15Many neurons show compartmentalized activity, in which activity does not spread 16 readily across the cell, allowing input and output to occur locally. However, the 17 functional implications of compartmentalized activity for the wider neural circuit are 18 often unclear. We addressed this problem in the Drosophila mushroom body, whose 19 principal neurons, Kenyon cells, receive feedback inhibition from a large, non-spiking 20 interneuron called APL. We used local stimulation and volumetric calcium imaging to 21show that APL inhibits Kenyon cells in both their dendrites and axons, and that both 22 activity in APL and APL's inhibitory effect on Kenyon cells are spatially localized, 23 allowing APL to differentially inhibit different mushroom body compartments. 24Applying these results to the Drosophila hemibrain connectome predicts that 25 individual Kenyon cells inhibit themselves via APL more strongly than they inhibit 26 other individual Kenyon cells. These findings reveal how cellular physiology and 27 detailed network anatomy can combine to influence circuit function. 28 29 effect on Kenyon cells, or how APL's physiology would affect the structure of 81 feedback inhibition in the mushroom body. 82
83We addressed these questions by volumetric calcium imaging in APL. By quantifying 84 the spatial attenuation of the effects of locally stimulating APL, we found that both 85 activity in APL and APL's inhibitory effect on Kenyon cells are spatially restricted, 86 allowing APL to differentially inhibit different compartments of the mushroom body. 87Combining these physiological findings with recent connectomic data predicts that 88 individual Kenyon cells inhibit themselves via APL more strongly than they do other 89 Kenyon cells. Our findings establish APL as a model system for studying local 90 computations and their role in learning and memory. 91
92Results 93
94Spatially restricted responses to electric shock in APL 95
96We first asked whether physiological responses to sensory stimuli are spatially 97 localized within APL. At very low odor concentrations, odor responses in APL can be 98 restricted to one lobe (Inada et al., 2017), but it remains unclear how sensory-evoked 99 activity is structured across APL at larger scales. We examined this question using 100 electric shock, a typical 'punishment' used for olfactory aversive conditioning. 101 102 APL responds to electric shock (Liu and Davis, 2008;, but it is 103 unknown if these responses are spatially restricted. To test this, we subjected flies to 104 electric shock and recorded activity volumetrically throughout the APL lobes using 105