Anterior Temporal Lobectomy Group Research Articles

The Effects of Human Hippocampal Resection on the Serial Position Curve

The purpose of this study was to examine the contribution of the human hippocampal formation to the classic serial position curve. Seventy-seven patients who underwent anterior temporal lobectomy (ATL) (47 left, 30 right) were administered a list learning task before and after surgery, and changes in the serial position curve were examined. Forty nonsurgical patients with complex partial seizures were tested at comparable intervals and served as controls. Changes in the serial position curve were seen only after left ATL, and almost exclusively among patients without hippocampal sclerosis. Patients without left hippocampal sclerosis, and who therefore underwent resection of hippocampus that was to a considerable degree structurally (and presumably functionally) intact, showed significant declines in recall from the primacy and middle portions of the list compared to all other groups. There was no change in the recency portion of the list. Patients with left hippocampal sclerosis showed only a modest decline in recall from the middle region compared only to the control group, and the right ATL groups did not show any significant changes in serial position recall. These findings demonstrate the contribution of the left hippocampus to those discrete portions of the serial position curve which rely on secondary memory, and have implications for assessing the effects of ATL on memory function.

Dysnomia after Left Anterior Temporal Lobectomy without Functional Mapping

The integrity of language function after a left (dominant) anterior temporal lobectomy performed without the use of functional mapping remains controversial. Much of the controversy concerns the degree to which analyses of group data obscure the identification of surgically induced dysnomia in individual patients. This study investigated postoperative language outcome in our entire series (n = 162) of nonretarded, left hemisphere speech dominant patients with intractable nonlesional epilepsy who underwent a left (n = 85) or right (n = 77) anterior temporal lobectomy without functional mapping. A comparison of preoperative to (6 mo) postoperative performance on a standardized test of nominal speech revealed the following: 1) a statistically significant but clinically modest difference in outcome between left and right anterior temporal lobectomy groups; 2) a subgroup (7%) of left anterior temporal lobectomy patients exhibited a postoperative dysnomia (a decline in nominal speech that exceeded the worst performance in the right anterior temporal lobectomy group); and 3) a postoperative decline in nominal speech after left anterior temporal lobectomy was specifically associated with a later age at the onset of epilepsy. Within the context of group data suggesting minimal risk to language function when functional mapping is not used, a small subgroup of individuals characterized by a later onset of epilepsy can be identified who exhibit a surgically induced dysnomia after a standard left anterior temporal lobectomy. Whether these results differ from the outcome of functional mapping remains to be determined.

Declarative memory following anterior temporal lobectomy in humans.

This study hypothesized that verbal memory decline following anterior temporal lobectomy (ATL) is associated with a lack of significant neuropathology in resected left, but not right, hippocampus and is limited to measures of episodic memory only. Tests of immediate (digit span), semantic (visual naming), and episodic memory as measured by the California Verbal Learning Test (CVLT) were administered before and 6 months after resection of the anterior left (n = 36) or right (n = 26) temporal lobe. There were no effects of hippocampal pathology on measures of immediate or semantic memory for either ATL group or for episodic memory for the right ATL group. Left ATL patients who demonstrated no/mild hippocampal sclerosis exhibited significantly greater postoperative decline in episodic memory compared with those with moderate/marked hippocampal sclerosis on multiple CVLT indices (recall measures, learning characteristics, and contrast measures).

Endogenous potentials after anterior temporal lobectomy

The scalp topography of endogenous potentials was studied in patients who had previously undergone unilateral anterior temporal lobectomy (ATL). These excisions include medial temporal lobe (MTL) structures that have been shown to generate large potentials during tasks that evoke P3 at the scalp. Following right or left ATL, patients showed no differences from unoperated control subjects in overall amplitude of P3 or any other potential measured. The topography of P3 was very similar in both ATL groups and the control subjects, with no differences in laterality. These results suggest that the MTL is not the major generator of the P3 recorded at the scalp in the tasks studied here.