7 to 12 Hz activity in rat gustatory cortex reflects disengagement from a fluid self-administration task

The 7 to 12 Hz rhythm is a high-voltage oscillatory phenomenon recorded in many rat neocortical regions, largely analogous to the rodent and human somatosensory mu rhythm. Central to any interpretation of the functional significance of this pattern is the analysis of the behavioral context associated with it. Much of the debate on the function of mu, variously believed to represent either an environment-oriented or -isolated state, has relied primarily on its association with quiet immobility. In this report, we describe the relationship between the 7 to 12 Hz rhythm and a more complex behavioral setting, in which we were able to dissociated task orientation from disengagement. We trained head-restrained, water-restricted rats to perform a simple variant of a timed fluid self-administration task, while recording local field potentials from gustatory cortex (GC). Rats progressed through two behavioral states that were clearly distinguishable on the basis of lever-pressing regimes: a task-oriented state and a second state that reflected disengagement from the task. Concurrent GC neural recordings revealed bilaterally coherent oscillations in the 7 to 12 Hz range associated solely with the latter state. Consistent with published recordings of mu rhythm from somatosensory cortex, these rhythmic episodes were endogenously quenched when the rats prepared to lever-press; this inhibition of rhythmic episodes lasted through fluid delivery and consumption, making it clear that GC rhythms are not related to gustatory processing itself. By showing a direct relationship between the 7 to 12 Hz rhythm and disengagement from a task, these data provide strong and novel evidence that this gustatory rhythm in rats is associated with withdrawal from experimental contingencies.