A quantitative portrait of habituation in Stentor coeruleus

Habituation--the decrement in response to a series of stimuli--is a widespread form of learning observed across many organisms, including the unicellular organism Stentor coeruleus. A lesser-known feature of Stentor habituation, shared with animals, is potentiation: faster habituation to a second stimulus series despite partial or complete recovery of responsiveness before that series begins. This suggests that although the first-order habituation memory can decay during the recovery period between the two series, a persistent second-order memory mediates faster relearning. We investigate the response profile of Stentor across a range of stimulation frequencies and recovery periods to identify the timescales at which these memory traces operate. We introduce a statistical framework to infer both population and single-cell learning parameters, allowing us to quantify prior qualitative findings and examine relationships among parameters across cells. Two key findings are that potentiation is frequency-sensitive, and that recovery and potentiation are decoupled, consistent with a serial and hierarchical cascade of leaky integrator units underlying these processes. This quantitative portrait provides a foundation for mechanistic modeling of intracellular memory in Stentor.