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The fornix is a white matter bundle located in the mesial aspect of the cerebral hemispheres, which connects various nodes of a limbic circuitry and is believed to play a key role in cognition and episodic memory recall. As the most prevalent cause of dementia, Alzheimer’s disease (AD) dramatically impairs the quality of life of patients and imposes a significant societal burden on the healthcare system. As an established treatment for movement disorders, deep brain stimulation (DBS) is currently being investigated in preclinical and clinical studies for treatment of memory impairment in AD by modulating fornix activity. Optimal target and stimulation parameters to potentially rescue memory deficits have yet to be determined. The aim of this review is to consolidate the structural and functional aspects of the fornix in the context of neuromodulation for memory deficits. We first present an anatomical and functional overview of the fibres and structures interconnected by the fornix. Recent evidence from preclinical models suggests that the fornix is subdivided into two distinct functional axes: a septohippocampal pathway and a subiculothalamic pathway. Each pathway’s target and origin structures are presented, followed by a discussion of their oscillatory dynamics and functional connectivity. Overall, neuromodulation of each pathway of the fornix is discussed in the context of evidence-based forniceal DBS strategies. It is not yet known whether driving fornix activity can enhance cognition—optimal target and stimulation parameters to rescue memory deficits have yet to be determined.

Deep brain stimulation (DBS) to the fornix is an investigational treatment for patients with mild Alzheimer’s Disease. Outcomes from randomized clinical trials have shown that cognitive function improved in some patients but deteriorated in others. This could be explained by variance in electrode placement leading to differential engagement of neural circuits. To investigate this, we performed a post-hoc analysis on a multi-center cohort of 46 patients with DBS to the fornix (NCT00658125, NCT01608061). Using normative structural and functional connectivity data, we found that stimulation of the circuit of Papez and stria terminalis robustly associated with cognitive improvement ( R  = 0.53, p  < 0.001). On a local level, the optimal stimulation site resided at the direct interface between these structures ( R  = 0.48, p  < 0.001). Finally, modulating specific distributed brain networks related to memory accounted for optimal outcomes ( R  = 0.48, p  < 0.001). Findings were robust to multiple cross-validation designs and may define an optimal network target that could refine DBS surgery and programming. Deep brain stimulation has been investigated as a potential treatment for cognitive impairments in Alzheimer’s disease. Here the authors carry out post hoc analysis of multi-center cohorts to investigate the anatomical and functional correlates of effective deep brain stimulation, and find that stimulating circuit of Papez, fornix and bed nucleus of the stria terminalis, and a multi-region functional network, were associated with clinical improvement.

Recently, much attention has been focused on the definition and structure of the hippocampus and its subfields, while the projections from the hippocampus have been relatively understudied. Here, we derive a reliable protocol for manual segmentation of hippocampal white matter regions (alveus, fimbria, and fornix) using high-resolution magnetic resonance images that are complementary to our previous definitions of the hippocampal subfields, both of which are freely available at https://github.com/cobralab/atlases. Our segmentation methods demonstrated high inter- and intra-rater reliability, were validated as inputs in automated segmentation, and were used to analyze the trajectory of these regions in both healthy aging (OASIS), and Alzheimer's disease (AD) and mild cognitive impairment (MCI; using ADNI). We observed significant bilateral decreases in the fornix in healthy aging while the alveus and cornu ammonis (CA) 1 were well preserved (all p's<0.006). MCI and AD demonstrated significant decreases in fimbriae and fornices. Many hippocampal subfields exhibited decreased volume in both MCI and AD, yet no significant differences were found between MCI and AD cohorts themselves. Our results suggest a neuroprotective or compensatory role for the alveus and CA1 in healthy aging and suggest that an improved understanding of the volumetric trajectories of these structures is required. •Novel high-resolution manual segmentation of human alveus, fimbria, and fornix.•Validation (precision and accuracy) of manual atlases for use in automatic segmentation.•Application of automatic segmentation on AD/MCI and healthy aging datasets.•Results suggest neuroprotective role for alveus and hippocampal CA1 region.

