Role of intralaminar thalamus in gaze mechanisms: evidence from electrical stimulation and fiber-sparing lesions in cat.

Information on eye movement related activity in the internal medullary lamina (IML) of the thalamus is consistent with an IML role in the control of eye movements, or with its serving to convey information to the forebrain on impending events in the oculomotor system. Interpretation is impeded by the fact that eye movements evoked by electrical stimulation of the IML might be triggered by antidromic activation of projections to the IML from brainstem preoculomotor centers. In order to address this issue, thresholds for the elicitation of eye movements and cortical recruiting responses (as a marker for IML integrity) were studied before and after destruction of IML neuronal populations by the fiber-sparing neurotoxin ibotenic acid. Experiments were performed in alert cats chronically implanted with a scleral search coil for the monitoring of eye movements and an electrode-cannula assembly that permitted threshold determinations and drug injections at the stimulating site without any movement of probes in the tissue. Drastic elevation of the threshold for cortical recruiting responses, but not for eye movements following ibotenic acid destruction of IML neurons uncomplicated by myelin damage at the site of stimulation suggests an antidromic route for eye movements elicited by IML stimulation.