Cerebellar-dependent motor learning is based on pruning a Purkinje cell population response

Results

In an attempt to clarify how OV PCs generate STSA, we recorded SS of OV PCs from two rhesus monkeys during two different forms of STSA, termed inward and outward adaptation, respectively. During inward adaptation, the target is shifted back from its original location by a few degrees in the direction of the fovea, prompting a decrease in saccade amplitude. Conversely, during outward adaptation, the shift moves the target farther away from the fovea, causing an increase in saccade amplitude. Of >800 PCs in vermal lobuli VI and VIIA, whose SS could be isolated, 212 (135 in monkey N and 77 in monkey E) could be kept track of during a complete adaptation experiment, consisting of an initial block of visually guided saccades before adaptation and a subsequent block in which adaptation was allowed to develop until a maximal and stable level of gain change (>15%) was established. Individual PCs were randomly assigned to inward adaptation or to outward adaptation. Of the 212 PCs for which complete datasets were available, 128 were studied during outward adaptation, and 84 were studied during inward adaptation.

Fig. 1 depicts the SS as well as the accompanying complex spike (CS) responses of exemplary PCs, three of them recorded during inward adaptation (Fig. 1 A–C: PCs 1, 2, 3) and three recorded during outward adaptation (Fig. 1 D–F: PC 4,5,6). Four of the PCs (Fig. 1 A–D: PCs 1, 2, 3, 4) exhibited saccade-related bursts, strongest in individually varying directions, whereas the fifth one (Fig. 1E: PC 5) displayed a weak dip right after saccade onset (PC 5) and the sixth one (Fig. 1F: PC 6) displayed a strong saccade-related pause. All PCs showed clear changes of their responses during saccadic adaptation. Although the saccade-related bursts of two PCs observed during inward adaptation decreased with decreasing saccade gain resulting from adaptation (PCs 1 and 2), the saccade-related burst of PC 4, observed during outward adaptation, grew over the course of adaptation. Much more profound changes were exhibited by PCs 3, 5, and 6. PC 3 started inward adaptation as a saccade-related burst neuron and ended it as a clear saccade-related pause neuron. In contrast, PC 5 started outward adaptation as an omni-pause neuron and emerged from adaptation as a saccade-related burst neuron. PC 6, which was tested during outward adaptation, showed the most dramatic changes. It started out with a saccade-related pause, interrupting a high frequency background discharge. Although the background activity decreased in the course of adaptation, the saccade-related pause disappeared, and at adaptation offset, a strong postsaccadic response had developed. The simple and complex spike waveforms that underlay the detection of SS and CS events are presented at high temporal resolution in supporting information (SI) Fig. S1 for PCs 1–5 (not available for PC 6). As the waveforms did not change in the course of the experiments, we are confident that the changes of the responses described before were not artifacts of poor isolation of these PCs. The same conclusion can be drawn for PC 6 in view of the consequence of extinction for the firing pattern of PC 6. As shown in Fig. S2, the peculiar changes of the firing pattern observed during adaptation completely reverted during the course of extinction from gain-increase adaptation until—at the end of the extinction period—the original baseline pattern exhibited by this PC before adaptation had commenced was displayed again.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

Change in PC responses during STSA. (A–C) Three examples of PC-SS and CS responses recorded during inward STSA. (D–F) Three examples of PC-SS and CS responses recorded during outward STSA. For each column, the peristimulus histograms (PSTH) represent the mean saccade-related activity of the cell at different times during STSA from the beginning (topmost) to the end (bottommost) of adaptation, each PSTH is based on 50 trials, as a function of the time relative to saccade onset (the vertical dashed line on each PSTH). The number given on top of each PSTH indicates the mean saccadic gain for the group of trials underlying the PSTH. For PCs 1, 2, 4, and 5, the bottommost row depicts the amplitude tuning of a neuron before (blue curve) and during (red curve) adaptation. The arrow indicates the course of adaptation. The CS responses were taken from a recent study on the influence of STSA on CS discharge patterns (20).

