Faculty Bibliography

The goal of this review is to compare two divergent lines of research on signal-centered behavior: the orienting reflex (OR) and autoshaping. A review of conditioning experiments in animals and humans suggests that the novelty hypothesis of the OR is no longer tenable. Only stimuli that represent biological 'relevance' elicit ORs. A stimulus may be relevant a priori (i.e., unconditioned) or as a result of conditioning. Exposure to a conditioned stimulus (CS) that predicts a positive reinforcer causes the animal to orient to it throughout conditioning. Within the CS-US interval, the initial CS-directed orienting response is followed by US-directed tendencies. Experimental evidence is shown that the development and maintenance of the conditioned OR occur in a similar fashion both in response-independent (classical) and response-dependent (instrumental) paradigms. It is proposed that the conditioned OR and the signal-directed autoshaped response are identical. Signals predicting aversive events repel the subject from the source of the CS. It is suggested that the function of the CS is not only to signal the probability of US occurrence, but also to serve as a spatial cue to guide the animal in the environment

The interaction of the commissural and associational systems to the CA1 region of the hippocampus was studied by recording extracellular field potentials and single unit activity in anesthetized rats. Associational fibers were activated by stimulating the stratum oriens of the CA1 region contralateral to recording: this stimulation activated the Schaffer collaterals by antidromically firing the pyramidal cells of the CA3 region on the side of recording. Commissural fibers were stimulated where they emerge from CA3 region. Both pathways excited both the basal and apical dendrites of the CA1 pyramidal cells. Commissural activation in stratum oriens was more efficient than associational path stimulation, while the opposite was seen in stratum radiatum. Responses elicited by associational path activation had their peak negativity 100--150 micrometers deeper in stratum radiatum than commissurally evoked responses. Both pathways were able to discharge pyramidal neurons. Both homonymous and heteronymous double pulse stimulation showed response facilitation. Simultaneous activation of both pathways induced a greater amplitude population spike than predicted by algebraic summation of the independent responses. Over 80% of the responsive CA1 cells could be fired by either pathway. These results show a considerable convergence of the commissural and associational pathways on CA1 pyramidal cells, although their predominant locus of excitation might be different

Rats with lesions severing either the subcallosal fornix (Fo) or the medial half of the fimbria (Fi) were used. They were compared with control (Co) animals in a working memory task (serial alternation) and a reference memory task (cue-guided alternation). Neither task required spatial mapping strategy. Damaging the Fi, but not the Fo, caused a severe deficit in the serial alternation task. Analysis of individual performance revealed that Fi rats either adopted a 'side strategy, ' resulting in worse than chance performance. This active perseveration required intact working memory mechanism. In the cue-guided alternation task, Fo animals proved superior to Co and Fi rats. These findings are inconsistent with notions that the exclusive role of the hippocampus is spatial mapping or storing of recent memories. They indicate also differential involvement of the fimbria and fornix fibers in behavior

We studied the response properties of interneurons of the dentate gyrus to stimulation of the commissural and perforant path inputs in urethane-anesthetized rats. Commissural stimulation inhibited the population spike evoked by concurrent stimulation of the perforant path. Stimulation thresholds of the interneurons were significantly lower than thresholds of the projection cells. The latency of 3 interneurons was shorter than the onset of the commissurally evoked field response. The results suggest that commissural fibers directly excite interneurons of the dentate gyrus, and the activity of the projection cells may be inhibited in a feed-forward manner

Rats with implanted stimulating and recording microelectrodes were trained in a straight alley to repeatedly press a bar placed at one end of the alley and to run for water reward available at the other end. Stimulating the commissural input evoked field EPSP and population spike in the CA1 region while no population spike was observed in the dentate area. The amplitude of the CA1 population spike was maximum during running and smallest during drinking. Field potentials in the dentate gyrus changed in an opposite manner (drink greater than groom greater than press greater than run). Perforant path evoked cell discharges in the dentate gyrus which were maximal during drinking and smallest during running. The behavior dependent changes of the evoked potentials covaried with the frequency and power of the simultaneously recorded theta activity. Perforant path stimulation during the negative-going phase of the dentate theta cycle evoked significantly greater granule cell responses than stimuli during the positive-going phase. These observations suggest that the medical septum exerts a potent biasing effect on the efficacy of other afferent to the hippocampus

The electrical activity of the dorsal and ventral divisions of the hippocampus was investigated quantitatively during the development of discriminative operant training and defensive conditioning. Signal-related (respondent, sign-tracking) and goal-directed (operant, 'voluntary') activities and their hippocampal electrical correlates were compared. Signal-related behaviour was accompanied by significantly higher frequency spectral peaks than lever press behaviour. Spectral peaks were higher during lever pressing in early stages of training as compared to criterion performance. In defensive conditioning overt sign-tracking behaviour disappeared but spectral peak shifts toward higher values upon signal presentation persisted. Comparison of dorsal and ventral hippocampal activity revealed high coherence values and similar frequency shifts during behaviour transitions. These findings argue against suggestions that a close relationship exists between elementary motor acts and hippocampal EEG

The interaction of the commissural and perforant path systems was studied by recording extracellular field potentials and single unit activity in the dentate gyrus in urethane-anesthetized rats. Conditioning commissural volleys suppressed extracellular synaptic potentials, population spikes and single unit discharges evoked by perforant path stimulation. Commissural stimulation (single or repetitive) failed to induce a population spike, however strong the stimulation. About half of the cells fired monosynaptically to perforant path volleys and 20% to commissural volleys. Half of the commissurally driven units fired before or coincided with field potential onset. The antidromic mechanism of these short latency unitary spikes was shown by the collision test. Commissural activation reduced spontaneous cell firing without previous excitation in 25% of the neurons. Less than 6% of the cells responded to stimulation of both inputs, indicating little convergence between the two pathways. We contend that a simple from of recurrent inhibition fails to explain the above findings, and the possibility of feed-forward inhibition by commissural activation has been raised

The potentiation effect of short tetani (5 sec, 50 Hz) of commissural fibers having dendritic synapses on CA1 pyramidal cells of the hippocampus was studied in freely behaving rats. High frequency stimulation produced a large increase in amplitude of the population spike and, to a lesser extent, of the extracellular synaptic responses. This increase was maintained for 4 days after a single train. The results imply that the efficiency of the synaptic transmission in the commissural path--CA1 pyramidal synapse can be modified