Discrete Behavioral Responses

Much of the research on how the brain codes and influences behavior has included studies of the neural bases of learning and memory. Physiological psychologists, neurophysiologists, neuroanatomists, and neuropsychologists have made major interdisciplinary contributions in this field, using a variety of experimental techniques.

One such productive research strategy in studying learning-related phenomena in the nervous system is the use of simple mammalian models. The model-systems approach assumes that the most promising path to an understanding of complex human learning and memory phenomena is the study of neural processes associated with simple learning and memory tasks in nonhumans. The governing assumption in such work is that the manner in which the brain codes learning and memory events is less complex in simple learning tasks than in more elaborate ones. This reduction in task complexity has made it possible to begin an analysis of the brain pathways and structures governing learning and memory processes.

Much useful data about brain substrates has emerged from classical conditioning of discrete behavioral responses. This approach involves presenting a conditioned stimulus (CS) just before a second, unconditioned stimulus (US). Before training, the CS typically elicits no overt response while the US reliably elicits a discrete, reflexive response called the unconditioned response (UR). After a number of CS-US pairings, however, the animal begins to execute the discrete response after presentation of the CS but before presentation of the US. This learned, anticipatory response is called the conditioned response (CR). Many investigations of the neural substrates of learning and memory have used this simple learning paradigm. The classical eyeblink-conditioning paradigm has significantly advanced the understanding of how the brain encodes learning processes. Two eyeblink-conditioning preparations are presented here: nictitating membrane/eyeblink conditioning in the rabbit and eyeblink conditioning in the freely moving rat.

Nictitating Membrane/Eyelid Conditioning in the Rabbit

In the early 1960s, Gormezano developed a classical conditioning preparation in rabbits that has proved valuable for the study of the neural bases of conditioning (see Gormezano, Kehoe, and Marshall, 1983 and Woodruff-Pak and Steinmetz, 2000). During simple classical delay conditioning, a tone or light CS is paired with a shock or air puff US. The CS initially causes no movement, while the US elicits movement of the nictitating membrane (the NM, a third eyelid found in some species) and closure of the outer eyelids. After 100 to 150 of these pairings, an NM or eyelid movement is elicited by the CS even in trials when the air puff or shock US is not presented. Because many of the parametric features of this behavioral paradigm have been well documented by Gormezano and associates, it has been adopted by a number of laboratories for use as a model system for the study of the neural substrates of learning (see Steinmetz, 2000 and Steinmetz et al., 2001, for reviews).

Data from a variety of animal preparations and from the human amnesia literature suggested that the hippocampus, a limbic system structure, was involved in coding learning and memory. Because of these observations, Thompson and associates attempted to assess hippocampal involvement in classical NM conditioning (e.g., Berger and Thompson, 1978). Recordings from the hippocampus revealed neurons that altered their firing patterns during paired but not unpaired presentations of the CS and US. Even before CRs were observed, neurons in the hippocampus began discharging in a pattern that preceded and "modeled" the amplitude and time course of the learned behavioral response (i.e., the unit activity looked as though it could be producing the behavioral response). But lesions of the hippocampus failed to abolish learning or retention of the simple motor response, even when the lesions included all of the neo-cortex as well as the hippocampus. These data indicated that the hippocampus was probably involved in coding the classically conditioned NM response but that it was not essential for producing CRs. More recent data suggest that in addition to possibly modulating the conditioning process during simple delay conditioning, an intact hippocampus may be necessary for more complex classical conditioning preparations like trace conditioning and discrimination-reversal conditioning. Furthermore, studies in humans suggest that awareness of the training contingencies may be an important determinant of the involvement of the hippocampus in conditioning (e.g., Manns, Clark, and Squire, 2000).

The interpositus nucleus of the cerebellum appears to be essential for classical conditioning (see Steinmetz, 2000 and Thompson, 1986, for reviews). The cerebellum is a structure that plays a major role in motor control. Recordings from the interpositus nucleus as well as portions of cerebellar cortex revealed populations of neurons that formed amplitude-time course models of the CR during paired CS-US presentations. Furthermore, electrolytic or chemical lesions of the interpositus nucleus (as small as one cubic millimeter) permanently abolished CRs in trained rabbits. Cerebellar lesions before training prevented the formation of CRs. The interpositus is known to output to the red nucleus, which in turn sends projections to brain-stem nuclei that control the musculature involved in generating NM movements and eyelid closure.

