The internal sense of daytime versus nighttime, against which light is evaluated, is generated by a microcircuit within the network of circadian neurons. Using a paradigm in which flies report their time-of-day estimates, we found that the circadian system assists in evaluating light conditions relative to time of day, and that mismatch between prediction and reality shapes behavioral responsiveness to light. What if conditions in the environment suddenly changed and no longer matched the expectation set by the clock? A mismatch between reality and prediction (bright light during the night or darkness during daytime) could be interpreted as an error signal and lead to behavioral modification. Predictability, enabled by the circadian clock, needs to be balanced with flexibility. For example, it instructs the timing of cell division ( 7) and hormone secretion ( 8) and enables animals to seek food and shelter before nightfall ( 9). The clock organizes various physiological processes on a ~24-hour scale ( 6).
The day-night oscillations are represented on the cellular level by the circadian clock, a molecular program shaped by environmental light-key clock proteins are light sensitive, so that under light-oscillating conditions their levels rise and fall rhythmically ( 5). We took advantage of arguably the most predictable occurrence in the natural environment, the back-and-forth between bright days and dark nights, to explore this question. While sensory circuits have been successfully mapped in many systems, there is almost no understanding of how sensory expectations are encoded by the brain. The mechanisms that give rise to this integrated signal are not clear, but are thought to require convergence, on dopaminergic neurons, between ongoing sensory signals and expectations generated from previous outcomes ( 1).
Discrepancy between expectation and reality is famously reflected in the activity of dopaminergic neurons ( 2), which have been proposed to generate an error signal ( 3) that can bias behavior ( 4). To do this, it must continually check ongoing sensations against expectations, an idea known as predictive coding ( 1). The brain assigns valence to incoming sensory stimuli, allowing responsiveness to be context dependent. We propose that a dynamic model of environmental light resides in the shifting connectivities of the LNv-DN1a circuit, which helps animals evaluate ongoing conditions and choose a behavioral response. Switching between the two states requires daily remodeling of LNv and DN1a axons such that the maximum presynaptic area in one population coincides with the minimum in the other. The internal daytime-nighttime context is generated by two interconnected and functionally opposing populations of circadian neurons-LNvs generating the daytime state and DN1as generating the nighttime state. We found that light elicits an acute increase in locomotion (startle) that is modulated in a time-of-day–dependent manner: Startle is potentiated during the nighttime, when light is unexpected, but is suppressed during the daytime.
We studied how sensory information can be contextualized, by examining light-evoked locomotor responsiveness of Drosophila relative to time of day. Behavioral responsiveness to external stimulation is shaped by context.
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