Rubber hand illusion

Rubber hand illusion

The rubber hand illusion (RHI) is a perceptual experience that often occurs when an administered tactile stimulation of a person’s real hand hidden from view, occurs synchronously with a corresponding visual stimulation of an observed rubber hand placed in full vision of the person in a position corresponding to where their real hand might normally be. The perceptual illusion is that the person feels a sense of “ownership” of the rubber hand which they are looking at. Most studies have focused on the underlying neural properties of the illusion and the experimental manipulations that lead to it. The illusion could also be used for exploring the sense of limb and prosthetic ownership for people after amputation. Cortical electrodes such as those used in sensorimotor stimulation surgery for pain may provide an opportunity to further understand the cortical representation of the illusion and possibly provide an opportunity to modulate the individual’s sense of body ownership. Thus, the RHI might also be a critical tool for the development of neurorehabilitative interventions that will be of great interest to the neurosurgical and rehabilitation communities 1).


The widely used rubber hand illusion (RHI) paradigm provides insight into how the brain manages conflicting multisensory integration regarding bodily self-consciousness. Previous functional neuroimaging studies have revealed that the feeling of body ownership is linked to activity in the premotor cortex, the intraparietal areas, the occipitotemporal cortex, and the insula. Matuz-Budai et al. from Pécs, investigated whether the individual differences in the sensation of body ownership over a rubber hand, as measured by the subjective report and the proprioceptive drift, are associated with structural brain differences in terms of cortical thickness in 67 healthy young adults. Matuz-Budai et al. found that individual differences measured by the subjective report of body ownership are associated with the cortical thickness in the somatosensory regions, the temporoparietal junction, the intraparietal areas, and the occipitotemporal cortex, while the proprioceptive drift is linked to the premotor cortex and the anterior cingulate cortex. These results are in line with functional neuroimaging studies indicating that these areas are indeed involved in processes such as cognitive-affective perspective-taking, visual processing of the body, and the experience of body ownership and bodily awareness. Consequently, these individual differences in the sensation of body ownership are pronounced in both functional and structural differences 2)


1)

Ramakonar H, Franz EA, Lind CR. The rubber hand illusion and its application to clinical neuroscience. J Clin Neurosci. 2011 Dec;18(12):1596-601. doi: 10.1016/j.jocn.2011.05.008. Epub 2011 Oct 13. PMID: 22000838.
2)

Matuz-Budai T, Lábadi B, Kohn E, Matuz A, Zsidó AN, Inhóf O, Kállai J, Szolcsányi T, Perlaki G, Orsi G, Nagy SA, Janszky J, Darnai G. Individual differences in the experience of body ownership are related to cortical thickness. Sci Rep. 2022 Jan 17;12(1):808. doi: 10.1038/s41598-021-04720-8. PMID: 35039541.

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.
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