Rajan Kashyap - Academia.edu (original) (raw)

Conference Presentations by Rajan Kashyap

Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. Howe... more Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. However, the neural underpinnings of repetition priming for famous faces in terms of effective connectivity are not known. Here we investigated this problem using dynamic causal modelling of latency-corrected event-related brain potentials (RERPs). Source waveforms of RERP-derived sources in the Occipital Lobe, Fusiform Gyrus, Mediotemporal Lobe, Prefrontal Cortex and Anterotemporal Lobe of each hemisphere entered into models with only forward (F) or also with backward (FB) connections. Based on the framework of predictive coding formulated for repetition suppression, modulations of F and FB connections were expected for sources that displayed priming effects in their source waveforms. Hence, neural sources in each hemisphere were fitted with either F or FB connections. Inter-hemispheric connections were considered between homologous areas and were allowed to be modulated in an incremental manner resulting in a model space that comprised of 24 models. Bayesian model averaging across models revealed effective bidirectional connectivity between the Fusiform Gyrus (face perception) and Prefrontal Cortex (decision-making) in both hemispheres to be modulated by priming. In the left

17th International Biophysics Congress (IUPAB), sponsored by International Union for Pure and Applied Biophysics (IUPAB), organized by Biophysical Society of China (BSC) and Institute of Biophysics, Chinese Academy of Sciences (IBP, CAS) at Beijing, 2011

The effect of self gravity and its interaction with mutual or external gravity are categorized as... more The effect of self gravity and its interaction with mutual or external gravity are categorized as one of the topic in biophysics and have been named as ‘self gravitation bio’ by the Biophysical Society in 2008. Concept of self gravitation bio has been developed initially by the author3 as ‘biomechanics of intrinsic gravity’, which is regarded as an important milestone for understanding various little understood biological phenomena.
At embryonic age of about 4 weeks or about 28 days from fertilization, the embryo measures about 4 mm in length and begins to curve into a C shape. The heart bulges, further develops, and begins to beat in a regular rhythm. Earlier within the zona pellucid, the undifferentiated mass of morula develops in a solid round shaped ball. We initiated our quantitative study of embryological mass through Finite element method (FEM) for demonstration of compressive action of self gravity and counter mechanism due to metabolic energy, as hypothesized by author3 (Fig 1).

However to maintain strength of self gravity independent, it requires some sorts of anti-gravitational mechanism or media. On this principle, it is not difficult to see that embryonic mass is floating over the amniotic fluid, behaving as if a separate frame of reference (accelerated, non-accelerated and inertial) due to maintenance of up-thrust of the fluid against major external gravitational force (similar to playing ball within the running train and on the stationary ground). There is no problem for the embryo now to behave as self gravitating mass due to up-thrust of the amniotic fluid. Such mechanism has been named and described as metabolically inert infrastructure (MII) by the author3. Therefore it is not difficult to identify heart as the centre of self gravitating embryo at that stage of development (Fig 2).

Considering heart as core and periphery of embryological mass as extreme nodes, it follows that the self gravitational forces describes well the Cauchy sequence of topological vector space. The embryological space behaves like a banach space. We calculated and demonstrated the space quantitatively.

