Acquisition, Extinction, and Reacquisition of a Cerebellar Cortical Memory Trace

Responses to conditioned and unconditioned stimuli. A–D, Typical field potential responses recorded from the cerebellar surface (averages of 10 sweeps). Climbing fiber responses indicated by asterisks. E–H, Simple and complex spikes (asterisks) recorded from single Purkinje cells. A, E, Periocular stimulation (1 pulse, 3 mA) elicits two complex spikes after 11 and 23 ms, respectively, corresponding to the second and third positive field potentials in the surface recording (stimulus indicated by arrow). B, F, Stimulation of the inferior olive (1 pulse, 100 μA) elicits a complex spike response after 4 ms, corresponding to the first positive field potential response. C, G, Stimulation of climbing fibers in the inferior cerebellar peduncle (1 pulse, 100 μA) elicits a complex spike response after 2 ms, corresponding to the first positive field potential response but no simple spikes. D, H, Stimulation of mossy fibers in the middle cerebellar peduncle (1 pulse, 100 μA) elicits a positive field potential response after 1 ms, and presentation of 15 consecutive stimuli at 50 Hz (bar below graph) reliably causes an increase in simple spike activity but does not elicit any complex spikes.

Acquisition of Purkinje cell CR. A, Two sample records from a Purkinje cell exposed to paired CS–US presentations (bars above graph), from trial 1 (top) and 640 trials later (bottom). The acquired Purkinje cell response had a latency of ∼60 ms. B, Raster plot of the simple spike activity recorded from the same cell during 640 trials of paired CS–US presentations. The inhibitory response during the CS period (framed) gradually developed as training progressed. C, Average raster plot based on 11 Purkinje cell records during acquisition. The plot is built up of squares, the shadings of which indicate average firing rate across all cells (for details, see Materials and Methods). The light area that gradually appears represents the Purkinje cell CR, i.e., an inhibitory response with a firing rate below background level (100%). The darker areas indicate increased simple spike activity. D, Simple spike activity in 11 individual Purkinje cell records shown as plots of firing frequency (percentage of background activity) during the whole 300 ms CS period. There is a consistent decrease in all cells regardless of whether periocular, olivary, or climbing fiber stimulation was used as US. In one cell, acquisition was observed twice: first using a mossy fiber CS and later using a forelimb CS (see Table 1). E, Temporal analysis of simple spike activity. The 300 ms CS period was divided into three 100 ms periods that are plotted individually. During acquisition, there was only a slight decrease in firing rate during the first 100 ms compared with the change observed in the rest of the CS period. F, Spontaneous simple spike activity during the 600 ms period before CS presentation in 11 individual Purkinje cell records, shown as percentage of background activity when recording started.

Timing of Purkinje cell responses. A–D, Peristimulus time histograms of conditioned Purkinje cell responses to CS-alone presentations (bars above graphs indicate CS duration). The CS–US interval is the darkened part each histogram. Bin width is 10 ms, and simple spike activity was averaged over 50 trials in A–C and 20 trials in D. The mark on the y-axis indicates 100 Hz simple spike frequency. A–C, Three examples of Purkinje cell responses to CS-alone stimulation in which CRs were acquired using a 300 ms mossy fiber CS and a 300 ms CS–US interval to different unconditioned stimuli (i.e., stimulation of the periorbital area, inferior olive, or climbing fibers). D, This cell was trained with a longer (600 ms) CS and a shorter (200 ms) CS–US interval. E, Average response profiles during CS period for Purkinje cells trained with 200 and 300 ms CS–US intervals, respectively, showing longer latency to both onset and maximum response (dotted lines) in the second group (for additional details, see Materials and Methods and Results).

Purkinje cell activity during extinction. A, Two example records from a Purkinje cell exposed to unpaired CS–US stimulation (bar above graph indicates the 300 ms CS period), from trial 1 (top) and 440 trials later (bottom). The Purkinje cell CR had an onset latency of 60 ms and a duration of 300 ms. B, Raster plot of simple spike activity recorded from the same cell as above during 440 trials of unpaired CS–US stimulation. The inhibitory response during the CS period (framed) gradually extinguished as training progressed. C, Average raster plot of simple spikes recorded from nine Purkinje cells during paired CS–US stimulation. The plot is built of squares, the shadings of which indicate average firing rate across all cells (for details, see Materials and Methods). The light area that gradually disappears represents the Purkinje cell CR, i.e., an inhibitory response with firing rate below background level (100%). D, Simple spike activity in nine individual Purkinje cells shown as plots of firing frequency (percentage of background activity) during the whole 300 ms CS period. There is a consistent increase of simple spike activity in all cells. E, Temporal analysis of training induced changes of simple spike activity. The 300 ms CS period was divided into three 100 ms periods and plotted separately. F, Spontaneous simple spike activity during the 600 ms period before CS presentation in nine individual Purkinje cell records, shown as percentage of background activity when recording started.