1. Ch. 14: The Cutaneous SensesPP

2. Plasticity of Cortical Maps
Cortical organization
Figure d14.5:
Somatosensory cortex
Experience-dependent plasticity
Similar to Auditory system: training owl monkeys to discriminate between frequencies.
William Jenkins & Michael Merzenich (1987)
Keeping in mind that particular regions of the brain corresponds to a specific part of the body, research have found that a specific area of the brain can increase in size depending on the amount of times a certain function pertaining to that specific area of brain had been used which is known as the experience-dependent plasticity.
This concept is similar to the experiment we learned in chapter 11 about the auditory system. They found that training that involves particular frequency increased the spaced devoted to that frequency in Auditory area A1 that responds best to tones.
Can anyone think of another example or tell me how a musician vs. a non-musician associate with the experience-dependent plasticity?
Another example of the auditory system is that musical training enlarges area of the auditory cortex that responds best to piano tones (pp. 284). Researchers compared the cortical response to piano tones of musicians who had been playing their instruments for about 12 to 28 yrs to people who have never played instruments. Results showed that 25 percent of the A1 was more active than in non- musicians.
In 1987 Jenkins & Merzenich had trained monkeys to complete a task that involved the extensive use of a particular location on one fingertip. They measured the cortical areas corresponding to each of the monkey’s fingers before the training and after the training. The training was held for 3 months. They found that after stimulation of fingertip over the 3 months had expanded.
Miriam can you tell me what the blue shade represents and why this is important?
Again, an experiment with violinist showed that after training to play the violin with their right hand, but uses the left fingers to hold the string had greater cortical areas responding to this specific function increased; enhancing their ability to play the violin.

3. Perceiving Details Measuring tactile acuity: Brial reading
Two-point threshold - minimum separation needed between two points to perceive them as two entities
Grating acuity - placing a grooved stimulus on the skin and asking the person to indicate the orientation of the grating
Raised pattern identification –using such
patterns to determine the smallest size that
can be identified
Reading Braille depend on the ability to detect details on the skin. As a matter a fact, Braille readers depend on identifying the tactile differences to determine the letter.
In order to perceive tactile detail researchers have come up with three different methods that determine tactile acuity.
The first one is two-point threshold is the minimum separation between two points on the skin that when stimulated is perceived as two units. This is measured by gently pressing on the skin with two points and asking the person whether they felt one or two points.
The Grating acuity is measured by pressing a grooved stimulus onto the skin and asking the person to indicate the orientation of the grating.
Raised pattern identification involves using patterns to determine the smallest size that can be identified.

4. Receptor Mechanisms for Tactile Acuity
There is a high density of Merkel receptors (SA1) in the fingertips.
Merkel receptors: densely packed on the fingertips (similar to cones in the fovea)
Tactile acuity thresholds are determined by Merkel receptors which are found in the fingertips and determine the density between two points. Merkel receptors are packed on the fingertips which are similar to the cones found in the fovea.
If you have two points that are very close to each other, lets say 12mm, touch your skin at the same time you would perceive the touch as one but increasing the distance
This graphs shows the correlations between density of Merkel receptors and tactile acuity. The bottom numbers show

5. Cortical Mechanisms for Tactile Acuity
Body areas w/high acuity have larger areas of cortical tissue
Receptor field for neuron in cutaneous system parallels receptive field for visual system .
Areas with higher acuity have smaller receptive fields on the skin.
Body areas with high acuity have larger areas of cortical tissue devoted to them. So just like the receptive field on the retina is stimulated influenced the firing rate of the neuron, the firing rate of neuron is also influenced by the receptive field in the cutaneous system.
We can see from the diagram that areas with higher acuity have smaller receptive fields on the skin. The monkey’s finger has the smallest receptive field and is known to have better acuity. This means that two points that are close together on the fingers might fall on receptive field that don’t overlap; suggesting that the fingers are more sensitive; enhancing the ability to feel two close together points on the skin as two separate points.
Now areas of the hand or the arm have receptive fields that overlap and two points with the same distance between each other is perceived as one stimulation since cortical neurons are too close.
The graph shows areas of the body that are highly sensitive. For example the fingers have lower threshold suggesting high sensitivity and calf have higher threshold suggesting low sensitivity/acuity.

