fMRI - current topics
The central question common to all investigations is: at what stage in the auditory pathway is the purely sensory representation of acoustic stimuli transformed into the perceptual code, which in the end is the basis for what we really hear?
Brain activation in response to low-frequency sound and infrasound
Infrasound in the human environment may be generated by large industrial plants or wind farms, and there is an increasing concern about possible negative effects on well-being and health. However, the perception mechanisms for infrasound stimuli in the human auditory system are not completely understood yet. The aim of our work is a detailed analysis of the interrelation of psychoacoustic results (detection thresholds, loudness scaling, and annoyance ratings), questionnaire responses characterising the individual sensitivity to very low- frequency sound, and corresponding brain activation maps from functional MRI.
This work is part of the joint research project 15HLT03 Ears II, funded by the EU within the European Metrology Programme for Innovation and Research (EMPIR). Coordinator is PTB Braunschweig.
Loudness and loudness integration
Loudness is the perceptual correlate of the intensity of a sound. It is still not completely understood how the main physical signal parameters sound level, temporal structure and spectral contents are transformed into the perceptual measures like loudness or audibility of a signal in a masking background. We investigate effects of spectral and temporal loudness summation, with respect to the question, at what stage in the auditory system the formation of the loudness percept is completed.
Periodicity and pitch
Pitch is the perceptual correlate of the periodicity of a sound, within the frequency range from about 30 Hz to 6 kHz. We investigate the representation of pitch and changes of pitch, and the audibility of a periodic sound in a masking noise. The central idea is that at some stage in cortex, the pitch percept is universally represented, irrespective of the specific features that are the basis of its formation.
Neuropsychological and brain imaging studies indicate a left hemispherical preference for intelligible speech in most listeners. However, the reason of this specialisation, the starting point of hemispheric preference, and the exact nature of the hierarchy of listening to spoken language, from the processing of basic speech sounds as in the previous studies to the comprehension of complex sentences, are still not completely understood. We investigate the representation of spoken language in auditory cortex, with stimuli ranging from simple isolated speech sounds to complete sentences of various complexity.
General aspects of auditory fMRI
There are several problems with fMRI for auditory experiments. Switching of the gradient coils in the strong external magnetic field results in very loud noises, caused by mechanical vibrations due to Lorentz forces acting on the coils. Some precautions have to be introduced; first is to avoid hearing damage, and second is to ensure that the recorded brain activity is caused by the actual experimental stimulus, and not by the scanner noise itself. There are now MR compatible, high-fidelity headphones available that have been fitted into conventional ear defenders. These give an attenuation of external noise between 15 and 40 dB. Additional damping of scanner noise can be achieved passively by sound absorbing material around the subject’s head and inside the walls of the scanner bore, but also actively by means of active noise cancellation (ANC) to compensate for the scanner noise.
This lack of immediate responses can be turned into an advantage for auditory fMRI by introduction of the paradigm of “sparse temporal sampling”. It allows for a separation of the brain activation due to scanner noise from the one due to the actual stimulus of interest. Instead of continuous data acquisition during epochs of stimulus and epochs of rest, stimulation and data acquisition are separated in time, with periods of several seconds of stimulus presentation without any scanning.