Papers by Rajan Kashyap

Psychological medicine, Mar 21, 2024

Frontiers in Human Neuroscience, Sep 2, 2022

Background: Conventionally, transcranial direct current stimulation (tDCS) aims to focalize the c... more Background: Conventionally, transcranial direct current stimulation (tDCS) aims to focalize the current reaching the target region-of-interest (ROI). The focality can be quantified by the dose-target-determination-index (DTDI). Despite having a uniform tDCS setup, some individuals receive focal stimulation (high DTDI) while others show reduced focality ("non-focal"). The volume of cerebrospinal fluid (CSF), gray matter (GM), and white matter (WM) underlying each ROI govern the tDCS current distribution inside the brain, thereby regulating focality. Aim: To determine the regional volume parameters that differentiate the focal and non-focal groups. Methods: T1-weighted images of the brain from 300 age-sex matched adults were divided into three equal groups-(a) Young (20 ≤ × < 40 years), (b) Middle (40 ≤ × < 60 years), and (c) Older (60 ≤ × < 80 years). For each group, inter and intra-hemispheric montages with electrodes at (1) F3 and right supraorbital region (F3-RSO), and (2) CP5 and Cz (CP5-Cz) were simulated, targeting the left-Dorsolateral Prefrontal Cortex (DLPFC) and-Inferior Parietal Lobule (IPL), respectively. Both montages were simulated for two current doses (1 and 2 mA). For each individual head simulated for a tDCS configuration (montage and dose), the current density at each region-ofinterest (ROI) and their DTDI were calculated. The individuals were categorized into two groups-(1) Focal (DTDI ≥ 0.75), and (2) Non-focal (DTDI < 0.75). The Frontiers in Human Neuroscience 01 frontiersin.org Kashyap et al. 10.3389/fnhum.2022.952602 regional volume of CSF, GM, and WM of all the ROIs was determined. For each tDCS configuration and ROI, three 3-way analysis of variance was performed considering-(i) GM, (ii) WM, and (iii) CSF as the dependent variable (DV). The age group, sex, and focality group were the between-subject factors. For a given ROI, if any of the 3 DV's showed a significant main effect or interaction involving the focality group, then that ROI was classified as a "focal ROI." Results: Regional CSF was the principal determinant of focality. For interhemispheric F3-RSO montage, interaction effect (p < 0.05) of age and focality was observed at Left Caudate Nucleus, with the focal group exhibiting higher CSF volume. The CSF volume of focal ROI correlated positively (r ∼ 0.16, p < 0.05) with the current density at the target ROI (DLPFC). For intrahemispheric CP5-Cz montage, a significant (p < 0.05) main effect was observed at the left pre-and post-central gyrus, with the focal group showing lower CSF volume. The CSF volume correlated negatively (r ∼-0.16, p < 0.05) with current density at left IPL. The results were consistent for both current doses. Conclusion: The CSF channels the flow of tDCS current between electrodes with focal ROIs acting like reservoirs of current. The position of focal ROI in the channel determines the stimulation intensity at the target ROI. For focal stimulation in interhemispheric F3-RSO, the proximity of focal ROI reserves the current density at the target ROI (DLPFC). In contrast, for intrahemispheric montage (CP5-Cz), the far-end location of focal ROI reduces the current density at the target (IPL). KEYWORDS transcranial direct current stimulation (tDCS), realistic volumetric approach-based simulator for transcranial electric stimulation (ROAST), Systematic-Approach-for-tDCS-Analysis (SATA), current dose, brain volume, focality, age and sex Highlights individuals with greater amounts of CSF in those pockets show reduced tDCS focality in the target region.-We suggest our computational modeling approach for the determination of tDCS focality to be used in healthy individuals and recommend future studies to extend it in patients with stroke or neurodegeneration who exhibit various locations and extent of CSF pockets.

BackgroundEmerging evidence support the view that brain stimulation might improve essential tremo... more BackgroundEmerging evidence support the view that brain stimulation might improve essential tremor (ET) by altering brain dynamics and facilitating brain plasticity. Yet, we are still missing a mechanistic explanation of the whole brain dynamics underlying these plasticity defining changes.MethodIn this study, we explored the effect of low-frequency repetitive transcranial magnetic stimulation (rTMS) over primary motor cortex (M1) on dynamic functional connectivity (DFC) in patients with ET. Resting state fMRI (RsfMRI) was acquired before and after a single session of rTMS in 30 patients with ET and compared with RsfMRI of 20 age, gender and education matched healthy controls (HCs). We have measured the effect of brain stimulation using network topological re-organization through whole brain integration and segregation, brain stability and capacity of neural propagation through metastability and intrinsic ignition.ResultsPatients with ET had altered DFC measures compared to controls...

European Journal of Neuroscience, 2018

Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. Howe... more Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. However, the neural underpinnings of repetition priming for famous faces in terms of effective connectivity is not known. Here we investigated this problem using dynamic causal modelling of latency-corrected event-related brain potentials (RERPs). Source waveforms of RERP-derived sources in the Occipital Lobe, Fusiform Gyrus, Mediotemporal Lobe, Prefrontal Cortex and Anterotemporal Lobe of each hemisphere entered into models with only forward (F) or also backward (FB) connections. Based on the framework of predictive coding formulated for repetition suppression, modulations of F and FB connections were expected for sources that displayed priming effects in their source waveforms. Hence, neural sources in each hemisphere were fitted with either F or FB connections. Interhemispheric connections were considered between homologous areas and were allowed to be modulated in an incremental manner resulting in a model-space that comprised of 24 models. Bayesian model averaging across models revealed effective bidirectional connectivity between the Fusiform Gyrus (face perception) and Prefrontal Cortex (decision making) in both hemispheres to be modulated by priming. In the left hemisphere, there is also a substantial involvement from the Mediotemporal Lobe, indicating the facilitation of automatic retrieval of the famous person's names. Furthermore, there is evidence that the priming is supported by connections from the right to the left Fusiform Gyri possibly in the service of inter-hemispheric cooperation. Altogether, the study indicates that along with top-down modulations, efficient processing within and across the two hemispheres is crucial for famous face priming.

