deconvoluted steady-state responses, but the secondary source activities were inconsistent. Linear summation of the deconvoluted source waveforms accounted for more than 96% of the steady-state variance. This indicates that the primary activity of the auditory cortex remains constant up to high stimulation rates and is not specifically enhanced around 40 Hz.

Sustained Magnetic Fields Reveal Separate Sites for Sound Level and Temporal Regularity in Human Auditory Cortex

Magnetoencephalography was used to investigate the relationship between the sustained magnetic field in auditory cortex and the perception of periodic sounds. The response to regular and irregular click trains was measured at three sound intensities. Two separate sources were isolated adjacent to primary auditory cortex: One, located in lateral Heschls gyrus, was particularly sensitive to regularity and largely insensitive to sound level. The second, located just posterior to the first in planum temporale, was particularly sensitive to sound level and largely insensitive to regularity. This double dissociation to the same stimuli indicates that the two sources represent separate mechanisms; the first would appear to be involved with pitch perception and the second with loudness. The delay of the offset of the sustained field was found to increase with interclick interval up to 200 ms at least, which suggests that the sustained field offset represents a sophisticated offset-monitoring mechanism rather than simply the cessation of stimulation.

Neural Correlates of Auditory Perceptual Awareness under Informational Masking

Our ability to detect target sounds in complex acoustic backgrounds is often limited not by the ears resolution, but by the brains information-processing capacity. The neural mechanisms and loci of this “informational masking” are unknown. We combined magnetoencephalography with simultaneous behavioral measures in humans to investigate neural correlates of informational masking and auditory perceptual awareness in the auditory cortex. Cortical responses were sorted according to whether or not target sounds were detected by the listener in a complex, randomly varying multi-tone background known to produce informational masking. Detected target sounds elicited a prominent, long-latency response (50–250 ms), whereas undetected targets did not. In contrast, both detected and undetected targets produced equally robust auditory middle-latency, steady-state responses, presumably from the primary auditory cortex. These findings indicate that neural correlates of auditory awareness in informational masking emerge between early and late stages of processing within the auditory cortex.

Human Cortical Activity during Streaming without Spectral Cues Suggests a General Neural Substrate for Auditory Stream Segregation

The brain continuously disentangles competing sounds, such as two people speaking, and assigns them to distinct streams. Neural mechanisms have been proposed for streaming based on gross spectral differences between sounds, but not for streaming based on other nonspectral features. Here, human listeners were presented with sequences of harmonic complex tones that had identical spectral envelopes, and unresolved spectral fine structure, but one of two fundamental frequencies (f0 ) and pitches. As the f 0 difference between tones increased, listeners perceived the tones as being segregated into two streams (one stream for each f 0) and cortical activity measured with functional magnetic resonance imaging and magnetoencephalography increased. This trend was seen in primary cortex of Heschls gyrus and in surrounding nonprimary areas. The results strongly resemble those for pure tones. Both the present and pure tone results may reflect neuronal forward suppression that diminishes as one or more features of successive sounds become increasingly different. We hypothesize that feature-specific forward suppression subserves streaming based on diverse perceptual cues and results in explicit neural representations for auditory streams within auditory cortex.

Transient bold activity locked to perceptual reversals of auditory streaming in human auditory cortex and inferior colliculus

Our auditory system separates and tracks temporally interleaved sound sources by organizing them into distinct auditory streams. This streaming phenomenon is partly determined by physical stimulus properties but additionally depends on the internal state of the listener. As a consequence, streaming perception is often bistable and reversals between one- and two-stream percepts may occur spontaneously or be induced by a change of the stimulus. Here, we used functional MRI to investigate perceptual reversals in streaming based on interaural time differences (ITD) that produce a lateralized stimulus perception. Listeners were continuously presented with two interleaved streams, which slowly moved apart and together again. This paradigm produced longer intervals between reversals than stationary bistable stimuli but preserved temporal independence between perceptual reversals and physical stimulus transitions. Results showed prominent transient activity synchronized with the perceptual reversals in and around the auditory cortex. Sustained activity in the auditory cortex was observed during intervals where the ΔITD could potentially produce streaming, similar to previous studies. A localizer-based analysis additionally revealed transient activity time locked to perceptual reversals in the inferior colliculus. These data suggest that neural activity associated with streaming reversals is not limited to the thalamo-cortical system but involves early binaural processing in the auditory midbrain, already.

Activity Associated with Stream Segregation in Human Auditory Cortex is Similar for Spatial and Pitch Cues

Streaming is a perceptual mechanism by which the brain segregates information from multiple sound sources in our environment and assigns them to distinct auditory streams. Examples for streaming cues are differences in frequency spectrum, pitch, or space, and potential neural correlates for streaming based on spectral and pitch cues have been identified in the auditory cortex. Here, magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) were used to evaluate if response enhancement in auditory cortex associated with streaming represents a general pattern that is independent of the stimulus cue. Interaural time differences (ITDs) were used as a spatial streaming cue and were compared with streaming based on fundamental frequency (f(0)) differences. The MEG results showed enhancement of the P(1)m after 60-90 ms that was similar during streaming based on ITD and pitch. Sustained fMRI activity was enhanced at identical sites in Heschls gyrus and planum temporale for both cues; no topographical specificity for space or pitch was found for the streaming-associated enhancement. These results support the hypothesis of an early convergence of the neural representation for auditory streams that is independent of the acoustic cue that the streaming is based on.