Intermittent theta burst stimulation (iTBS) is a well-established treatment for major depressive disorder (MDD), but predicting clinical outcomes remains challenging. Heart rate deceleration induced by iTBS has emerged as a potential biomarker for treatment response, yet the role of white matter (WM) properties in mediating these effects is largely unexplored. In this quadruple-blind, crossover study, we investigated the relationship between WM microstructure, iTBS-driven heart rate modulation, and antidepressant effects. Using correlational tractography, we focused on four major WM tracts—the cingulum, fornix, superior longitudinal fasciculus, and uncinate fasciculus—to examine short-term microstructural changes in relation to therapeutic outcomes. At baseline, findings revealed that fractional anisotropy (FA) in the fornix and right dorsal cingulum was negatively correlated with heart rate deceleration, while radial and mean diffusivity (MD, RD) in the fornix were positively correlated. In the right ventral cingulum, FA showed a positively correlation, while MD and RD were negatively correlated with symptom improvement. Longitudinally, FA increases in the left cingulum were significantly associated with greater symptom alleviation post-treatment. Notably, the correlation between iTBS-induced heart rate modulations and clinical improvement after six weeks, previously demonstrated in this cohort, was identified, while WM microstructural properties in the fornix and cingulum demonstrated predictive value for both heart rate modulation and treatment response. WM changes in the cingulum, evident as early as four weeks, highlight its unique neuroplasticity potential along iTBS intervention. Together, these findings provide novel insights into the structural connectivity patterns influencing iTBS outcomes, offering a novel foundation for more personalized therapeutic strategies in MDD.

Deep brain stimulation (DBS) of the fimbria–fornix—a region that provides input to the hippocampus—is shown to restore hippocampus-dependent memory and hippocampal long-term potentiation and neurogenesis in a mouse model of Rett syndrome, suggesting that DBS, which is already used in the treatment of several neurological conditions, could be a viable approach to mitigating cognitive impairment in Rett syndrome and other disorders of childhood intellectual disability. Deep brain stimulation in Rett syndrome Rett syndrome is a genetic disorder that causes profound intellectual disability and other impairments. Huda Zoghbi and colleagues now show that in a mouse model of the disorder, a two-week course of daily deep-brain stimulation of the fimbria-fornix — part of the brain that provides input to the hippocampus — restored hippocampal-dependent memory when tested three weeks after the end of the treatment. It also restored hippocampal long-term potentiation and neurogenesis. These findings indicate that deep-brain stimulation, which is already used in the treatment of motor diseases such as Parkinson's disease and dystonia, could be a viable approach to mitigating cognitive impairment in Rett syndrome and other disorders of childhood intellectual disability. Deep brain stimulation (DBS) has improved the prospects for many individuals with diseases affecting motor control, and recently it has shown promise for improving cognitive function as well. Several studies in individuals with Alzheimer disease and in amnesic rats have demonstrated that DBS targeted to the fimbria–fornix 1 , 2 , 3 , the region that appears to regulate hippocampal activity, can mitigate defects in hippocampus-dependent memory 3 , 4 , 5 . Despite these promising results, DBS has not been tested for its ability to improve cognition in any childhood intellectual disability disorder. Such disorders are a pressing concern: they affect as much as 3% of the population and involve hundreds of different genes. We proposed that stimulating the neural circuits that underlie learning and memory might provide a more promising route to treating these otherwise intractable disorders than seeking to adjust levels of one molecule at a time. We therefore studied the effects of forniceal DBS in a well-characterized mouse model of Rett syndrome (RTT), which is a leading cause of intellectual disability in females. Caused by mutations that impair the function of MeCP2 (ref. 6 ), RTT appears by the second year of life in humans, causing profound impairment in cognitive, motor and social skills, along with an array of neurological features 7 . RTT mice, which reproduce the broad phenotype of this disorder, also show clear deficits in hippocampus-dependent learning and memory and hippocampal synaptic plasticity 8 , 9 , 10 , 11 . Here we show that forniceal DBS in RTT mice rescues contextual fear memory as well as spatial learning and memory. In parallel, forniceal DBS restores in vivo hippocampal long-term potentiation and hippocampal neurogenesis. These results indicate that forniceal DBS might mitigate cognitive dysfunction in RTT.

