Biology subjects

Assem, M.

Publications and source records attributed to Assem, M..

9 recordsLinked to original sources

bioRxiv2026

Aperiodic neural activity links electromagnetic and hemodynamic representations of domain-general cognitive demand across the cortical hierarchy

The human brain demonstrates remarkable flexibility and capacity for domain-general cognitive control, allowing us to perform diverse and complex tasks. Central to this ability is the multiple- demand (MD) network, a domain-general system that is robustly engaged during demanding tasks in fMRI studies. However, the electrophysiological signatures underlying these domain-general responses remain elusive. While recent research has implicated aperiodic neural activity as a promising candidate, the limited spatial resolution of non-invasive electrophysiology has left it unresolved how this aperiodic signal relates to demand-related activity within the MD network and whether this relationship reflects a broader organizational principle across the cortex. To address these questions, we used a multimodal fusion framework to integrate fMRI and magnetoencephalography (MEG) data acquired while participants performed a diverse set of cognitive control tasks. We found that raw MEG- fMRI correspondence was strongest in unimodal sensorimotor cortices and progressively decreased toward transmodal association cortex during cognitive control tasks, revealing a hierarchical decline in correspondence between the electromagnetic and hemodynamic signals measured by these technologies. However, the proportion of this variance that was attributable to cognitive demand and carried by aperiodic signals showed the reverse gradient, systematically increasing along the sensorimotor-association axis. In particular, in the MD network, aperiodic broadband power showed the strongest demand-specific cross-modal commonality, outperforming canonical oscillatory components. These findings reveal two opposing hierarchical gradients: overall MEG-fMRI correspondence across all electrophysiological signals decreased toward association cortex, whereas the proportion attributable to aperiodic signals associated with cognitive demand increased. Our results identify aperiodic neural activity as a key electrophysiological substrate of cognitive control and a bridge linking electromagnetic and hemodynamic representations across the cortical hierarchy.

neuroscience↗

bioRxiv2026

Cognitive control networks in human and macaque

A much-replicated finding in human brain imaging is a distributed 'multiple-demand' or MD system, increasing in activity for many kinds of cognitive demand, and centrally involved in cognitive control. MD regions are proposed to encode a distributed mental model of critical task events, bound together in the roles and relationships needed to direct action selection. Though previous data hint at a corresponding network in the macaque, there has been no direct comparison to human data. Here we used functional magnetic resonance imaging to measure whole brain activation in a multi-step saccadic maze task, compared to a control requiring similar moves but without goal-based decisions. Human data were a close match to the canonical MD network, extended to include adjacent regions and in particular much of the canonical dorsal attention network. Data from two monkeys suggested correspondences in dorsomedial frontal, insula/orbitofrontal and posterior temporal cortex. In lateral frontal cortex there was just a single, largely dorsal activation patch, in contrast to multiple distinct human patches. In one animal, as in human, there was accompanying activation of dorsal and medial parietal cortex. In macaque as in human, together with previous data, our findings suggest an extended and strongly interconnected brain network recruited by increased cognitive challenge.

neuroscience↗

bioRxiv2025

The Taco Setup: A Novel TMS-fMRI Setup for High Resolution Whole Brain Imaging

Simultaneous TMS-fMRI holds significant opportunities for advancing basic and translational neuroscience. However, current configurations face technical limitations, particularly the need to accommodate TMS hardware within the MRI environment. Previous solutions have used low numbers of radio-frequency (RF) channels, limiting fMRI data quality and whole-brain coverage. Here, we introduce a novel 22-channel "Taco" TMS-fMRI configuration that re-purposes flexible RF coils, wrapping them around both the participants head and the TMS coil to preserve whole brain signal reception. Guided by precision fMRI principles, we optimized acquisition protocols to achieve to achieve high temporal signal-to-noise ratio (tSNR) across the cortex. Data from three pilot participants demonstrate robust signal quality, including in regions proximal to the TMS coil. This setup offers a relatively simple and cost-effective approach to integrating precision fMRI into TMS-fMRI research.

