Morphometry of Corpus Callosum in South Indian Population

Background: The corpus callosum (CC) is a connecting bridge between two cerebral hemispheres and helps in interhemispheric integration of information. Purpose: The primary objective of the study is to explain the topographical position of CC in relation to the brain in the South Indian population, contributing to the reference values of measurements of CC, which helps in planning surgical interventions. Also, the reference values help in cross-referencing with other populations and ethnic group. Methods and Material: In the study, 40 formalin fixed, full brain specimens were cut in midsagittal plane and CC was measured along with its relation to the brain. The major diameters considered were longitudinal dimension of corpus callosum (LC), distance of CC from frontal pole to genu (AS), distance of CC occipital pole to splenium (PS), and longitudinal dimension of brain (LB) from frontal pole to occipital pole. Statistical Analysis Used: The statistical analysis was carried with the mean, standard deviation, 95% CI, and the range measured for each measurement. The Pearson coefficient was evaluated between dimensions of brain and CC. Multivariate linear regression analysis was done to correlate between LB as a dependent variable, and LC, vertical dimension of brain (CD), and PS as independent variables. Results: Pearson’s ratio showed a positive correlation between LB and PS (0.61), and also between LB and LC (0.59). The ratio of LC/LB was 0.45 and LC/CD was 0.69, which are stable in all brains studied. Conclusions: The study concludes that CC maintains a stable proportion with its parts (genu, rostrum, body, and splenium) and with the horizontal dimension of the brain. Further, measured values help in cross-referencing with other population.

Neural Correlates of Subsequent Memory-Related Gaze Reinstatement

Mounting evidence linking gaze reinstatement—the recapitulation of encoding-related gaze patterns during retrieval—to behavioral measures of memory suggests that eye movements play an important role in mnemonic processing. Yet, the nature of the gaze scanpath, including its informational content and neural correlates, has remained in question. In this study, we examined eye movement and neural data from a recognition memory task to further elucidate the behavioral and neural bases of functional gaze reinstatement. Consistent with previous work, gaze reinstatement during retrieval of freely viewed scene images was greater than chance and predictive of recognition memory performance. Gaze reinstatement was also associated with viewing of informationally salient image regions at encoding, suggesting that scanpaths may encode and contain high-level scene content. At the brain level, gaze reinstatement was predicted by encoding-related activity in the occipital pole and BG, neural regions associated with visual processing and oculomotor control. Finally, cross-voxel brain pattern similarity analysis revealed overlapping subsequent memory and subsequent gaze reinstatement modulation effects in the parahippocampal place area and hippocampus, in addition to the occipital pole and BG. Together, these findings suggest that encoding-related activity in brain regions associated with scene processing, oculomotor control, and memory supports the formation, and subsequent recapitulation, of functional scanpaths. More broadly, these findings lend support to scanpath theory's assertion that eye movements both encode, and are themselves embedded in, mnemonic representations.

The Mechanism of Macular Sparing

Patients with homonymous hemianopia sometimes show preservation of the central visual fields, ranging up to 10°. This phenomenon, known as macular sparing, has sparked perpetual controversy. Two main theories have been offered to explain it. The first theory proposes a dual representation of the macula in each hemisphere. After loss of one occipital lobe, the back-up representation in the remaining occipital lobe is postulated to sustain ipsilateral central vision in the blind hemifield. This theory is supported by studies showing that some midline retinal ganglion cells project to the wrong hemisphere, presumably driving neurons in striate cortex that have ipsilateral receptive fields. However, more recent electrophysiological recordings and neuroimaging studies have cast doubt on this theory by showing only a minuscule ipsilateral field representation in early visual cortical areas. The second theory holds that macular sparing arises because the occipital pole, where the macula is represented, remains perfused after occlusion of the posterior cerebral artery because it receives collateral flow from the middle cerebral artery. An objection to this theory is that it cannot account for reports of macular sparing in patients after loss of an entire occipital lobe. On close scrutiny, such reports turn out to be erroneous, arising from inadequate control of fixation during visual field testing. Patients seem able to detect test stimuli on their blind side within the macula or along the vertical meridian because they make surveillance saccades. A purported treatment for hemianopia, called vision restoration therapy, is based on this error. The dual perfusion theory is supported by anatomical studies showing that the middle cerebral artery perfuses the occipital pole in many individuals. In patients with hemianopia from stroke, neuroimaging shows preservation of the occipital pole when macular sparing is present. The frontier dividing the infarcted territory of the posterior cerebral artery and the preserved territory of the middle cerebral artery is variable, but always falls within the representation of the macula, because the macula is so highly magnified. For physicians, macular sparing was an important neurological sign in acute hemianopia because it signified a posterior cerebral artery occlusion. Modern neuroimaging has supplanted the importance of that clinical sign but at the same time confirmed its validity. For patients, macular sparing remains important because it mitigates the impact of hemianopia and preserves the ability to read fluently. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Dynamics of absolute and relative disparity processing in human visual cortex

