Eyeblink conditioning
Encyclopedia
Eyeblink conditioning is a form of classical conditioning
that has been used extensively to study neural structures and mechanisms that underlie learning
and memory
. The procedure is relatively simple and usually consists of pairing an auditory or visual stimulus
(the conditioned stimulus (CS)) with an eyeblink
-eliciting unconditioned stimulus (US) (e.g. a mild puff of air to the cornea or a mild shock). Naïve organisms initially produce a reflexive, unconditioned response (UR) (e.g. blink or extension of nictitating membrane) that follows US onset. After many CS-US pairings, an association is formed such that a learned blink, or conditioned response (CR), occurs and precedes US onset. The magnitude of learning
is generally gauged by the percentage of all paired CS-US trial
s that result in a CR. Under optimal conditions, well-trained animals produce a high percentage of CRs (> 90%). The conditions necessary for, and the physiological mechanisms that govern, eyeblink CR learning have been studied across many mammalian species, including mice, rats, guinea pigs, rabbits, ferrets, cats, and humans. Historically, rabbits have been the most popular research subjects.
. Forward conditioning describes a presentation format in which the CS precedes the US in time. That is, from the perspective of the research subject, experiencing the US is contingent upon having just experienced the CS. EBC is usually, but not always, conducted in this manner. Other stimulus contingencies
include backward conditioning, in which US comes before CS, and simultaneous conditioning, in which CS and US are presented at the same time. In any case, the time between CS onset and US onset is the interstimulus interval
(ISI).
, the trace procedure also requires the hippocampus
.
) to the accessory abducens and abducens motor nuclei (see Cranial nerve nucleus
). Output from these nuclei control various eye muscles that work synergistically to produce an unconditioned blink response to corneal stimulation (reviewed, Christian & Thompson, 2003). Electromyogram (EMG) activity of the orbicularis oculi muscle
, which controls eyelid closure, is considered to be the most prominent and sensitive component of blinking (Lavond et al., 1990) and is, thus, the most common behaviorally-derived dependent variable in studies of EBC.
(Brodal, Walberg & Hoddevik, 1975; Thompson, 1989). Climbing fibers ultimately project to both the deep cerebellar nuclei and Purkinje cells (PCs) in the cerebellar cortex.
(PN) can support different CS modalities (auditory tone, light, etc.) for EBC as they receive projections from auditory, visual, somatasensory, and association systems (Glickstein et al., 1980; Brodal, 1981; Schmahmann & Pandya, 1989; 1991; 1993). When the CS is a tone, auditory information is received via the cochlear nuclei
(Steinmetz & Sengelaub, 1992). The PN give rise to mossy fiber (MF) axons that carry CS-related information (Steinmetz et al., 1987; Lewis et al., 1987; Thompson et al., 1997) to the cerebellum via the middle cerebellar peduncle, and terminate in both the cerebellar nuclei, and at granule cells (GR) of the cerebellar cortex (Steinmetz & Sengelaub, 1992). Granule cells give rise to parallel fiber
(PF) axons which synapse onto PCs.
Two cerebellar sites of CS-US convergence are 1) cells of the deep nuclear region, and 2) PCs of the cortex. In addition to receiving converging CS and US input via the PN and IO, respectively, cells of the cerebellar nuclei receive GABA
-ergic inhibitory input from PCs of the cerebellar cortex. Output from the interpositus nucleus includes projections to the red nucleus
, and the red nucleus sends projections to the facial and abducens nuclei. These nuclei supply the motor output component of the reflexive eyeblink. Therefore, in addition to being a site of stimulus convergence, the deep nuclei are also the cerebellum’s output structure.
lesions, which destroy neuronal cell bodies and spare passing fibers, provided evidence for a highly localized region of cerebellar nuclear cells that are essential for learning and performing CRs (Lavond et al., 1985). The population of cells critical for EBC appears to be restricted to a ~ 1 mm3 area of dorsolateral anterior INP ipsilateral to the conditioned eye. Lesions to this area of INP result in an inability to acquire eyeblink CRs in naïve animals. Additionally, the permanence of the localized lesion effect is remarkable. In well-trained animals, CRs abolished as a result of lesion are not reacquired, even after extensive training that spans over 8 months (Steinmetz et al., 1992). These results demonstrate that a highly localized region of cerebellum must be intact for CR learning to occur in EBC.
