Biology subjects

Lakshman, M.

Publications and source records attributed to Lakshman, M..

4 recordsLinked to original sources

bioRxiv2025

The ventral visual stream for reading converges on the transmodal language network

Reading bridges sensation and cognition. To derive meaning from written words, visual input is first processed in unimodal (i.e., sensory-specific) visual streams and then engages a distributed language network (LANG) that includes classic perisylvian language areas and supports transmodal (i.e., sensory-nonspecific) functions. A reading-relevant region in the inferotemporal cortex (ITC), sometimes called the visual word form area (VWFA), has been the subject of controversy because it displays properties of both systems: it responds to meaningless written pseudowords, implying a unimodal visual function, but also responds to meaningful speech, implying a transmodal function. We investigated whether precision functional mapping could help clarify this regions role in reading. We characterized a stream of visual regions along the ITC that responded preferentially to visual orthographic forms (i.e., written pseudowords, consonant strings, and real words). Network mapping revealed that only the most anterior region of this "orthographic stream" was connected to the LANG network and accordingly showed responses to meaningful speech. Furthermore, this anterior region was more selective, responding preferentially to text-based stimuli, whereas the more posterior regions of the stream were additionally activated by perceptually similar images (i.e., number strings, foreign script). Our results support that connections to the LANG network may drive specialization along the orthographic stream for writing. This basal language network region may represent an interface between visual and transmodal language systems, thus serving as a critical nexus for reading.

neuroscience↗

bioRxiv2025

Targeting intracranial electrical stimulation to network regions defined within individuals causes network-level effects

Intracranial electrical stimulation (ES) is routinely used therapeutically, diagnostically, and to provide causal evidence in neuroscience studies. However, our understanding of the brain network-level effects of ES remains limited. We applied precision functional mapping (PFM), based on functional magnetic resonance imaging (fMRI), to define large-scale networks within individual epilepsy patients. We show that single-pulse electrical stimulation (SPES) and high-frequency electrical stimulation (HFES) are more likely to evoke within-network responses and elicit network-related behavioral effects, respectively, when applied near to a PFM-defined network region. Network-level effects were more likely when stimulating sites in white matter, in close proximity to the targeted network, and within a region predominantly occupied by the targeted network. Further, network-specific modulation may be achievable by applying lower current intensities at these sites. Our findings support that modulation of specific networks is achievable by targeting ES to a functional anatomic "sweet spot" that can be identified using PFM.

neuroscience↗

bioRxiv2023

Social cognitive regions of human association cortex are selectively connected to the amygdala

Reasoning about someones thoughts and intentions - i.e., forming a theory of mind - is an important aspect of social cognition that relies on association areas of the brain that have expanded disproportionately in the human lineage. We recently showed that these association zones comprise parallel distributed networks that, despite occupying adjacent and interdigitated regions, serve dissociable functions. One network is selectively recruited by theory of mind processes. What circuit properties differentiate these parallel networks? Here, we show that social cognitive association areas are intrinsically and selectively connected to regions of the anterior medial temporal lobe that are implicated in emotional learning and social behaviors, including the amygdala at or near the basolateral complex and medial nucleus. The results suggest that social cognitive functions emerge through coordinated activity between amygdala circuits and a distributed association network, and indicate the medial nucleus may play an important role in social cognition in humans.

neuroscience↗

bioRxiv2023

Situating the parietal memory network in the context of multiple parallel distributed networks using high-resolution functional connectivity

A principle of brain organization is that networks serving higher cognitive functions are widely distributed across the brain. One exception has been the parietal memory network (PMN), which plays a role in recognition memory but is often defined as being restricted to posteromedial association cortex. We hypothesized that high-resolution estimates of the PMN would reveal small regions that had been missed by prior approaches. High-field 7T functional magnetic resonance imaging (fMRI) data from extensively sampled participants was used to define the PMN within individuals. The PMN consistently extended beyond the core posteromedial set to include regions in the inferior parietal lobule; rostral, dorsal, medial, and ventromedial prefrontal cortex; the anterior insula; and ramus marginalis of the cingulate sulcus. The results suggest that, when fine-scale anatomy is considered, the PMN matches the expected distributed architecture of other association networks, reinforcing that parallel distributed networks are an organizing principle of association cortex.

neuroscience↗