Comparisons of dorsal and ventral hippocampus cornu ammonis region 1 pyramidal neuron activity during trace eye-blink conditioning in the rabbit.

Previous studies demonstrating a critical role of the hippocampus during trace eye-blink conditioning have focused primarily upon the dorsal portion of the structure. However, evidence suggests that a functional differentiation exists along the septotemporal axis of the hippocampus. In the present study, the activity of 2588 single cornu ammonis region 1 pyramidal neurons of the dorsal hippocampus and ventral hippocampus were recorded during trace and pseudo-eye-blink conditioning of the rabbit. Learning-related increases in dorsal hippocampus neuron firing rates were observed immediately prior to behavioral criterion, and increased over the course of training. Activation of dorsal hippocampus neurons during trace conditioning was also greater than that of ventral hippocampus neurons, including during the trace interval, in well-trained animals. An unexpected difference in the patterns of learning-related activity between hemispheres was also observed. Neurons of the dorsal hippocampus ipsilateral and contralateral to the trained eye, exhibiting significant increases in firing rate [rate increasing neurons], demonstrated the greatest magnitude of activation early and late in training, respectively. Rate increasing neurons of the dorsal hippocampus also exhibited a greater diversity of response profiles, with 69% of dorsal hippocampus rate increasing neurons exhibiting significant increases in firing rate during the conditioned stimulus and/or trace intervals, compared with only 8% of ventral hippocampus rate increasing neurons (the remainder of which were significantly responsive during only the unconditioned stimulus and/or post-unconditioned stimulus intervals). Only modest learning-related activation of ventral hippocampus neurons was observed, reflected as an increase in conditioning stimulus-elicited rate increasing neuron response magnitudes over the course of training. No differences in firing rate between dorsal hippocampus and ventral hippocampus neurons during a 1-day pre-training habituation session were observed. Thus, dorsal hippocampus activation is more robust, suggesting a more substantial role for these neurons in the processing of temporal information during trace eye-blink conditioning.

Pervasiveness and magnitude of context effects: evidence for the relativity of absolute magnitude estimation.

To test the assertion that absolute magnitude estimation serves to minimize context effects, two experiments were conducted in which area stimuli were judged under differing conditions. In Experiment 1, four groups of subjects made magnitude estimations of triangles ranging in area from 1.5 to 3,072 cm2. No standard or modulus was used, and instructions were similar to those used in absolute magnitude estimation experiments. Each group first judged a different subrange of the stimuli (1.5-24; 48-768; 6-96; or 192-3,072 cm2) before making judgments of the remaining stimuli. In Experiment 2, two groups of subjects made magnitude estimations of triangles ranging in area from 1.5 to 12,288 cm2, with each group first judging a different subrange of stimuli (1.5-24 cm2 or 768-12,288 cm2). The design and instructions were virtually identical to those used in absolute magnitude estimation experiments. Our results indicate that the wording of the instructions is not crucial and that judgments are influenced in two ways that are not predicted by proponents of absolute magnitude estimation. First, the power functions fit to the initially presented subranges (e.g., 1.5-24 cm2), which were judged without contextual effects produced by previously presented stimuli, were inconsistent with one another. Second, judgments of the remaining stimuli were influenced by the subrange of stimuli judged initially. The prevalence of context effects in both experiments, in spite of instructional differences, suggests that although one should avoid using a standard and modulus, there is little else to be gained by adopting the absolute magnitude estimation procedure.