At what precise juncture does one’s individual being cease, giving way to the external environment?
While this might initially appear to be a peculiar inquiry with an evidently straightforward resolution, the intricate architecture of the human brain expends considerable effort in discerning this very boundary. Current scientific investigations have established a correlation between a particular cadence of neural oscillations within a specific cerebral region and the subjective experience of bodily ownership.
In a series of novel experimental protocols, juxtaposing the insights of Swedish and French researchers, 106 individuals engaged in a phenomenon known as the rubber hand illusion. This process involved the meticulous monitoring and strategic stimulation of their neural activity to ascertain resultant effects.
This well-established perceptual trick entails concealing one of the participant’s actual hands from their visual field, substituting it with an artificial counterpart. When tactile stimuli are applied to both the genuine and the synthetic appendage simultaneously and repeatedly, a disquieting sensation can emerge, fostering the perception that the prosthetic limb is an integral component of the individual’s physical self.
The rigorous testing, which in one experimental phase incorporated electroencephalography (electroencephalography) to capture intricate patterns of brain activity, illuminated that the perception of bodily ownership appears to originate from the specific frequency of alpha waves emanating from the parietal cortex. This particular neural hub is critically involved in constructing the body schema, processing multimodal sensory input, and consolidating a cohesive sense of self.
“We have successfully pinpointed a foundational neural mechanism that is instrumental in shaping our continuous, embodied existence,” states the principal investigator, Mariano D’Angelo, a distinguished neuroscientist affiliated with the Karolinska Institute in Sweden.
“These discoveries hold the potential to furnish novel perspectives on afflictions such as schizophrenia, wherein the fundamental sense of self is profoundly disrupted.”
In the initial phase of experimentation, participants were subjected to tactile stimulation on the index finger of both their real and surrogate hands by a robotic manipulator. This stimulation was administered either with perfect temporal synchronization or with a temporal discrepancy of up to 500 milliseconds between each tactile event.
As hypothesized, participants reported a heightened sense of the synthetic hand being an extension of their own physique when the tactile applications were synchronous. This subjective feeling diminished progressively as the temporal gap between perceived sensation and visual input widened.
The electroencephalographic recordings obtained during the second experimental iteration provided further granular detail to this phenomenon. The intrinsic frequency of alpha waves within the parietal cortex demonstrated a discernible correlation with the participants’ capacity to discern the temporal lag between sequential tactile stimuli.
Individuals exhibiting more rapid alpha wave activity appeared to more readily discount the artificial appendage, even when a minute temporal disparity was present in the tactile input. Conversely, those with slower alpha wave frequencies were more inclined to perceive the prosthetic hand as belonging to them, even when the tactile applications were spaced further apart.
Subsequently, the research team explored whether the oscillation frequency of these neural waves exerted direct control over the sensation of bodily ownership, or if these patterns might instead be a byproduct of an underlying, common factor.
Employing a third cohort of participants, a non-invasive methodology known as transcranial alternating current stimulation was utilized to selectively augment or decelerate the temporal frequency of the individuals’ alpha waves. The results unequivocally indicated that this manipulation had a tangible impact on the perceived realism of the artificial hand.
Accelerating an individual’s alpha wave oscillations resulted in a more robust and precise sense of bodily ownership, thereby increasing sensitivity to subtle temporal discrepancies. Conversely, decelerating these brain waves had an inverse effect, diminishing the participants’ ability to differentiate between their own physical form and external objects.
“Our discoveries illuminate the sophisticated mechanisms by which the brain orchestrates the integration of bodily signals, thereby forging a unified and coherent sense of self,” explains Henrik Ehrsson, a leading neuroscientist at Karolinska.
The research team posits that these findings could pave the way for novel therapeutic interventions or enhanced comprehension of neurological conditions characterized by a distorted body map, such as schizophrenia or the phantom limb sensations experienced by individuals post-amputation.
Furthermore, this research may contribute to the development of more lifelike prosthetic devices and more immersive virtual reality experiences.
