The cerebellum and memory

Differences in responses to 70 dB clicks of cerebellar units with simple versus complex spike activity: (i) in medial and lateral ansiform lobes and flocculus; and (ii) before and after conditioning blink conditioned responses with clicks as conditioned stimuli

Activity was recorded from 554 cerebellar units in eleven conscious cats to determine if responses to 70 dB clicks differed in units with simple and complex spike discharges. Effects of region of recording and behavioral state (with click used as a conditioned stimulus for conditioning) were also assessed. Cells with only simple spikes were distinguished from cells that had the following types of complex spike events: Type I-simple or initial spike followed > 1 ms by multiple spikes with baseline displacement (classical complex spikes), Type II--followed < or = 1 ms by spikes with or without baseline displacement (spikes in the absolute refractory period should arise from a separate site of initiation), and Type III-followed by spikes and displacement too close to the baseline noise to distinguish as Type I or II. Among the groups mean baseline activity was greatest in cells with Type I complex spikes, least in cells with Type III complex spikes, and greater in Type II cells than simple cells. Significant increases in activity within 32 ms of presenting clicks were found in the groups of Type II cells and simple cells. These appear to be the main auditory responsive cells of the cerebellar regions studied. Activity of Type II cells best reflected the temporal properties of the click; responses of simple cells had slower onsets (except in flocculus) and longer durations. Responses to click in Type II and simple cells differed in recordings from: (i) lateral ansiform lobe (lateral crus I and portions of crus II), (ii) medial ansiform lobe (medial crus I), and (iii) flocculus. The largest mean responses above baseline in the first 32 ms after click were found in Type II cells of the lateral ansiform lobe with onsets of 8-16 ms. Magnitudes of response differed before and after conditioning and backward conditioning. In the lateral ansiform lobe, the < 32 ms response to click was greater in Type II than simple cells in each state, but showed a greater increase above baseline after backward conditioning when conditioned responses were not produced than after conditioning. The onset of increased activity to click conditioned stimuli in Type II cells of the lateral ansiform region preceded the onset of the blink conditioned response after conditioning, consisted almost entirely of simple spikes, and reflected an increase in magnitude of response as opposed to an increased number of responsive units. After conditioning, an increased number of units in the flocculus responded to click conditioned stimuli in the 16-24 ms post stimulus period. Of the 16 cells with an onset of increased activity at this time, eight showed only simple spike activity. Seven of the remaining eight cells (all Type II) showed a significant increase in conditioned stimulus-evoked complex spiking above the low (usually < 1/s) baseline level of complex spike discharges. The findings support the conclusions that cerebellar units can respond rapidly enough to acoustic stimuli to play a role in auditory as well as motor processing and that the responses to 70 dB clicks differ among cells with simple and complex spike discharges. The differences are influenced substantially by the region of cerebellar recording and the behavioral state. The findings in cells of the flocculus offer the first evidence that complex as well as simple spike activity can contribute to an increased probability of discharge to click as a conditioned stimulus after conditioning.

A single photon emission computed tomography study of cerebral regional perfusion changes induced by a learning task in subjects with mild memory impairment

Fifteen nondemented subjects with memory complaints underwent serial single photon emission computed tomography (SPECT) studies with technetium-99m-d, l-hexamethyl-propylene amine oxime (HMPAO) as tracer. Scans were carried out under a baseline conditions and during the learning phase of the Memory Efficiency Profile (MEP), a combined visual perception and memory task developed by Rey. Results indicate a positive correlation between activation, as indexed by HMPAO uptake, and neuropsychological assessment. Right temporal activation was correlated with MEP immediate recall. The right cerebellum was correlated with both MEP immediate and delayed recall. This study suggests that SPECT can show cortical activation during cognitive performance in subjects with mild memory impairment.

On the cerebellum, cutaneomuscular reflexes, movement control and the elusive engrams of memory

This review focuses on the role of the cerebellum in regulating cutaneomuscular reflexes and provides a hypothesis regarding the way in which this action contributes to the coordination of goal-directed movements of the extremities. Specific attention is directed towards the cerebellum's role in conditioned and unconditioned eyeblink reflexes and limb withdrawal reflexes as models of its interactions with the cutaneomuscular reflex systems. The implications regarding the cerebellum as a storage site for motor engrams also is discussed in the context of these two behaviors. The proposed hypothesis suggests that the cerebellum regulates important features of the cutaneomuscular reflex circuits including the integration of their activity with descending pathways in a manner that implements these fundamental reflex circuits in the organization and control of goal-directed movements of the extremities.