Figure 3.

Circuits for high-order integration and value computation.

In the larval brain the MB and the LH receive convergent sensory inputs from olfactory, thermosensory and visual projection neurons (green and light blue) [16,14,15]. The LH neurons (light brown) are assumed to parse the information according to innate valence. The MB Kenyon cells (dark brown) are thought to decorrelate sensory signals via highly divergent connections, expose them to reinforcement-gated plasticity (red open circles), and generate learnt valence signals (purple). MB is characterized by a recurrent architecture: a GABAergic neuron (black) gathers signals at the KCs axons and feeds back onto the KCs dendrites [16,17]; in addition, KCs signal back to the teaching neurons (mostly DANs, red) [16,49] and so do the MB output neurons (purple and orange feedbacks) [16,18]. These multilayered paths provide neural substrates for integrating prediction to the teaching signals, in addition to other inputs from the SEZ, for adaptive memory update (See Box i [18]). Further down the circuit, MB and LH are likely integrated for valence signals that can be used as instructive signal for navigation (orange, See Box ii [54] and iii [35]). The SEZ (grey) transforms more [16,35] or less [3] integrated signals into behavior.

i.Top panel, the experimental exploration of the responses of some teaching DANs, combining calcium transient and optogenetic stimulations, confirms that DANs integrate external (multisensory nociceptive neurons Basin) and memory-related (MB outputs neurons) inputs combining the recording of cell activity and optogenetic stimulations. Bottom panel, In an artificial neural network incorporating the connectivity of the MB and the response tuning of the DANs, discriminative signal for conditioned stimuli that predict (CS+) or not (CS-) the unconditioned stimulus (US) emerges in the DANs after associative training [18].

ii. Reverse-correlation experiments [13,53, 54, 55] link olfactory and visual inputs to navigation [54]. Under fictive odor stimulation (the optogenetic activation of receptors for attractive odors continuously varying in intensity), redirecting turns are initiated by a decrease in the signal, while under real visual stimulation (aversive blue light varying in intensity) turns are initiated when the signal increases. The sensorimotor transformation estimated for the combination of inputs suggests a linear integration of olfactory and visual inputs, probably readable at the level of LH output.

iii. PDM-DN, a neuron reconstructed in EM, downstream of LH neurons and descending to the SEZ, is necessary for navigating towards an odor source [35]. The optogenetic activation of this neuron triggers stop and redirection in crawling animals by stopping the wave of body contraction that goes through the body and shutting off the activity of the corresponding motoneurons.