New findings on how brain handles tactile sensations

The traditional understanding in neuroscience is that tactile sensations from the skin are only assembled to form a complete experience in the cerebral cortex, the most advanced part of the brain. However, this is challenged by new research findings from Lund University in Sweden that suggest both that other levels in the brain play a greater role than previously thought, and that a larger proportion of the brain’s different structures are involved in the perception of touch.

“It was believed that a tactile sensation, such as touching a simple object, only activated a very small part of the cerebral cortex. However, our findings show that a much larger part is probably activated. The assembly of sensations actually starts in the brainstem”, said neuroscience researcher Henrik Jörntell at Lund University.

According to his colleague Fredrik Bengtsson, who also participated in the research, this is the first study to show how complex tactile sensations from the skin are coded at the cellular level in the brain.

“Our findings have given us a new key to understanding how the perception of touch in the skin is processed and communicated to the brain”, he said.

The Lund researchers have worked in collaboration with researchers in Paris to study how individual nerve cells receive information from the skin. They used a ‘haptic interface’, which created controlled sensations of rolling and slipping movements and of contact initiating and ceasing. Movements proved decisive for the perception of touch – something that was not previously technically possible to study.

The findings of the Swedish-French research group have been published in the distinguished journal Neuron. The work is based on animal experiments and is first and foremost basic research, which aims to increase knowledge of the function of the brain. However, there are also possible areas of application.

“Normal hand and arm prostheses do not give any feedback and therefore no sensation of being a ‘real’ hand or arm. However, there are new, advanced prostheses with sensors that can supply information to the amputated arm. Our research could contribute to the further development of such sensors”, said Henrik Jörntell.

The new findings could also have a bearing on psychiatric illness and brain diseases such as stroke and Parkinson’s disease. Detailed knowledge of how the brain and its various parts process information and create a picture of a tactile experience is important to understanding these conditions.

“If we know how a healthy brain operates, we can compare it with the situation in different diseases. Then perhaps we can help patients’ brains to function more normally”, said Henrik Jörntell.

Decoding touch

With their whiskers rats can detect the texture of objects in the same way as humans do using their fingertips. A study, in which some scientists of SISSA have taken part, shows that it is possible to understand what specific object has been touched by a rat by observing the activation of brain neurons. A further step towards understanding how the brain, also in humans, represents the outside world.

We know the world through the sensory representations within our brain. Such “reconstruction” is performed through the electrical activation of neural cells, the code that contains the information that is constantly processed by the brain. If we wish to understand what are the rules followed by the representation of the world inside the brain we have to comprehend how electrical  activation is linked to the sensory experience. For this reason, a team of researchers including Mathew Diamond, Houman Safaai and Moritz von Heimendahl of the International School for Advanced Studies (SISSA) of Trieste have analyzed the behavior and the activation of neural networks in rats while they were carrying out tactile object recognition tests.

During the experiments researchers observed the performance of rats – the animals were discriminating one texture from another – along with the activation of a group of sensory neurons. “For the first time the study has monitored the activity of multiple neurons, while until now, due to technical limitations, researchers had examined only individual neurons,” explains Diamond, who heads up the Tactile Perception and Learning Lab at SISSA. “The activity of such groups of neurons is represented in our model as multi-dimensional clouds, comprising as many dimensions as the number of cells under examination (up to ten). We have observed a different cloud for the contact with each different texture.”

By analyzing the “clouds”, Diamond and his colleagues were able to successfully decode the object contacted by the rodent. “Our method is so accurate that when the rat would mistake one object for another, the decoding would also indicate a different object from the one actually touched. And this happened because the representation made by the brain – and, as a consequence, our decoding – appeared like that of a different object. Hence the error.”

Diamond’s team has no intention of stopping here. “In real life, we generally recognize objects using more senses all together, in an integrated manner. We use touch and sight at the same time, for instance,” explains Diamond. “For this reason we are now working on new experiments employing more neurons, with more complicated stimuli, and more senses, to build ‘multimodal’ representations of objects.”

More in detail…

This kind of “mind reading” carried out on rats’ brain by Diamond and his colleagues is important to understand how the brain forms a representation of the world. “Each one of us perceives a physical world outside ourselves, yet actually all we have at our disposal to create an experience of the world is the representation that our brain makes of it through the input of sensory organs” says Diamond.

To understand that such a representation is at the very least partial it is enough to think of all the information about the world that escapes us all the time: for instance, we are blind to infrared and ultraviolet rays, we are unable to hear certain sound frequencies or smell some chemical substances or others. Some details pertaining to the physical world are completely invisible or, to put it better, imperceptible (others are interpreted incorrectly, like visual illusions, for example.)

This is a further demonstration that what we perceive is not the physical world in itself, but the neuronal activation the world evokes inside our brain.

How Our Sense of Touch is a Lot Like the Way We Hear

Sliman Bensmaia, PhD, assistant professor of organismal biology and anatomy at the University of Chicago, studies the neural basis of tactile perception, or how our hands convey this information to the brain. In a new study published in the Journal of Neuroscience, he and his colleagues found that the timing and frequency of vibrations produced in the skin when you run your hands along a surface, like searching a wall for a light switch, play an important role in how we use our sense of touch to gather information about the objects and surfaces around us.

The sense of touch has traditionally been thought of in spatial terms, i.e. receptors in the skin are spread out across a grid of sorts, and when you touch something this grid of receptors transmits information about the surface to your brain. In their new study, Bensmaia, two former undergraduates, and a postdoctoral scholar in his lab—Matthew Best, Emily Mackevicius and Hannes Saal—found that the skin is also highly sensitive to vibrations, and that these vibrations produce corresponding oscillations in the afferents, or nerves, that carry information from the receptors to the brain. The precise timing and frequency of these neural responses convey specific messages about texture to the brain, much like the frequency of vibrations on the eardrum conveys information about sound.

Neurons communicate through electrical bits, similar to the digital ones and zeros used by computers. But, Bensmaia said, “One of the big questions in neuroscience is whether it’s just the number of bits that matters, or if the specific sequence of bits in time also plays a role. What we show in this paper is that the sequence of bits in time does matter, and in fact for some of the skin receptors, the timing matters with millisecond precision.”

Researchers have known for years that these afferents respond to skin vibrations, but they studied their responses using so-called sinusoidal waves, which are smooth, repetitive patterns. These perfectly uniform vibrations can be produced in a lab, but the kinds of vibrations produced in the skin by touching surfaces in the real world are messy and erratic.