The representation of peripheral neural activity in the middle-latency evoked field of primary auditory cortex in humans

Short sweeps with increasing instantaneous frequency (up-chirps) designed to compensate for the propagation delay along the human cochlea enhance the magnitude of wave V of the auditory brainstem responses, while time reversed sweeps (down-chirps) reduce the magnitude of wave V [Dau, T., Wegner, O., Mellert, V., Kollmeier, B., J. Acoust. Soc. Am. 107 (2000) 1530-1540]. This effect is due to synchronisation of frequency channels along the basilar membrane and it indicates that cochlear phase delays are preserved up to the input of the inferior colliculus. The present magnetoencephalography study was designed to investigate the influence of peripheral synchronisation on the activation in primary auditory cortex. Spatio-temporal source analysis of middle-latency auditory evoked fields (MAEFs) elicited by clicks and up- and down-chirps showed that up-chirps elicited significantly larger MAEF responses compared to clicks or down-chirps. Both N19m-P30m magnitude and its latency are influenced by peripheral cross-channel phase effects. Furthermore, deconvolution of the empirical source waveforms with spike probability functions simulated with a cochlear model indicated that the source waves for all stimulus conditions could be explained with the same unit-response function, i.e. a far field recorded cortical response of a very small cell assembly along the medio-lateral axis of Heschls gyrus that receives input from a small number of excitatory fibres. The conclusion is that (i) phase delays between channels in the auditory pathway are preserved up to primary auditory cortex, and (ii) MAEFs can be described by a convolution of a unit-response function with the summary neural activity pattern of the auditory nerve.

Local Intra-arterial Fibrinolysis of Thromboemboli Occurring During Neuroendovascular Procedures With Recombinant Tissue Plasminogen Activator

Background and Purpose— There is a lack of systematic data regarding local intra-arterial fibrinolysis (LIF) of thromboemboli occurring during neuroendovascular procedures with the use of recombinant tissue plasminogen activator (rtPA). We report our technique for treating LIF of intracerebral thromboemboli occurring during neuroendovascular procedures. Methods— Nine of 723 patients (1.2%) who underwent neuroendovascular procedures during the period from January 1997 to September 2002 suffered thromboembolic complications. These patients were treated by LIF with a maximum dose of 0.9 mg rtPA per kilogram body weight. Recanalization was categorized as successful (Thrombolysis in Myocardial Infarction [TIMI] grade 2 or 3) versus unsuccessful (TIMI grade 0 or 1), and clinical outcome was categorized as independent (Rankin Scale score 0 to 2) versus dependent or dead (Rankin Scale score 3 to 6). Results— The minimum time between thrombus detection and beginning of LIF was 10 minutes, and the maximum time was 90 minutes. Successful recanalization was achieved in 4 of 9 patients (44%). All 9 patients suffered cerebral ischemic infarctions, and none of the patients sustained intracerebral hemorrhage. Two patients (22%) died from malignant brain infarctions. Four patients (44%) remained moderately disabled, and 3 patients (33%) were severely disabled 3 months after LIF. Conclusions— Although we used relatively high doses of rtPA, the recanalization rates and clinical outcome of LIF in our patients were not satisfactory. Strategies for the prevention of thromboemboli during neuroendovascular procedures must be improved, and novel fibrinolytic or thrombolytic techniques should be developed.

Clozapine-Induced Obsessive-Compulsive Syndromes Improve in Combination With Aripiprazole

Patients with schizophrenia often experience comorbid obsessive-compulsive syndromes (OCSs). Within these patients, a significant subgroup developed secondary OCS during treatment with antiserotonergic, atypical antipsychotic agents such as clozapine. Although cognitive behavioral therapy and antiobsessive antidepressants brought up inconsistent results, in some cases, dose reductions of clozapine in combination approaches were able to alleviate OCS. One suggestive agent for antiobsessive add-on treatment is aripiprazole, a partial agonist at dopamine and serotonin receptors.Here, we summarize the courses of 7 patients (6 men; mean age, 37 years; mean duration of psychotic illness, 17 years). They had been treated with clozapine for 9 years. The distressing and treatment-resistant comorbidity with OCS emerged approximately 4 years after the start of clozapine therapy. During combined treatment with mean doses of 22.9 mg of aripiprazole for 9.7 weeks, we assessed a small yet statistically not significant improvement of the psychotic disorder, whereas a marked reduction of obsessions and significant improvements of compulsions could be observed. The mean total Yale Brown Obsessive Compulsive Rating Scale decreased from 18.7 to 12.4 (P = 0.003).These data support the findings of 2 previous case reports and point toward an antiobsessive potency of aripiprazole. The relevant disabling comorbidities of psychosis and OCS need further investigation with multimodal neurobiological approaches. The proposed strategy should be further evaluated in prospective controlled trials with severity of comorbid OCS as a primary end point.