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We investigated the process of amodal completion in a same-different experiment in which test pairs were preceded by sequences of two figures. The first of these could be congruent to a global or local completion of an occluded part in the second figure, or a mosaic interpretation of it. We recorded and analyzed the magnetoencephalogram for the second figures. Compared to control conditions, in which unrelated primes were shown, occlusion and mosaic primes reduced the peak latency and amplitude of neural activity evoked by the occlusion patterns. Compared to occlusion primes, mosaic ones reduced the latency but increased the amplitude of evoked neural activity. Processes relating to a mosaic interpretation of the occlusion pattern, therefore, can dominate in an early stage of visual processing. The results did not provide evidence for the presence of a functional ‘stage’ in completion per se, but characterize the mosaic interpretation as a qualitatively special one that can rapidly emerge in visual processing when context favors it. [ABSTRACT FROM AUTHOR]

With sufficiently fast data sampling, ubiquitous sharp transients appear in magnetoencephalography (MEG) data. Initially, no known collective neuronal activity could explain MEG signal generation well above 100 Hz, so it was assumed that these transients were entirely composed of background electronic noise that could be eliminated by filtering and averaging. Recent studies at the cellular level provided evidence for synchronous synaptic input to dendrites and volleys of near-simultaneous action potentials. MEG studies have also identified high-frequency oscillations well above 200 Hz after averaging large number of somatosensory evoked responses. In this study, we searched for evidence of high-frequency neuronal activity in the raw MEG signal using the highest sampling rate available with our hardware. Two human subjects participated in three experiments using visual cues to define planning, preparation, and execution or inhibition of saccades. Tomographic analysis identified "MEG spikes" that were widely distributed across the cortex, cerebellum, and brainstem during cue presentations and saccades. Here we demonstrate how these MEG spikes can be recorded and localized in real time and show that task demands influence their properties. The MEG spikes were organized into feedforward and corollary discharge sequences that could, when combined with the slower activity-linked processing in discrete brain areas over long periods, lasting hundreds of milliseconds. Preparation for impending saccade began as soon as relevant information became available. Cues providing partial information initiated competing motor programs for as yet undecided future actions that were maintained until cues with new information resolved the uncertainty. [ABSTRACT FROM AUTHOR]

We trained four right-handed male subjects to detect small changes in the frequency of 21 Hz electrical stimulation applied to digits 2+3+4 of the right hand for 4 hours. Before and after the training, we recorded magnetoencephalographic (MEG) signals when the stimulation was applied to digit 2-5 separately using a whole-head MEG system. We applied tomographic analysis to the MEG data to track the evolution of activity over the entire brain every 1.6 ms and evaluated the change for each digit before and after the training. In summary: (1) subjects improved in discrimination performance with training; (2) activations were identified in the left primary somatosensory (L-SI) and medial parietal precuneus (PCu) areas, but no systematic changes were observed in location and strength of activation of these two areas; (3) after training in L-SI, the 21 Hz spectral power increased for digits 3 and 4 over the stimulation period, while the 10 Hz spectral power increased for digit 3 around stimulus onset and offset; (4) only digit 3 showed significant change of correlation between L-SI and PCu areas around the stimulus onset and offset, coincident with the increased 10 Hz spectral power. Our results suggest that short-term plasticity is associated with changes in timing and interaction between cortical areas [ABSTRACT FROM AUTHOR]