Some years ago, we adopted classical conditioning of
the eyeblink response as a prototypic model of Pavlovian
conditioning and the rabbit as our initial preparation,
largely because so much work had been done with this animal
and paradigm. All mammals studied, including human, exhibit
the same basic properties of associative learning in eyeblink
conditioning.
Several brain systems become massively engaged in the
paradigm, particularly the hippocampus and cerebellum;
if the US (unconditioned stimulus) is sufficiently aversive,
the amygdala is also engaged. However, using the basic
delay procedure (conditioned stimulus and unconditioned
stimulus overlap and coterminate) only lesions of the
cerebellum abolish the CR (conditioned response). The
critical lesion is to the anterior cerebellar interpositus
nucleus ipsilateral to the trained eye. The lesion has
no effect on the reflex eyeblink. Neuronal unit activity
increases massively in this critical region of the nucleus
with training and electrical stimulation of the region
elicits eyeblink in untrained animals; the circuit is
hard-wired from interpositus to behavior.
In a long series of studies using eletrophysiological
recordings, lesions, stimulation, and anatomical pathway
tracing, we identified the essential (necessary and sufficient)
circuit for this form of learning and memory. Reversible
inactivation (drugs or cooling) of the following structures
does not prevent learning at all: motor nuclei, red nucleus,
9 superior cerebellar peduncle (output from cerebellum).
Reversible inactivation of the anterior interpositus nucleus
completely prevents learning and inactivation of the cerebellar
cortex impairs but does not prevent learning. So the anterior
interpositus appears to be the critical structure and
the probable locus of the basic or primary memory trace.
Blocking protein synthesis in the interpositus nucleus
completely prevents learning.
Use of mutant and KO (knock out) mice has demonstrated
that eyeblink conditioning can develop, albeit with some
impairment, in the complete absence of functional cerebellar
cortex (ped mouse) so long as the interpositus nucleus
is not lesioned (if it is, there is no learning at all).
In the cerebellar cortex, several mutant and KO mice studies
support the following observation: if cerebellar cortical
LTD is impaired, so is eyeblink learning.
Recordings from identified cerebellar Purkinje neurons
in the behaving animals are convenient because the CS
evoked mossy-parallel fiber activation is recorded as
simple spikes and the US evoked climbing fiber activation
is recorded as complex spikes and the two types are easily
separable. In trained animals, although several patterns
of learning- induced simple spike responses are observed
(and many Purkinje neurons are of course not interested
in the form of learning), the most common pattern is a
decrease in simple spike frequency in the CS period. This
result is completely consistent with ]to's phenomenon
of LTD (Long Term Depression) in cerebellar cortex (decrease
in parallel fiber synaptic efficacy on Purkinje neuron
dendrites as a result of repeated pairing of the CS (parallel
fiber activation] and US [climbing fiber activation]).
For complex spikes, those Purkinje neurons that are influenced
by the US consistently show evoked complex spikes to the
onset of the US on US alone trials and to the US impaired
trials early in training. As learning develops, these
US evoked complex spikes are suppressed, just as is US-evoked
activity in the inferior olive. Since we argue that this
circuit (trigeminal to inferior olive to cerebellum as
climbing fibers) is the essential reinforcing or teaching
pathway-these results are completely consistent with the
elegant formulation by Rescoria and Wagner for acquisition
of classically conditioned responses.
Cerebellar cortical lesion studies and studies on mutant
and KO mice with cerebellar cortical abnormalities all
produce the same two effects: (1) the CR peak latency
is no longer adaptive, i.e., no longer occurs at the onset
of the US; instead it has a significantly shorter latency,
as we showed many years ago (McCormick and Thompson, 1984,
Science, 223, 296-299); and (2) acquisition of
the behavioral CR is slower and develops to a lesser extent.
We have argued as noted above, that the basic or primary
memory trace is established in the interpositus nucleus;
we suggest that secondary traces develop in cerebellar
cortex that serve to modulate the interpositus (via Purkinje
neuron inhibition of interpositus neurons) to achieve
adaptive timing and normal learning.
In a series of experiments we were able to identify the
brain circuit necessary for the behavioral phenomenon
of blocking, discovered by Kamin. If animals are first
trained, e.g., to a tone CS (corneal airpuff US) until
they are well trained and then given additional training
to a compound tone-light CS (corneal airpuff US), and
then tested to the light CS, they show no learning to
the light. In contrast, if animals are only given training
to compound tone-light CS, they learn to respond to the
tons and the light. Prior training to the tone blocks
subsequent learning to the light in the compound stimulus
training. In cognitive terms, the light adds no new information
and so is ignored.
We are not satisfied with such "mentalistic" explanations
and set out to identify the essential circuit for blocking
in eyeblink conditioning. As it happens, the interpositus
has a strong direct GABAergice inhibitory projection to
the inferior olive, as well as a strong excitatory projection
to the red nucleus (and from there to motor nuclei). Since
neuronal activity in the interpositus grows markedly over
training in the CS period preceding the onset of the behavioral
CR, we reasoned that the growing inhibition of the inferior
olive would shut down its climbing fiber projection to
the cerebellum normally evoked by the US onset; so after
the animal is well trained to tone, additional training
to tone-light will not result in additional learning because
the reinforcing or teaching input to the cerebellum, the
climbing fiber system, is shut down at the inferior olive.
This argument is consistent with the more general formulation
of the Rescoria-Wagner algorithm. Recordings from the
inferior olive supported this possibility, as did recordings
of complex spikes from Purkinje neurons in cerebellar
cortex (evoked by climbing fibers).
In the critical test, we completed the blocking paradigm
with an additional group given tone training first, then
compound tone-light training for 5 days with infusions
of picrotoxin in the inferior olive to block GABA inhibition
from the interpositus (a control group received the same
training but with only vehicle infused in the inferior
olive). The control for blocking received no prior tone
training. The result was as expected. The group not given
prior tone training showed clear learning to the light
CS; the blocking group with vehicle infusion in the inferior
olive showed blocking, i.e., no learning to light. The
critical group, receiving picrotoxin in the inferior olive
during compound tone-light training after tone training
showed responding to light just like the control group
that had no prior tone training. Blocking GABA inhibition
in the inferior olive completely blocked the behavioral
phenomenon of blocking. This is a most satisfying result
in that we were able to show that the cerebellar circuit
itself instantiated the phenomenon of blocking-blocking
is an emergent property of the network itself and not
a result of some specialized molecular processes.
