The Roles of Left Versus Right Anterior Temporal Lobes in Semantic Memory: A Neuropsychological Comparison of Postsurgical Temporal Lobe Epilepsy Patients

Abstract

The presence and degree of specialization between the anterior temporal lobes (ATLs) is a key issue in debates about the neural architecture of semantic memory. Here, we comprehensively assessed multiple aspects of semantic cognition in a large group of postsurgical temporal lobe epilepsy (TLE) patients with left versus right anterior temporal lobectomy (n = 40). Both subgroups showed deficits in expressive and receptive verbal semantic tasks, word and object recognition, naming and recognition of famous faces and perception of faces and emotions. Graded differences in performance between the left and right groups were secondary to the overall mild semantic impairment; primarily, left resected TLE patients showed weaker performance on tasks that required naming or accessing semantic information from a written word. Right resected TLE patients were relatively more impaired at recognizing famous faces as familiar, although this effect was observed less consistently. These findings unify previous partial, inconsistent results and also align directly with fMRI and transcranial magnetic stimulation results in neurologically intact participants. Taken together, these data support a model in which the 2 ATLs act as a coupled bilateral system for the representation of semantic knowledge, and in which graded hemispheric specializations emerge as a consequence of differential connectivity to lateralized speech production and face perception regions.

Figure 1.

Resection overlap map for the 17 left and 17 right TLE patients. Overlap of the resection areas defined by the Seghier et al. (2008) method. Left TLE patients overlap is shown on the right of the image, right TLE patients overlap is shown on the left of the image. Color bars indicate the number of patients with resection in that area. Warmer colors = greater overlap, cooler colors = less overlap.

Figure 2.

Summary of accuracy (A) and reaction time (B) data from the neuropsychological battery. Data for the control participants are shown in grey and data from the left and right TLE patients are shown in white and black, respectively. Accuracy data reported as percentages and correct response times are reported in milliseconds. Significant differences between the groups based on one-way between group ANOVAs are noted with asterisks; the color of the asterisk denotes the direction of the effect.

Figure 3.

Synonym judgment frequency × imageability analysis. Breaking down performance on the synonym judgment task by frequency (high, low) and imageability (high, medium, low). The color of the line denotes the group (controls, grey; left TLE, white; right TLE, black). The line style denotes the condition (solid line = high imageability, hashed line = medium imageability, dotted line = low imageability). Error bars denote standard error.

Figure 4.

Specific-level picture naming results. (A) Picture naming results for high and low frequency items across the 3 groups for accuracy (left) and reaction time (right). Error bars show standard error. (B) Results from the error analysis are separated by error category.