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Conditioned reflexes exemplify associative learning

This is an excerpt from Neurophysiological Basis of Human Movement, Second Edition, by Mark L. Latash, PhD.

One of the subtypes of nondeclarative memory is associative learning. This type of learning involves creating a relationship between two stimuli. Typical examples include classical conditioning and operant conditioning. In operant conditioning, a relationship between an action by an animal and an external stimulus (commonly, a food reward) is learned. In other words, the animal’s own behavior acts as one of the stimuli. Classical conditioning involves a special group of phenomena termed conditioned reflexes.

It is known that putting food in the mouth leads to salivation, especially when the food is dry. Food stimulates the mucous membrane of the mouth, sensory nerves transfer the stimulation to the salivatory brain center, and the brain center reacts to the stimulation with a command to the salivary glands. This phenomenon occurs even in the smallest cubs. Such mechanisms are termed unconditioned or inborn reflexes. The famous Russian physiologist I.P. Pavlov discovered that if a hungry dog hears a bell or whistle, sees a bulb of a certain color, or senses something else half a minute before food arrives every time it eats, it gradually starts to salivate not when it gets the food and not even when it sees the food but when the additional signal is turned on. In such an experiment the researcher witnesses the birth of a new version of the salivation reflex elaborated with artificial means. This new version is not an inborn, general reflex but is a reflex reflecting an enrichment of the personal experience of the dog. These reflexes are termed conditioned in contrast to the inborn, unconditioned reflexes.

Pavlov suggested a theory of brain functioning based on unconditioned and conditioned reflexes. This theory considered the brain as a purely reactive organ whose behavior is defined by environmental stimuli, and it viewed behavior as the process of “equilibrating” the body with the environment. Unfortunately for this theory and fortunately for human beings (as well as many other animals), higher animals do not act as purely reactive systems but instead explore the environment, formulating needs and trying to satisfy them rather than waiting for appropriate stimuli from the environment to trigger conditioned reflexes. Activity is the driving force of the functioning of the human brain.

By the middle of the 20th century, the importance of activity was understood by another great Russian scientist, Nikolai Bernstein. Bernstein formulated the goals of behavior as overcoming the environment rather than equilibrating the body with it. Bernstein created the physiology of initiative (imprecisely referred to in Western publications as the physiology of activity), a whole new field of study that tries to explain behavior based on internal needs and goals of an animal (or a human). Commonly, these needs and goals are in conflict with stimuli from the environment.

Bernstein created the physiology of initiative to a large degree based on his earlier studies of the automation of labor and athletic movements. He found out that during movement automation, the variability of movement trajectories and other characteristics was not eliminated. Movements do not become identical or machine-like, although the ultimate motor outcome does become highly reproducible. For example, during shaving with a sharp razor blade or when shooting at a target, success depends on fractions of millimeters or angular seconds. Only the high variability of the behaviors of individual elements of automated movements allows humans to reach such a high accuracy when performing repetitions in conditions of ever-present and unexpected forces. Therefore, memory traces of such movements cannot represent movement formulas such as combinations of patterns of muscle forces, muscle activation levels, or joint trajectories. As we will discuss in chapter 19, the researchers of motor control do not agree on the physical or physiological nature of the control variables the central nervous system uses during natural voluntary movements. However, it is safe to say that skill formation by the central nervous system is an active process whose implications for the neurophysiology of memory have not been adequately explored.

Another procedure used in memory studies is operant conditioning. It involves rewarding an animal (with a small portion of a favorite food) for correct behavioral responses. The response may be under an apparent control by the animal, such as when choosing a correct turn in a maze, or it may not, such as during spinal reflex responses. Even the simplest, monosynaptic spinal reflexes can learn and store memory traces in operant conditioning experiments, as has been shown in the ingenious studies by Wolpaw and his group (Wolpaw 1987; Wolpaw and Carp 1993). Although it may require thousands of repetitions, eventually an animal’s central nervous system learns to modify an apparently uncontrollable phenomenon such as the amplitude of a monosynaptic response.

Problem #18.4

  • Pavlov built towers of silence in which his dogs were deprived of any stimuli except those used for conditioned reflexes. Use the theory of conditioned reflexes and the physiology of initiative to predict the behavior of the dogs.

Problem #18.5

  • What physiological mechanisms could be involved in changing the amplitude of the monosynaptic responses in Wolpaw’s experiments?

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