Posterior parietal cortex

Posterior parietal cortex

The posterior parietal cortex (the portion of parietal neocortex posterior to the primary somatosensory cortex) plays an important role in planned movements, spatial reasoning, and attention.

Damage to the posterior parietal cortex can produce a variety of sensorimotor deficits, including deficits in the perception and memory of spatial relationships, inaccurate reaching and grasping, in the control of eye movement, and inattention. The two most striking consequences of PPC damage are apraxia and hemispatial neglect.


Pereira et al. from Geneva showed that perceptual consciousness and monitoring involve evidence accumulation. They performed single-unit recording in a participant with a microelectrode in the posterior parietal cortex, while they detected vibrotactile stimuli around the detection threshold and provided confidence estimates. They find that detected stimuli elicited neuronal responses resembling evidence accumulation during decision-making, irrespective of motor confounds or task demands. They generalized these findings in healthy volunteers using electroencephalography. Behavioral and neural responses are reproduced with a computational model considering a stimulus as detected if accumulated evidence reaches a bound, and confidence as the distance between maximal evidence and that bound. They concluded that gradual changes in neuronal dynamics during evidence accumulation relates to perceptual consciousness and perceptual monitoring in humans 1)


Spatial remapping, the process of updating information across eye movements, is an important mechanism for trans-saccadic perception. The right posterior parietal cortex (PPC) is a region that has been associated most strongly with spatial remapping. The aim of a project of Ten Brink et al. was to investigate the effect of damage to the right PPC on direction specific transsaccadic memory. They compared trans-saccadic memory performance for central items that had to be remembered while making a left- versus rightward eye movement, or for items that were remapped within the left versus right visual field.

They included 9 stroke patients with unilateral right PPC lesions and 31 healthy control subjects. Participants memorized the location of a briefly presented item, had to make one saccade (either towards the left or right, or upward or downward), and subsequently had to decide in what direction the probe had shifted. We used a staircase to adjust task difficulty (i.e., the distance between the memory item and probe). Bayesian repeated measures ANOVAs were used to compare left versus right eye movements and items in the left versus right visual field.

In both conditions, patients with right PPC damage showed worse trans-saccadic memory performance compared to healthy control subjects (for the condition with left- and rightward gaze shifts, BF10 = 3.79; and when items were presented left or right, BF10 = 6.77), regardless of the direction of the gaze or the initial location of the memory item. At the individual level, none of the patients showed a direction specific deficit after leftward versus rightward saccades, whereas two patients showed worse performance for items in the left versus right visual field.

Damage in the right PPC did not lead to gaze direction specific impairments in trans-saccadic memory, but instead caused more general spatial memory impairments 2).


1)

Pereira M, Megevand P, Tan MX, Chang W, Wang S, Rezai A, Seeck M, Corniola M, Momjian S, Bernasconi F, Blanke O, Faivre N. Evidence accumulation relates to perceptual consciousness and monitoring. Nat Commun. 2021 May 31;12(1):3261. doi: 10.1038/s41467-021-23540-y. PMID: 34059682.
2)

Ten Brink AF, Fabius JH, Weaver NA, Nijboer TCW, Van der Stigchel S. Trans-saccadic memory after right parietal brain damage. Cortex. 2019 Jun 28;120:284-297. doi: 10.1016/j.cortex.2019.06.006. [Epub ahead of print] PubMed PMID: 31376588.

Superior longitudinal fasciculus

Superior longitudinal fasciculus

The superior longitudinal fasciculus (also called the superior longitudinal fascicle or SLF) is a pair of long bi-directional bundles of neurons connecting the front and the back of the cerebrum. Each association fiber bundle is lateral to the centrum ovale of a cerebral hemisphere and connects the frontaloccipitalparietal, and temporal lobes. The neurons pass from the frontal lobe through the operculum to the posterior end of the lateral sulcus where numerous neurons radiate into the occipital lobe and other neurons turn downward and forward around the putamen and radiate to anterior portions of the temporal lobe.


