Figure 4.

Control of behavior. The circuit diagrams on the right present an extremely simplified view of the brain, illustrating targets of optogenetic intervention (the circuit elements in yellow). Signals from external and internal sensors are used to construct internal representations of the animal’s state. Behavior is generated when states are mapped onto actions and the resulting action representations recruit motor systems. The probabilistic rules determining which states are mapped onto which actions are subject to short- and long-term modulation. (a) Flies sense ambient CO₂ levels as an index of stressful overcrowding. Optogenetic activation of CO₂-sensing neurons creates the illusion of crowded conditions and elicits avoidance behavior (Suh et al. 2007). (b) Pheromonal, gustatory, and visual signals indicate the presence of a potential mating partner. In the male brain, the state “presence of receptive female” is mapped onto the action “courtship,” which leads to the activation of a motor program involving a unilateral wing beat, the so-called courtship song. (c) In the female brain, the state “presence of receptive female” is not mapped onto the action “courtship.” However, the motor program for male-specific courtship is still present in the female, as it can be recruited optogenetically, bypassing sensory input altogether (Clyne & Miesenböck 2008).