Biology subjects

Lambon Ralph, M. A.

Publications and source records attributed to Lambon Ralph, M. A..

22 records · Page 2Linked to original sources

bioRxiv2019

The neural bases of resilient cognitive systems: Evidence of variable neuro-displacement in the semantic system.

The purpose of this study was to initiate exploration of an equally-important research goal: what are the neurocomputational mechanisms that make these cognitive systems \"well engineered\" and thus resilient across a range of performance demands and to mild levels of perturbation or even damage? We achieved this aim by investigating the neural dynamics of the semantic network with two task difficulty manipulations. We found that intrinsic resilience-related mechanisms were observed in both the domain-specific semantic representational system and the parallel executive control networks. Functional connectivity between these regions was also increased and these increases were related to better semantic task performance. Our results suggest that higher cognitive functions are made resilient by flexible, dynamic changes (variable neuro-displacement) across both domain-specific and multi-demand networks. Our findings provide strong evidence that the compensatory functional alterations in the impaired brain might reflect intrinsic mechanisms of a well-engineered neural system.

neuroscience↗

bioRxiv2019

Connectivity Gradient in the Human Left Inferior Frontal Gyrus: Intraoperative Cortico-Cortical Evoked Potential Study

In the dual-stream model of language processing, the exact connectivity of the ventral stream to the anterior temporal lobe remains elusive. To investigate the connectivity among the inferior frontal gyrus (IFG) and the lateral part of the temporal and parietal lobes, we integrated spatiotemporal profiles of cortico-cortical evoked potentials (CCEPs) recorded intraoperatively from 14 patients who had had resective surgeries for brain tumor or epileptic focus. The 4D visualization of the combined CCEP data showed that the pars opercularis (Brocas area) connected to the posterior temporal cortices and the supramarginal gyrus, while the pars orbitalis connected to the anterior lateral temporal cortices and the angular gyrus. Quantitative topographical analysis of CCEP connectivity confirmed an anterior-posterior gradient of connectivity from IFG stimulus sites to the temporal response sites. Reciprocality analysis indicated that the anterior part of the IFG is bi-directionally connected to the temporal or parietal area. The present study revealed that each IFG subdivision has a different connectivity to the temporal lobe with an anterior-posterior gradient and supports the classical connectivity concept of Dejerine that the frontal lobe is connected to the temporal lobe through the arcuate fasciculus and also a double-fan-shaped structure, anchored at the limen insulae.

neuroscience↗

bioRxiv2019

Evidence for a deep, distributed and dynamic semantic code in human ventral anterior temporal cortex

How does the human brain encode semantic information about objects? This paper reconciles two seemingly contradictory views. The first proposes that local neural populations independently encode semantic features; the second, that semantic representations arise as a dynamic distributed code that changes radically with stimulus processing. Combining simulations with a well-known neural network model of semantic memory, multivariate pattern classification, and human electrocorticography, we find that both views are partially correct: semantic information is distributed across ventral temporal cortex in a dynamic code that possesses stable feature-like elements in posterior regions but with elements that change rapidly and nonlinearly in anterior regions. This pattern is consistent with the view that anterior temporal lobes serve as a deep cross-modal "hub" in an interactive semantic network, and more generally suggests that tertiary association cortices may adopt dynamic distributed codes difficult to detect with common brain imaging methods.

neuroscience↗

bioRxiv2019

Overarching principles and dimensions of the functional organisation in the inferior parietal cortex

The parietal cortex (PC) is implicated in a confusing myriad of different cognitive processes/tasks. Consequently, understanding the nature and organisation of the core underlying neurocomputations is challenging. According to the Parietal Unified Connectivity-biased Computation (PUCC) model two properties underpin PC function and organisation. Firstly, PC is a multi-domain, context-dependent buffer of time-and space-varying input, the function of which, over time, becomes sensitive to the statistical temporal/spatial structure of events. Secondly, over and above this core buffering computation, differences in long-range connectivity will generate graded variations in task engagement across subregions. The current study tested these hypotheses using a group ICA technique with two independent fMRI datasets (task and resting state data). Three functional organisational principles were revealed. Factor 1: inferior PC was sensitive to the statistical structure of sequences for all stimulus types (pictures, sentences, numbers). Factor 2: a dorsal-ventral variation in generally task-positive vs. task-negative (variable) engagement. Factor 3: An anterior-posterior dimension in inferior PC reflecting different engagement in verbal vs. visual tasks, respectively. Together the data suggest that the core neurocomputation implemented by PC is common across domains, with graded task engagement across regions reflecting variations in the connectivity of task-specific networks that interact with parietal cortex.

neuroscience↗