Associative neural and behavioral change in Hermissenda: consequences of nervous system orientation for light and pairing specificity.

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Abstract
The locomotor behavior of Hermissenda, following 3 training sessions of exposure to paired light and rotation presentations, exhibited light- and pairing-specific changes whose magnitude depended on the orientation of the animals during testing. This diminished phototactic behavior reflected both an increase in the latency for initiation of movement as well as a decrease in velocity. Assessment of animals'' locomotion in the vertical orientation, rather than the horizontal, revealed a general decrease in locomotor latencies and greater light and pairing specificity of the associative behavioral change. The behavioral changes were closely paralleled by intracellular recordings from photoreceptors. Type B photoreceptors from animals exposed to paired light-rotation presentations generally responded with greater impulse frequencies to a broad range of light intensities than did cells from animals exposed to random light-rotation presentations, 24 and 48 h after the conclusion of training. These impulse-frequency differences were accompanied by facilitation of paired-condition cells'' long-lasting depolarization following offsets of light, increased input resistances of B-cells in darkness, but no differences in spontaneous activity during the dark nor resting potential in darkness. These associative changes observed for single type B photoreceptors were also manifest in the interaction of pairs of type B photoreceptors as well as pairs of type B and A photoreceptors, 24 h after training. The greater light specificity of changes in locomotor behavior for the vertical testing orientation was accompanied by greater light/dark impulse activity ratios for paired B photoreceptors, recorded in the vertical orientation. This greater neural light specificity for the vertical orientation is consistent with the tonic synaptic inhibition of B photoreceptors by caudal hair cells of the animal''s statocyst, which occurs in the vertical orientation. Type B photoreceptors define primary loci of neural change, which are related to the observed associative behavioral change.