Saccadic adaptation changes saccade amplitude. Hence, the changes in discharge rates during saccadic adaptation exhibited by these exemplary PCs might simply reflect the amplitude dependence of their discharge. However, this is not the case because discharge rates during adaptation never lay on the amplitude tuning curves collected before adaptation (Fig. 1 A, B, D, and E, Lower). Could the changes observed during adaptation simply reflect an instability of the discharges that become manifest owing to the long time that saccadic adaptation requires in monkeys? This does not seem to be the case, as suggested by observations on a group of 12 PCs with saccade-related bursts, studied when monkeys were asked to make repetitive nonadapted saccades over periods comparable with those needed for adaptation, however, without adaptation taking place. None of these 12 PCs showed significant changes of their response in the absence of adaptation, although a few individual PCs exhibited a subtle and nonsignificant tendency of their peak discharge to be delayed. On the other hand, of the 212 PCs tested during saccadic adaptation, 120 (57%) exhibited significant changes, whereas 43% of these PCs were not influenced by adaptation.

Specifically, during inward adaptation, 54 PCs (64%) of the 84 tested changed, whereas 30 of them showed no change (36%). Thirty-seven PCs exhibited decreases of saccade-related bursts (44%), with the burst sometimes disappearing completely. Twelve PCs, which did not show any saccade-related responses before adaptation, exhibited a saccade-related pause later (14%), and five (6%) showed idiosyncratic changes (e.g., burst to pause, Fig. 1A, PC3). On the other hand, during outward adaptation, 66 PCs (52%) of the 128 tested changed. Of them, 43 PCs (65% of the 66 neurons), which had not shown any saccade-related burst before adaptation, gradually developed such a modulation during adaptation. The discharge of 23 PCs (35% of the 66 neurons) changed in an idiosyncratic manner [pause to burst, pause to postsaccadic responses (e.g., PC 6 in Fig. 1F)]. Forty PCs of the whole group showed saccade-related bursts before and after adaptation (31%) without any significant change. The remaining 22 neurons (17%) did not show any saccade-related responses (burst, pause, or postsaccadic activity), either before or after adaptation, or were neurons that showed a saccade-related pause before as well as after adaptation. It seems that saccadic adaptation prompts changes in the discharge patterns of PCs, which can be perplexingly different. However, although the adaptation modifies the discharge patterns, it leaves the level of background activity unaffected (Fig. S3). The inconsistency of the changes of the discharge patterns precludes the deduction of a simple rule on the relationship of discharge changes to adaptation. The absence of such a rule for individual PCs may be less surprising if one recalls the fact that several hundred PCs converge on individual target neurons in the deep cerebellar nuclei (DCN) (15). Hence, we may assume that the DCN target neuron will not be able to discern discharge patterns of individual PCs. In other words, it should be the collective discharge of all PCs, feeding a particular DCN target neuron that matters—largely independently of the specifics of individual contributions. Actually, we were recently able to provide support for the idea that information from PCs is read out in a population format in a study of normal visually guided saccades. In this study, unlike the saccade-related bursts of individual PCs, the calculated collective instantaneous discharge rate (population burst) of larger group of PCs gave a precise description of the timing of saccades (9). With these considerations in mind, we computed population responses for the two groups of PCs studied during inward and outward adaptation, respectively, and scrutinized their behavior during saccadic adaptation (Fig. 2). Population responses were calculated by sorting the responses of individual PCs according to saccade duration (bin width, 2.5 ms). For each saccade duration, we then computed compound perisaccadic histograms with a bin width of 1 ms, smoothed by a Gaussian filter with a standard deviation of 5 ms. Burst onset and offset were defined by a 4 × baseline criterion (9). The preadaptation PB is shown in Fig. 2A based on all n = 212 PCs (8,226 saccades) and in Fig. S4 separately for the two groups of PCs subjected to inward and outward adaptation, respectively. In both figures, the PB is plotted as it develops over time (x axis) for saccades of 45–75 ms durations (y axis). In full accordance with our previous report (9), the two figures show that—independently of the composition of the PC groups—the PB starts 50 ms before saccade onset, peaks at saccade onset, and ends at the time the saccade ends, independently of the duration of the saccade. The tight temporal relationship between burst end and saccade end is expressed by the almost perfect linear regression between the two variables (Fig. S5A, burst end = 14.8088 ms + 0.7882* saccade end, r² = 0.9976, P < 0.001). Both inward and outward adaptations were accompanied by gradual changes of the PB, which differed qualitatively for the two forms of adaptation. During outward adaptation, both the onset and the peak of the PB gradually moved to later points in time (Fig. 2B, based on 13,228 saccades, and 2E, based on 17,682 saccades) until, at the end of adaptation, both had been delayed by 24 and 31 ms, respectively, relative to the preadaptation PB. On the other hand, the tight relationship between saccade end and PB end was fully maintained (Fig. S5B, burst end = 0.6556 ms + 0.99* saccade end, r² = 0.97, P < 0.001; paired t test, saccade end vs. saccade duration, P > 0.5). In contrast, during inward adaptation, the whole PB moved to the left, peaking earlier and ending earlier (Fig. 2C, based on 7,611 saccades, and Fig. 2F, based on 13,354 saccades). It is particularly