Recordings from prefrontal cortex in awake behaving monkeys revealed sustained periods of firing during delay periods in visual-guided tasks designed to assess working memory demands (Williams and Goldman-Rakic, 1995; Ichihara-Takeda and Funahashi, 2007). These persistent patterns of firing are maintained in the absence of visual cues and were interpreted as a cellular correlate of the PFC’s ability to hold and manipulate information in anticipation of a future action. Work from Steriade and others also described a slow oscillation (0.5–2Hz) in prefrontal cortex during periods of quiet wakefulness or sleep (Steriade et al., 1993). The slow oscillation consisted of periods of hyperpolarized down-states interrupted by brief transitions into depolarized up-states during which action potentials often arose. One hallmark of this activity is the relatively constant amount of depolarization produced during an up-state (~10–15 mV) that persists for several seconds. During active waking periods or REM sleep, neurons reside mostly at up-state membrane potentials consistent with enhanced sensory input and synaptic drive. In acute slices of prefrontal cortex, neurons usually do not display spontaneous oscillations of membrane potentials (termed bistability) or action
