Spontaneous activity resembling tone-evoked activity in the primary auditory cortex of guinea pigs

In the primary auditory cortex (AI), a pure tone evokes propagating activity along a strip of the cortex. We have previously shown that focal activation of AI triggers autonomously propagating activity that resembles tone-evoked activity (Song et al., 2006). Because a focal spontaneous activity is expected to trigger similar activity propagation, spontaneous activity resembling tone-evoked activity may exist in AI. Here we tested this possibility by optical imaging of AI in guinea pigs. After obtaining tone-evoked activities, we made long-duration optical recordings (9-40s) and isolated spontaneous activities from respiration and heartbeat noises using independent component analyses. Spontaneous activities were found all over AI, in all animals examined. Of all spontaneous events, 33.6% showed significant correlation in spatio-temporal pattern with tone-evoked activities. Simulation using a model that captures the temporal feature of spontaneous response in single channels but sets no constraint among channels, generated no spontaneous events that resembled tone-evoked activations. These results show the existence of spontaneous events similar in spatio-temporal pattern to tone-evoked activations in AI. Such spontaneous events are likely a manifestation of cortical structures that govern the pattern of distributed activation in AI.