Single-cell activity in cat motor cortex. I. Modifications during classical conditioning procedures

Neuronal signalling of fear memory

The learning and remembering of fearful events depends on the integrity of the amygdala, but how are fear memories represented in the activity of amygdala neurons? Here, we review recent electrophysiological studies indicating that neurons in the lateral amygdala encode aversive memories during the acquisition and extinction of Pavlovian fear conditioning. Studies that combine unit recording with brain lesions and pharmacological inactivation provide evidence that the lateral amygdala is a crucial locus of fear memory. Extinction of fear memory reduces associative plasticity in the lateral amygdala and involves the hippocampus and prefrontal cortex. Understanding the signalling of aversive memory by amygdala neurons opens new avenues for research into the neural systems that support fear behaviour.

Sensitization induced receptive field plasticity in the auditory cortex is independent of CS-modality

Sensitization training with an auditory stimulus produces a general increase in response magnitude across the entire receptive field (RF) of neurons in the primary auditory cortex of the guinea pig (Bakin, J.S. and Weinberger, N.M., Brain Res., 536 (1990) 271-286). To determine if this effect reflects an auditory system-specific process or is caused by a process independent of the training stimulus modality, RFs in primary auditory cortex were characterized before and immediately after adult guinea pigs were given sensitization training with either an auditory or a visual training stimulus. General increases in auditory response magnitude across the RF were observed in 7 out of 7 auditory sensitization cases and 4 out of 5 visual sensitization cases. There were no statistical differences between the effects of auditory and visual sensitization training. These findings indicate that the general increases observed following sensitization training are the result of processes independent of CS modality, in contrast to the highly specific RF modifications that are caused by classical conditioning. The findings suggest that the 2 forms of RF plasticity, CS-specific re-tuning due to associative conditioning and polymodal general increases in gain due to non-associative sensitization, may reflect neural mechanisms involved in selective attention and vigilance, respectively.

Inhibitory action of a conditioning procedure on visual responsive neurons of the nucleus reticularis thalami in rats

In urethane anesthetized rats neuronal responses of the visual part of nucleus reticularis thalami (vTR) to light were compared with those during pairing light as a conditioned stimulus (CS) with the electrical stimulation of the rat's tail (US). The intensity of the US was adjusted to the minimum required to evoke a slight freezing behavior in the awake rat. The firing rate of most vTR neurons decreased in the period between light and US application (P less than 0.01). Significant response modulations to light were observed in 39% of the units, in most of them they persisted over an extinction period of 15 min. In addition, neurons which were predominantly inhibited by conditioning sometimes changed from regular spiking to a burst pattern. The results support the hypothesis that conditioning related facilitation of geniculate neurons observed in previous experiments can be explained at least partly by disinhibition of geniculate units from vTR.

Conditioning-related changes of unit activity in the dorsal lateral geniculate nucleus of urethane-anaesthetized rats

Changes in geniculate unit activity of urethane-anaesthetized and freely moving rats were investigated during conditioning. The conditioned stimulus (CS) was a flash which was paired with an electrical stimulation of the tail as unconditioned stimulus (US). The discharge rates evoked by the CS during forward conditioning were significantly higher in responding units than those evoked by reversal of CS and US (backward conditioning) or by pseudoconditioning. Tail stimulation alone did not cause significant changes in the firing rate of most of the neurons. In 25% of the investigated neurons the facilitation of activity evoked by forward conditioning persisted during an extinction period of more than 15 min. The effect of conditioning on neuronal activity appeared to be comparable in urethane-anaesthetized rats and in freely moving ones which responded to the US with a slight freezing behavior.

"Learned" changes in the responses of the rat barrel field neurons

The effect of pairing two vibrissa stimulations on unit responses of the barrel field of the somatosensory cortex were studied in partially restrained but awake and undrugged rats. Before pairing, one of the stimulations (S2) evoked a stable, short-latency and excitatory response from the recorded unit. Depending on the neuron, the other stimulation (S1), preceding S2 by 500 ms, did or did not have an effect before pairing. In a number of cases, the S1-S2 association produced significant changes in the unit responses: (1) the appearance of an excitatory response to S1 when that stimulus was ineffective before pairing; (2) the modification of pre-existing responses to S1 and/or S2. In all instances these modifications consisted in the decrease or disappearance of the "afferent inhibition" and/or the appearance of long-latency excitatory components. These effects appeared after some 30-100 trials and persisted in some cases up to 20 min after interruption of pairing. Our observations provide the first physiological data on the plasticity of the vibrissa projections in the chronic adult rodent. Though the underlying plastic neural elements and mechanisms remain to be specified, these phenomena suggest that "learned" changes in unit activity may occur in sensory systems and not only in "non-specific" ones.