or Lidocaine
. These methods are advantageous primarily because the experimenter can essentially turn neural tissue on and off, per se. The effect of each of these inactivation protocols on CR learning and execution has been tested throughout the cerebellum and associated brainstem structures. When applied to the INP, temporary inactivation completely prevents learning of CRs in naïve animals, and learning occurs normally during post-inactivation training (Clark et al., 1992; Krupa et al., 1993; Nordholm et al., 1993; Krupa & Thompson, 1997). Additionally, INP inactivation in well-trained animals results in a complete depression of conditioned responding, which returns to plateau levels when the INP comes back online (Clark et al., 1992).
, and have revealed a population of cells that discharge prior to the initiation of the learned eyeblink CR and fire in a pattern of increased response frequency that predicted and modeled the temporal form of the behavioral CR (McCormick et al., 1981; 1982; 1983; Thompson, 1983; 1986; Foy et al., 1984; McCormick & Thompson, 1984a; b; Berthier & Moore, 1990; Gould & Steinmetz, 1996). Similar results were found in the rat INP (Freeman & Nicholson, 2000; Stanton & Freemen, 2000; Rogers et al., 2001), thus demonstrating that underlying circuitry for this form of learning may be conserved across species. Although samples of single-unit activity from the INP and surrounding nuclei have revealed a multitude of response patterns during EBC (Tracy, 1995), many of the cells in the anterior dorsolateral INP significantly increase their firing rate in a precise temporal pattern that is delayed from CS onset and precedes CR onset (Foy et al., 1984; Berthier & Moore, 1990). This pattern of responding is indicative of a structure that is capable of encoding learning and/or executing learned behavioral responses.
critical to EBC have been posited to exist downstream from the cerebellum. Some proposed loci include the red nucleus (Tsukahara, Oda, and Notsu, 1981), the trigeminal nucleus and associated structures (Desmond & Moore, 1983), or the Facial motor nucleus
(Woody et al., 1974). All of these structures have been ruled out as potential sites of plasticity critical to learning the eyeblink CR (Krupa, Thompson, and Thompson, 1993; Clark and Lavond, 1996; Krupa, Weng, and Thompson, 1996).
However recent studies (Nilaweera et al., 2006) found that temporary block of cerebellar output prevented normal acquisition of conditioned responses. The authors concluded that this form of associative learning in the rabbit eyeblink system requires extra-cerebellar learning and/or cerebellar learning that depends on the operation of cerebellar feedback loops.
Muscimol
and found significant acquisition deficits, but animals eventually learned. Clark et al. (1997) replicated these results with a cooling probe in HVI. Attwell, Rahman, and Yeo (2001) discovered similar disruption of with HVI inactivation. They infused the AMPA receptor antagonist CNQX into HVI during acquisition training and found that CNQX
-infused rabbits did not learn the eyeblink CR. However, post-acquisition CNQX infusions did not affect retention. These results are perplexing, given that animals ultimately learned the eyeblink CR in all other cerebellar cortical lesion and inactivation studies. One reason why this effect is so strong may be that Attwell et al., (2001) trained animals for only 4 days at an ISI that is outside of a range known to be optimal for learning [150-300 msec is an optimal CS-US interval and the magnitude of learning decreases as the ISI is increased (Schneiderman and Gormezano, 1964; Smith, Coleman, and Gormezano, 1969)].
In a single unit recording study where the individual Purkinje cells were shown to be located in the area controlling blinks and to receive climbing fibre input on US presentations, only inhibitory responses were found. In a recent study of similarly characterized Purkinje cells which were followed for up to more than fifteen hours, it was found that repeated presentations of the CS and US caused the gradual development of a pause in Purkinje cell firing. This pause response, called a Purkinje cell CR, was also obtained when direct mossy fibre stimulation was used as the CS and direct climbing fibre stimulation as the US. Unpaired presentations of the CS and US caused extinction of the Purkinje cell CR. When paired presentations were reintroduced after extinction, Purkinje cell CRs reappeared rapidly, mirroring the "savings" phenomenon demonstrated at the behavioral level. Purkinje cell CRs were also adaptively timed.