noteworthy that the PB not only ended earlier relative to the preadaptation burst but also considerably earlier than the adapted saccade, independently of saccade duration at 30–32 ms after saccade onset. In other words, inward adaptation completely breaks the tight relationship between PB offset and saccade end (Fig. S5C, burst end = 34.076 ms + 0.0568* saccade end, r² = 0.4167, P = 0.23; paired t test, saccade end vs. saccade duration, P < 0.001) that characterizes unadapted saccades as well as increased amplitude saccades resulting from outward adaptation. How can STSA change the PB so smoothly and consistently, although individual PCs change in a seemingly incoherent pattern? An answer is provided by a look at the timing of those PCs that fired saccade-related bursts after adaptation. In the case of inward adaptation, these were the same PC (n = 67, PC_in) that also fired saccade-related bursts before adaptation. However, the group of PCs studied during outward adaptation consisted of two main subgroups; the first comprised PCs that fired saccade-related bursts before as well as at the end of adaptation, PC_out-11,(n = 40, 31%), whereas the second group (n = 43, 34%) consisted of those PCs that were not saccade-related before adaptation but fired saccade-related bursts at the end of adaptation, PC_out-01. It must be noted that the reverse was not observed, i.e., no saccade-related PC lost its burst as a consequence of outward adaptation. Fig. 3A, C, and E, show the distributions of the times of peak discharge rates for the three groups of PCs, separately for inward and for outward adaptation and separately for the periods immediately preceding the beginning (groups PC_in and PC_out-11 only) and end of adaptation (all groups). PCs studied in the outward adaptation task with bursts before adaptation (PC_out-11) shifted their peak significantly (before: 7.37 ± 16.8 ms vs. after: 15.25 ± 15.52 ms, t test, P < 0.05) by on average 8 ms to later times relative to saccade onset (Fig. 3A). Those that had developed new bursts at the end of adaptation (PC_out-01) fired on average even later, namely 39.41 ± 13.10 ms after saccade onset (Fig. 3C), which was significantly (P < 0.001) later than the mean of the PC_out-11 at the end of adaptation. In other words, outward adaptation shifts the population activity to later points in time by delaying preexisting bursts and by generating new, very late bursts. Delaying preexisting bursts was not accompanied by significant changes in discharge rates (Fig. 3B). In contrast to outward adaptation, inward adaptation shifted the time of peak discharges to an earlier point (before: 6.41 ± 18.27 ms vs. after: −0.1 ± 15.26 ms, t test, P < 0.03). This shift was accompanied by a decrease in firing rate (before: 96.16 spikes per s ± 20.08 spikes per s vs. after: 88.25 spikes per s ± 17.53 spikes per s, t test, P < 0.01). In full correspondence with the behavior exhibited by the population burst for inward adaptation, the burst ends of individual PCs also shifted by, on average, 13 ms to earlier points in time relative to the saccade end (Fig. 3H, before: −15.06 ± 10.402 ms; after: −28.2 ± 11.45 ms; t test, P < 0.001). On the other hand, outward adaptation did not affect the close temporal coincidence of burst end and saccade end that characterizes the unadapted state (Fig. 3G, before: −13.825 ± 9.886 ms; after: −16.5783 ± 10.174 ms; t test, P > 0.1). In sum, the conspicuous adaptation-induced changes characterizing the PB can be led back to distinct changes in the timing of individual saccade-related bursts as well as the birth of new, late bursts in the case of outward adaptation. Hence, the other, less frequent patterns of PC responses observed during adaptation, such as the appearance or disappearance of saccade-related pauses, probably only fine-tune the PB, rather than adding qualitative features, not explicable by the contributions of bursting PCs. Regardless, the distribution of burst ends of individual PCs is far too wide to allow them to determine the timing of saccades. This requires the population signal, which—as discussed next—shows disparate changes that precisely parallel similarly disparate changes in saccade kinematics, resulting from the two forms of adaptation tested.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Effect of STSA on PC PB. (A) PB before STSA. PB plotted as a function of saccade time (x axis; 0, saccade onset) and duration (y axis), measured in discrete time steps (x axis: 1 ms, y axis: 2.5 ms). PBs during outward (B) and inward adaptation (C) were collected at a level of adaptation, when the gain change achieved lay between 5% and 10%; (D and E) PBs after stable outward (D) and inward adaptation (E) (gain change >15%). Black dashed, red, and yellow lines depict, respectively, saccade onset, saccade offset, and PB end.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Distributions of times of peak discharge, mean spiking, rates, and times of burst offset for saccades before and at the end of adaptations. (A and B) Neurons tested for outward adaptation, showing a saccade-related burst before as well as at the end of adaptation (PC_out-11). Distributions of time of peak discharge rates in milliseconds (A) and mean firing rate (B) before (white distributions and thin curves) and at the end (gray distributions and thick curves) of outward adaptation. (C and D) Neurons tested for outward adaptation, which did not exhibit any saccade-related burst before adaptation (PC_out-01). Format as in A and B. (D) The white distribution and the thin curve plot the background activity of the PC_out-01 before adaptation. (E and F) Neurons tested for inward adaptation(PC_in). Format as in A and B. (G and H) Distributions of times of burst ends before (white distributions and thin curves) and at the end of adaptation (gray distributions and thick curves). (G) All neurons tested for outward adaptation (PCout-01 and PCout-11). (H) All neurons tested for inward adaptation.