Cortical processing of binocular disparity is believed to begin in V1 where cells are sensitive to absolute disparity, followed by the extraction of relative disparity in higher visual areas. While much is known about the cortical distribution and spatial tuning of disparity-selective neurons, the relationship between their spatial and temporal properties is less well understood. Here, we use steady-state Visual Evoked Potentials and dynamic random dot stereograms to characterize the temporal dynamics of spatial mechanisms in human visual cortex that are primarily sensitive to either absolute or relative disparity. Stereograms alternated between disparate and non-disparate states at 2 Hz. By varying the spatial frequency content of the disparate fields from a planar surface to corrugated ones, we biased responses towards absolute vs. relative disparities. Reliable Components Analysis was used to derive two dominant sources from the 128 channel EEG records. The first component (RC1) was maximal over the occipital pole while the second component (RC2) was maximal over right lateral occipital electrodes. In RC1, first harmonic responses were sustained, tuned for corrugation frequency, and sensitive to the presence of disparity references, consistent with prior psychophysical sensitivity measurements. By contrast, the second harmonic, associated with transient processing, was not spatially tuned and was indifferent to references, consistent with it being generated by an absolute disparity mechanism. In RC2, the sustained response component showed similar tuning and sensitivity to references. However, sensitivity for absolute disparity dropped off, and transient signals were mainly driven by the lowest corrugation frequencies.

Gray matter volume differences between transgender men and cisgender women: A voxel-based morphometry study

Objective: Gender dysphoria (GD) is characterized by distress due to inconsistency between gender identity and biological sex. Individuals with GD often desire to be the other gender, which is called transgender. Although altered brain volumes in transgender people, particularly transgender women, have been reported, the particular brain regions have been inconsistent among studies. This study aimed to investigate neuroanatomical differences in transgender men without physical interventions. Method: T1-weighted magnetic resonance images (MRIs) were acquired in 21 transgender men and 21 cisgender women matched for biological sex and age. Whole-brain comparisons using voxel-based morphometry (VBM) were performed to identify gray matter volume (GMV) differences between transgender men and cisgender women. Results: Transgender men showed greater GMV in the right posterior cingulate gyrus ( PFWE-corr = 3.06×10-6) and the left occipital pole ( PFWE-corr = 0.017) and lower GMV in the left middle temporal gyrus ( PFWE-corr = 0.017) than cisgender women. Even after including serum sex hormone levels as covariates, the posterior cingulate gyrus was still significant ( PFWE-corr < 0.05). In contrast, the occipital pole and the middle temporal gyrus were not significant after controlling for the sex hormone levels ( PFWE-corr > 0.05), especially affected by testosterone but not estradiol. Conclusion: These findings suggest that transgender men have altered brain structure. We suggest that larger posterior midline structures may contribute to sensitivity to self-referential processing through altered visual perception in transgender people.

Neural correlates of subsequent memory-related gaze reinstatement

Mounting evidence linking gaze reinstatement- the recapitulation of encoding-related gaze patterns during retrieval- to behavioral measures of memory suggests that eye movements play an important role in mnemonic processing. Yet, the nature of the gaze scanpath, including its informational content and neural correlates, has remained in question. In the present study, we examined eye movement and neural data from a recognition memory task to further elucidate the behavioral and neural bases of functional gaze reinstatement. Consistent with previous work, gaze reinstatement during retrieval of freely-viewed scene images was greater than chance and predictive of recognition memory performance. Gaze reinstatement was also associated with viewing of informationally salient image regions at encoding, suggesting that scanpaths may encode and contain high-level scene content. At the brain level, gaze reinstatement was predicted by encoding-related activity in the occipital pole and basal ganglia, neural regions associated with visual processing and oculomotor control. Finally, cross-voxel brain pattern similarity analysis revealed overlapping subsequent memory and subsequent gaze reinstatement modulation effects in the parahippocampal place area and hippocampus, in addition to the occipital pole and basal ganglia. Together, these findings suggest that encoding-related activity in brain regions associated with scene processing, oculomotor control, and memory supports the formation, and subsequent recapitulation, of functional scanpaths. More broadly, these findings lend support to Scanpath Theory’s assertion that eye movements both encode, and are themselves embedded in, mnemonic representations.

