The establishment of synaptic connections and their refinement during development require neural activity. Increasing evidence suggests that spontaneous bursts of neural activity within an immature network are mediated by gamma-aminobutyric acid via a paradoxical excitatory action. Our data show that in the developing hippocampus such synchronous burst activity is generated in the hilar region by transiently coupled cells. These cells have been identified as neuronal elements because they fire action potentials and they are not positive for the glial fibrillary acidic protein staining. Oscillations in hilar cells are "paced" by a hyperpolarization-activated current, with properties of Ih. Coactivated interneurons synchronously release GABA, which via its excitatory action may serve a neurotrophic function during the refinement of hippocampal circuitry.

In instrumental learning, Thorndike's law of effect states that stimulus-response relations are strengthened if they occur prior to positive reinforcement and weakened if they occur prior to negative reinforcement. In this study, we demonstrate that neural correlates of Thorndike's law may be observed in the primary auditory cortex, A1. Adult owl monkeys learned to discriminate tones higher than a standard frequency. Responses recorded from implanted microelectrodes initially exhibited broad spectral selectivity over a four-to-five octave range. With training, frequency discrimination thresholds changed from close to one octave to about 1 12 octave. Physiological recordings during the week in which the monkey came under behavioral control signaled by a drop in measured threshold had stronger responses to all frequencies. During the same week, A1 neural responses to target stimuli increased relative to standard and nontarget stimuli. This emergent difference in responsiveness persisted throughout the subsequent weeks of behavioral training. These data suggest that behavioral responses to stimuli modulate responsiveness in primary cortical areas. In instrumental learning, an animal progressively associates its actions with future outcomes (1). One principle in instrumental learning is Thorndike's law of effect, which states that stimulus-response relationships are strengthened if their pairing leads to future positive reinforcement and weakened if their pairing leads to future negative reinforcement. This learning should be reflected by changes in associated neural response. In the current experiment, a chronic multisite recording implant technology (2) was used to document possible neural substrates of instrumental learning in the primary auditory cortex of the awake primate.Learning-induced representational change in mature primary sensory cortex occurs in a task-dependent manner (3-8) and can include changes in receptive field size (3,6,7,9), observed cortical column size (4, 8), and cortical representational area associated with behaviorally important sensory inputs (4, 7). One study (6), especially relevant to the experiments reported here, used operant training to drive cortical representational changes in the primary auditory cortex, A1, of adult owl monkeys. Animals oriented in a behavioral apparatus and attended to a series of tone pairs. If the elements of a pair differed in frequency, and the animal removed itself from the apparatus within time limits, it was rewarded with a food pellet. Misses resulted in a brief time-out. In trained animals, the cortical area that responded to the frequency range of the target and standard tones expanded. This change in representational area was inversely correlated with the progressively improved frequency discrimination thresholds.In the present study, owl monkeys were again engaged in auditory frequency discrimination, but in a different behavioral task. Animals oriented in a listening posture and received a series of single tones, which were initially at a constan...

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