Internal bias vs. external sensory cues: the role of ''handedness'' in navigation decisions of invertebrate model systems

How do the internal properties of individual animals combine with external environmental stimuli to produce a physical output? Answering this is important for understanding how the brain functions, and we examine the question using simple invertebrate animals, the Drosophila larva and C. elegans. Both animals have well-mapped brain circuits and limited behavioral repertoires, and exhibit exploratory behavior as they search for food, while at the same time responding to external stimuli. We focus on one specific internal property, the tendency of an animal to turn leftward or rightward (``handedness''). We find statistically significant bias in individual turning and drifting directions, but that turn and drift handedness are uncorrelated. This lack of correlation explains why even strongly left- or right-turning crawlers on average have similar diffusion rates as unbiased turners. Both handedness types are also weakly persistent, with frequent shifts to new biases. We show that the internal bias (handedness) strongly affects, and in fact dominates, individual turning decisions in the presence of a temperature gradient, suggesting that inherent traits of individuals must be considered to achieve a more complete understanding of navigation and brain processing.