• Mechanisms of Valuation: Encoding of Outcome Variables in Orbitofrontal Cortex and Ventral Striatum

      Cooch, Nisha Kaul; Schoenbaum, Geoffrey (2014)
      Adaptive decision making requires that we consider not only the inherent value of our options but also more specific features of those options, which can be used to compute the current value of each choice in a dynamic environment. General value information is conveyed via `model-free' signaling, whereas details of outcomes that are independent of value are conveyed via `model-based' representations of outcomes. Orbitofrontal cortex (OFC) and ventral striatum (VS), which function as part of the reward system, have been implicated in value-guided behaviors, but their specific contributions to model-free and model-based signaling remains elusive. Several studies suggest that OFC is not critical for distinguishing differentially valued outcomes of a common currency. Instead, encoding of specific features of outcomes in OFC appears to provide the flexibility required for advantageous choice selection in several value-guided behaviors. However, VS has been shown to be essential when either value or specific feature information is necessary for adaptive behavior. In the following set of experiments, we tested the hypothesis that OFC signals model-based information, which is incorporated with model-free signals in VS. To isolate model-free and model-based signals, we independently manipulated size (value) and flavor (specific feature) of rewards while recording single units in the rat OFC (Experiment 1) or VS (Experiment 2). Our data provide evidence for model-based signals in OFC and evidence for a hybrid of model-based and model-free signals in VS. Whereas OFC lesions disrupted model-based representations of flavor in VS, they did not eliminate all model-based signaling. We therefore conclude that OFC provides some, but not all, of the model-based information in VS.
    • The Nucleus Accumbens as a Switchboard: Heterosynaptic Suppression in the Ventral Striatum

      Calhoon, Gwendolyn Gabrielle; O'Donnell, Patricio (2013)
      Many brain circuits control behavior through the integration of information provided by separate inputs onto a common target neuron. Medium spiny neurons (MSNs) in the ventral striatum (VS) receive converging excitatory afferents from the prefrontal cortex (PFC), hippocampus (HP), and thalamus, among others, and the integration of these inputs is critical for shaping goal-directed behaviors. Although under baseline conditions the membrane activity of MSNs is controlled largely by the HP, the PFC can elicit up states in MSNs during periods of high frequency activity, such as that which occurs during decision making epochs. Moreover, during epochs of high PFC activity, the VS loses synchrony with the HP. It is therefore possible that PFC inputs locally attenuate responses to other glutamatergic inputs to the VS. We investigated whether strong, transient PFC activation can disengage the VS from the HP by measuring the effect of high frequency PFC stimulation on MSN responses to stimulation of other synaptic inputs. Using in vivo intracellular recordings, we found that delivering trains of stimuli to the PFC suppresses HP- and thalamus-evoked synaptic responses in the VS, partially through a GABAergic mediator. These findings indicate that high frequency PFC activity overrides HP control of MSN up state transitions, and provide evidence of heterosynaptic inhibition in this system in vivo. This interaction may enable the PFC to exert influence on basal ganglia loops during decision-making instances with minimal disturbance from ongoing contextual inputs.