6. Perceiving Vibration
Pacinian corpuscle (PC) is primarily responsible for sensing vibration.
PC’s nerve fibers respond best to high rates of vibration than low rates.
The structure of the PC: responsible for the response to vibration & fibers without the PC respond only to continuous pressure.
The system is also responsible for perceiving vibration and the mechanoreceptors that that are responsible for sensing vibration is Pacinian corpuscle.
The cutaneous system is capable of sensing vibrations coming from mechanical devices such as the car, electric toothbrush or cellphone.
Nerve fibers associated with PC’s respond best to high rates of vibration. The reason why the PC’s fibers fire well to rapid vibration has to do with the rapid changes in pressure. PC have series of layers that transmits rapidly when pressure is applied but not when the pressure is continuous, thus the structure of the PC is responsible for the response to the vibrations.
In the 60’s Werner Lowenstein conducted an experiment where he studied the Pacinian corpuscle. Lowenstein stimulated or pushed on the PC location which on the figure is location A and found that location A caused the rapid response, but not at point B. He concluded that the PC must be responsible for detecting vibration at point A and not at continuous pressure.

7. Perceiving Texture
Katz (1925) suggested: perception of texture depends on two cues
Spatial cues are determined by the size, shape, and distribution of surface elements.
Temporal cues are determined by the rate of vibration as skin is moved across finely textured surfaces.
Two types of receptors may be responsible for the duplex theory of texture perception
In 1925 Katz proposed that perception of texture depends on two cues
Spatial cues which are determined by the size, shape, and distribution of surface elements and temporal cues which are determined by the rate of vibration as skin is moved across finely textured surfaces. An example of spatial is perceiving a texture such as reading Braille dots. Temporal cues happen when the skin moves across a textured surface such as fine sandpaper & provide information about the vibration that resulted as the movement over the surface occurred.
Katz said that there are two types of receptors involved in texture perception known as the duplex theory of texture perception.

8. Perceiving Texture .
Hollins and Reisner (2000) shows support for the role of temporal cues.
To detect differences between fine textures, participants needed to move their fingers across the surface.
Hollin and Risnner study had participants touch surfaces without moving their fingers to judge the roughness using the procedure of magnitude estimation.
Participants hardly sensed any difference, but when they were able to move their fingers across the surface, they were able to detect the difference between the fine textures. So, it is possible to sense the roughness of the surface when vibrations were generated at the surface of the fingers.
You can see in the graph that fine textures judgments are higher than those who were not allowed to move their fingers across the surface. However very fine texture judgment of roughness is detectable when participants are not allowed to move their fingers across the surface.

9. Adapting Stimulus Hollins used selective adaptation procedure
Two conditions: 10Hz & 250Hz
Results: condition 250Hz could not detect texture difference after a while
A year later Hollins and coworkers used the selective adaptation procedure to see how inactivation of the receptor by adaptation affected perception.
Participants were presented two different conditions of adaptation were they were asked to run their finger over a standard and test fine textures. Their task was to determine which texture was finer. The first condition participant’s skin was vibrated with a 10 Hz (vibration per second) stimulus for 6 minutes. The second condition was 250 Hz adaption. Remember that Pacinian corpuscle respond best at high frequencies and the Meissner corpuscle response best at low frequencies.
Results showed that participants could tell the difference between the two textures when they had not been adapted or had received the 10 Hz or 250 Hz adaptation condition.
Hollin noticed that participants in the second condition who adapted to frequency of 250 affected their texture perception and could not sense the texture difference. This suggest that adapting to PC receptor eliminates the ability to sense fine texture by moving the fingers over a surface.
This means this study supports the spatial cues and fine textures by temporal cues or duplex theory of perception.