Inhibitory Cerebello-Olivary Projections and Blocking Effect in Classical Conditioning J J Kim, D J Krupa, R F Thompson Science, vol. 279, (1998)

Blocking: Observed data CS A US UR (A-US) First CS (A) paired with US –A-US First CS (A) and second CS (B) paired with US –Compound conditioning –AB-US UR (AB-US) CS A CS B US CS B CRCS A Test A alone –Normal CR Test B alone –Very little or no CR

Blocking: Interpretation If US is fully predicted by A (A-US), then adding B does not provide new information –Save on unnecessary computation Weaker pre-conditioning of A-US causes a stronger effect of B-US –Inverse proportionality of A-US and B-US CS B CR CS A CS B CRCS A Redundant

NMR = nictitating membrane response Eyeblink Conditioning Speaker (CS A) Light (CS B) Air nozzle (US) Thread to eyelid Eyelid movement measurement device Eyelid closes (UR, CR)

Postulated eyeblink conditioning circuit CS-US association (Purkinje cells in HVI spike when CR is learned) Blocking inhibition GABA antagonists (eg. Picrotoxin [PTX]) prevent blocking Interpositus nucleus The question: Is this circuit correct?

Experiment 1: Procedure Standard classical conditioning –Tone CS –Airpuff US –Eye closing UR becomes CR during training 54 Purkinje cells recorded during conditioning –31 in lobule HVI <--- most likely activity site for eyeblink conditioning –12 in anterior lobe HV, 6 in HVIIA, 5 in paramedian lobule

Experiment 1: Eyelid & Cell Responses Naïve animals (5 cells)Trained animals (11 cells) CS-US trials US only trials CR No purkinje cell response Purkinje cell spikes UR

Experiment 1: Control, Conclusions, Comments Control case: strictly unpaired tone and airpuff trials –20 out of 45 cells responded to the airpuff with complex spikes –Indicates that tone and airpuff must be paired for spike suppression Conclusion: as eyeblink conditioning occurs the inferior olive’s ability to convey US information to the cerebellum is suppressed –This is not really shown - just the involvement of Purkinje cells Comments –UR amplitude in response to US-only trials is higher for trained animals: why?

Trained eyelid response to picrotoxin (PTX) US only CS + US before PTX infusion CS + US after PTX infusion Purkinje cell spikes No purkinje cell spikes Purkinje cell spikes CR UR 3 cells recorded Well-trained rabbits –how many? PTX injected into inferior olive

Experiment 2: Procedure Preparation –Rabbits implanted with guide cannulae above contralateral inferior olive CS A US UR (A-US) Phase I: Tone-airpuff conditioning –7 sessions, 1 per day (10 blocks x 10 trials) UR (AB-US) CS A CS B US Phase II: Tone-light-airpuff conditioning –Simultaneously introduce one of two fluids: GABA antagonist: picrotoxin (PTX) Placebo: artificial cerebrospinal fluid (ACSF) –5 sessions, 1 per day (10 blocks x 10 trials) CS B ??? US Light-airpuff test –Light CS + airpuff US testing (B-US) –5 sessions, 1 per day (10 blocks x 10 trials)

Experiment 2: Test groups Main group –ACSF: 6 rabbits –PTX: 12 rabbits Control group –5 rabbits Phase IPhase IILight-airpuff Phase IILight-airpuff

Experiment 2: Results Phase I Tone CS Light-airpuff test Light CS Phase II Tone + light CS Normal acquisition ACSF/PTX maintain response Control case similar to PTX ACSF shows blocking, then re-learning Blocking Control case acquisition PTX does not affect UR amplitude US-only Partial response

Is the circuit correct? Experiment 1 --> something stops Purkinje cell spiking –Purkinje cell spiking correlated with CR PTX infusion in inferior olive restores Purkinje cell spiking –Inferior olive and GABA are involved Experiment 2 --> PTX infusion prevents blocking –PTX seems to prevent GABA inhibition of inferior olive Blocking inhibition Interpositus nucleus

Specific Comments Mechanism for inverse relationship between strength of A-US and B-US is not explained Slow acquisition during Experiment 2 light-airpuff test (compared to Phase I) not explained Decrease of blocking over time is not explained –Due to simultaneous extinction of A-US and acquisition of B-US?

The End

Cerebellar Cortex (Ghez & Tach, 2000) Molecular layer Granule-cell layer Purkinje cell layer Parallel fibres Climbing fibre Purkinje cell Purkinje cell axon Mossy fibre Golgi cell Basket cell Stellate cell Granule cell