The description of human white matter pathways experienced a tremendous improvement, thanks to the advancement of neuroimaging and dissection techniques. The downside of this progress is the production of redundant and conflicting literature, bound by specific studies’ methods and aims. The Superior Longitudinal System (SLS), encompassing the arcuate (AF) and the superior longitudinal fasciculi (SLF), becomes an illustrative example of this fundamental issue, being one of the most studied white matter association pathways of the brain. Vavassori et al. provided a complete illustration of this white matter fiber system’s current definition, from its early descriptions in the nineteenth century to its most recent characterizations. They proposed a review of both in vivo diffusion magnetic resonance imaging-based tractography and anatomical dissection studies, enclosing all the information available up to date. Based on these findings, they reconstructed the wiring diagram of the SLS, highlighting a substantial variability in the description of its cortical sites of termination and the taxonomy and partonomy that characterize the system. They aimed to level up discrepancies in the literature by proposing a parallel across the various nomenclature. Consistent with the topographical arrangement already documented for commissural and projection pathways, they suggested approaching the SLS organization as an orderly and continuous wiring diagram, respecting a medio-lateral palisading topography between the different frontalparietaloccipital, and temporal gyri rather than in terms of individualized fascicles. A better and complete description of the fine organization of white matter association pathways’ connectivity is fundamental for a better understanding of brain function and their clinical and neurosurgical applications 1).


The aim of a study was to examine the arcuate fasciculus (AF) and superior longitudinal fasciculi (SLF), which together form the dorsal language stream, using fiber dissection and diffusion imaging techniques in the human brain.

Twenty-five formalin-fixed brains (50 hemispheres) and 3 adult cadaveric heads, prepared according to the Klingler method, were examined by the fiber dissection technique. The authors’ findings were supported with MR tractography provided by the Human Connectome Project, WU-Minn Consortium. The frequencies of gyral distributions were calculated in segments of the AF and SLF in the cadaveric specimens.

The AF has ventral and dorsal segments, and the SLF has 3 segments: SLF I (dorsal pathway), II (middle pathway), and III (ventral pathway). The AF ventral segment connects the middle (88%; all percentages represent the area of the named structure that is connected to the tract) and posterior (100%) parts of the superior temporal gyrus and the middle part (92%) of the middle temporal gyrus to the posterior part of the inferior frontal gyrus (96% in pars opercularis, 40% in pars triangularis) and the ventral premotor cortex (84%) by passing deep to the lower part of the supramarginal gyrus (100%). The AF dorsal segment connects the posterior part of the middle (100%) and inferior temporal gyrus (76%) to the posterior part of the inferior frontal gyrus (96% in pars opercularis), ventral premotor cortex (72%), and posterior part of the middle frontal gyrus (56%) by passing deep to the lower part of the angular gyrus (100%).

This study depicts the distinct subdivision of the AF and SLF, based on cadaveric fiber dissection and diffusion imaging techniques, to clarify the complicated language processing pathways 2).

Superior longitudinal fasciculus classification.


1)

Vavassori L, Sarubbo S, Petit L. Hodology of the superior longitudinal system of the human brain: a historical perspective, the current controversies, and a proposal. Brain Struct Funct. 2021 Apr 21. doi: 10.1007/s00429-021-02265-0. Epub ahead of print. PMID: 33881634.
2)

Yagmurlu K, Middlebrooks EH, Tanriover N, Rhoton AL Jr. Fiber tracts of the dorsal language stream in the human brain. J Neurosurg. 2015 Nov 20:1-10. [Epub ahead of print] PubMed PMID: 26587654.

Medical student

Medical student

For students beginning their medical education, the neuroscience curriculum is frequently seen as the most difficult, and many express an aversion to the topic. A major reason for this aversion amongst learners is the perceived complexity of neuroanatomy 1).

The National Undergraduate Neuroanatomy Competition was established in 2013 as a means for students to display this commitment as well as academic ability.

A bespoke 22 item questionnaire was designed to determine career outcomes and the role of competition attendance in job applications. It was distributed using the SurveyMonkey website to the 87 attendees at the 2013 and 2014 competitions.

