Interstimulus interval effects on the habituation of the contraction response in the worm Eisenia foetida

Habituation is a form of learning characterized by a decreased response to repeated stimuli. Research has shown that the interval between stimuli (interstimulus interval [ISI]) influences the development and retention of habituation. Experiments 1-4 examined earthworm contraction-response habituation to light using ISIs ranging from 2 to 256 s. Experiment 1 showed an inverted-U in response probability across ISIs: very short (2-8 s) and very long (128-256 s) ISIs produced greater within-session decrements than intermediate ISIs (16-88 s). Experiment 2 found that 128-256 s yielded faster, larger decrements than 88 s, with group differences persisting 24 hr later when all animals were tested at 4 s. Experiment 3 focused on ISIs of 2-64 s with extended training (50 trials): on Day 1, 32 s yielded the least within-session decrement, and under the common 4-s Day 2 test, only the 64-s group showed evidence of retention of habituation. Experiment 4 replicated the key patterns when comparing ISIs of 4, 32, and 128 s: 128 s produced the largest short- and long-term decrements. Experiments 5 and 6 introduced an air puff and demonstrated stimulus specificity of the 128-s ISI condition. Day 2 responding was lowest when the Day 2 stimulus matched Day 1 (light-light; puff-puff), whereas switching stimuli (light-puff; puff-light) reduced or eliminated this carryover. Overall, the ISI-dependent patterns and stimulus-specificity effects observed here confirm the value of this animal model as a preparation for studying habituation. (PsycInfo Database Record (c) 2025 APA, all rights reserved).