Abstracts

Rationale: Verbal memory and word finding difficulty are common in left (dominant) TLE patients and are at risk for further decline following left temporal lobe resection for seizure control.  Verbal memory impairment has been attributed to hippocampal dysfunction/resection, and word finding or “naming” difficulty has been attributed to the adverse effects of seizures on lateral temporal cortex and then removal of critical neocortical language cortex. Although the hippocampus is understood to be central to episodic memory, several findings suggest that the hippocampus might also play a critical role in naming, i.e., a semantic memory process. These include poorer naming ability in patients with left hippocampal sclerosis (HS) compared to those with a structurally normal hippocampus, and greater naming decline following resection in patients with a structurally normal left hippocampus than in patients with compromised hippocampal integrity due to HS.  Nevertheless, the role of the dominant hippocampus in naming remains unsettled, as extra-hippocampal areas are typically affected by surgical interventions, including those that target medial temporal structures. To more directly assess the potential role of the hippocampus in naming, we tested naming during stimulation of hippocampal and adjacent sites in patients with implanted depth electrodes. Methods: Participants were 18 refractory, dominant (17 left, 1 right dominant) TLE patients with hippocampal depth electrodes (11 women, mean age: 35.6 years, SD=13.5; mean education: 14.1 years, SD = 3.5.). Naming tasks included visual object naming and auditory description naming (e.g., “what a king wears on his head”). Whenever possible, adjacent depth electrode sites were tested as well. Stimulation parameters were: 2-8 mA, 50 Hz, pulse width 0.1 at 3-5 seconds duration. In the last six patients, we additionally tested memory for pictured items presented during stimulation via post stimulation free recall. Binomial test assessed positive vs. negative results with hippocampal stimulation. Results: Naming was tested at 39 depth electrode sites across patients. The vast majority of stimulation trials produced negative results, with only 3 patients (16.6%; p = .007) showing naming impairment with hippocampal stimulation: in 2 patients, auditory and visual naming were impaired, and in one patient, only auditory naming was impaired. Three patients had positive naming findings at sites adjacent to the hippocampus. Hippocampal stimulation did not impair memory in any of the patients tested. Conclusions: These overall, negative naming results suggest that the dominant hippocampus is not a critically involved in naming, and therefore, hippocampal resection is likely not the cause of postoperative naming decline. However hippocampal stimulation also failed to impair memory, which is well established to be mediated by the hippocampus. Thus, it is possible that, unlike the disruptive effects of discrete cortical stimulation, hippocampal disruption might require stimulation at more than one site, or might require a different set of stimulation parameters. Funding: NIH R01 NS35140 (MH)