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bioRxiv2025

Category-biased patches encircle core domain-general regions in the human lateral prefrontal cortex

The fine-grained functional organization of the human lateral prefrontal cortex (PFC) remains poorly understood. Previous fMRI studies delineated focal domain-general, or multiple-demand (MD), PFC areas that co-activate during diverse cognitively demanding tasks. While there is some evidence for category-selective (face and scene) patches, in human and non-human primate PFC, these have not been systematically assessed. Recent precision fMRI studies have also revealed sensory-biased PFC patches adjacent to MD regions. To investigate if this topographic arrangement extends to other domains, we analysed two independent fMRI datasets (n=449 and n=37) utilizing the high-resolution multimodal MRI approaches of the Human Connectome Project (HCP). Both datasets included cognitive control tasks and stimuli spanning different categories: faces, places, tools and body parts. Contrasting each stimulus category against the remaining ones revealed focal interdigitated patches of activity located adjacent to core MD regions. The face and place results were robust, replicating across different executive tasks, experimental designs (block and event-related) and at the single subject level. In one dataset, where participants performed both category and sensory tasks, place patches overlapped with visually biased regions, while face patches were positioned between visual and auditory biases. Our results paint a refined view of the fine-grained functional organization of the PFC, revealing a recurring motif of interdigitated domain-specific and domain-general circuits. This organization offers new constraints for models of cognitive control, cortical specialization and development.

neuroscience↗

bioRxiv2023

Dynamic layer-specific processing in the prefrontal cortex during working memory

The dorsolateral prefrontal cortex (dlPFC) is reliably engaged in working memory (WM). Evidence from non-human primates indicates that the dlPFC comprises different cytoarchitectonic layers that play distinct roles in WM subprocesses; yet the functional role of the dlPFCs laminar circuitry in human WM is not well understood. In this study, participants completed a delayed-match-to-sample WM task while undergoing functional magnetic resonance imaging (fMRI) at ultra-high resolution, which allowed us to examine layer-specific responses of the dlPFC to manipulations in WM load and motor response. We conducted univariate and multivariate analyses across all periods of the WM task: encoding, delay and retrieval. First, we observed that superficial layers activate stronger than deep layers to higher WM load during the delay period. This aligns with earlier work showing preferential superficial layer activation to WM manipulation and as such may indicate lamina-specific activation of the frontoparietal network to heightened task demands more generally. Second, we found that superficial layers show higher decoding of WM load differences than deep layers during the retrieval period. In this context, we could show that decoding of WM load in the superficial layer exhibited dynamic changes across the encoding, delay and retrieval period of the task, indicative of separate WM control processes that occur on the WM content. Last, we found that superficial and deep layers are both non-differentially involved in the motor response, contradicting earlier findings of a preferential deep layer activation in humans. Taken together, our results provide new insights into the functional laminar circuitry of the dlPFC during WM and provide further support for a dynamic account of dlPFC coding.

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bioRxiv2022

Basis of executive functions in fine-grained architecture of cortical and subcortical human brain networks

Theoretical models suggest that executive functions rely on both domain-general and domain-specific processes. Supporting this view, prior brain imaging studies have revealed that executive activations converge and diverge within broadly characterized brain networks. However, the lack of precise anatomical mappings has impeded our understanding of the interplay between domain-general and domain-specific processes. To address this challenge, we used the high-resolution multimodal MRI approach of the Human Connectome Project to scan participants performing three canonical executive tasks: n-back, rule switching, and stop signal. The results reveal that, at the individual level, different executive activations converge within 9 domain-general territories distributed in frontal, parietal and temporal cortices. Each task exhibits a unique topography characterized by finely detailed activation gradients within domain-general territory shifted towards adjacent resting-state networks; n-back activations shift towards the default mode, rule switching towards dorsal attention and stop signal towards cingulo-opercular networks. Importantly, the strongest activations arise at multimodal neurobiological definitions of network borders. Matching results are seen in circumscribed regions of the caudate nucleus, thalamus and cerebellum. The shifting peaks of local gradients at the intersection of task-specific networks provide a novel mechanistic insight into how partially-specialised networks interact with neighbouring domain-general territories to generate distinct executive functions.

