ABSTRACT
The human and rodent ventral striatal local field potentials show striking oscillations in the gamma band (~ 40-100 Hz), which have been linked to aspects of behaviour such as reward anticipation and delivery, movement initiation, learning from feedback, and decision-making. These oscillations show a rich temporal organization, whose relationship with behavioural variables is not well understood. Here, we show that, in rats performing a conditioned approach task, low-gamma and high-gamma oscillations during an immobile reward anticipation epoch were largely insensitive to outcome value, even though rats distinguished behaviourally between different outcomes, and single units encoded outcome value. Behaviour was highly stereotyped, yet we observed large variability from trial to trial in the occurrence and timing of these oscillations. Furthermore, higher-order features such as high-gamma power leading low-gamma power, and phase-amplitude coupling to lower-frequency bands, were only marginally modulated by outcome value. Moreover, these patterns closely resembled those found during off-task rest periods in which no rewards could be earned. These observations suggest a new interpretation of ventral striatal gamma oscillations as reflecting a default or resting state, with only minor and highly variable modulation by specific task-related variables.