Responses were received by 40 competitors (response rate 46.0%). Twenty-four (60.0%) responders intend to pursue a career in either neurosurgery (n=18) or neurology (n=6). This included 10 (25.0%) responders who had successfully entered either neurosurgery (n=9) or neurology (n=1). The performance of these 10 (n=11, 57.0% ± 13.6) was significantly better than the other responders (n=30, 46.5% ± 13.5) (p=0.036). Seventeen (42.5%) responders either included their attendance at NUNC in a post-Foundation job application or intend to.

The National Undergraduate Neuroanatomy Competition provides the opportunity for medical students to demonstrate their interest in neurosurgery. It has the potential to be used as a tool for recognizing medical students suitable for neurosurgery training 2).

Osler created the first residency program for specialty training of physicians, and he was the first to bring medical students out of the lecture hall for bedside clinical training. Historically, medical student education in neurological surgery has generally limited student involvement to assisting in research projects with minimal formal clinical exposure before starting sub-internships and application for the neurosurgery match. Consequently, students have generally had little opportunity to acquire exposure to clinical neurosurgery and attain minimal proficiency 3).

Neurosurgery seeks to attract the best and brightest medical students; however, there is often a lack of early exposure to the field, among other possible barriers.

Medical students show varying clinical practical skills when entering their final year clinical clerkship, which is the final period to acquire and improve practical skills prior to their residency. Behling et al. developed a one-on-one mentoring program to allow individually tailored teaching of clinical practical skills to support final year students with varying skill sets during their neurosurgical clinical clerkship.

Each participating student (n = 23) was paired with a mentor. At the beginning students were asked about their expectations, teaching preferences, and surgical interests. Regular meetings and evaluations of clinical practice skills were scheduled every 2 weeks together with fixed rotations that could be individually adjusted. The one-on-one meetings and evaluations with the mentor gave each student the chance for individually tailored teaching. After completion of the program, each student evaluated their experience.

The mentoring program was well-received by participating students and acquisition or improvement of clinical practical skills was achieved by most students. A varying practical skill level and interest in the field of surgery was seen.

A neurosurgical one-on-one mentoring program is well received by final year medical students and allows for individually tailored learning of clinical practical skills 4).

Lubelski et al. sought to identify successful practices that can be implemented to improve medical student recruitment to neurosurgery.

United States neurosurgery residency program directors were surveyed to determine the number of medical student rotators and medical students matching into a neurosurgery residency from their programs between 2010 and 2016. Program directors were asked about the ways their respective institutions integrated medical students into departmental clinical and research activities.

Complete responses were received from 30/110 institutions. Fifty-two percent of the institutions had neurosurgery didactic lectures for 1st- and 2nd-year medical students (MS1/2), and 87% had didactics for MS3/4. Seventy-seven percent of departments had a neurosurgery interest group, which was the most common method used to integrate medical students into the department. Other forms of outreach included formal mentorship programs (53%), lecture series (57%), and neurosurgery anatomy labs (40%). Seventy-three percent of programs provided research opportunities to medical students, and 57% indicated that the schools had a formal research requirement. On average, 3 medical students did a rotation in each neurosurgery department and 1 matched into neurosurgery each year. However, there was substantial variability among programs. Over the 2010-2016 period, the responding institutions matched as many as 4% of the graduating class into neurosurgery per year, whereas others matched 0%-1%. Departments that matched a greater (≥ 1% per year) number of medical students into neurosurgery were significantly more likely to have a neurosurgery interest group and formal research requirements. A greater percentage of high-matching programs had neurosurgery mentorship programs, lecture series, and cadaver training opportunities compared to the other institutions.

In recent decades, the number of applicants to neurosurgery has decreased. A major deterrent may be the delayed exposure of medical students to neurosurgery. Institutions with early preclinical exposure, active neurosurgery interest groups, research opportunities, and strong mentorship recruit and match more students into neurosurgery. Implementing such initiatives on a national level may increase the number of highly qualified medical students pursuing neurosurgery 5).


A medical student training camp was created to improve the preparation of medical students for the involvement in neurological surgery activities and sub-internships.

A 1-day course was held at Weill Cornell Medicine, which consisted of a series of morning lectures, an interactive resident lunch panel, and afternoon hands-on laboratory sessions. Students completed self-assessment questionnaires regarding their confidence in several areas of clinical neurosurgery before the start of the course and again at its end.

