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Optical recording methods using voltage-sensitive dyes were used to monitor activity in rat somatosensory cortex. We measured evoked signals in response to whisker stimulation before (control) and after the addition of the epileptogenic agent, bicuculline, and also detected spontaneous interictal events that occurred after bicuculline. Bicuculline led to an increase in the size, duration, cortical extent, and, surprisingly, the latency of the evoked responses. These enhanced evoked responses appeared to originate in the region of the control response and propagate outward. In contrast, the spontaneous signals appeared to originate at random cortical positions and had a more variable cortical extent. A transition signal measured just after the addition of bicuculline was larger than the control response but localized and rapid in time course. In most cases, there was a good correlation between the optical recordings and field potential measurements made with a ball electrode on the cortical surface, but there were occasional instances where the optical signal disappeared while the ball electrode signal was unchanged.

Food-avoidance conditioning in the mollusk Pleurobranchaea results in suppression of the feeding response to food stimiuli. In conditioned animals, identified interneurons of the central pattern generator (CPG) for feeding behavior, the Int-2s, respo',nd to a food stimulus with greater and more long-lasting excitation than controls. Enhanced Int-2 responsiveness to food stimuli is associated with markedly heightened Int-2 excitability. Sustained activity in the Int-2s arrests motor output of the,oscillatory CPG in the protraction/retraction movement cycle of feeding through tonic excitation of a population of retractor interneurons and inhibition of protractors. The CPG, locus of the learning mechanism is permissive of sensory excitation of alternative behavior and leaves the possibility open for release of the suppressed behavior in a fully aroused state.Identification and characterization of learning mechanisms is an important goal-in understanding the organization of animal behavior. Orie animal in which the neurophysiology of learning has. been u.usefully studied is the marine slug Pleurobranchaea, a predatory and voracious feeder on other invertebrates and carrion. Pleurobranchaea readily learns to avoid specific food stimuli and suppress its feeding behavior when food is presented in association with a noxious electric shock (1-3). Leaining occurs efficiently over 1-10 training trials and is influenced by motivational state (4); memory persists over many days (2). This type of learning has clear adaptive significance for the foraging strategies and food selection of omnnivores and predators (5).Neural correlates of learning were previously reported (6, 7). In the hungry untrained animal, food applied to the chemosensory oral veil normally causes strong synaptic excitation and spiking of the paracerebral neurons (PCNs) of the brain (cerebropleural ganglion). PCN activity is important in the initiation and maintenance of feeding behavior (8). In contrast, in food-avoidance-trained animals food stimuli cause profound synaptic inhibition of the PCNs coincident with suppression of feeding behavior (6, 7). When the inhibitory pathway was described, it was found to have widespread synaptic effects throughout the motor network for feeding behavior (9-12). Moreover, activity in this pathway suppressed feeding behavior (11, 12).The following experiments were undertaken to elucidate the roles of the inhibitory neurons in learning. The neurons of the inhibitory pathway were found to be more active in conditioned animals, a result largely due to enhanced excitability in one of the neuron groups: the interneuron 2 population (Int-2s). Evidence that they substantially contribute to learned suppression of feeding establishes these neurons as loci where feeding behavior is regulated by learning experience. The mode of action whereby the Int-2s inhibit behavior provides an interesting mechanism with wider implications for the organization of behavior. MATERIALS AND METHODSConditioning Procedures. Pleurobranchaea cal...

Optical monitoring methods have reached the level of development where activity from a network of cells can be recorded in a minimally-dissected behaving animal. The spike activity in the buccal ganglion of Navanax was monitored during feeding and activity in the Aplysia abdominal ganglion was monitored during the gill-withdrawal reflex. Approximately 30 neurons in the Navanax buccal ganglion were active during feeding and between 250 and 400 neurons in the Aplysia abdominal ganglion were active during the gill-withdrawal reflex. A reasonably complete understanding of the neuronal basis of the gill withdrawal may not be possible with presently available scientific methods. Substantial improvements in signal-to-noise ratio in optical measurements will be necessary before the majority of synaptic potentials can be detected optically. Understanding circuits that involve more than a few neurons will be a challenge to neurobiologists.