Humans survive and thrive through social exchange. Yet, social dependency also comes at a cost. Perceived social isolation, or loneliness, affects physical and mental health, cognitive performance, overall life expectancy, and increases vulnerability to Alzheimer’s disease-related dementias. Despite severe consequences on behavior and health, the neural basis of loneliness remains elusive. Using the UK Biobank population imaging-genetics cohort ( n  = ~40,000, aged 40–69 years when recruited, mean age = 54.9), we test for signatures of loneliness in grey matter morphology, intrinsic functional coupling, and fiber tract microstructure. The loneliness-linked neurobiological profiles converge on a collection of brain regions known as the ‘default network’. This higher associative network shows more consistent loneliness associations in grey matter volume than other cortical brain networks. Lonely individuals display stronger functional communication in the default network, and greater microstructural integrity of its fornix pathway. The findings fit with the possibility that the up-regulation of these neural circuits supports mentalizing, reminiscence and imagination to fill the social void. Here, using pattern-learning analyses of structural, functional, and diffusion brain scans in ~40,000 UK Biobank participants, the authors provide population-scale evidence that the default network is associated with perceived social isolation.

•Mesoscale mapping of connectivity in the human hippocampus.•Mapping of fimbrial fibers onto hippocampal substructures.•Mapping of inter- and translamellar networks in the hippocampus.•Dissection of 50 individual pathways between the head and body subregions.•Identification of 12 separate lamellae in the body of the hippocampus. The human hippocampus is essential to cognition and emotional processing. Its function is defined by its connectivity. Although some pathways have been well-established, our knowledge about anterior-posterior connectivity and the distribution of fibers from major fiber bundles remains limited. Mesoscale (250 μm isotropic acquisition, upsampled to 125 μm) resolution MR images of the human temporal lobe afforded a detailed visualization of fiber tracts, including those that related anterior-posterior substructures defined as subregions (head, body, tail) and subfields (cornu ammonis 1–3, dentate gyrus) of the hippocampus. Fifty pathways were dissected between the head and body, highlighting an intricate mesh of connectivity between these two subregions. Along the body subregion, 12 lamellae were identified based on morphology and the presence of interlamellar fibers that appear to connect neighboring lamellae at the edge of the external limb of the granule cell layer (GCL). Translamellar fibers (i.e. longitudinal fibers crossing more than 2 lamellae) were also evident at the edge of the internal limb of the GCL. The dentate gyrus of the body was the main site of connectivity with the fimbria. Unique pathways were dissected within the fimbria that connected the body of the hippocampus with the amygdala and the temporal pole. A topographical segregation within the fimbria was determined by fibers’ hippocampal origin, illustrating the importance of mapping the spatial distribution of fibers. Elucidating the detailed structural connectivity of the hippocampus is crucial to develop better diagnostic markers of neurological and psychiatric conditions, as well as to devise novel surgical interventions.

Mechanism-based treatments for schizophrenia are needed, and increasing evidence suggests that oxidative stress may be a target. Previous research has shown that N-acetylcysteine (NAC), an antioxidant and glutathione (GSH) precursor almost devoid of side effects, improved negative symptoms, decreased the side effects of antipsychotics, and improved mismatch negativity and local neural synchronization in chronic schizophrenia. In a recent double-blind randomized placebo-controlled trial by Conus et al., early psychosis patients received NAC add-on therapy (2700 mg/day) for 6 months. Compared with placebo-treated controls, NAC patients showed significant improvements in neurocognition (processing speed) and a reduction of positive symptoms among patients with high peripheral oxidative status. NAC also led to a 23% increase in GSH levels in the medial prefrontal cortex (GSHmPFC) as measured by 1H magnetic resonance spectroscopy. A subgroup of the patients in this study were also scanned with multimodal MR imaging (spectroscopy, diffusion, and structural) at baseline (prior to NAC/placebo) and after 6 months of add-on treatment. Based on prior translational research, we hypothesized that NAC would protect white matter integrity in the fornix. A group × time interaction indicated a difference in the 6-month evolution of white matter integrity (as measured by generalized fractional anisotropy, gFA) in favor of the NAC group, which showed an 11% increase. The increase in gFA correlated with an increase in GSHmPFC over the same 6-month period. In this secondary study, we suggest that NAC add-on treatment may be a safe and effective way to protect white matter integrity in early psychosis patients.

