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 rotat ion (unconditioned stimulus; US) with intertrial intervals (ITIs) of 6 0 and 120 s, but not 30 s. Likewise, contiguous in vitro stimulation o f the visual and vestibular receptors, an analog of behavioral conditi oning, resulted in an increase in the input resistance (i.e., excitabi lity, a correlate of conditioning) of the B photoreceptors of the Herm issenda'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 ligh t-induced generator potential and hence voltage-dependent Ca2+ influx during the light CS, (b) a depression of the Ca2+ current that persist ed throughout shorter ITIs, and (c) a steady-state inactivation of the Ca2+ current as a result of a sustained depolarization persisting fro m the previous trial. These results are consistent with a 2-process th eory 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.