Brain stimulation, recording, and lesion methods have helped to delineate possible pathways involved in projecting the CS and US to the cerebellum. It appears that an acoustic CS may be projected to a number of primary brain-stem auditory nuclei that, in turn, relay parallel projections to lateral regions of the pontine nuclei. Cells in the lateral pontine nuclear regions may then project axons to the cerebellum. The air puff US appears to be projected from the cornea of the eye to the trigeminal nucleus in the brain stem to the inferior olivary complex (also in the brain stem). Cells in the inferior olive then appear to send axons to regions of the cerebellum. Data from temporary lesions (using the GABA agonist, muscimol, or brain cooling methods) and recording and stimulation studies have provided evidence that neuronal plasticity is established in cerebellar regions that receive convergent CS and US input and then relayed to brain-stem nuclei responsible for generating the motor response. Sites in the interpositus nucleus and discrete regions of the cerebellar cortex receive this convergent input; plasticity mechanisms at these sites are under investigation. Figure 1 shows a schematic diagram of neural circuitry that seems to play a role in classical eyelid conditioning.

The rabbit classical NM conditioning paradigm has produced a wealth of data concerning the neural substrates of a simple form of motor learning. The careful control over stimulus presentation and response elicitation afforded by this learning preparation has allowed the analysis of critical stimulus pathways and potential regions of stimulus convergence, thus advancing the study of the cellular bases of this form of learning.

Eyeblink Conditioning in the Freely Moving Rat

Rats have enjoyed a booming popularity as subjects in more recent classical eyeblink conditioning experiments. Several laboratories have begun using variations of a preparation described by Ronald Skelton (1988) to conduct classical eyeblink conditioning experiments in freely moving rats. This preparation uses tones or lights as CSs and periorbital stimulation as a US. Connections to the rat are made through a commutator that allows the rat free movement within the conditioning chamber.

Using rats instead of rabbits in eyeblink conditioning studies has some advantages: rats are less costly to purchase and maintain; more is known about the neuroanatomy of the rat; rats have a wider repertoire of other behaviors that can be studied at the same time as eyeblink conditioning; and the shorter lifespan of rats is better suited to developmental and aging studies.

Studies concerning the basic circuitry underlying eyeblink conditioning in the rat indicated that the basic neural substrates of this form of conditioning in rats are nearly identical to those of the rabbit. The cerebellum plays a critical role in encoding the learning and memory of the response. Other brain areas, like the hippocampus and neostriatum, are also involved in the conditioning process, and these regions seem to play a modulatory role in the learning and memory of this simple behavior.

The rat preparation has proved quite valuable for two lines of research. First, Stanton and colleagues have very successfully used the rat eyeblink conditioning preparation to study neural and behavioral correlates of development of this simple form of learning. For example, their elegant studies have shown that the development of conditioned responses parallels closely the development of the cerebellum and related brain circuits. Second, the rat eyeblink conditioning preparation has been used successfully to study the behavioral and neurological effects of early alcohol exposure, a model of the human condition known as fetal alcohol syndrome. In this model, neonatal rats are given binge levels of alcohol over a few days. Once the rats reach adulthood, researchers use eyeblink conditioning and neural recording and neuroanatomy methods to study the long-term effects of neonatal alcohol exposure on behavioral and neural function. Such studies indicate that neonatal alcohol exposure results in a permanent loss of neurons in the cerebellar cortex and the deep cerebellar nuclei and that that loss of these neurons, in turn, affects physiological and behavioral response during eyeblink conditioning, which requires the cerebellum.

Conclusion

The two different classical eyeblink conditioning discussed above have provided basic data on how the brain codes the learning of simple behavioral responses. Future work in this area will likely continue in two directions: further delineation of essential cellular processes that actually code the conditioning process (e.g., possible mechanisms in the cerebellum and brain stem that account for the conditioning); and studies aimed at delineating the interactions between higher (e.g., cerebral cortex) and lower (e.g., brain stem and cerebellum) brain areas during classical conditioning. Genetic knockout and mutant preparations, reversible brain inactivation methods, basic molecular biology techniques, used in conjunction with eyeblink classical conditioning should advance our understanding of the neurobiology of learning and memory, especially by providing key data about how the brain codes simple learning tasks like classical conditioning of discrete responses.

See also:ACTIVE AND PASSIVE AVOIDANCE LEARNING: BEHAVIORAL PHENOMENA; CLASSICAL CONDITIONING: BEHAVIORAL PHENOMENA; GUIDE TO THE ANATOMY OF THE BRAIN: CEREBELLUM; NEURAL SUBSTRATES OF CLASSICAL CONDITIONING: CARDIOVASCULAR RESPONSES;NEURAL SUBSTRATES OF CLASSICAL CONDITIONING: FEAR CONDITIONING, FREEZING; NEURAL SUBSTRATES OF CLASSICAL CONDITIONING: FEAR-POTENTIATED STARTLE

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Joseph E.Steinmetz