Once the firehose of sensory information reaches the brain, a bewildering process of making sense of this data begins. The objective is to create an accurate internal model of the world that you can base your next decisions upon. To do this, all of the many bits of data flowing along the sensory neurons have to be knitted together somehow. Thanks to the unfortunate circumstances of minor strokes, brain researchers have been able to track down many of the important steps in this information processing.
You might have heard of the experiences of limb amputees who, for a time, experience the ‘phantom limb’ effect. The neurons having been severed still report back to the brain that their stimulation means the limb still exists, and for a period of time the amputee has to deal with the ghost limb that is not really there. In another bizarre situation, a stroke victim has a perfect understanding that their left arm belongs to them, but insists that their right arm belongs to a relative living 1000 miles away. This malady is called asomatognosia by neurophysiologists.
From many studies of how pinpoint strokes affect brain function, neuroanatomists have identified specific regions of the brain that allow us to integrate our sensory information and create a coherent model of the outside world as it exists in space and time. The first thing the brain has to do is to have a ‘sense’ of its own body and how it is located in space. It also has to identify this ‘self’ as being different from that of other people. If it cannot do this accurately, it cannot decide how to move in space, anticipate the consequences of that movement, or how to anticipate and empathize with the actions of other people. Nearly all of this activity seems to be relegated to a single area in the brain.
The temporoparietal junction (TPJ) takes information from the limbic system (emotional state) and the thalamus (memory) and combines it with information from the visual, hearing and internal body sensory systems to create an integrated internal model of where your body is located in space. The TPJ has left and right ‘lobes’ that control your ability to pay attention (right) and to anticipate other people’s emotions and desires (left). Patients with schizophrenia have abnormal levels of stimulation in the TPJ and cannot discern the intentions of other people. Stimulation of the right TPJ by placing electrodes in unesthetized patients leads to out-of-body experiences, schizophrenic behavior, and the phantom limb effect. The right TPJ tries to create a coherent body image from many different, and sometimes contradictory sensory inputs. When this process breaks down because the contradictory information is too strong to inhibit or ignore, you experience that you actually have two distinct bodies in space. This seems to be the direct, neural basis for out-of-body experiences.
But there is an even stranger brain region whose stimulation leads to an error in deciding where the body and self ends in space, and where the outside world begins.
The posterior cingulate body plays a huge role in self-location and body ownership. What this means is that we experience our body as having a definite location in space, and that this location is where you, the ‘Self’ is located. Strokes in this region cause asomatognosia patients not to recognize a limb as belonging to them. But you don’t have to be a stroke victim to experience this dislocation of body and self.
If you sit at a table facing a barrier that lets you see an artificial, life like right hand but not your real right hand, and you rhythmically stroke the real hand, eventually your brain gets fooled into believing that the artificial right hand is actually yours. If someone suddenly stabs the artificial hand, you will actually jump reflexively as though, for just an instant, the brain got confused about which was your real right hand being attacked!
The Posterior Superior Parietal Lobule gives us a sense of the boundary between our physical body and the rest of the world. When activity in this brain region is reduced, the individual seems to lose a sense of where their body ends and the rest of the world begins. The feeling is one of having ‘merged with the universe’ and your body is in some way infinite. Mindfulness practices such as meditation can modify the stimulation of this region and give the practitioner exactly this dramatic experience.
So you see, once sensory data gets to the brain, it is in for an amazing ride through many brain regions that help us build up the person or self that we feel we are through space and time.
By the way, for a fascinating introduction to these topics, read V.S. Ramachandran and Sandra Blakeslee’s book ‘Phantoms in the Brain: Probing the mysteries of the human mind’
Here is an interesting 2013 research paper in the journal Frontiers in Psychology ‘Alterations in the sense of time, space, and body in the mindfulness-trained brain: a neurophenomenologically-guided MEG study’ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3847819/
The connection between meditation and brain region function and stimulation is covered in this article: Mindfulness Practices and Meditation. https://neurowiki2012.wikispaces.com/Mindfulness+Practices+and+Meditation
But now let’s consider how the brain actually makes its models.
Check back here on Friday, December 2 for the next installment!