Systematic Approach for tDCS Analysis- Software & Manual

Brain Stimulation, 2021

INTRODUCTION The observed variability in the effects of transcranial direct current stimulation (... more INTRODUCTION The observed variability in the effects of transcranial direct current stimulation (tDCS) is influenced by the amount of current reaching the targeted region-of-interest (ROI). Age and sex might affect current density at target ROI due to their impact on cortical anatomy. The present tDCS simulation study investigates the effects of cortical anatomical parameters (volumes, dimension, and torque) on simulated tDCS current density in healthy young, middle-aged, and older males and females. METHODOLOGY Individualized head models from 240 subjects (120 males, 18-87 years of age) were used to identify the estimated current density (2 mA current intensity, 25 cm2 electrode) from two simulated tDCS montages (CP5_CZ and F3_FP2) targeting the inferior parietal lobule (IPL) and middle frontal gyrus (MFG), respectively. Cortical parameters including segmented brain volumes (cerebrospinal fluid [CSF], grey and white matter), cerebral-dimensions (length/width &length/height) and brain-torque (front and back shift, petalia, and bending) were measured using the magnetic resonance images (MRIs) from each subject. The present study estimated sex differences in current density at these target ROIs mediated by these cortical parameters within each age group. RESULTS For both tDCS montages, females in the older age group received higher current density than their male counterparts at the target ROIs. No sex differences were observed in the middle-aged group. Males in the younger age group had a higher current density than females, only for the parietal montage. Across all age groups, CSF, and grey matter volumes significantly predicted the current intensity estimated at the target sites. In the older age group only, brain-torque was a significant mediator of the sex difference. CONCLUSIONS Our findings demonstrate the presence of sex differences in the simulated tDCS current density, however this pattern differed across age groups and stimulation locations. Future studies should consider influence of age and sex on individual cortical anatomy and tailor tDCS stimulation parameters accordingly.

NeuroImage, 2019

There is significant interest in using resting-state functional connectivity (RSFC) to predict hu... more There is significant interest in using resting-state functional connectivity (RSFC) to predict human behavior. Good behavioral prediction should in theory require RSFC to be sufficiently distinct across participants; if RSFC were the same across participants, then behavioral prediction would obviously be poor. Therefore, we hypothesize that removing common resting-state functional magnetic resonance imaging (rs-fMRI) signals that are shared across participants would improve behavioral prediction. Here, we considered 803 participants from the human connectome project (HCP) with four rs-fMRI runs. We applied the common and orthogonal basis extraction (COBE) technique to decompose each HCP run into two subspaces: a common (group-level) subspace shared across all participants and a subject-specific subspace. We found that the first common COBE component of the first HCP run was localized to the visual cortex and was unique to the run. On the other hand, the second common COBE component of the first HCP run and the first common COBE component of the remaining HCP runs were highly similar and localized to regions within the default network, including the posterior cingulate cortex and precuneus. Overall, this suggests the presence of run-specific (state-specific) effects that were shared across participants. By removing the first and second common COBE components from the first HCP run, and the first common COBE component from the remaining HCP runs, the resulting RSFC improves behavioral prediction by an average of 11.7% across 58 behavioral measures spanning cognition, emotion and personality.

Dysphagia following stroke consists a complex set of symptoms that arises from impaired functioni... more Dysphagia following stroke consists a complex set of symptoms that arises from impaired functioning of the skeletal muscles involved in swallowing. This impairment results from reduced ability of the muscles to generate the optimum force needed to perform the required actions, the symptom known as muscle weakness. It is commonly believed that muscle weakness in dysphagia results from a decrease in muscle activity due to reduced voluntary motor unit recruitment. Therefore, strengthening exercises are predominantly chosen to rehabilitate dysphagia; which however may not be a universally applicable solution. This is because muscle overactivity characterized by increased involuntary motor unit recruitment is also a characteristic of stroke patients and can interfere with generation of required muscle force. Presently, muscle overactivity in dysphagia diagnosis and management remains unexplored in contrast to limb muscles that are also structurally classified as skeletal muscles (like sw...

Advances in Cognitive Neurodynamics (V)

The late components of ERPs pose difficult problem in source localization because latency jitter ... more The late components of ERPs pose difficult problem in source localization because latency jitter smears peak arising from conventional averaging. RIDE, a technique developed to solve smearing in ERP components, improves scalp topography. Our study assessed whether the benefits of RIDE at surface level could improve source localization of RIDE-transformed ERPs (RERPs) from a face priming task. Separate source models for ERP and RERP were created and sources were localized. Jackknife averaging reduced the residual variance compared to source model fitting on individual subject data. Priming effects (PE) in the sources were calculated for three early and three late time windows. Significant improvement in PE is found from RERP sources especially in the late sources. We found early PE in right hemisphere and late PE in left hemisphere, indicating earlier face processing in right. Also, the PE in right hemisphere outnumbered the left, signifying right hemispheric dominance of face recognition.

