Optogenetics uses light exposure to manipulate physiology in genetically modified organisms. Abundant tools for optogenetic excitation are available, but the limitations of current optogenetic inhibitors present an obstacle to demonstrating the necessity of neuronal circuits. Here we show that anion channelrhodopsins can be used to specifically and rapidly inhibit neural systems involved in Drosophila locomotion, wing expansion, memory retrieval and gustation, thus demonstrating their broad utility in the circuit analysis of behavior.

Perceptual decision making is an active process where animals move their sense organs to extract task-relevant information. To investigate how the brain translates sensory input into decisions during active sensation, we developed a mouse active touch task where the mechanosensory input can be precisely measured and that challenges animals to use multiple mechanosensory cues. Male mice were trained to localize a pole using a single whisker and to report their decision by selecting one of three choices. Using high-speed imaging and machine vision, we estimated whisker–object mechanical forces at millisecond resolution. Mice solved the task by a sensory-motor strategy where both the strength and direction of whisker bending were informative cues to pole location. We found competing influences of immediate sensory input and choice memory on mouse choice. On correct trials, choice could be predicted from the direction and strength of whisker bending, but not from previous choice. In contrast, on error trials, choice could be predicted from previous choice but not from whisker bending. This study shows that animal choices during active tactile decision making can be predicted from mechanosensory and choice-memory signals, and provides a new task well suited for the future study of the neural basis of active perceptual decisions. SIGNIFICANCE STATEMENT Due to the difficulty of measuring the sensory input to moving sense organs, active perceptual decision making remains poorly understood. The whisker system provides a way forward since it is now possible to measure the mechanical forces due to whisker–object contact during behavior. Here we train mice in a novel behavioral task that challenges them to use rich mechanosensory cues but can be performed using one whisker and enables task-relevant mechanical forces to be precisely estimated. This approach enables rigorous study of how sensory cues translate into action during active, perceptual decision making. Our findings provide new insight into active touch and how sensory/internal signals interact to determine behavioral choices.

Optogenetics employs light exposure to manipulate physiology in genetically modified organisms. There are abundant tools for optogenetic excitation of neuronal activity, but the limitations of current activity photo-inhibitors present an obstacle to demonstrating the necessity of specific neuronal circuits. Here we show that anion channelrhodopsins can be used to specifically and rapidly inhibit a range of systems involved in Drosophila locomotion, wing expansion, memory retrieval and gustation, demonstrating their broad utility to the circuit analysis of behavior. Potent optogenetic inhibition of behaviorPage of 1 24 Potent optogenetic inhibition of behavior Page of 2 24 Figure 1. Guillardia theta anion channelrhodopsins are potent inhibitors of motor function and neuronal spiking. A. Flies expressing GtACR1 in cholinergic neurons (Cha-Gal4>UAS-GtACR1) fell from a vertical surface when illuminated. Light intensities in μW/mm 2 are given in each panel. The proportion of climbing flies is given as a percentage of flies outside the floor area of the chamber. At the highest setting, flies fell in response to green, blue and red light, but responded only to green light at lower intensities. Each solid trace is the mean of 3 experiments (48 flies), line color corresponds to the color of illumination from a projector, error ribbons are 95% confidence intervals. B. Flies expressing GtACR2 (Cha>GtACR2) fell in response to blue and green light at the highest intensity; green light had little effect at 19 μW/mm 2 , blue light (14 μW/mm 2 ) was sufficient to causing falling. C. Representative Muybridge series from a high-speed video (1000 frames per second) during light onset (top row) show that green light illumination (peak 525 nm, 38 μW/mm 2 ) of a fly expressing GtACR1 in cholinergic neurons elicited a rapid onset of immobility. Frames are spaced at 50 ms intervals, numbers indicate time relative to illumination start in ms. Difference images between consecutive frames (bottom row) map the motion. Coloured bar indicates frames where the fly is illuminated. Potent optogenetic inhibition of behavior Page of 3 24 Potent optogenetic inhibition of behavior Page of 7 24 Video acquisition Videos were captured at 30 frames per second using an AVT Guppy F-046B CCD camera (Stemmer Imaging, UK), equipped with a 12 mm CCTV-type lens and connected to a computer via an IEEE 1394 cable. Experiments were conducted under infrared (IR) light; an IR longpass filter (Edmund Optics, Singapore) was used to reduce de-Potent optogenetic inhibition of behavior Page of 14 24Figure 7. GtACRs have minimal toxicity. A1. Central brain cells were observed to express yellow fluorescent protein-tagged GtACR1 (green, endogenous fluorescence) in Crz>GtACR1 fly brains prior to all-trans-retinal (ATR) feeding. A2-3. Antibody staining for Crz protein (α-Crz, red) confirmed GtACR1-expressing cells were Crz + . B1-3. Crz+ cell morphology was normal in Crz>GtACR1 flies after ATR feeding and one day of illumination with 19 μW/mm 2 green light. C1-3. Crz+ cell m...

Perceptual decision making is an active process where animals move their sense organs to extract task-relevant information. To investigate how the brain translates sensory input into decisions during active sensation, we developed a mouse active touch task where the mechanosensory input can be precisely measured and that challenges animals to use multiple mechanosensory cues. Mice were trained to localise a pole using a single whisker and to report their decision by selecting one of three choices. Using high-speed imaging and machine vision we estimated whisker-object mechanical forces at millisecond resolution. Mice solved the task by a sensory-motor strategy where both the strength and direction of whisker bending were informative cues to pole location. We found competing influences of immediate sensory input and choice memory on mouse choice. On correct trials, choice could be predicted from the direction and strength of whisker bending, but not from previous choice. In contrast, on error trials, choice could be predicted from previous choice but not from whisker bending. This study shows that animal choices during active tactile decision making can be predicted from mechanosenory and choice-memory signals; and provides a new task, well-suited for future study of the neural basis of active perceptual decisions.

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