Exploring the domain of the cerebellar timing system
Summary (2 min read)
1. Studies with patients with cerebellar lesions
- These results required a reconccptualization of the domain of cerebellar function.
- This structure has generally been linked to motor functions, or sensorimotor learning.
- The fact that the patients were impaired on a purely perceptual task suggested that its domain should be specified in terms of a particular mental operation, namely the representation of the temporal relationships between events.
- The authors have hypothesized that this computational capability is invoked across a wide range of tasks that require this form of representation.
2. The cerebellum and sensorimotor learning
- It remains difficult to specif3.' the learning domain of the cerebellum (see Ivry, 1993) .
- This hypothesis emphasizes the task domain of the cerebellum and focuses on the fact that the climbing fiber pathway provides a salient error signal for shaping appropriate skeletal responses.
- By this way of thinking, the cerebellum is associated with NMR conditioning because this type of learning is only adaptive if it is appropriately timed (Kecle and lvr3'.
- That is, learning an association and forming the temporal representation of that association can not be thought of as distinct.
- Due to negative feedback loops and physiological processes such as slow IPSPs, other consequences of the mossy fiber activity may not be evident for hundreds of milliseconds.
3. Potential limitations of cerebellar timing
- Second, behavioral studies in humans suggest that there may be a qualitative change around 2-4 scc in their capacity to represent temporal infommtion.
- Below this duration, successive events are seen as belonging to a conunon temporally-defined group, regardless of whether this group has a rh}ethmic structure or lacks such organization.
- Ps.vchophysical studies have also indicated an increase in the Weber fraction on duration discrimination tasks for intervals longer than 2 sec (Getty.
- On the motor side, Mates et al. (1994) have shox~aa that when tapping with a periodic pacing signal, people shift from a predictive to an reactive mode as the target interval becomes longer.
- For intervals less than 2-3 sec. the subjects' responses tended to anticipate the tones.
4. Animal models of temporal discrimination
- To date, this animal research has ignored the cerebellum, perhaps because this structure has been assumed to be limited to the motor domain while these tasks focus on perceptual and memor), processes.
- Moreover, while some studies have used stimuli that are less than 1 sec (Allan and Gibbon, 1991) , the maiority of this work has involved stimuli that are considerably longer, frequently ranging tip to 40 sec.
- As noted above, the authors have hypothesized that the ccrcbcllar timing system is limited to relatively short durations.
- The authors working model is that this timing process is relatively immune to cognitive influences.
- The onset of a stimulus may automatically activate different sets of neurons, and mcmor), demands are minimal.
4.1. EXPERIMENT I
- To obtain psychometric fi~nctions, the test phase included both the training durations and probe durations.
- For the SR task, there were nine probe durations ranging from 200 to 850 ms, with seven of these durations falling between the endpoint values.
- On 50% of the trials, one of the two training durations was selected and correct responses were reinforced.
- On the other 50% of the trials, one of the nine probe durations was selected and no reward was possible.
- The rats completed 36 sessions of the test phase prior to surgery.
4.1.2. Results And l)iscnssion.
- The effect of task and the interaction were not significant.
- In contrast, the consistency measure revealed a dissociation between the two tasks.
- This result is in accord with the hypothesis that the temporal range of the cerebellum is limited to short intervals.
- For the first two postsurgery sessions on the SR task, the consistency scores were -5.25 and -4.88.
- Second, their animals received many more trials post-surger).,' than are commonly used in lcsion studies.
4.2. EXPERIMENT 2
- Third, all sessions were four hours in duration and alternated by session between the two tasks.
- Wc did not include any mixed sessions since an auditory stimulus was used for both tasks.
- On each day, one group was tested on the SR task and the other group was tested on the ID task.
- All animals received extensive training on the two tasks prior to surgeD'.
- After a one-week recover3.' period, they were tested for an additional 26 sessions post-surgery.
4.2.2. Results And Discttssion.
- Lesions (Ivry and Keele, 1989) . and included tests of both time production and time perception.
- Rather than focus on particular tasks (e.g., reflex conditioning, motor control, perceptual processing), it can be useful to consider the operation performed by a structure and then explore how widely that operation is employed.
- In the case of the cerebellum, it appears that its timing capability can help account for why this structure is essential for coordinated movement, speech production, sensorimotor learning, as well as certain perceptual functions (see Ivry, 1993) .
- A timing system that is used for coordinating the actions of multiple joints or perceiving the velocity of a moving stimulus is unlikely to be useful for tasks that span many seconds or minutes.
- Researchers utilizing tasks such as the peak procedure have suggested vet3' different evolutionary pressures for why rats might benefit from being able to determine when 40 see has elapsed.
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Citations
791Â citations
Cites background from "Exploring the domain of the cerebel..."
...[43]), as well as timing in the milliseconds range [39], thereby suggesting the existence of a unitary psychological and neurobiological interval timing mechanism....
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779Â citations
Cites background from "Exploring the domain of the cerebel..."
...The cerebellum could also be involved [16;17], and shows particularly appropriate circuitry for the measurement of brief intervals [18-21]....
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Cites background from "Exploring the domain of the cerebel..."
...distraction of attention in dual task scenarios ( Rammsayer & Lima, 1991) can have differential influence (but see Macar, Grondin, & Casini, 1994), while lesions to specific brain areas elicit differential impairments ( Clarke, Ivry, Grinband, Roberts, & Shimizu, 1996 )....
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References
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