Trial-spacing effects in Hermissenda suggest contributions of associative and nonassociative cellular mechanisms

In behaving Hermissenda, a preparatory conditioned response developed across repeated pairings of light (conditioned stimulus; CS) and rotation (unconditioned stimulus; US) with intertrial intervals (ITIs) of 60 and 120 s, but not 30 s. Likewise, contiguous in vitro stimulation of the visual and vestibular receptors, an analog of behavioral conditioning, resulted in an increase in the input resistance (i.e., excitability, a correlate of conditioning) of the B photoreceptors of the Hermissenda's eye, but only with ITIs greater than 60 s. Calcium signaling in the B cell, critical to the induction of this neuronal plasticity, was attenuated with shorter ITIs owing to (a) a reduction of the light-induced generator potential and hence voltage-dependent Ca²⁺ influx during the light CS, (b) a depression of the Ca²⁺ current that persisted throughout shorter ITIs, and (c) a steady- state inactivation of the Ca²⁺ current as a result of a sustained depolarization persisting from the previous trial. These results are consistent with a 2-process theory of associative learning in which a primary process (Ca²⁺ influx) may be opposed by a secondary process (depression of the Ca²⁺ current) during short ITIs.