Eyelid Classical Conditioning Research Paper

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Classical conditioning as Pavlov (1927) discovered it involves the creation of an association between a previously neutral stimulus (a bell, a lab coat) and a reflex-evoking stimulus (meat powder) which causes an unconscious reflexive response (salivation). With repeated pairings the neutral stimulus (becoming the ‘conditional stimulus’ as Pavlov knew it or the conditioned stimulus as we know it) comes to evoke a learned response (the conditioned response) which is similar to but not exactly like the reflexive response elicited by the meat powder (the unconditioned stimulus). A picture of Pavlov’s salivating dogs is a common enough image to the public in cartoons and science fiction. Strictly speaking, Pavlovian conditioning is classical conditioning of an autonomic or visceral response.

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1. Somatic Conditioning

In contrast, classical conditioning of a somatic response involves a striated muscle reflex rather than smooth or cardiac muscles. The classic example is Twitmeyer’s patellar tendon reflex, familiar to most of us in a physical exam: a tap to the knee with a physician’s hammer (the unconditioned stimulus) causes a knee jerk reflex (the unconditioned response). The conditioning situation is similar to Pavlov’s autonomic conditioning, with a previously neutral stimulus such as a sound acquiring the eliciting properties of the unconditioned stimulus—the sound alone comes to elicit a knee jerk. The difference from Pavlov is that somatic conditioning of the patellar tendon reflex is a skeletal muscle response. For various reasons mostly related to its application, the patellar tendon reflex is awkward to implement. Theoretically, however, any reflexive motor response can be classically conditioned. The eyelid reflex provides a convenient, easily accessible, easily implemented means of studying classical conditioning of a somatic muscle response (e.g., Gormezano et al. 1986, Schneiderman et al. 1962, Woodruff-Pak and Steinmetz 2000). First widely used in studies of classical conditioning in humans, an eye blink response is typically elicited with a puff of air directed at the cornea (the unconditioned stimulus) or shock to the face. Either stimulus reliably elicits an eye blink. Animal studies of eye blink conditioning often use an air puff when the subject is restrained, but under conditions where the animal is freely moving a face shock may be used instead. Studies using human eye blink conditioning favor using an air puff.

2. Essential Properties Of Classical Conditioning

It is important to note that classical conditioning using Pavlov’s salivation or the eye blink paradigms are structured in the same way and have the same essential properties found in all classical conditioning. That is, in each paradigm there are identifiable conditioned and unconditioned stimuli, unconditioned reflexes and learned conditioned responses. The relationships between these stimuli and responses are also the same. Thus, for example, the conditioned stimulus must precede the unconditioned stimulus (forward conditioning) by at least about 100 ms. The learned conditioned response is very similar to but not identical to the evoked reflexive response. In both types of classical conditioning, acquisition shows a typical sigmoid curve and reaches an asymptotic level. In general, strong stimuli result in faster learning. Subsequent training where the unconditioned response is withheld (extinction procedure) causes diminution of the learned response, spontaneous recovery at the beginning of subsequent days of testing, and eventually the absence of conditioned responses. Additional training that pairs the conditioned and unconditioned stimuli rapidly restores the conditioned response. These are only a few of the salient properties that are shared in both types of classical conditioning.

3. Learning Conditioned Responses

In studies with animals, optimal learning is usually reached in about 130 trials when each trial has the onset of the tone conditioned stimulus preceding the onset of the air puff unconditioned stimulus by 250 ms. The interval between trials might average 30 s. Intertrial intervals of 9 s or less do not support conditioning. In studies with humans, optimal learning is usually reached in about 50 trials when each trial has the onset of the tone conditioned stimulus preceding the onset of the air puff unconditioned stimulus by 500 ms. The interval between trials might average 60 s. Humans must additionally be distracted during conditioning, for example, by watching silent movies. Recent studies show, however, that humans who learn can accurately describe the stimulus contingencies.

Learned responses are defined as eye blinks that occur after the onset of the tone conditioned stimulus and before the air puff unconditioned stimulus on paired trials. A further criterion is that the response must be 0.5 mm or larger to be counted as a response. On test trials where only the tone conditioned stimulus is given, conditioned responses are defined as any 0.5 mm or larger response after the tone until the end of the trial, usually a period of 250 ms after the air puff would have begun. In both cases, reflexive alpha responses that occur too quickly after the tone conditioned stimulus should be excluded as not being true conditioned responses. Typically for rabbits any response latency less than 25 ms is considered to be an alpha response. For humans, any response latency less than 100 ms might be considered to be an alpha response. A common criterion for learning is the first time that eight conditioned responses occur in nine trials (89 percent responding). An additional criterion for stable performance (overtraining) requires continued performance at some level, for example 70 percent responding with learned responses. With rabbits 100 percent responding is often achieved. With humans the rate is usually not perfect. Once learned, the association shows good retention over months and perhaps years.

