Network Modulation in Alzheimer's Disease

Overview

The purpose of this study is to assess the effects of non-invasive brain stimulation on memory and language ability in patients with two phenotypic variations of underlying Alzheimer disease pathology: amnestic mild cognitive impairment (aMCI) and logopenic variant of primary progressive aphasia (lvPPA). This study will use repetitive Transcranial Magnetic Stimulation (rTMS) to stimulate nodes of networks that are thought to be affected in these two conditions. Specifically, a node of the Default Mode Network (DMN)-the angular gyrus (AG)-will be stimulated in aMCI patients; and a node of the language network-the posterior inferior frontal gyrus (pIFG) will be stimulated in patients with lvPPA. We will use functional connectivity MRI (fcMRI) to assess changes in functional network architecture following the stimulation. We will also assess putative cognitive improvements resulting from the stimulation by in-depth language testing in lvPPA patients and in-depth memory testing in aMCI patients.

Specific Aims Aim 1: To investigate the feasibility of selective rTMS modulation of the Default Mode Network (DMN) vs. language network in patients with aMCI and lvPPA. We will assess the reliability of the rTMS/fcMRI approach to network modulation by studying the same subjects over repeated sessions, with the same methodology, including appropriate sham stimulation conditions. Hypothesis 1a: Changes in network functional connectivity following excitatory rTMS to AG (a node of the DMN) and pIFG (a node of the language network) across identical stimulation sessions separated in time, will show minimal within subject variability in aMCI and lvPPA patients, respectively, thus confirming the reliability of the approach and the reproducibility of prior findings demonstrating connectivity changes within the DMN and language networks. Hypothesis 1b: High frequency rTMS to pIFG will reliably increase pIFG-SMG-pMTG functional connectivity within the language network; high frequency rTMS to AG will reliably increase functional connectivity between pIPL and HF in the DMN. Hypothesis 1c: The changes in network functional connectivity will be selective, in that stimulation to AG (in the DMN) will not modulate connectivity within the language network, while stimulation to pIFG (in the language network) will not modulate DMN connectivity. Aim 2: To assess the effects of rTMS modulation on memory and language function in patients with aMCI and lvPPA. Hypothesis 2a: Excitatory rTMS to the language network will be associated with improved language performance in patients with lvPPA. Hypothesis 2b: Excitatory rTMS to the DMN will be associated with improved memory performance in patients with aMCI. Hypothesis 2c: Cognitive improvements will persist for at least two weeks following cessation of rTMS. Aim 3: To identify the functional network mechanisms underlying cognitive improvement in patients with aMCI and lvPPA after rTMS. Hypothesis 3a: Alterations in intrinsic functional connectivity (as assessed with fcMRI) within each network following rTMS will correlate with improved cognitive performance. Hypothesis 3b: Patients with a greater degree of atrophy and tau pathology within each network (assessed in a prior study) at baseline will predict a lesser response to rTMS. We will recruit up to 20 patients with aMCI and up to 20 patients with lvPPA (with amyloid and tau imaging or CSF biomarkers supportive of underlying AD pathology). We will first perform a baseline functional connectivity MRI (fcMRI), the purpose of which will be to obtain individualized network maps which can be used to target the networks of interest during future sessions. This will be followed by up to two weeks of five consecutive days of stimulation (Monday through Friday). aMCI patients will receive stimulation to the angular gyrus (AG) node of the DMN, and lvPPA patients will receive stimulation to the posterior inferior frontal gyrus (pIFG) node of the language network. During one of the two weeks stimulation will be active, and during the other week stimulation will be SHAM. Order of active and SHAM stimulation blocks will be counterbalanced. We will concomitantly assess the effects of rTMS modulation on memory and language function in patients with aMCI and lvPPA, and in so doing identify the functional network mechanisms underlying cognitive improvement after rTMS. Language and memory assessments will be performed on days 1 (Monday) , 3 (Wednesday) and 5 (Friday) of the week. Finally, to assess the distributed effects of the stimulation upon functional connectivity, fcMRI will be performed on days 1 (Monday) and 5 (Friday). We anticipate these results will lead to an impactful manuscript showing proof-of-concept data: 1) that it is possible to improve memory in aMCI and language in lvPPA using rTMS targeted selectively to the relevant brain network, and: 2) regarding mechanisms underlying the effects of rTMS on network function.