Background: In Transcranial Direct Current Stimulation (tDCS) the injected current gets distribut... more Background: In Transcranial Direct Current Stimulation (tDCS) the injected current gets distributed across the brain areas. The motive is to stimulate the target region-of-interest (ROI), while minimizing the current in non-target ROIs. For this purpose, determining the appropriate current-dose for an individual is difficult. Aim: To introduce Dose-Target-Determination-Index (DTDI) to quantify the focality of tDCS and examine the dose-focality relationship in three different populations. Method: Here, we extended our previous toolbox i-SATA to the MNI reference space. After a tDCS montage is simulated for a current-dose, the i-SATA(MNI) computes the average (over voxels) current density for every region in the brain. DTDI is the ratio of average current density at target ROI to the ROI with maximum value (peak region). Ideally target ROI should be the peak region, so DTDI shall range from 0 to 1. Higher the value, the better the dose. We estimated the variation of DTDI within and ac...

Scientific Reports

There is significant interest in understanding the pathophysiology of Obsessive–Compulsive Disord... more There is significant interest in understanding the pathophysiology of Obsessive–Compulsive Disorder (OCD) using resting-state fMRI (rsfMRI). Previous studies acknowledge abnormalities within and beyond the fronto-striato-limbic circuit in OCD that require further clarifications. However, limited information could be inferred from the conventional way of investigating the functional connectivity differences between OCD and healthy controls. Here, we identified altered brain organization in patients with OCD by applying individual-based approaches to maximize the identification of underlying network-based features specific to the OCD group. rsfMRI of 20 patients with OCD and 22 controls were preprocessed, and individual-fMRI-subspace was derived for each subject within each group. We evaluated group differences in functional connectivity using individual-fMRI-subspace and established its advantage over conventional-fMRI methodology. We applied prediction-based approaches to highlight ...

Journal of Neural Engineering

BackgroundTranscranial Direct Current Stimulation (tDCS) is a technique where a weak current is p... more BackgroundTranscranial Direct Current Stimulation (tDCS) is a technique where a weak current is passed through the electrodes placed on the scalp. The distribution of the electric current induced in the brain due to tDCS is provided by simulation toolbox like Realistic-volumetric-Approach-based-Simulator-for-Transcranial-electric-stimulation (ROAST). However, the procedure to estimate the total current density induced at the target and the intermediary region of the cortex is complex. The Systematic-Approach-for-tDCS-Analysis (SATA) was developed to overcome this problem. However, SATA is limited to standardized headspace only. Here we develop individual-SATA (𝓲-SATA) to extend it to individual head.MethodT1-weighted images of 15 subjects were taken from two Magnetic Resonance Imaging (MRI) scanners of different strengths. Across the subjects, the montages were simulated in ROAST. 𝓲-SATA converts the ROAST output to Talairach space. The x, y and z coordinates of the anterior commiss...

Psychological medicine, Mar 21, 2024

Frontiers in Human Neuroscience, Sep 2, 2022

European Journal of Neuroscience, 2018

Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. Howe... more Repetition priming, that is, the repeated processing of a stimulus, facilitates performance. However, the neural underpinnings of repetition priming for famous faces in terms of effective connectivity is not known. Here we investigated this problem using dynamic causal modelling of latency-corrected event-related brain potentials (RERPs). Source waveforms of RERP-derived sources in the Occipital Lobe, Fusiform Gyrus, Mediotemporal Lobe, Prefrontal Cortex and Anterotemporal Lobe of each hemisphere entered into models with only forward (F) or also backward (FB) connections. Based on the framework of predictive coding formulated for repetition suppression, modulations of F and FB connections were expected for sources that displayed priming effects in their source waveforms. Hence, neural sources in each hemisphere were fitted with either F or FB connections. Interhemispheric connections were considered between homologous areas and were allowed to be modulated in an incremental manner resulting in a model-space that comprised of 24 models. Bayesian model averaging across models revealed effective bidirectional connectivity between the Fusiform Gyrus (face perception) and Prefrontal Cortex (decision making) in both hemispheres to be modulated by priming. In the left hemisphere, there is also a substantial involvement from the Mediotemporal Lobe, indicating the facilitation of automatic retrieval of the famous person's names. Furthermore, there is evidence that the priming is supported by connections from the right to the left Fusiform Gyri possibly in the service of inter-hemispheric cooperation. Altogether, the study indicates that along with top-down modulations, efficient processing within and across the two hemispheres is crucial for famous face priming.

Systematic Approach for tDCS Analysis- Software & Manual

Brain Stimulation, 2021

NeuroImage, 2019

Advances in Cognitive Neurodynamics (V)

Scientific Reports

Journal of Neural Engineering

Journal of Motor Behavior