Biomedical subjects

Karsten Specht

Publications and source records attributed to Karsten Specht.

22 records · Page 2Linked to original sources

PubMed2003

Functional segregation of the temporal lobes into highly differentiated subsystems for auditory perception: an auditory rapid event-related fMRI-task.

With this study, we explored the blood oxygen level-dependent responses within the temporal lobe to short auditory stimuli of different classes. To address this issue, we performed an attentive listening event-related fMRI study, where subjects were required to concentrate during the presentation of different types of stimuli. Because the order of stimuli was randomized and not predictable for the subject, the observed differences between the stimuli types were interpreted as an automatic effect and were not affected by attention. We used three types of stimuli: tones, sounds of animals and instruments, and words. We found in all cases bilateral activations of the primary and secondary auditory cortex. The strength and lateralization depended on the type of stimulus. The tone trials led to the weakest and smallest activations. The perception of sounds increased the activated network bilaterally into the superior temporal sulcus mainly on the right and the perception of words led to the highest activation within the left superior temporal sulcus as well as in left inferior frontal gyrus. Within the left temporal sulcus, we were able to distinguish between different subsystems, showing an extending activation from posterior to anterior for speech and speechlike information. Whereas posterior parts were involved in analyzing the complex auditory structure of sounds and speech, the middle and anterior parts responded strongest only in the perception of speech. In summary, a functional segregation of the temporal lobes into several subsystems responsible for auditory processing was visible. A lateralization for verbal stimuli to the left and sounds to the right was already detectable when short stimuli were used.

Acoustic Stimulation↗

PubMed2003

Focused attention in a simple dichotic listening task: an fMRI experiment.

Whole-head functional magnetic resonance imaging (fMRI) was used in nine neurologically intact subjects to measure the hemodynamic responses in the context of dichotic listening (DL). In order to eliminate the influence of verbal information processing, tones of different frequencies were used as stimuli. Three different dichotic listening tasks were used: the subjects were instructed to either concentrate on the stimuli presented in both ears (DIV), or only in the left (FL) or right (FR) ear and to monitor the auditory input for a specific target tone. When the target tone was detected, the subjects were required to indicate this by pressing a response button. Compared to the resting state, all dichotic listening tasks evoked strong hemodynamic responses within a distributed network comprising of temporal, parietal, and frontal brain areas. Thus, it is clear that dichotic listening makes use of various cognitive functions located within the dorsal and ventral stream of auditory information processing (i.e., the 'what' and 'where' streams). Comparing the three different dichotic listening conditions with each other only revealed a significant difference in the pre-SMA and within the left planum temporale area. The pre-SMA was generally more strongly activated during the DIV condition than during the FR and FL conditions. Within the planum temporale, the strongest activation was found during the FR condition and the weakest during the DIV condition. These findings were taken as evidence that even a simple dichotic listening task such as the one used here, makes use of a distributed neural network comprising of the dorsal and ventral stream of auditory information processing. In addition, these results support the previously made assumption that planum temporale activation is modulated by attentional strategies. Finally, the present findings uncovered that the pre-SMA, which is mostly thought to be involved in higher-order motor control processes, is also involved in cognitive processes operative during dichotic listening.

Acoustic Stimulation↗

PubMed2002

Recovery of semantic word processing in transcortical sensory aphasia: a functional magnetic resonance imaging study.

In a previous functional magnetic resonance imaging (fMRI) study with normal subjects, we demonstrated regions related to conceptual-semantic word processing around the first frontal sulcus (BA 9) and the posterior parietal lobe (BA 7/40) in agreement with several previous reports. We had the possibility, using the same fMRI paradigm, to study two consecutive cases with left middle cerebral artery (MCA) infarction (RC and HP) and lesions affecting either solely the pre-frontal (HP) or both the pre-frontal and posterior parietal part of the network activated in normal subjects (RC). Both patients showed transcortical sensory aphasia (TSA) on acute assessment. This contradicts classical disconnection accounts of the syndrome stating intact conceptual representations in TSA. Their recovery of language comprehension was associated with activation of a left hemispheric network. Mainly activations of left perilesional pre-frontal regions (RC), left Wernicke's area (RC and HP) or the left posterior middle and inferior temporal cortex (HP) were demonstrated in the TSA patients. The latter findings suggest that in our cases of TSA functional take-over has occurred in regions with related functions ('redundancy recovery') rather than in previously unrelated areas ('vicarious functioning'). Our data support distributed models of conceptual-semantic word processing and multiple left hemispheric representations of closely related functions.

Adult↗

PubMedDate not supplied

"Soundmorphing": a new approach to studying speech perception in humans.

A problem in current studies of brain activation in speech perception is that most studies use stimuli that are sampled from different categories. This study presents a new approach in creating acoustic stimuli by filtering real words in different ways, thus this enables one to 'morph' the sounds gradually from something like tones to real words while preserving temporal characteristics ("soundmorphing"). This will make it possible to explore steps between tone and speech processing on a gradual scale. This study aimed to investigate more closely the network, involved in the perception of speech and the decoding of auditory (speech-)stimuli. Sets of auditory stimuli were created by dividing real words into several frequency-bands and creating a randomised rearrangement of these components. Four different sets of stimuli were used, containing 1, 2, 3 or 4 frequency bands (total range: 0-2500 Hz). Only the latter one contained intelligible words. During the fMRI session, these four trial types were presented pseudo-randomised. In all conditions, significant activations of auditory cortex were observed. The number of activated voxels and their significance increased over the four trial types. In addition, Broca's area, SMA, left thalamus, and right cerebellum were activated in the most complex and most speech-like condition. Comparing these complexes with the tone-like trials, only the left auditory cortex, left SMA, and cingulate gyrus became significant. These activations reflect the bilateral processing of the auditory stimuli, but only the left temporal areas demonstrated an increasing BOLD response with an increasing number of presented frequency components.

Acoustic Stimulation↗