Spatiotemporal structure of sensory-evoked and spontaneous activity revealed by mesoscale imaging in anesthetized and awake mice

The functional role of cortical spontaneous activity is still not well understood. For instance, it remains controversial, how spontaneous activity relates to sensory evoked patterns, with some studies showing striking similarities between both, while other studies reporting differences. To investigate those discrepancies, we systematically compared several characteristics of visual, auditory and tactile evoked activity elicited with different stimulus strengths and spontaneous activity motifs in lightly anesthetized and awake mice using mesoscale wide-field voltage-sensitive dye and glutamate imaging respectively. In particular, we analyzed: amplitude, speed, and propagation trajectory in spontaneous and evoked activity patterns. We found that spontaneous activity had similar amplitude, and speed when compared to evoked activity elicited with low stimulus strengths. However, this similarity gradually decreased when the strength of stimuli eliciting evoked responses increased. Moreover, stronger stimuli elicited larger amplitude evoked activity with higher speed and with more stereotypical propagation trajectory. Similarly, amplitude of spontaneous activity was positively correlated with the speed of propagation. Interestingly, the trajectories of evoked activity covered a smaller cortical area compared to spontaneous activity. Altogether, these results show that the relationship between spontaneous and sensory evoked activity depends on the strength of sensory stimuli. Thus, this study helps to reconcile the previously reported conflicting findings when comparing evoked and spontaneous activity.