Spatio-temporal cortical patterns evoked in man by a discrimination task

Magnetoencephalographic (MEG) recordings were obtained from a 37 sensor array during performance of a discrimination task. Pairs of stimuli were presented to the somatosensory and auditory systems which fell into two classes. Subjects were instructed to respond to one class and not the other. A “virtual electrode” estimation procedure was applied to generate time varying waveforms from locations within the brain as if from direct electrodes. Multivariate statistical procedures were then applied to the single trial MEG and virtual electrode estimates. This produces strong statistical inferences from the data from each subject, produces functional timing and localization for each subject, and demonstrates clear differences between subjects. It is an integrated experimental/analytic paradigm which is applicable to the study of perception, cognition, affect, and a wide variety of psychopathologies. minant analysis was applied to the sampled single trial MEG data to (1) determine whether information was extracted and (2) to produce a discriminant function by which the 37 channel data stream can be collapsed to a single data stream to produce functional timing information. For all 5 subjects, these results conform to the following model for the sequence of activities required to perform the discrimination task: 1. imary sensory processing. scrimination between stimulus classes, this being the imperative function upon which correct task performance depends. sponse preparation. tor activation. minant analysis was applied to the sampled single trial virtual electrode estimates to (1) determine whether information was extracted and (2) to produce functional localization information by identifying virtual electrode sites which are differentially activated under different experimental conditions. For sensory modality, the parietal cortex was the primary site of differential activation at 36 msec post stimulus. At 54 msec, differential activation had spread anteriorly along the cortical convexity and at 95 msec had spread to the medial cortical surface. For stimulus class, differential activation was seen in the cerebellum and in the parietal, parieto temporal, premotor and anterior temporal cortices. For response type (GO/NOGO), differential activation was seen primarily in the parietal and premotor cortices. trial classification accuracies from raw MEG data produced significant p-values for all subjects for sensory modality (p < 10−100) and for 4 out of 5 subjects for stimulus class and trial type (p < 10−4). Single trial classification accuracies from virtual electrode estimates produced significant p-values for all subjects for sensory modality (p < 10−60) and for 4 out of 5 subjects for stimulus class (p < 10−4) and trial type (p < 10−6). Sensory modality classification accuracy was significant for 4 out of 5 subjects for the time epoch 400 msec following stimulus presentation (p < 10−5) and for 1 subject for the time period 1.8 seconds following stimulus presentation (p < 10−12).