, 2001) and slower-frequency synchronization of neural activity representing unattended stimuli (Cohen and Maunsell, 2009; Mitchell et al., 2009). In sum, our results suggest that synchronous oscillations allow dynamic selection of currently relevant neural ensembles. This may be particularly

important in prefrontal cortex, where neurons have highly diverse properties and thus a particular ensemble must be formed from neurons that are also members of other ensembles (Rigotti et al., 2010). JQ1 The dynamic nature of synchronized oscillations may provide a substrate for the ensembles that allows their rapid selection and deselection and, hence, cognitive flexibility. Two macaque monkeys, one male (CC, Macaca fascicularis) and one female (ISA, Macaca mulatta), were trained on a cued task-switching paradigm ( Figure 1A). Neural activity was simultaneously recorded during task performance from two frontal regions: the dorsolateral prefrontal cortex (PFC, area 9/46) and the anterior cingulate cortex (ACC, areas 24c and 32). Only data from the dorsolateral prefrontal cortex are reported here. The recording well targeting PFC was placed in the left hemisphere and was centered approximately 28 mm anterior to the interaural plane and 21 mm lateral from the midline. Stereotaxic positioning

of the well was Epacadostat guided by structural magnetic resonance imaging. Neural activity was recorded during 34 sessions (11 for monkey CC, 23 for monkey ISA). Arrays of

up to sixteen epoxy-coated tungsten electrodes (FHC) were lowered into the PFC during each recording session (median number of electrodes with well-isolated all single neuron activity was 5.5 per session). Electrodes were lowered in pairs by a custom-built microdrive assembly and spaced at least 1 mm apart. Electrodes were lowered acutely each day through an intact dura and allowed to settle before recording. This ensured stable isolation of the single neuron activity. After each recording session, the electrodes were retracted and the microdrive assembly was removed from the well. A Plexon Multichannel Acquisition Processor (MAP; Plexon) was used to perform electrophysiological recordings. The signal from each electrode was filtered by the preamplifier between 154 Hz and 8.8 kHz to isolate spiking activity and between 3.3 and 88 Hz to isolate the local field potential. Both spiking activity and local field potentials were referenced to earth ground (although the same results were observed when rereferencing locally, within PFC). The raw spiking waveforms were digitized at 40 kHz and subsequently sorted into single units offline, based on waveform shape characteristics and principal components analysis (Offline Sorter, Plexon). During recording, electrodes were lowered to maximize the signal-to-noise ratio of spiking activity and were not guided by the task relevance of neural responses.