A significant increase in self-assessed confidence was observed in all skill areas surveyed. Overall, rising fourth year students who were starting sub-internships in the subsequent weeks reported a substantial increase in their preparedness for the elective rotations in neurosurgery.

The preparation of medical students for clinical neurosurgery can be improved. Single-day courses such as the described training camp are an effective method for improving knowledge and skill gaps in medical students entering neurosurgical careers. Initiatives should be developed, in addition to this annual program, to increase the clinical and research skills throughout medical student education 6).

Medical students in Canada must make career choices by their final year of medical school. Selection of students for a career in neurosurgery has traditionally been based on marks, reference letters and personal interviews. Studies have shown that marks alone are not accurate predictors of success in medical practice; personal skills and attributes which can best be assessed by reference letters and interviews may be more important. A study was an attempt to assess the importance of, and ability to teach, personal skills and attitudes necessary for successful completion of a neurosurgical training program.

questionnaire was sent to 185 active members of the Canadian Neurosurgical Society, asking them to give a numerical rating of the importance of 22 personal skills and attributes, and their ability to teach those skills and attributes. They were asked to list any additional skills or attributes considered important, and rate their ability to teach them.

Sixty-six (36%) questionnaires were returned. Honesty, motivation, willingness to learn, ability to problem solve, and ability to handle stress were the five most important characteristics identified. Neurosurgeons thought they could teach problem solving, willingness to consult informed sources, critical thinking, manual dexterity, and communication skills, but honesty, motivation, willingness to learn and ability to handle stress were difficult or impossible to teach.

Honestymotivationwillingness to learnproblem solving and Stress management are important for success in a neurosurgical career. This information should be transmitted to medical students at “Career Day” venues. Structuring letters of reference and interviews to assess personal skills and attributes will be important, as those that can’t be taught should be present before the start of training 7).


1)

Larkin MB, Graves E, Rees R, Mears D. A Multimedia Dissection Module for Scalp, Meninges, and Dural Partitions. MedEdPORTAL. 2018 Mar 22;14:10695. doi: 10.15766/mep_2374-8265.10695. PubMed PMID: 30800895; PubMed Central PMCID: PMC6342347.
2)

Hall S, Stephens JR, Myers MA, Elmansouri A, Geoghegan K, Harrison CH, E N, D A, Parton WJ, Payne DR, Seaby E, Border S. The career impact of the National Undergraduate Neuroanatomy Competition. World Neurosurg. 2019 Sep 25. pii: S1878-8750(19)32516-1. doi: 10.1016/j.wneu.2019.09.086. [Epub ahead of print] PubMed PMID: 31562974.
3) , 6)

Radwanski RE, Winston G, Younus I, ElJalby M, Yuan M, Oh Y, Gucer SB, Hoffman CE, Stieg PE, Greenfield JP, Pannullo SC. Neurosurgery Training Camp for Sub-Internship Preparation: Lessons From the Inaugural Course. World Neurosurg. 2019 Apr 1. pii: S1878-8750(19)30926-X. doi: 10.1016/j.wneu.2019.03.246. [Epub ahead of print] PubMed PMID: 30947014.
4)

Behling F, Nasi-Kordhishti I, Haas P, Sandritter J, Tatagiba M, Herlan S. One-on-one mentoring for final year medical students during the neurosurgery rotation. BMC Med Educ. 2021 Apr 22;21(1):229. doi: 10.1186/s12909-021-02657-0. PMID: 33882933.
5)

Lubelski D, Xiao R, Mukherjee D, Ashley WW, Witham T, Brem H, Huang J, Wolfe SQ. Improving medical student recruitment to neurosurgery. J Neurosurg. 2019 Aug 9:1-7. doi: 10.3171/2019.5.JNS1987. [Epub ahead of print] PubMed PMID: 31398709.
7)

Myles ST, McAleer S. Selection of neurosurgical trainees. Can J Neurol Sci. 2003 Feb;30(1):26-30. PubMed PMID: 12619780.
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