Classical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...
that has been used extensively to study neural structures and mechanisms that underlie learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
and memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
. The procedure is relatively simple and usually consists of pairing an auditory or visual stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
(the conditioned stimulus (CS)) with an eyeblink
Blink
Blinking is the rapid closing and opening of the eyelid. It is an essential function of the eye that helps spread tears across and remove irritants from the surface of the cornea and conjunctiva. Blink speed can be affected by elements such as fatigue, eye injury, medication, and disease...
-eliciting unconditioned stimulus (US) (e.g. a mild puff of air to the cornea or a mild shock). Naïve organisms initially produce a reflexive, unconditioned response (UR) (e.g. blink or extension of nictitating membrane) that follows US onset. After many CS-US pairings, an association is formed such that a learned blink, or conditioned response (CR), occurs and precedes US onset. The magnitude of learning
Learning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
is generally gauged by the percentage of all paired CS-US trial
Trial
A trial is, in the most general sense, a test, usually a test to see whether something does or does not meet a given standard.It may refer to:*Trial , the presentation of information in a formal setting, usually a court...
s that result in a CR. Under optimal conditions, well-trained animals produce a high percentage of CRs (> 90%). The conditions necessary for, and the physiological mechanisms that govern, eyeblink CR learning have been studied across many mammalian species, including mice, rats, guinea pigs, rabbits, ferrets, cats, and humans. Historically, rabbits have been the most popular research subjects.
CS-US Contingency
The order in which stimuli are presented is an important factor in all forms of classical conditioningClassical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...
. Forward conditioning describes a presentation format in which the CS precedes the US in time. That is, from the perspective of the research subject, experiencing the US is contingent upon having just experienced the CS. EBC is usually, but not always, conducted in this manner. Other stimulus contingencies
Contingencies
Contingencies is the bimonthly magazine of the American Academy of Actuaries, providing a large and diverse readership with general interest and technical articles on a wide range of issues related to the actuarial profession...
include backward conditioning, in which US comes before CS, and simultaneous conditioning, in which CS and US are presented at the same time. In any case, the time between CS onset and US onset is the interstimulus interval
Interstimulus interval
The interstimulus interval is the temporal interval between the offset of one stimulus to the onset of another. For instance, Max Wertheimer did experiments with two stationary, flashing lights that at some interstimulus intervals appeared to the subject as moving instead of stationary...
(ISI).
The Delay and Trace Procedures
In delay EBC, the CS onset precedes the US onset and the two stimuli overlap and coterminate. In the trace EBC, the CS precedes the US and there is a stimulus free period (trace interval) between CS offset and US onset. While both of these procedures require the cerebellumCerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, the trace procedure also requires the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
.
The Blink Reflex
When a US is delivered to the cornea of the eye, sensory information is carried to the trigeminal nucleus and relayed both directly and indirectly (via reticular formationReticular formation
The reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
) to the accessory abducens and abducens motor nuclei (see Cranial nerve nucleus
Cranial nerve nucleus
A cranial nerve nucleus is a collection of neurons in the brain stem that is associated with one or more cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nuclei. Lesions occurring at these nuclei can lead to effects resembling those seen by...
). Output from these nuclei control various eye muscles that work synergistically to produce an unconditioned blink response to corneal stimulation (reviewed, Christian & Thompson, 2003). Electromyogram (EMG) activity of the orbicularis oculi muscle
Orbicularis oculi muscle
The orbicularis oculi is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short fibrous band, the medial palpebral ligament.From...
, which controls eyelid closure, is considered to be the most prominent and sensitive component of blinking (Lavond et al., 1990) and is, thus, the most common behaviorally-derived dependent variable in studies of EBC.
The US Pathway
The trigeminal nucleus also sends efferent projections to the inferior olive (IO), and this represents the US pathway for EBC. The critical region of the IO for eyeblink conditioning is the dorsal accessory olive (Brodal, 1981), and climbing fibers (CF) from this region send information about the US to the cerebellumCerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
(Brodal, Walberg & Hoddevik, 1975; Thompson, 1989). Climbing fibers ultimately project to both the deep cerebellar nuclei and Purkinje cells (PCs) in the cerebellar cortex.
The CS Pathway
The pontine nucleiPontine nuclei
The pontine nuclei are a part of the pons involved in motor activity. Corticopontine fibres carry information from the primary motor cortex to the ipsilateral pontine nucleus in the ventral pons, and the pontocerebellar projection then carries that information to the contralateral cerebellum via...
