Touch is our most under-rated sense. We relish what we eat, we are intrigued or repelled by what we smell, and vision and hearing connect us to the world around us and our fellow humans. And touch? Well, it's just there, isn't it?
Actually no! When was the last time you saw a distressed child ask for more words of advice as opposed to asking for a cuddle? Touch comforts. We use it to caress and to strike, to detect (in the dark or in places we can't see into), to feel for shape, size, texture, weight, volume – and we do all this using our hands and fingers in different ways, as the diagram shows you (image adapted from Sekuler & Blake, 2002).
Touch is not only an important sense to gain information about the world (as hearing and vision also are), it is also a very sophisticated sense using all of the sensory processing strategies as do other sensory systems.
In this simulation we are going to explore three aspects of tactile processing.
(a) We'll first look at the sensory "receptive fields". A receptive field simply tells us from what part of the body a sensory neuron gets information and this is important to help us localize objects around us or on us. The RF will depend on the type of sensory neuron: for touch neurons, it's some part of the body surface; for visual neurons, some part of the retina in the eye; for auditory neurons, some part of the inner ear. In the simulation here we'll look at these receptive fields (RFs) for neurons in the hand.
(b) As that RF information flows from the neurons in the hand, through a succession of way-stations (called nuclei), up to the cortex of the brain it is altered. This is because information from different neurons is not kept separate but is often integrated. So in the second simulation we will see the structure of RFs of neurons in the cortex. By comparing these RFs to RFs for neurons in the hand from the first simulation, we can identify the pattern of integration in the brain of information originally emanating from different neurons in the hand.
(c) Sensory systems provide a lot more information than simply whether there is some there to see, hear, touch – for example, in our vision, neurons from the eye allow us to distinguish between different wavelengths of light, and this forms the basis of what is ultimately colour perception. So in this touch simulation, we will look at the information that tactile sensory neurons provide to allow us to identify the objects we touch. They do so through the timing of the electrical signals, the Action Potentials, they produce as we move our fingers carefully over an object we're trying to identify solely through touch.
In this simulation you will "record" the neural activity from Aβ nerve fibers with skin receptors on the hand.
In the Receptive fields tab below, you will explore the receptive field locations and sizes for three different nerve fibers. Click on the hand to simulate touching the hand at that location for one second. The oscilloscopes show the amplified voltage trace recorded extracellularly from an electrode close to each of the nerve fibers. Each of the near-vertical lines represents an action potential from that nerve. If your speakers are on, you will also be able to hear the neural activity from the nerve fiber of your choice (use the radio buttons next to each neurons' name to choose). If a click falls within the receptive field for any of the nerves, a coloured dot will appear at that spot. Once you've found the receptive fields, make sure to explore their borders so that you can measure their size by dragging the ruler on the bottom left of the hand.
In the Sensitivity tab you will further explore the sensitivity profile for one of the receptive fields. Not all of the areas of the receptive field produce the same neural response. You will map the response curve for three cross sections of the receptive field by clicking on different points of three coloured bands. If you click several times on the exact same spot, will you get the same number of spikes? Why? Once you've mapped enough of the three response curves (i.e. there are no large intervals in between dots on the coloured bands), you will gain access to the real world examples on the following tab.
The Examples tab shows some real world examples of receptive fields, redrawn from: Johansson, R. S. (1978). Tactile sensibility in the human hand: receptive field characteristics of mechanoreceptive units in the glabrous skin area. The Journal of physiology, 281, 101.
Find the receptive field for each of three neurons by clicking on different points of the hand. The neural responses for each click will appear on the oscilloscopes, and you can choose which neuron's response you would like to listen to at any point in time.
Once you've found the location on the hand that triggers a response for a particular neuron, measure the diameter of the receptive field by dragging the ruler on the lower left corner of the hand on top of the receptive field. Please enter the appropriate size for each neuron's receptive field on the table below. What relationship do you observe between the location of the receptive fields on the hand and their size?
Once you've measured the three diameters, you will gain access to the next tab to learn about sensitivity.
This shows the receptive field on the palm of the hand from the previous tab, zoomed in. The previous tab simplified the receptive fields by making it a binary response: the neuron responded equally when touched inside its receptive field and did not respond otherwise. In reality the response varies within the receptive field, and we will explore how it can vary in this tab.
Please map the response curve for each of the horizontal coloured bands in the receptive field, to discover changes in the rate of the neuron's response. Once you've mapped the three curves, a visual representation of the sensitivity profile will appear and you will gain access to some real world examples in the next tab.
In reality, the sensitivity profiles of tactile nerve fibers are a little bit more complex than just concentric circles. These are some examples of human hand receptive fields:
Johansson, R. S. (1978). Tactile sensibility in the human hand: receptive field characteristics of mechanoreceptive units in the glabrous skin area. The Journal of physiology, 281, 101.
In this part of the simulation you will "record" the neural activity from neurons in the primary somatosensory cortex, with receptive fields along the right arm. Choose which electrode you wish to record from first by clicking on one of the three semi-transparent electrodes on the brain. Your mission is to find and measure the diameter of the receptive fields for each of the three neurons.
Click on the arm to simulate touching the arm at that location for one second. The neural response for each click will appear on the oscilloscope, and if a click falls within the receptive field for the neuron you are currently recording from, a coloured dot will appear at that spot. Once you've found the receptive fields, make sure to explore their borders so that you can measure their size by dragging the ruler on the bottom left of the hand.
Please enter the appropriate size for each neuron's receptive field on the table below. What relationship do you observe between the location of the receptive fields on the arm and their size?
In this part of the simulation you will "record" the neural activity from an afferent fiber as well as neurons in area 3b and 1 of the primary somatic sensory cortex, all with receptive fields on the tip of the left index finger. The black dots on the paper represent an embossed braille letter.
Click on one of the arrows to the left of the paper to smoothly glide the finger along the page. You can choose which neuron's activity you would like to hear by selecting the desired radio button in the box.
You can click on each arrow as many times as you want to generate a new trial. A "spatial event plot" for each neuron will appear on the smaller papers in the box.
If your eyes were closed, would you be able to tell which braille letter was on the paper by just listening to one of these neurons? If so, which one(s)?