The neuromagnetic response to spoken sentences: co-modulation of theta band amplitude and phase.

Speech elicits a phase-locked response in the auditory cortex that is dominated by theta (3-7 Hz) frequencies when observed via magnetoencephalography (MEG). This phase-locked response is potentially explained as new phase-locked activity superimposed on the ongoing theta oscillation or, alternatively, as phase-resetting of the ongoing oscillation. The conventional method used to distinguish between the two hypotheses is the comparison of post- to prestimulus amplitude for the phase-locked frequency across a set of trials.

Discrimination of speech stimuli based on neuronal response phase patterns depends on acoustics but not comprehension.

Speech stimuli give rise to neural activity in the listener that can be observed as waveforms using magnetoencephalography. Although waveforms vary greatly from trial to trial due to activity unrelated to the stimulus, it has been demonstrated that spoken sentences can be discriminated based on theta-band (3-7 Hz) phase patterns in single-trial response waveforms. Furthermore, manipulations of the speech signal envelope and fine structure that reduced intelligibility were found to produce correlated reductions in discrimination performance, suggesting a relationship between theta-band phase patterns and speech comprehension.

Hemispheric asymmetry in mid and long latency neuromagnetic responses to single clicks.

We examine lateralization in the evoked magnetic field response to a click stimulus, observing that lateralization effects previously demonstrated for tones, noise, frequency modulated sweeps and certain syllables are also observed for (acoustically simpler) clicks. These effects include a difference in the peak latency of the M100 component of the evoked field waveform such that the peak consistently appears earlier in the right hemisphere, as well as rightward lateralization of field amplitude during the rise of the M100 component. Our review of previous findings on M100 lateralization, taken together with our data on the click-evoked response, leads to the hypothesis that these lateralization effects are elicited by stimuli containing a sharp sound energy onset or acoustic transition rather than specific types of stimuli.

A theory of the visual system biology underlying development of spatial frequency lateralization.

The spatial frequency hypothesis contends that performance differences between the hemispheres on various visuospatial tasks are attributable to lateralized processing of the spatial frequency content of visual stimuli. Hellige has proposed that such lateralization could arise during infant development from the earlier maturation of the right hemisphere combined with the increasing sensitivity of the visual system to high spatial frequencies. This proposal is intuitively appealing but lacks an explicit theory with respect to the underlying visual system biology.

The effects of multi-task learning and time-varying hemispheric asymmetry on lateralisation in a neural network model.

While various functional and cognitive capabilities appear to differ in both degree and direction of lateralisation, the factors underlying these differences are poorly understood. It is hypothesised that time-varying asymmetry in plasticity between homologous regions in the cerebral hemispheres, coupled with asynchronous development of capabilities, may account for the lateralisation differences in two ways. First, the lateralisation of an earlier acquired behaviour may influence the lateralisation of a later developing behaviour.