Two neuronal systems are involved in a classical conditioning in the rat

Unit activity of the hippocampus, the centrum-medianum-parafascicularis and medialis dorsalis nuclei of the thalamus, was recorded in chronic rats during a classical conditioning; neocortical electroencephalographic and somatic responses were also recorded. Conditioned unit responses of the different groups of neurons were compared according to the precocity of their appearance, their stability, their latency and the differentiation between the positive (reinforced) and the negative (non-reinforced) conditioned stimuli. Conditioned unit responses of type I hippocampal neurons (probably pyramids) and of neurons located in the centrum-medianum-parafascicularis nucleus did not differentiate between the positive and negative conditioned stimuli; they progressed rapidly, then declined and disappeared. They were contemporary with an initial conditioning stage which was characterized by undifferentiated arousal responses (desynchrony of electroencephalographic activity) to both conditioned stimuli. Conditioned unit responses of type II hippocampal neurons (probably granule cells or interneurons of the fascia dentata) and of neurons located in the medialis dorsalis nucleus progressed slowly and differentiated between the conditioned stimuli during a late conditioning stage which was characterized by the regression of arousal responses and the differentiation of somatic responses. These data strongly suggest that two neuronal systems, each having a different role, are involved in classical conditioning in the rat.

Conditioning of single units in visual association cortex: cell-specific behavior within a small population

These experiments examine the interrelationships between the activity of adjacent neurons during learning. Does learning depend on coherent behavior in a population of neurons or does it depend on particular neurons engaging in a particular activity at specific times? A second purpose was to examine specificity in response modification as a function of reinforcement contingency. Cells from visual association cortex of locally anesthetized, paralyzed cats and rabbits were studied with extracellular microelectrodes capable of recording single and multiunit activity, as well as local field potentials. Multiunit records were fractionated by amplitude "windows" discrimination. Pavlovian discriminative conditioning procedures were used to evaluate selective plasticity. Cells that were activated by at least two different visual stimuli were selected. Only one of the effective stimuli was paired with foot-shock (reinforcement). Of the 180 cells or cell clusters studied, 27% exhibited conditioned modification to the reinforced stimulus (CS+) and 19% changed their response pattern to the unreinforced stimulus (CS-). None of the well isolated cells showed conditioning to both CS+ and CS-. Thus, cellular plasticity was specific to reinforcement contingency. These results provide a first demonstration of reinforcement-dependent functional distinctiveness at the neuronal level. Some cells showed no alteration of response pattern despite a most prolonged conditioning procedure. Neighboring cells, responsive to the same stimuli, revealed increases or decreases in firing rate, selective changes in the latency or amplitude of single response peaks, or the appearance of one or more new peaks as a function of conditioning. Rarely did adjacent cells show the same type of alteration when alteration occurred; there was no general tendency toward coherent firing patterns as conditioning proceeded.

Response plasticity of single neurons in rabbit auditory association cortex during tone-signalled learning

Single unit activity was monitored in rabbit auditory association cortex (AC) throughout the acquisition of classically conditioned, nictitating-membrane response. The CS was a tone burst at the characteristic frequency of each neuron. Rabbits which were pseudoconditioned or received conditioning trials but did not learn the response served as control groups. Significant alterations in CS-evoked firing rate were termed 'response plasticity'. Neurons in conditioned animals were more than twice as likely to show response plasticity during the 250 ms CS-US interval than neurons in control animals. Such differences were evident both in the early (0-60 ms) and late (60-250 ms) portions of the CS-US interval. Most early changes appeared at 21-40 ms after CS onset. Response plasticity was most commonly manifested as an increase or decrease in CS-evoked firing rate with little change in the response pattern (PST histogram shape). In some neurons, subcomponents of response patterns (early or late portions of the CS-US interval) were observed to change independently of each other. Spontaneous rate and UCS-evoked activity were not modified with learning. Early in training (transition trials), neural activity evoked by the tone CS in conditioned animals was not different from that in controls. Response plasticity was most pronounced after the CR was first learned (trained trials) and stabilized once the Cr was well established (overtrained trials). Recording sites of neurons showing conditioning-related response plasticity were co-extensive with those of cells that did not.

Classical conditioning of kindled seizures

As a model of reflex epilepsy, rats were classically conditioned to show paroxysmal spike activity to an auditory stimulus. A tone stimulus was paired with seizures induced by kindling. After repeated pairings, paroxysmal discharges were elicited at tone onset. The duration of the tone, proximity to the stimulation, and the number of pairings appeared to be important in the conditioning process.

Long-lasting facilitation of a synaptic potential following tetanization in the in vitro hippocampal slice

Field potentials evoked by stimulation of afferent fibers in stratum radiatum were recorded in the CA1 region of the hippocampal slice maintained in vitro. Stimulation rates of 3-50/sec produced a large increase in amplitude of the population spike in CA1. This increase was maintained for several hours after the tetanization. The facilitation phenomenon appeared to be specific to the synapse of stratum radiatum afferents onto CA1 pyramidal cells since: (1) stimulation outside the radiatum layer did not produce the effect, (2) antidromic field potentials recorded in CA3 were unchanged, (3) EPSP threshold in CA1 was unchanged, and (4) alveus tetanization did not produce a facilitatory effect.