Visualization of ERP Components in Individual Subjects During a Speech-processing Task

The high temporal resolution of event-related potentials (ERPs) make them well suited for visualizing the temporal sequence of neural activity associated with speech processing. The objective of this study was to develop a methodology that could be applied in clinical settings to precisely identify the onset and offset of ERP components and lateralization events in individual subjects as they processed speech stimuli. Subjects were presented prerecorded sequences of the word “2” or “5” and instructed to count the number of times they heard the target stimulus, which occurred 10 percent of the time. ERPs were recorded from eleven monopolar leads referenced to linked earlobes:  Fz, Cz, Pz, four sites near T7-P7 (T3-T5) and four near T8-P8 (T4-T6). EEG was sampled for 36 target stimuli at 100Hz for 950 ms after stimulus onset and then averaged into six blocks of 6 waveforms each. Statistical significance was determined for each sample point using binomial tests on block-averaged data. The criteria employed to decide if a particular voltage was significantly positive or negative at each ERP sample point was that all six block averages were required to have the same sign (p = .032, two-tailed).  The likelihood of a Type II error was reduced by requiring at least two successive-significant points to have the same sign (p = .001). Displaying a time series of the significance-test results revealed many individual differences accompanying the perception, recognition and counting of speech stimuli.  Of special interest were: (1) negative potentials in the left hemisphere terminating simultaneously with the onset of positive potentials in the right, suggesting a transition between language-processing stages and (2) a long-period ERP component, measured from bipolar recordings comparing left and right hemispheres directly, which overlapped in time with short-period ERP components, measured from monopolar sites referenced to linked earlobes. Viewing ERP components with point-by-point significance tests offers a complementary method to fMRI, which has better spatial resolution, for observing the neural processes associated with speech processing in individual subjects.