(PN) can support different CS modalities (auditory tone, light, etc.) for EBC as they receive projections from auditory, visual, somatasensory, and association systems (Glickstein et al., 1980; Brodal, 1981; Schmahmann & Pandya, 1989; 1991; 1993). When the CS is a tone, auditory information is received via the cochlear nuclei
Cochlear nuclei
The cochlear nuclei are two heterogeneous collections of neurons in the mammalian brainstem that receive input from the cochlear nerve, which carry sound information from the cochleae...
(Steinmetz & Sengelaub, 1992). The PN give rise to mossy fiber (MF) axons that carry CS-related information (Steinmetz et al., 1987; Lewis et al., 1987; Thompson et al., 1997) to the cerebellum via the middle cerebellar peduncle, and terminate in both the cerebellar nuclei, and at granule cells (GR) of the cerebellar cortex (Steinmetz & Sengelaub, 1992). Granule cells give rise to parallel fiber
Parallel fiber
Parallel fibers arise from granule cells in the cerebellar cortex. They form excitatory synapses onto the dendrites of Purkinje cells ....
(PF) axons which synapse onto PCs.
CS-US Convergence in the Cerebellum
Two cerebellar sites of CS-US convergence are 1) cells of the deep nuclear region, and 2) PCs of the cortex. In addition to receiving converging CS and US input via the PN and IO, respectively, cells of the cerebellar nuclei receive GABA
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
-ergic inhibitory input from PCs of the cerebellar cortex. Output from the interpositus nucleus includes projections to the red nucleus
Red nucleus
The red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
, and the red nucleus sends projections to the facial and abducens nuclei. These nuclei supply the motor output component of the reflexive eyeblink. Therefore, in addition to being a site of stimulus convergence, the deep nuclei are also the cerebellum’s output structure.
Critical Role of the Interpositus Nucleus
Professor Richard F. Thompson and his colleagues initially identified the cerebellum as the essential structure for learning and executing eyeblink CRs. Some scientists think that the Interpositus nucleus is the site critical to learning, retaining, and executing the conditioning blink response.Lesion Studies
The first evidence for the role of the cerebellum in EBC came from McCormick et al. (1981). They found that a unilateral cerebellar lesion which included both cortex and deep nuclei permanently abolished CRs. In subsequent studies, it was determined that lesions of the lateral interpositus and medial dentate nuclei were sufficient to prevent acquisition of CRs in naïve animals (Lincoln et al., 1982) and abolished CRs in well-trained animals (McCormick & Thompson, 1984). Finally, the use of Kainic acidKainic acid
Kainic acid is a natural marine acid present in some seaweed. It is a specific agonist for the kainate receptor used as an ionotropic glutamate receptor which mimics the effect of glutamate...
lesions, which destroy neuronal cell bodies and spare passing fibers, provided evidence for a highly localized region of cerebellar nuclear cells that are essential for learning and performing CRs (Lavond et al., 1985). The population of cells critical for EBC appears to be restricted to a ~ 1 mm3 area of dorsolateral anterior INP ipsilateral to the conditioned eye. Lesions to this area of INP result in an inability to acquire eyeblink CRs in naïve animals. Additionally, the permanence of the localized lesion effect is remarkable. In well-trained animals, CRs abolished as a result of lesion are not reacquired, even after extensive training that spans over 8 months (Steinmetz et al., 1992). These results demonstrate that a highly localized region of cerebellum must be intact for CR learning to occur in EBC.
Reversible Inactivation Studies
Reversible inactivation of the INP has provided further evidence for its role in EC. Methods used to temporarily inactivate nervous tissue include use of a cooling probe (< 10°C), and locally infusing MuscimolMuscimol
Muscimol is the major psychoactive alkaloid present in many mushrooms of the Amanita genus. Unlike psilocin , which is a serotonergic psychedelic and agonist for the 5-HT2A receptor set, muscimol is a potent, selective agonist for the GABAA receptor set and is a deliriant as a opposed...
or Lidocaine
Lidocaine
Lidocaine , Xylocaine, or lignocaine is a common local anesthetic and antiarrhythmic drug. Lidocaine is used topically to relieve itching, burning and pain from skin inflammations, injected as a dental anesthetic or as a local anesthetic for minor surgery.- History :Lidocaine, the first amino...
