The ability to feel touch is one of the fundamental sensory experiences of being human. When you reach out and touch an object, a cascade of signals travel from sensors in your skin, through your nerves, to your spinal cord and up to your brain. Without this ability to sense the external environment through touch, we would be unable to interact with the world around us.
But given that touch sensations originate in the periphery of the body, an interesting question arises – can the brain itself actually feel touch?
How does the brain process touch sensations?
To understand whether the brain itself can feel sensations like touch, we first need to understand how the brain normally processes touch from the body’s skin and nerves. Here is a brief overview of what happens:
- Touch receptors and sensory neurons in the skin detect sensations like pressure, vibration, and temperature.
- Signals travel along nerve fibers to the spinal cord.
- In the spinal cord, the signals synapse with second-order neurons that carry the signals up to the brain.
- Touch signals travel up the spinal cord pathways and arrive at the thalamus, a structure that acts as a relay station for sensory information.
- From the thalamus, touch signals get sent to two main areas of the cerebral cortex that process touch sensations:
- The primary somatosensory cortex, located in the brain’s parietal lobe, creates a kind of map of sensation from different parts of the body.
- The secondary somatosensory cortex, located in the frontal lobe, integrates sensory signals with other contextual information.
So in summary, dedicated pathways carry touch signals from the skin into sensory processing regions of the brain. But what happens if those external signals are interrupted? Can the brain itself actually feel touch without input from the body?
Stimulating the brain to create artificial sensations
Neuroscientists have found that it is possible to directly stimulate the brain to create sensory perceptions – including the sensation of touch – even in cases where external sensory input is blocked. Some key examples of this include:
- Direct electrical stimulation of the somatosensory cortex can elicit tactile sensations like pressure, vibration, and movement across an individual’s skin, even for body parts that don’t actually exist like phantom limbs.
- Magnetic stimulation of the somatosensory cortex with transcranial magnetic stimulation (TMS) has also been shown to create tactile sensations.
- People with nerve damage or limb amputations who have electrodes implanted in their brains can perceive sensations of touch when the electrodes stimulate the somatosensory cortex.
In these cases, neural activity in sensory regions of the brain is artificially driven independent of actual touch receptors, demonstrating that the brain is capable of perceiving sensations of touch even without peripheral sensory input.
Phantom touch experiences
Further evidence that the brain itself can create sensations of touch comes from cases of phantom touch experiences. Phantom touch refers to tactile sensations that are perceived but not actually being induced by external stimulation.
Examples of phantom touch include:
- Some amputees continue to vividly feel their missing limb, a phenomenon known as phantom limb sensations. These percepts include touch sensations, even though the missing limb is not there to send touch signals.
- Individuals who have had limbs amputated may feel phantom sensations of touch when their face or remaining stump is touched, suggesting cross-wiring in the brain’s body map.
- Some people who are paralyzed because of spinal cord injury report being able to feel touch on paralyzed body parts when seeing those body parts being touched, suggesting visual input activates the sensory cortex.
While not fully understood, these phantom touch experiences provide further evidence that the brain itself contributes strongly to tactile perception, even creating sensations of touch on its own in the absence of actual external stimulation.
What brain imaging tells us
Modern brain imaging techniques have provided great insight into how the brain processes real versus imagined or illusory touch. Some key findings include:
- Functional MRI (fMRI) shows that real tactile stimulation activates areas of the somatosensory cortex in patterns that map onto the corresponding body area being touched.
- When people are asked to imagine touch or experience phantom touch, similar regions of the somatosensory cortex show activation, though often more weakly than for real touch.
- The secondary somatosensory cortex shows distinct patterns of activation for perceived versus imagined touch.
- Even in people with total loss of a limb, the corresponding somatosensory cortex for that missing body part gets activated by phantom sensations, imagined touch for that body part, or observing someone else being touched there.
Overall, brain imaging indicates that phantom and imagined touch sensations originate from activity within the same sensory processing regions of the brain as real tactile perceptions.
