Variations in evoked cortical activity also occurred in spatial pattern. Both spontaneous and evoked cortical activity manifested as propagating waves (Fig. 4). Sensory evoked waves followed a consistent propagation pattern, initiating from the location of the thalamic afferents and spreading to large areas of the cortex. In barrel cortex, whisker evoked waves initiated from within the corresponding barrel and propagated to the entire barrel cortex. In images obtained from multiple trial averaging, this general propagating pattern can be nicely seen (Fig. 4, image row AVG). An overall propagating velocity of , ( animals) was obtained. However, when single trial data were examined, the propagating velocity varied largely, ranging from 0 (no propagation) to about . As shown in Fig. 4, large trial-to-trial variations were observed in the propagating patterns. The propagation pattern of individual trials was compared with that of the multiple trial averages. While all trials showed a similar overall spatiotemporal pattern, significant trial-to-trial variations can be seen in individual trials. In the bottom row of Fig. 4, the contour lines of the propagation patterns for trials 5, 6, 8, and 10 were superimposed, in order to compare them with the contours of the average pattern. The contour lines were highly variable, suggesting that the propagation of an evoked wave was highly dynamic. Since spontaneous waves occurred frequently, they may interact with the evoked response and contribute to anisotropic propagation patterns. In trial 5, the evoked activity appeared to merge with a spontaneous event and propagated faster in the direction toward the spontaneous event.