In theory, the observed LFP oscillation patterns could arise from volume conductance from a different source than ventral striatum itself, such as nearby piriform cortex, where gamma oscillations are prominent (Berke, 2005; Kay and Freeman, 1998). To address whether ventral striatal LFPs were locally relevant, we compared the spike times of putative FSIs to LFP activity. Relationships between spike times and particular phases of the ongoing LFP, or spike-field coherency (Fries et al., 2001; Mitra and Bokil, 2008), can be examined by the spike-triggered average (STA) which shows the average LFP in some time window around the time of a spike. Two example STAs for ventral striatal FSIs against the LFP recorded from the same tetrode are shown in Figures 11A,B. The STA of the neuron in Figure 11A showed strong ∼60 Hz modulation, while the STA of the neuron in Figure 11B showed no such modulation but tended to fire at the trough of a ∼12 Hz rhythm instead. In general, different neurons exhibited STA modulations at a range of frequencies, with a substantial number showing no apparent phase