Understanding referred sensations
Another phenomenon that sheds light on the brain’s tactile perceptions is referred sensations. Referred sensations happen when you feel a sensation somewhere in the body other than the actual source of the stimulation.
Common examples of referred sensations include:
- Feeling pain in your left arm or jaw during a heart attack.
- Feeling a painful tingling down your arm when you pinch a nerve in your neck.
- Feeling an itchiness in your ear canal when healing from a throat infection.
In these cases, the actual irritation is in one part of the body, but you perceive sensation elsewhere. This happens because of crossed signals in the sensory pathways between peripheral nerves and the brain.
Referred sensations reveal that tactile perception relies heavily on how the brain interprets and localizes sensory signals, highlighting the brain’s constructive role in generating sensations of touch.
Differences between touch on skin versus directly on the brain
Given that the brain contributes substantially to tactile perception, an interesting question is – does the brain respond the same way to touch on the skin versus direct stimulation of the cortical surface? There are some important differences:
- Touch on the skin involves peripheral receptors and sensory pathways carrying signals to the brain. Cortical stimulation bypasses this.
- Brain stimulation requires higher intensities to evoke sensations compared to light touch on the skin.
- Sensations from brain stimulation have less spatial precision compared to skin stimulation.
- Brain stimulation more often creates subjective experiences of vibration, fluttering, tingling, etc., whereas skin touch evokes defined sensations of pressure, hot/cold, etc.
- The natural sensations from skin touch have strong emotional content and nuance, while brain stimulation has a more artificial feel.
Therefore, while the brain can produce tactile sensations when artificially stimulated, there are significant qualitative differences from natural touch on the skin mediated by the body’s sensory systems.
Do sensations require both bottom-up and top-down processing?
Modern neuroscience views sensation and perception as an interactive process with both bottom-up and top-down components.
Bottom-up processing refers to signals moving from sensory receptors, through nerve pathways, to sensory processing regions of the brain. This provides the raw data of sensation.
Top-down processing refers to higher level factors like attention, expectation, context, emotions, and memories that shape perception. This provides interpretation and meaning.
To create rich, natural sensations likely requires the coordinated interplay of both bottom-up signals from the body and top-down modulatory systems in the brain.
Isolated stimulation of sensory cortex can elicit basic touch sensations, but lacks the detailed nuance contributed by peripheral sensory pathways. Phantom sensations demonstrate top-down influences but may feel distorted or incomplete without concurrent bottom-up signals.
Full perceptual experience arises through the integrated combination of both sensory signals and cognitive processes.
Implications for artificial touch sensations
The fact that the brain plays an active role in generating touch sensations, rather than passively receiving signals from the periphery, has important implications for creating artificial touch perceptions.
Some key insights include:
- Devices that stimulate the brain’s touch centers may enable sensory restoration for paralysis or prosthetics.
- Mimicking patterns of natural nerve signals will be important for brain implants to feel natural.
- Leveraging top-down processes like vision, expectation, and attention can enhance perception of artificial stimulation.
- Multisensory integration will help artificial stimuli feel more natural and precisely localized.
- Brain stimulation combined with natural sensory signals may enable the best integration.
Understanding that both the body and brain contribute to touch perception provides key principles for technologies aimed at artificially restoring or enhancing tactile sensations.
Conclusion
In summary, the brain does appear capable of directly generating sensations of touch under certain conditions. This includes when sensory cortex is artificially stimulated, in phantom touch experiences, and through referred sensations.
However, perceptions of touch elicited by brain stimulation differ in quality from natural tactile sensations mediated by the body’s sensory pathways. Furthermore, current evidence suggests that full perceptual experience requires integration of bottom-up signals from sensory receptors with top-down processing in the brain.
So in a sense, the brain can “feel” touch on its own, but full, natural tactile sensation likely requires complementary activity in both the brain and the peripheral nervous system working in tandem.
Understanding these mechanisms of tactile perception in the brain is shedding light on how to create more natural sensations using artificial stimulation, with promising applications for neuroprosthetics and sensory augmentation technologies.
