ABSTRACT
Combined magnetoencephalography and positron emission tomography identified a prior source of activity in the left middle frontal gyrus during uncued movements of the right index finger. Voluntary movements gave rise to a change in the cortical electrical potential known as the Bereitschaftspotential or Readiness Potential, recorded as early as 1500 ms before the onset of movement. The Readiness Field is the magnetic field counterpart to the Bereitschaftspotential. In the present study, magnetoencephalography identified four successively active sources of fluctuation in the Readiness Field in the period from 900 ms before, to 100 ms after, the onset of the movement. The first source to be active was registered between 900 and 200 ms prior to the onset of the movement. This source of initial activity was mapped by positron emission tomography to the middle frontal gyrus, Brodmann area 9. The three sources subsequently to be active were mapped to the supplementary motor area, premotor cortex, and motor cortex (M1), all in the left hemisphere.
Subject(s)
Brain Mapping , Contingent Negative Variation/physiology , Frontal Lobe/physiology , Magnetoencephalography , Motor Activity/physiology , Tomography, Emission-Computed , Adult , Dominance, Cerebral/physiology , Female , Humans , Male , Middle Aged , Motor Cortex/physiologyABSTRACT
Auditory event related magnetic fields were measured using an odd-ball paradigm in which the rare event was a tone of short duration, D2, and the frequent one a tone of longer duration, D1. The subjects were required to attend to and count the number of rare stimuli. In the average across target stimuli a mismatch field (MMF) occurs and the dependence of the MMF, especially its latency, on the tone duration D2 is examined in detail. The location of an equivalent current dipole for the MMF-source is found and turns out to be at variance with earlier results. In addition to the MMF we propose a new component, here called M2", which in time overlaps the magnetic equivalent of the P200 signal and which has a source location (equivalent current dipole) lying rather close to the MMF-source. The two sources are, however, active at latencies differing by a time equal to D2. We speculate that M2" indicates the onset of the process: "evaluation of tone-duration" while the MMF indicates the end of this process.
Subject(s)
Attention/physiology , Contingent Negative Variation/physiology , Evoked Potentials, Auditory/physiology , Magnetoencephalography , Pitch Discrimination/physiology , Time Perception/physiology , Acoustic Stimulation , Brain Mapping , Cerebral Cortex/physiology , Dominance, Cerebral/physiology , Humans , Reaction Time/physiology , Reference Values , Signal Processing, Computer-AssistedABSTRACT
The intensity dependence of the 100 msec magnetic field signal evoked by contralateral application of a tone burst stimulus has been examined for both hemispheres and for a number of frequencies. In all cases the component of the magnetic field normal to the skull was measured; in some cases this component was oriented in the outward direction (group 1 and some group 2 subjects), in the other cases in the inward direction (group 2). The experimental results were analysed in terms of an equivalent current dipole model. The analysis gave rise to the introduction of a transit time (tau 0 approximately 60 msec) from the onset of the stimulus to the activation of the current dipole and to the introduction of a functional relationship between the dipole moment and a 'reduced' intensity, and between the latency and the 'reduced' intensity. Further, the reproducibility of the signal was verified.
