Asymmetrical hippocampal connectivity in mesial temporal lobe epilepsy: evidence from resting state fMRI

Background

Recent studies have shown that the human right-hemispheric auditory cortex is particularly sensitive to reduction in sound quality, with an increase in distortion resulting in an amplification of the auditory N1m response measured in the magnetoencephalography (MEG). Here, we examined whether this sensitivity is specific to the processing of acoustic properties of speech or whether it can be observed also in the processing of sounds with a simple spectral structure. We degraded speech stimuli (vowel /a/), complex non-speech stimuli (a composite of five sinusoidals), and sinusoidal tones by decreasing the amplitude resolution of the signal waveform. The amplitude resolution was impoverished by reducing the number of bits to represent the signal samples. Auditory evoked magnetic fields (AEFs) were measured in the left and right hemisphere of sixteen healthy subjects.

Results

We found that the AEF amplitudes increased significantly with stimulus distortion for all stimulus types, which indicates that the right-hemispheric N1m sensitivity is not related exclusively to degradation of acoustic properties of speech. In addition, the P1m and P2m responses were amplified with increasing distortion similarly in both hemispheres. The AEF latencies were not systematically affected by the distortion.

Conclusions

We propose that the increased activity of AEFs reflects cortical processing of acoustic properties common to both speech and non-speech stimuli. More specifically, the enhancement is most likely caused by spectral changes brought about by the decrease of amplitude resolution, in particular the introduction of periodic, signal-dependent distortion to the original sound. Converging evidence suggests that the observed AEF amplification could reflect cortical sensitivity to periodic sounds.     

The occurrence of periodic distortions in the extrusion of polymeric melts

In the extrusion of polymeric melts at high flow rates, often flow instabilities observed as surface distortions of the extrudate occur. The flow instability ‘spurt’ in piston-driven flows is accompanied by persistent oscillations in the pressure. In this paper spurt is explained in terms of constitutive instabilities (mechanical failure of the polymeric fluid itself), while the no-slip boundary condition at the wall of the die is maintained. The influence of compression on the onset of spurt is investigated. The polymeric melt is modelled as a JSO-fluid, but two other constitutive models are also considered. Numerical computations disclose that persistent oscillations in the pressure as well as in the volumetric flow rate occur for a bounded range of prescribed plunger speeds. The occurrence of the persistent oscillations is also explained by a linearized stability analysis. The frequency of the persistent oscillations is determined both from the linearized stability theory as well as by means of a Fourier spectral analysis. In conclusion, the theory is validated by a qualitative comparison with experimental results. Received March 3, 1998