Medial frontal theta is entrained to rewarded actions

Rodents lick to consume fluids. The reward value of ingested fluids is likely to be encoded by neuronal activity entrained to the lick cycle. Here, we investigated relationships between licking and reward signaling by the medial frontal cortex [MFC], a key cortical region for reward-guided learning and decision-making. Multi-electrode recordings of spike activity and field potentials were made in male rats as they performed an incentive contrast licking task. Rats received access to higher and lower value sucrose rewards over alternating 30 sec periods. They learned to lick persistently when higher value rewards were available and to suppress licking when lower value rewards were available. Spectral analysis of spikes and fields revealed evidence for reward value being encoded by the strength of phase-locking of a 6-12 Hz theta rhythm to the rats’ lick cycle. Recordings during the initial acquisition of the task found that the strength of phase-locking to the lick cycle was strengthened with experience. A modification of the task, with a temporal gap of 2 sec added between reward deliveries, found that the rhythmic signals persisted during periods of dry licking, a finding that suggests the MFC encodes either the value of the currently available reward or the vigor with which rats act to consume it. Finally, we found that reversible inactivations of the MFC in the opposite hemisphere eliminated the encoding of reward information. Together, our findings establish that a 6-12 Hz theta rhythm, generated by the rodent medial frontal cortex, is synchronized to rewarded actions.Significance StatementThe cellular and behavioral mechanisms of reward signaling by the medial frontal cortex [MFC] have not been resolved. We report evidence for a 6-12 Hz theta rhythm that is generated by the MFC and synchronized with ongoing consummatory actions. Previous studies of MFC reward signaling have inferred value coding upon temporally sustained activity during the period of reward consumption. Our findings suggest that MFC activity is temporally sustained due to the consumption of the rewarding fluids, and not necessarily the abstract properties of the rewarding fluid. Two other major findings were that the MFC reward signals persist beyond the period of fluid delivery and are generated by neurons within the MFC.

Diversity of Adjustments to Reward Downshifts in Vertebrates

This review focuses on reward-schedule effects, a family of learning phenomena involving surprising devaluations in reward quality or quantity (as in incentive contrast), and reward omissions (as in appetitive extinction), as studied in three taxonomic groups of vertebrates: mammals, birds, and amphibians. The largest database of dependable data comes from research with mammals in general, and with rats in particular. These experiments show a variety of behavioral adjustments to situations involving reward downshifts. For example, rats show disruption of instrumental and consummatory behavior directed at a small reward after receiving a substantially larger reward (called successive negative contrast, SNC)—a reward-schedule effect. However, instrumental SNC does not seem to occur when animals work for sucrose solutions—a reversed reward-schedule effect. Similar modes of adjustment have been reported in analogous experiments with avian and amphibian species. A review of the evidence suggests that carry-over signals across successive trials can acquire control over behavior under massed practice, but emotional memory is required to account for reward-schedule effects observed under widely spaced practice. There is evidence for an emotional component to reward-schedule effects in mammals, but similar evidence for other vertebrates is scanty and inconsistent. Progress in the comparative analysis of reward-schedule effects will require the intense study of a set of selected species, in selected reward-downshift situations, and aiming at identifying underlying neural mechanisms.

Mediational Role of Hormones in Incentive Contrast

Frustration can be defined as an emotional state generated by the omission or devaluation in the quantity or quality of an expected appetitive reward. Thus reactivity to a reward is affected by prior experience with the different reinforcer values of that reward, a phenomenon known as incentive relativity, which can be studied by different paradigms to induce frustration. In this work we will focus in successive negative contrast (SNC), involving a downshift of the reward, and in the complete omission of the reward, a phenomenon known as extinction. Also, we will discuss the role of the neuroendocrine mechanisms involved in these processes. Specifically, we analyze the action of monoamines, adrenal and sexual hormones.