An analysis of eye movement records collected together with the SS records underlying the PB discussed before shows that outward as well as inward adaptation led to clear deviations of the adapted saccades from the main sequence for unadapted saccades. In the case of outward adaptation, peak saccade velocity stayed constant at the preadaptation level, whereas saccade duration increased in the course of adaptation beyond the duration determined by the preadaptation main sequence (individual example, Fig. 4 A and B, Left; population data, Fig. 4C). As the time of peak saccade velocity and, therefore, also the duration of the saccade acceleration phase stayed relatively constant (Fig. S6), adaptation led to an increase in the duration of the deceleration phase and thereby to an increase in the skewness of the saccade (individual example, Fig. 4A Left; population data, Fig. 4C). On the other hand, in the case of inward adaptation, it was saccade duration that did not change, resulting in a duration of the final, reduced amplitude saccade, which exceeded the duration predicted by the preadaptation main sequence (individual example, Fig. 4A Right; population data, Fig. 4B Right). The amplitude reduction was the sole consequence of a steady decrease in saccade velocity over the course of adaptation down to 80% of the velocity predicted by the preadaptation main sequence at the end of adaptation (Fig. 4C). As the time of peak velocity did not change, the relative shares of acceleration and deceleration also remained unaltered by adaptation and the saccade stayed relatively symmetrical (Fig. S6B). During outward STSA, it appears that not only the saccade duration but also the velocity of the movement might be encoded by the PB. This is suggested by Fig. 5A (and Fig. S7) and B, which illustrate that the deceleration phase of outward-adapted saccades shows a velocity bump that is remarkably close to the bump in the PB.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 4.

Effect of STSA on saccade kinematics. (A) Example of eye position and eye velocity of saccades collected before (black curves) and at the end (colored curves) of adaptation. (B) Example of main sequence before vs. during adaptation; saccade duration and peak velocity are plotted as a function of saccade amplitude. The black curves represent the main sequence of saccades collected before adaptation, whereas the colored curves show saccades collected during adaptation (±SEM). A and B are based on the same dataset. (C) Percentage of change in saccade duration and peak of velocity as a function of gain change. Shown are the overall mean ±SEM of all adaptation sessions (82 adaptations in two monkeys) needed to record the 212 neurons considered in the analysis.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 5.

Profile of PC PB. (A) Mean velocity (±SE) profile of nonadapted 45 ms saccades (black) when compared with the mean velocity profile of adapted saccades after outward adaptation (red); the red bar above the profiles depicts the period when both profiles are significantly different (running paired t test, P < 0.0005, corrected for multiple comparison). As a consequence of outward adaptation, mean saccade duration increased from 45 to 58 ms. (B) Profile of SS population responses before adaptation of 45-ms-duration saccades (black curve), at the end of inward adaptation (blue curve), saccade duration still 45 ms, and outward adaptation (red curve) (saccade duration increase from 45 to 60 ms) relative to saccade onset. Means (±SE) are depicted.