Traditionally, the unconditioned response was thought to be stable and impervious to influence by the training stimuli. However, reflex facilitation is seen before conditioned responses appear. Reflex facilitation occurs when the size of the reflexive response to the air puff unconditioned stimulus increases in the presence of the tone conditioned stimulus. It has also been shown that there is sensitization of the air puff unconditioned response. This sensitization is seen as an increase in the size of the air puff unconditioned response after repeated exposure to tone conditioned stimuli and air puff unconditioned stimuli that have never been presented together.

4. Motor Response In Eyeblink Conditioning

The fact that the motor response is so well defined is one of the distinguishing features between operant and classical conditioning. The actual motor response in eye blink conditioning involves sensations of the air puff from receptors on the cornea and the region of the face around the eye (Prince 1964). This sensation involves the trigeminal nerve (cranial nerve V ) and its projections to the trigeminal nuclei. The trigeminal nuclei most involved are the principal (sensory) and descending (inferior or spinal) trigeminal nuclei. The former is characterized as being involved in the sense of touch and position, while the latter is characterized as being involved in light touch and pain, although these designations overlap. The reflex arc involves projections from the trigeminal nuclei to cranial motor nuclei that project motor neurons to the muscles involved in the eye blink. In its simplest terms the principal motor nucleus for the eye blink is the facial cranial nucleus (cranial nerve VII) which innervates the orbicularis occuli muscles that effect the eye blink. The eye blink involves more than just this connection but in humans this is the pathway usually considered. In animals other pathways may receive most of our attention. In common experimental subjects such as the rabbit and cat which have a ‘third eye lid’—the nictitating membrane—the cranial motor nuclei most involved are the abducens (cranial nucleus VI) and the accessory abducens nuclei, which innervate the rectus bulbus muscle in the rabbit. The nictitating membrane response for the rabbit involves retraction of the eyeball into the orbit, which presses Hardner’s gland, which in turn causes nictitating membrane extension. In the rabbit, nictitating membrane movement is a passive phenomenon. In the cat, nictitating membrane extension is an active process, where there is a projection to the superior oblique muscles that control the nictitating membrane. In addition to these connections, the eye blink response (especially in the case of the nictitating membrane in the rabbit) involves cranial projections to the extraocular eye muscles through the oculomotor (cranial nucleus III), trochlear (cranial nucleus IV), and abducens (cranial nucleus VI) innervations. Simultaneous activations of these extraocular muscles pull the eye ball into the socket.

5. Measuring The Eyeblink Response

From the foregoing it should be obvious that there are a couple of ways to measure the response to a puff of air directed at the eye. The most direct method is to measure the movement of the eyelids or nictitating membrane. This is accomplished by using a transducer—a device to translate movement into an electronic signal. Most commonly, this is accomplished with a minitorque potentiometer with a lever attached to the axle. Although it has been done, researchers are generally hesitant to directly attach a potentiometer lever to the eyelids of human subjects, for fear of poking the subject in the eye. This is especially a concern when the subject’s movements are unpredictable, as in Alzheimer’s patients. In animal subjects this measurement is made by placing a small loop of suture into the eyelid or nictitating membrane for attachment of the potentiometer lever or by temporarily gluing the potentiometer lever directly to the eyelid. The great advantage of using a potentiometer is that the actual movement is recorded. Since an eye blink presumably performs a protective feature, monitoring the actual eyelid movement gives an indication of its functionality.