. These methods are advantageous primarily because the experimenter can essentially turn neural tissue on and off, per se. The effect of each of these inactivation protocols on CR learning and execution has been tested throughout the cerebellum and associated brainstem structures. When applied to the INP, temporary inactivation completely prevents learning of CRs in naïve animals, and learning occurs normally during post-inactivation training (Clark et al., 1992; Krupa et al., 1993; Nordholm et al., 1993; Krupa & Thompson, 1997). Additionally, INP inactivation in well-trained animals results in a complete depression of conditioned responding, which returns to plateau levels when the INP comes back online (Clark et al., 1992).
Neural Recording Studies
Recordings of multiple-unit neuronal activity from rabbit INP during eyeblink conditioning have been possible with Chronic Electrode ImplantsChronic Electrode Implants
Chronic Electrode Implants are electronic devices implanted into the brain. They may record electrical impulses in the brain or they may stimulate neurons with electrical impulses from an external source.- Clinical applications and direction :...
, and have revealed a population of cells that discharge prior to the initiation of the learned eyeblink CR and fire in a pattern of increased response frequency that predicted and modeled the temporal form of the behavioral CR (McCormick et al., 1981; 1982; 1983; Thompson, 1983; 1986; Foy et al., 1984; McCormick & Thompson, 1984a; b; Berthier & Moore, 1990; Gould & Steinmetz, 1996). Similar results were found in the rat INP (Freeman & Nicholson, 2000; Stanton & Freemen, 2000; Rogers et al., 2001), thus demonstrating that underlying circuitry for this form of learning may be conserved across species. Although samples of single-unit activity from the INP and surrounding nuclei have revealed a multitude of response patterns during EBC (Tracy, 1995), many of the cells in the anterior dorsolateral INP significantly increase their firing rate in a precise temporal pattern that is delayed from CS onset and precedes CR onset (Foy et al., 1984; Berthier & Moore, 1990). This pattern of responding is indicative of a structure that is capable of encoding learning and/or executing learned behavioral responses.
Critical Sites for Learning Downstream?
Alternative sites of Synaptic plasticitySynaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
critical to EBC have been posited to exist downstream from the cerebellum. Some proposed loci include the red nucleus (Tsukahara, Oda, and Notsu, 1981), the trigeminal nucleus and associated structures (Desmond & Moore, 1983), or the Facial motor nucleus
Facial motor nucleus
The facial motor nucleus is a collection of neurons in the brainstem that belong to the facial nerve . These lower motor neurons innervate the muscles of facial expression and the stapedius.-Anatomy:...
(Woody et al., 1974). All of these structures have been ruled out as potential sites of plasticity critical to learning the eyeblink CR (Krupa, Thompson, and Thompson, 1993; Clark and Lavond, 1996; Krupa, Weng, and Thompson, 1996).
Summary
Taken together, results from lesion, inactivation, and neural recording studies seem to demonstrate that the dorsolateral portion of the anterior interpositus nucleus (INP) of the cerebellum, ipsilateral to the trained eye, is an essential site for CR acquisition and expression in EBC (Lincoln et al., 1982; Lavond et al., 1984a,b).However recent studies (Nilaweera et al., 2006) found that temporary block of cerebellar output prevented normal acquisition of conditioned responses. The authors concluded that this form of associative learning in the rabbit eyeblink system requires extra-cerebellar learning and/or cerebellar learning that depends on the operation of cerebellar feedback loops.
