PowerPoint Presentation

Introduction

• The rostral intralaminar nuclei of the thalamus (RIL) have been implicated in a variety of global and specific cognitive functions including cortical activation, working memory, visuospatial attention, visuomotor control, as well as response selection and motor initiation.

• Motor functions have been attributed to the connections between two RIL nuclei, the central lateral (CL) and the paracentral (PC), and the basal ganglia, ventral lateral thalamus, and the primary and secondary motor cortex.

• Bilateral electrolytic lesions of the CL produce deficits in balance and equilibrium without affecting spatial orientation (Jeljeli et al, 2000, Neruosci Res, 38:155-64) . Larger bilateral lesions of the intralaminar thalamus impair voluntary movements to external stimuli or motor intention (Burk & Mair, 2001, Behav Brain Res, 123:49-63).

• The present study seeks to extend these findings in two ways: 1) producing unilateral lesions in order to determine whether these motor functions are lateralized at the level of the intralaminar nuclei, and 2) examining the effects of intralaminar lesions that are restricted to the CL and PC using a visually cued reaction time task.

•Two-Choice, Reaction Time Task

• Long Evens hooded rats were trained to maintain a nosepoke in a central hole until a green LED illuminated for 0.15 sec to the left or right of the head after a variable foreperiod (0.2-0.8 sec).

• Training continued for 128 trials per session until each rat reached a criterion of 80% correct responses.

• Reaction Time = delay between the onset of the target and the withdrawal of the rat’s nose from the central hole.

• Movement Time = time between withdrawal from the central hole and the recording of a nosepoke at a side hole.

• Errors were followed by a 3.0 sec time-out period.

Incorrect: responding to the side hole opposite the target.

Late error: not responding within 3.0 sec of the target onset.

Premature: responding before the target onset.

• Stereotaxic coordinates of two unilateral injections of 0.1 uL 0.6M ibotenic acid referenced to bregma were:

AP -3.3, ML 1.2, DV -5.8 and AP -2.3, ML 1.3, DV –6.0) .

• Testing resumed after two postoperative days and continued for 15 days.

• Sections 40 micron thick were stained with cresyl-violet.

• The CL and PC received extensive damage in all five rats.

• Damage to other structures:

Substantial: mediodorsal (medially) and laterodorsal (dorsally) Slight: central part of the mediodorsal nucleus, posterior nuclear group and the rostral part of the lateral posterior thalamic nucleus

• Differences in lesion location or size were not associated with the severity of the deficits or the side on which behavioral deficits occurred. The data presented is for all rats are combined.

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Apparatus Task Timeline

Results

• Unilateral lesions of the CL and PC produced a decrease in correct responses that recovered after ten days of testing. This deficit in correct responding is accounted for by an increase in responding in the opposite direction (incorrect responses).

• Side of Effect refers to decrease in correct responses made contralateral (3 rats) or ipsilateral (2 rats) to the lesion. All rats showed the same pattern of postlesion task performance with respect to the side of the effect on other measures.

• Reaction times (time to initiate a response) did not change after the lesions, but movement times (time to execute a response) did increase when rats made correct responses on the side of the deficit.

• Intertrial responses to both side holes increased slightly.

Conclusion

• These data suggest that unilateral lesions in the rostral intralaminar nuclei, CL and PC, do produce lateralized deficits in correct responding on a visually cued reaction-time task. Histological analysis, however, does not distinguish between rats displaying ipsilateral and contralateral deficits.

• These impairments appear to be related to difficulties in executing a response caused by a slowing of movement or a bias to respond in the opposite direction. The deficit is not associated with impairments in initiating cued responses or making spontaneous responses in the same direction.

• These findings are consistent with those of previous studies that conclude that the rostral intralaminar nuclei play a role in motor but not sensory function.