Background illumination effects upon in vitro conditioning in Hermissenda

In the marine snail Hermissenda, associative learning can be accomplished by paired presentations of light and vestibular stimulation. It is generally assumed that associative learning depends upon the intensity or salience of the conditioned or unconditioned stimulus (CS and US, respectively). Accordingly, during Hermissenda conditioning a stronger dark adaptation is expected to render the CS (the light) more salient and hence facilitate association. We studied the influence of background illumination level using an in vitro pairing procedure in Hermissenda. This procedure allows one to assess the effect of conditioning upon a single cell, the B photoreceptor, which is implicated in this learning process. After 15 min of adaptation to a dim background light, B photoreceptors maintained a basal rate of firing, while after adaptation to complete darkness, they stopped firing. Paired and unpaired groups received 10 training trials in either a completely dark or a dim light environment. Although a trial to trial cumulative increase in excitability was found in the paired group trained in darkness, only the paired group trained under dim background light showed a higher input resistance and cell excitability 10 min after training. These results suggest that the background dim illumination was not needed for the induction but played a role in the maintenance of the pairing effect. Possible mechanisms for such a modulatory effect are discussed.