The 22q11.2 deletion syndrome (22q11DS) is associated with a wide spectrum of cognitive and psychiatric symptoms. Despite the considerable work performed over the past 20 years, the genetic etiology of the neurodevelopmental phenotype remains speculative. Here, we report de novo heterozygous truncating variants in the HIRA (Histone cell cycle regulation defective, S. Cerevisiae, homolog of, A) gene associated with a neurodevelopmental disorder in two unrelated patients. HIRA is located within the commonly deleted region of the 22q11DS and encodes a histone chaperone that regulates neural progenitor proliferation and neurogenesis, and that belongs to the WD40 Repeat (WDR) protein family involved in brain development and neuronal connectivity. To address the specific impact of HIRA haploinsufficiency in the neurodevelopmental phenotype of 22q11DS, we combined Hira knock-down strategies in developing mouse primary hippocampal neurons, and the direct study of brains from heterozygous Hira+/− mice. Our in vitro analyses revealed that Hira gene is mostly expressed during neuritogenesis and early dendritogenesis stages in mouse total brain and in developing primary hippocampal neurons. Moreover, shRNA knock-down experiments showed that a twofold decrease of endogenous Hira expression level resulted in an impaired dendritic growth and branching in primary developing hippocampal neuronal cultures. In parallel, in vivo analyses demonstrated that Hira+/− mice displayed subtle neuroanatomical defects including a reduced size of the hippocampus, the fornix and the corpus callosum. Our results suggest that HIRA haploinsufficiency would likely contribute to the complex pathophysiology of the neurodevelopmental phenotype of 22q11DS by impairing key processes in neurogenesis and by causing neuroanatomical defects during cerebral development.

Aims Repetitive transcranial magnetic stimulation (rTMS) could improve the clinical manifestations in Alzheimer's disease (AD), but its impact on deep brain tissue related to memory remains unclear. This study explored whether rTMS targeting cortical gray matter could regulate the white matter (WM) and exert modulatory effects on the network through WM bundles. Methods Seventy‐three AD patients underwent 14‐day rTMS over the left dorsolateral prefrontal cortex (44 real, 25 sham). Granger causality analysis assessed changes in effective connectivity (EC) between the fornix and whole‐brain voxels. Furthermore, the effects of rTMS treatment on fiber tracking parameters were analyzed. Results After rTMS therapy, patients with AD showed increased EC based on fornix in the real‐stimulation group. Functional network projections indicated that these clusters belonged to the frontoparietal network, the somatomotor network, as well as three white matter networks. Additionally, increased EC associated with fornix exhibited lateralization on the right side. Diffusion tensor imaging results showed no significant differences after the 14‐day rTMS treatment. Conclusion In conclusion, a 14‐day rTMS treatment in AD could regulate fornical function by increasing cortical‐fornix EC, indicating neuroplasticity changes in response to therapy. Trial Registration Chinese Clinical Trial Registry (https://www.chictr.org.cn/index.html; ChiCTR2200062564) We enrolled Alzheimer's disease participants who received 14‐day repetitive transcranial magnetic stimulation (rTMS) over the left dorsolateral prefrontal cortex (44 real rTMS, 25 sham). Real rTMS enhanced fornix‐based connectivity to frontoparietal, somatomotor, and white matter networks, highlighting the fornix as a mediator and potential biomarker of therapeutic response.