Role of the Cerebellar Cortex
Two areas of cortex that are known to be involved in eyeblink conditioning are lobule HVI (Lavond et al., 1987; Lavond and Steinmetz, 1989; Yeo and Hardiman, 1992) and the anterior lobe ((ANT) Garcia, Steele, and Mauk, 1999). The importance of cerebellar cortex in EBC, relative to INP, is a matter of debate in the scientific community.Lesion Studies
Several studies have attempted to assess the role of the cerebellar cortex in eyeblink CR learning, and early studies focused on large aspiration lesions of cerebellar cortex. Lavond and Steinmetz (1989) completely removed lobules HVI/HVIIa and significant portions of ANT, sparing INP, and found significant acquisition deficits. Compared to controls, lesioned animals took seven times longer to reach learning criterion. Significant percentages of CRs were eventually reached by the cortically-lesioned animals, but the CRs were low in amplitude and poorly timed. Finally, large lesions of cerebellar cortex after learning do not abolish learned CRs (Lavond et al., 1987). One common factor in all of these cortical ablation studies was that portions of cortex were spared; making it possible to assume that other areas of cortex were compensating for the loss of tissue.The pcd Mouse
Classical conditioning of a Purkinje cell deficient mutant mouse strain helped to determine the extent to which spared regions in cerebellar cortex were compensating for lesioned regions in the studies mentioned above. These mice are born with PCs that die after about 3 weeks of life. Because PCs are the sole output neuron of the cortex, this model effectively lesions all of cerebellar cortex. Results of conditioning were similar to the cortical aspiration mice. Mice took significantly longer to produce CRs, and the timing and gain of the response were distorted (Chen et al., 1996). Therefore, although eyeblink CR learning deficits are associated with cerebellar cortex lesions, the structure does not appear, ultimately, to be essential for CR learning or retention.Reversible Inactivation Studies
Results from cerebellar cortical inactivation studies are similar to those reported for lesion studies. For example, Krupa (1993) inactivated lobule HVI with the GABAA receptor agonistAgonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...
Muscimol
Muscimol
Muscimol is the major psychoactive alkaloid present in many mushrooms of the Amanita genus. Unlike psilocin , which is a serotonergic psychedelic and agonist for the 5-HT2A receptor set, muscimol is a potent, selective agonist for the GABAA receptor set and is a deliriant as a opposed...
and found significant acquisition deficits, but animals eventually learned. Clark et al. (1997) replicated these results with a cooling probe in HVI. Attwell, Rahman, and Yeo (2001) discovered similar disruption of with HVI inactivation. They infused the AMPA receptor antagonist CNQX into HVI during acquisition training and found that CNQX
CNQX
CNQX is a competitive AMPA/kainate receptor antagonist. Its chemical formula is C9H4N4O4.-External links:**...
-infused rabbits did not learn the eyeblink CR. However, post-acquisition CNQX infusions did not affect retention. These results are perplexing, given that animals ultimately learned the eyeblink CR in all other cerebellar cortical lesion and inactivation studies. One reason why this effect is so strong may be that Attwell et al., (2001) trained animals for only 4 days at an ISI that is outside of a range known to be optimal for learning [150-300 msec is an optimal CS-US interval and the magnitude of learning decreases as the ISI is increased (Schneiderman and Gormezano, 1964; Smith, Coleman, and Gormezano, 1969)].
Neural Recording Studies
Electrophysiological recording studies of cerebellar cortex have helped to better understand the role that PCs play in the eyeblink CR learning process. McCormick and Thompson (1984b) recorded PC activity during eyeblink training and found cell populations that discharged in a pattern apparently related the behavioral CR, while other PC populations discharged in patterns that coincided with either presentation of the CS or US. Similar results were found by Berthier and Moore (1986) with single unit recording of PCs in lobule HVI. They found that populations of neurons fire in relation to various aspects of eyeblink training, including CS and US presentation and CR execution. (Berthier and Moore, 1986; Gould and Steinmetz, 1996). Recently, similar stimulus- and response-related PC activity has been found in ANT (Green and Steinmetz, 2005). Finally, electrophysiological recordings of PCs in HVI and ANT have revealed a difference in the overall population responses of PCs. The majority of PCs show excitatory patterns of activity during eyeblink conditioning in HVI (Berthier and Moore, 1986; Gould and Steinmetz, 1996; Katz and Steinmetz, 1997), and inhibitory patterns of activity in ANT (Green and Steinmetz, 2005).In a single unit recording study where the individual Purkinje cells were shown to be located in the area controlling blinks and to receive climbing fibre input on US presentations, only inhibitory responses were found. In a recent study of similarly characterized Purkinje cells which were followed for up to more than fifteen hours, it was found that repeated presentations of the CS and US caused the gradual development of a pause in Purkinje cell firing. This pause response, called a Purkinje cell CR, was also obtained when direct mossy fibre stimulation was used as the CS and direct climbing fibre stimulation as the US. Unpaired presentations of the CS and US caused extinction of the Purkinje cell CR. When paired presentations were reintroduced after extinction, Purkinje cell CRs reappeared rapidly, mirroring the "savings" phenomenon demonstrated at the behavioral level. Purkinje cell CRs were also adaptively timed.