This study aimed to investigate whether transcranial magnetic stimulation (TMS) can induce selective working memory (WM) deficits of visual-object versus visual-spatial information in normal humans. Thirty-five healthy subjects performed two computerized visual n-back tasks, in which they were required to memorize spatial locations or abstract patterns. In a first series of experiments, unilateral or bilateral TMS was delivered on posterior parietal and middle temporal regions of both hemispheres after various delays during the WM task. Bilateral temporal TMS increased reaction times (RTs) in the visual-object, whereas bilateral parietal TMS selectively increased RTs in the visual-spatial WM task. These effects were evident at a delay of 300 ms. Response accuracy was not affected by bilateral or unilateral TMS of either cortical region. In a second group of experiments, bilateral TMS was applied over the superior frontal gyrus (SFG) or the dorsolateral prefrontal cortex (DLPFC). TMS of the SFG selectively increased RTs in the visual-spatial WM task, whereas TMS of the DLPFC interfered with both WM tasks, in terms of both accuracy and RTs. These effects were evident when TMS was applied after a delay of 600 ms, but not one of 300 ms. These findings confirm the segregation of WM buffers for object and spatial information in the posterior cortical regions. In the frontal cortex, the DLPFC appears to be necessary for WM computations regardless of the stimulus material.

Background: There is no consensus about which hippocampal subfields become atrophic earliest in the course of Alzheimer's disease (AD).Methods: Thirty AD patients, 41 mild cognitive impairment (MCI) patients, and 38 healthy controls (HCs) underwent cerebral magnetic resonance imaging (with an automated segmentation protocol for the volumetric analysis of hippocampal subfields) and a test of immediate and delayed recall of a 15‐word list.Results: The volumes of the presubiculum and subiculum presented the most remarkable reduction in the patient's groups. In the MCI group, only the volumes of presubiculum and subiculum predicted performance on the memory tests. In AD patients, the volumes of all hippocampal subfields (with the notable exception of the CA1) predicted memory scores.Conclusions: Our data point to a prevalent atrophy of the presubicular‐subicular complex from the early phases of AD. This finding is consistent with neuropathological observations in AD patients and probably reflects the severe degeneration of the perforant pathway while penetrating the hippocampus through the subicular field in its course from the entorhinal cortex to the dentate gyrus.

Objectives: Investigating in a case-control study whether the performance scores of a group of patients with Parkinson disease (PD) without dementia on tests of declarative memory could be predicted by hippocampal volume reduction (as assessed by automatic segmentation of cerebral magnetic resonance [MR] images) or by the rate of microstructural alterations (as evaluated by diffusion tensor analysis of MR images). Method: Twenty-five individuals with PD and 25 matched healthy control subjects underwent a 3-T MRI protocol with whole-brain T1-weighted and diffusion tensor imaging and a neuropsychological assessment. Images were processed to obtain indices of macrostructural (volume) and microstructural (mean diffusivity [MD]) variation of bilateral hippocampi. Neuropsychological evaluation included tests of verbal memory (15-minute delayed recall of a 15-word list) and visuospatial memory (20-minute delayed reproduction of Rey complex figure). Results: MD in the hippocampi of patients with PD was significantly increased with respect to that of the group of control subjects. Moreover, patients with high hippocampal MD values obtained low memory scores. In contrast, no difference emerged between patients with PD and healthy control subjects for hippocampal size, and no relationship could be found between hippocampal volumes and memory scores. Conclusions: These data confirm that the declarative memory impairment in patients with PD without dementia may be predicted by the rate of microstructural alterations in the hippocampal formation as detected by diffusion tensor imaging analysis.