Mechanisms of Cerebral Microbleeds

Cerebral microbleeds (CMB) are a common MRI finding, representing underlying cerebral microhemorrhages (CMH). The etiology of CMB and microhemorrhages is obscure. We conducted a pathological investigation of CMH, combining standard and immunohistological analyses of postmortem human brains. We analyzed 5 brain regions (middle frontal gyrus, occipital pole, rostral cingulate cortex, caudal cingulate cortex, and basal ganglia) of 76 brain bank subjects (mean age ± SE 90 ± 1.4 years). Prussian blue positivity, used as an index of CMH, was subjected to quantitative analysis for all 5 brain regions. Brains from the top and bottom quartiles (n = 19 each) were compared for quantitative immunohistological findings of smooth muscle actin, claudin-5, and fibrinogen, and for Sclerosis Index (SI) (a measure of arteriolar remodeling). Brains in the top quartile (i.e. with most extensive CMH) had significantly higher SI in the 5 brain regions combined (0.379 ± 0.007 vs 0.355 ± 0.008; p &lt; 0.05). These findings indicate significant coexistence of arteriolar remodeling with CMH. While these findings provide clues to mechanisms of microhemorrhage development, further studies of experimental neuropathology are needed to determine causal relationships.

Functional Connectivity of the Precuneus Reflects Effectiveness of Visual Restitution Training in Chronic Hemianopia

Visual field defects in chronic hemianopia can improve through visual restitution training, yet not all patients benefit equally from this long and exhaustive process. Here, we asked if resting-state functional connectivity prior to visual restitution could predict training success. In two training sessions of eight weeks each, 20 patients with chronic hemianopia performed a visual discrimination task by directing spatial attention towards stimuli presented in either hemifield, while suppressing eye movements. We examined two effects: a sensitivity change in the attended (trained) minus the unattended (control) hemifield (i.e., a training-specific improvement), and an overall improvement (i.e., a total change in sensitivity after both sessions). We then identified five visual resting-state networks and evaluated their functional connectivity in relation to both training effects. We found that the functional connectivity strength between the anterior Precuneus and the Occipital Pole Network was positively related to the attention modulated (i.e., training-specific) improvement. No such relationship was found for the overall improvement or for the other visual networks of interest. Our finding suggests that the anterior Precuneus plays a role in training-induced visual field improvements. The resting-state functional connectivity between the anterior Precuneus and the Occipital Pole Network may thus serve as an imaging-based biomarker that quantifies a patient’s potential capacity to direct spatial attention. This may help to identify hemianopia patients that are most likely to benefit from visual restitution training.

Abstract TP152: Sensory Function Improvement is Related to White Matter Changes During Rehabilitation

Objective: Structural changes related to recovery of sensory function are not well understood. Previously, we demonstrated a relationship between improved sensory function and increased cortical thickness in ipsilesional occipitoparietal and contralesional temporoparietal areas following rehabilitation of patients with chronic stroke(Pundik et al, 2018). The objective of the current work was to evaluate changes in white matter (WM) tracts originating in these cortical regions where thickness changes were related to enhanced sensory function after rehabilitation in chronic stroke. Methods: Chronic stroke survivors (n=18) received 12 weeks of upper limb sensory-motor rehabilitation. Outcome measures were sensory acuity (monofilament test, a sum over 9 locations on the hand; best score 25.47) and DTI measures of diffusivity [mean (MD), radial (RD), axial (AD)] and fractional anisotropy (FA). DTI analysis included: probabilistic tractography from seeds where cortical thickness changes correlated with sensory recovery, creation of a group-based tract by combining individual tracts, tract-based analysis using Spearman correlation, significance at p<0.05 with Bonferroni correction, and voxel-based analysis permutation inference for general linear modeling. Results: Subjects were 53.9±12.1 years old, 1.9±1.2 years after stroke and 27.8% female. Monofilament score improved from 46.6±13.1 to 43.9±12.9. Improved sensory acuity correlated with increased AD (rho=0.69, p=0.002), RD (rho=0.59, p=0.0093) and MD (rho=0.63, p=0.005) in the tract originating in the ipsilesional occipital pole. Voxel-based analysis between changes in MD and in monofilament identified a statistically significant cluster (family-wise error corrected p=0.03) within a subcortical region of the tract originating in the ipsilesional occipital pole. Conclusion: Improved sensory acuity after rehabilitation was related to DTI-measured microstructural changes in WM tracts. These findings further support the notion of structural plasticity associated with sensory function improvement during rehabilitation in chronic stroke.