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Eyeblink conditioning
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Eyeblink conditioning
Eyeblink conditioning (EBC) 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 trials 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.
The order in which stimuli are presented is an important factor in all forms of classical conditioning. 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). Animals are usually trained with a shorter ISI than humans, which can make interspecies comparisons difficult.
In delay EBC, the CS onset precedes the US onset and the two stimuli overlap and coterminate, with the stimuli converging in the cerebellar cortex and interpositus nucleus. 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 cerebellum, the trace procedure also requires the hippocampus and medial prefrontal cortex.
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 formation) 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.
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 cerebellum (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 pontine nuclei (PN) can support different CS modalities (auditory tone, light, etc.) for EBC as they receive projections from auditory, visual, somatosensory, 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 in the cerebellum, 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.
David A. McCormick, as a graduate student with Professor Richard F. Thompson, initially identified the cerebellum as the essential structure for learning and executing eyeblink CRs. Some scientists think that the interposed nucleus is the site critical to learning, retaining, and executing the conditioning blink response.
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Eyeblink conditioning
Eyeblink conditioning (EBC) 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 trials 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.
The order in which stimuli are presented is an important factor in all forms of classical conditioning. 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). Animals are usually trained with a shorter ISI than humans, which can make interspecies comparisons difficult.
In delay EBC, the CS onset precedes the US onset and the two stimuli overlap and coterminate, with the stimuli converging in the cerebellar cortex and interpositus nucleus. 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 cerebellum, the trace procedure also requires the hippocampus and medial prefrontal cortex.
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 formation) 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.
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 cerebellum (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 pontine nuclei (PN) can support different CS modalities (auditory tone, light, etc.) for EBC as they receive projections from auditory, visual, somatosensory, 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 in the cerebellum, 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.
David A. McCormick, as a graduate student with Professor Richard F. Thompson, initially identified the cerebellum as the essential structure for learning and executing eyeblink CRs. Some scientists think that the interposed nucleus is the site critical to learning, retaining, and executing the conditioning blink response.