Lecture 1-Human Senror

7/28/2019 Lecture 1-Human Senror

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Lecture #1:Human Sensors

Lecture Intent

This lecture gives an introduction to some of the sensors that are part of the human perceptual

system. This perceptual introduction includes some hands-on experiments for the students to use.

Introduction to Biological Sensors

In the study of sensors and sensing technology, we often encounter physical limits to the

measurement of certain signals. For example, temperature measurement can be limited by

thermodynamics - the presence of temperature fluctuations in all finite objects at finite

temperatures. More often, the information which can be derived from a temperaturemeasurement is limited by environmental disturbances - often referred to as environmental noise.

For example, the jungle environment naturally produces a broad spectrum of audio background

noise, and it is not generally useful to have sensitive hearing capabilities which are continuallyswamped by background noise. In the case of light sensors, the limitations to use in daytime are

usually due to the accuracy of the focusing elements, or perhaps the clarity of the atmosphere

(more of a problem these days, it seems). At night, the optical systems of nocturnal animals can

be limited by the very few numbers of photons that are available to detect. Human eyes arecapable of detecting statistical variations in the intensity of light from stars at night. The "flicker"

that you can see in faint stars on clear nights in the mountains (especially in winter!) is due to acombination of atmospheric turbulence and the fact that there are so few photons that statistical

effects in the arrival rate are detectable.

The natural evolutionary process in nature has been a remarkably efficient filter for useful

sensing capabilities. There are many examples of highly adapted sensing capabilities which offerreal performance advantages. And, there are many examples of rather primitive sensing

capabilities which can be utilized effectively by the more sophisticated software systems that are

also part of most natural systems. Some good examples of high-performance sensing systems

include chemical sensing of pheromones in moths, ultrasound sensing in bats (predators) andmoths (prey), and vision in predators such as cats and birds. Some examples of low-performance

sensors include tilt sensing in humans and tactile sensing on various parts of the anatomy.

To begin to get a feel for sensing technology and capabilities, I want to spend some time in thefirst lecture experimenting with human sensory capabilities. As we perform these experiments,

we want to think about many of the system issues associated with sensing, such as signal

conditioning, wiring, time response and hysteresis, packaging, and the role of software/hardware


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tradeoffs in system operation. We will find that the human sensing systems provide several really

interesting examples of ways to convert physical signals into information, and many interesting

examples of how to trade off various aspects of system performance and operational cost toachieve the desired result.

Mechanoreceptors

Please take a moment and notice how your fingertips sense the tabletops. Notice the different

mechanisms for sensing location, roughness, table edge, table temperature. And, try to payattention to how you use your fingers to detect certain signals, such as roughness or temperature.

In class, we usually pass around a selection of different sandpapers with coarseness ranging from

very fine to very coarse. The students are asked to handle the sandpaper as they normally would,and then to see how well they can distinguish the different pieces with their eyes closed. For

example, if the sandpaper is squeezed between the tips of thumb and finger and held still, is it

possible to tell rough from smooth? If so, what characteristics of the sandpaper are really being

detected - roughness or stiffness or thickness. Also, pay attention to the kinds of finger motionsthat are used to distinguish these samples, and think about the characteristics of the sensors in the

fingers that are being taken advantage of.

One important characteristic of the sensors in your skin is referred to as the Rate of Adaptation.

Most human mechanoreceptor cells respond to a change in the external stimulus (pressure,temperature, etc) by producing voltage pulses across neurons. Immediately after the change in

external stimulus, these pulses begin to appear. Over some time, the pulse rate declines and

eventually returns to the original passive level. The rate of adaptation is the rate at which themechanoreceptor pulse rate returns to normal after a change in stimulus. Simply put, sensors

with adaptation do not provide information about static signals - only about changing signals. To

use such a sensor to sense a static quantity, like roughness, it is necessary to make the roughnessproduce a time-varying contact force on the tactile sensors in the fingers.

The mechanoreceptors in your skin may be separated into distinct categories:

Fast Adaptation

Pacinian Corpuscles are rapidly adapting mechanoreceptors in your skin and are often the most

sensitive cells to very small changes in the stimulus, such as the tactile force. These rapidlyadapting cells return to a normal rate of pulses in less than 0.1 second. These delicate

mechanoreceptors are generally found in the subcutaneous layer of the skin, where they areprotected from the abuses which may occur at the surface. These receptors are used in humanperception to detect surface roughness as the fingertips are dragged across a surface, or very

small vibrations in machines. Because of their location far below the surface and the role of the

skin in transmission of signals, it is not necessary or useful to have a high areal density of thesereceptors. The skin acts to distribute the applied forces over relatively large areas (maybe 10x the

thickness of the skin), so spacing closer than tenths of a millimeter would not add any additional

sensitivity.


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Moderate Adaptation

Meissner's Corpuscles and hair follicle receptors are good examples of mechanoreceptors with

moderate adaptation rates. These receptors can be located near the surface of the skin, and adaptto changes on time periods of order 1 second. Some experiments with the hair on your arm

should confirm these adaptation rates. Think about the sorts of things such sensors (locatedaround hair follicles) would be useful for in an outdoor setting. If you've been camping recently,you might recall that these sensors are the ones you use most effectively to detect insects on your

skin (mosquitoes, ticks, flies, etc). Since these insects are a threat to human survival at some

level, evolving a capability to detect and remove such insects would be of obvious value.