neuroscience↗

bioRxiv2021

Precise topology of adjacent domain-general and sensory-biased regions in the human brain

Recent functional MRI studies identified sensory-biased regions across much of the association cortices and cerebellum. However, their anatomical relationship to multiple-demand (MD) regions, characterized as domain-general due to their co-activation during multiple cognitive demands, remains unclear. For a better anatomical delineation, we used multimodal MRI techniques of the Human Connectome Project to scan subjects performing visual and auditory versions of a working memory (WM) task. The contrast between hard and easy WM showed strong domain generality, with essentially identical patterns of cortical, subcortical and cerebellar MD activity for visual and auditory materials. In contrast, modality preferences were shown by contrasting easy WM with baseline; most MD regions showed visual preference while immediately adjacent to cortical MD regions, there were interleaved regions of both visual and auditory preference. The results may exemplify a general motif whereby domain-specific regions feed information into and out of an adjacent, integrative MD core.

neuroscience↗

bioRxiv2019

A Domain-general Cognitive Core defined in Multimodally Parcellated Human Cortex

Numerous brain imaging studies identified a domain-general or "multiple-demand" (MD) activation pattern accompanying many tasks and may play a core role in cognitive control. Though this finding is well established, the limited spatial localization provided by traditional imaging methods precluded a consensus regarding the precise anatomy, functional differentiation and connectivity of the MD system. To address these limitations, we used data from 449 subjects from the Human Connectome Project, with cortex of each individual parcellated using neurobiologically grounded multi-modal MRI features. The conjunction of three cognitive contrasts reveals a core of 10 widely distributed MD parcels per hemisphere that are most strongly activated and functionally interconnected, surrounded by a penumbra of 17 additional areas. Outside cerebral cortex, MD activation is most prominent in the caudate and cerebellum. Comparison with canonical resting state networks shows MD regions concentrated in the fronto-parietal network but also engaging three other networks. MD activations show modest relative task preferences accompanying strong co-recruitment. With distributed anatomical organization, mosaic functional preferences, and strong interconnectivity, we suggest MD regions are well positioned to integrate and assemble the diverse components of cognitive operations. Our precise delineation of MD regions provides a basis for refined analyses of their functions.

neuroscience↗

bioRxiv2017

Multiple Demand (MD) system’s activity predicts individual differences in working memory and fluid intelligence

Numerous brain lesion and fMRI studies have linked individual differences in executive abilities and fluid intelligence to brain regions of the fronto-parietal "multiple-demand" (MD) network. Yet, fMRI studies have yielded conflicting evidence as to whether better executive abilities are associated with stronger or weaker MD activations and whether this relationship is restricted to the MD network. Here, in a large-sample (n=216) fMRI investigation, we found that stronger activity in MD regions - functionally defined in individual participants - was robustly associated with more accurate and faster responses on a spatial working memory task performed in the scanner, as well as fluid intelligence measured independently (n=114). In line with some prior claims about a relationship between language and fluid intelligence, we also found a weak association between activity in the brain regions of the left fronto-temporal language network during an independent passive reading task, and performance on the working memory task. However, controlling for the level of MD activity abolished this relationship, whereas the MD activity-behavior association remained highly reliable after controlling for the level of activity in the language network. Finally, we demonstrate how unreliable MD activity measures, coupled with small sample sizes, could falsely lead to the opposite, negative, association that has been reported in some prior studies. Taken together, these results demonstrate that a core component of individual differences variance in executive abilities and fluid intelligence is selectively and robustly positively associated with the level of activity in the MD network, a result that aligns well with lesion studies.

neuroscience↗