Eye movement impairment after central thalamus lesions

Animal studies suggest that the central thalamus is involved in the control of eye movements that are based on extraretinal signals. In humans, there is only one recent paper that describes two patients with a lesion located within the intramedullary nuclei, in which visual memory-guided saccades were normal but became inaccurate when the eyes were displaced between target disappearance and saccade triggering, that is, when an extraretinal position signal had to be taken into account. We studied visual and auditory saccades recorded monocularly with the magnetic search coil technique in three patients with an isolated lesion located in the central thalamus. Visual saccades proved to be normal, whereas for auditory stimuli, saccade duration and peak velocity were normal but the amplitude of the first saccade was asymmetrical: saccades ipsilateral to the lesion were significantly smaller than those directed to the contralateral side. The patients were able to make corrective saccades and, hence, to improve gain and to decrease gain asymmetry. We suggest that patients were able to localize auditory targets correctly, did not correctly take into account eye position during the saccade, probably as a consequence of an inaccurate efference copy (corollary discharge) signal. Our findings are in keeping with the hypothesis that the central thalamus deals with saccades that are based on extraretinal signals.