An alternative to directly measuring the movement is to record EMG activity from the eyelid. This can be done directly by inserting recording electrodes into the orbicularis muscles, or indirectly by placing surface electrodes over the skin of the muscles. Both methods have been used in animals, but in humans surface electrodes are favored because they are noninvasive. It should be noted, however, that surface electrodes also pick up information about eye position because the anterior chamber of the eye is charged. In any event, the signal obtained through EMG measurements is imprecisely related to the actual movement. One method is to treat EMG as if it were a neural signal and to send it through a height discriminator. The actual shape of the eye blink is surprisingly reasonable with such a crude measure. The greatest advantage, however, is that the latency of the eye blink is very accurate. However, since movement is easier for us to relate to, EMG signals are usually converted electronically into signals that mimic the output of a potentiometer. The advantage is that the measurement appears to be more familiar. The disadvantage is that the electronics involved in the conversion delay the latency measurement depending on the speed of the eye blink (and therefore it is not a constant delay) and the overall travel of the blink is distorted by the filtering characteristics of the conversion.

Probably the most favored method for recording eye blinks in humans involves an indirect measure. Here, a light is reflected off the eye and the returning signal gives an indication of the position of the eyelid. Two methods are commonly employed. The first method uses visible light emissions from an incandescent source (for example, a flashlight bulb) or a light emitting diode (LED). The advantage of using visible light is that it can be used to continuously monitor the position of the eyelid. The one drawback of this method is that it is affected by ambient light. In particular, 60 Hz noise from fluorescent lighting can be picked up with the light detector. The second method uses infrared (IR) emissions from an IR-LED. The advantage of IR emissions is that they are little affected by ambient light. The disadvantage of IR emissions is that IR emission is heat which can be uncomfortable or even harmful to the eye. To counteract the heating problem, IR emissions are typically turned on and off rapidly to limit the build up of heat. This technique requires electronic circuitry to only measure eyelid activity during the period when the IR device is turned on, a technique which introduces higher frequency noise problems.

There is no perfect measure of the eye blink response. The methods outlined here indicate some of the trade-offs that must go into making a decision about acquiring the data.

6. A Model System For The Biology Of Learning And Memory

One of the old issues about conditioning is whether the subject needs to make the actual physical response in order to make the association. At one time there were reports in the operant autonomic conditioning literature that paralyzing the skeletal muscles with curare did not prevent the formation of learning, but this could not be replicated later. Instead, somatic mediation (voluntary skeletal movements or relaxation) is the prime factor in supposedly autonomic conditioning. Recently, however, experiments with reversible inactivation of the cranial motor nuclei show that classical conditioning is learned without the subject actually making the eye blink response during acquisition training.

Pavlov himself appreciated the advantages of classical conditioning for studies of the neural bases of learning and memory. The advantages include the experimenter’s control over the stimuli for the association and the experimenter’s ready identification of the motor responses (Gormezano et al. 1986, Schneiderman et al. 1962, Prince 1964). Classical conditioning is a model system for understanding the biology of learning. Unfortunately, Pavlov failed to localize learning and memory, in large part because he took a ‘top-down’ approach, starting with the cerebral cortex, which most people incorrectly assume is the repository of even the simplest of memories. The detached spinal cord is capable of forming primitive associations that are classically conditioned. Recently, studies that have taken a ‘bottom-up’ approach starting with the circuitry for the basic reflexive response have shown that the association for classical conditioning of the somatic muscle response involved in eyelid conditioning is localized to the cerebellum as a type of motor learning (e.g., Thompson et al. 1997).

Today, classical conditioning of the eyelid response is increasingly favored for providing a quick, reliable test of cognitive functioning in human clinical settings (Woodruff-Pak and Steinmetz 2000, Green and Woodruff-Pak 2000). The reasons for this include the clear structure of the paradigm with identifiable control over the stimuli and responses, the extensive behavioral studies on the properties of classical conditioning, and recent advances in identifying neural structures responsible for or participating in classical conditioning.


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  2. Green J T, Woodruff-Pak D S 2000 Eyeblink classical conditioning: Hippocampal formation is for neutral stimulus associations as cerebellum is for association-response. Psychological Bulletin 126: 138–58
  3. Pavlov I P 1927 Conditioned Reflexes. Oxford University Press, Oxford, UK
  4. Prince J H (ed.) 1964 The Rabbit in Eye Research. Charles C Thomas, Springfield, IL
  5. Schneiderman N, Fuentes I, Gormezano I 1962 Acquisition and extinction of the classically conditioned eyelid response in the albino rabbit. Science 136: 650–2
  6. Thompson R F, Bao S, Berg M S, Chen L, Cipriano B C, Kim J J, Thompson J K, Tracy J A, Krupa D J 1997 Associative learning. In: Schmahmann J D (ed.) The Cerebellum and Cognition. Academic Press, San Diego, CA
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