Slow Adaptation

Ruffini Endings, Merkel's Cells, and Tactile Disks are examples of slow adapting

mechanoreceptors. These receptors are generally located near the surface of the skin, and are

responsible for much of the static perceptive capabilities. For example, the sensitivity totemperature at the skin is generally of a slow-adapting type, as are many tactile sensors useful for

maintaining grip on an object. The adaptation time scale for these cells can be from 10 to more

than 100 seconds. Experiment with grasping of an object in the air, like a pencil or a cup of

coffee. Close your eyes and think about how it is that you overcome the adaptation in thesesensors to avoid dropping objects.

In fact, it is interesting to give some thought to the whole process of grasping objects in the air.

You have all developed a set of skills for holding drinks in your hand with a minimum of effort.Think about how often you mistakenly crush the coffee cup in your hand, or about how often the

cup slips completely through your fingers. Aside from falling asleep, these events are extremely

rare. However, the task of holding a cup of liquid is an extremely complicated one. Think aboutall the forces that must be balanced and maintained, and remember that the sensors used in thistask have very odd temporal response, and that ALL of them eventually stop sending information

about the forces on the fingertips if those forces are constant. Nevertheless, all of you are able to

accomplish this task without much direct feedback control being applied - in fact it might becompletely unconscious!

Your Mechanoreceptors

By probing your hand with a toothpick, try to locate the most sensitive regions. You will

probably find that different parts of your hand are sensitive to larger or smaller signals or have

different adaptation rates. These experiments work best if you close your eyes. If you trust yourneighbor, even better results can be obtained through cooperation.

After a few minutes of experimentation, I'll ask the class to discuss about a few observations -

such as the location for the most sensitive parts of the hand, the fastest and slowest adaptingreceptors, and any other interesting features.

What is your observation?
http://www.stanford.edu/class/me220/data/lectures/lect01/answer1_1.htmlhttp://www.stanford.edu/class/me220/data/lectures/lect01/answer1_1.html


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The different adaptation rates are also distributed throughout the hand in various ways. Where

would you expect the slow and fast adapting receptors to be?

Answer

Previous experiments on human subjects by medical researchers have found that the most

sensitive mechanoreceptors are located in the fingertips, where skin indentations as small as 6microns are detected. Moving along the finger to the hand, the sensitivity to indentation is

reduced by a factor of two. Sensitivity at the heel of the hand is yet another factor of 2 smaller.

Using the toothpick, try to verify this general trend in sensitivity. Any comments?

Based on your experiments, where are they?

Another interesting feature of these mechanoreceptors is the spatial resolution. Break your

toothpick in half, and simultaneously touch two places on your hand. With your eyes closed, you

should be able to tell that there is a minimum resolvable separation between the contact points.See if you can see how this minimum resolvable separation is different in different places.

Interesting places to test include the tip of your tongue, the lips, the forehead, and the back of

your neck.


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Answer


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Measurements have shown that the slowest adapting cells tend to be on the tips of the fingers and

thumb, where they may be in densities as great as 100 per square centimeter. Some slow-

adapting cells may also be found throughout the rest of the hand at densities of 10 - 20 / sq. cm.The rapid adapting cells are less common, and are also found in the fingertips and the outer edge

of the palm. Why do you think these cells are distributed in this manner?

Do you notice any variation in the resolution for these locations?

Figure 1. Cat's sensory system carries the information in the form

of a constant magnitude pulse train. The magnitude of the pulse is

independent of the size of the stimulus. Pulse density varies with

the stimulus magnitude.

The output signal of mechanoreceptors is generally in the form of a stream of voltage pulses. Theamplitude of the signal being transmitted is represented as a pulse density. In this regard, the

nervous system is essentially transmitting digital signals, rather than analog signals. The pulses

are analogous to TTL pulses from a voltage-controlled oscillator in an electrical circuit. Thereare many advantages to transmission of digital data rather than analog data. Many of these same

advantages are present in telecommunications, and modern systems are beginning to use digital

telecommunications for most applications.


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One interesting thing to keep in mind are the limitations of human mechanoperception. Given the

finite density of these cells, and the location of some of them far below the skin surface, it is

interesting that we can derive so much information. If designing a replacement system (virtualreality, anyone?) it is important to remember that only detectable sensations are worth

simulating. This probably means that Virtual Reality gloves do not need to accurately reproduce

the entire stimulus - only those which your hand can detect. At the same time, it is interesting tosort out how you are able to derive so much information from such a limited sensing capability.

Answer

Measurements have shown that the sensitivity can vary from 1-2 mm on the tip of

the tongue to several cm on the back of the neck.


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Auditory System

Another very interesting human sensory capability is the hearing system. The human ear consists

of several parts, the ear canal, the middle ear, and the inner ear. The functioning of all of theseelements is not fully understood. In many cases, one or more parts of the ear can undergo

structural damage, and some functionality is preserved.


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Fig 2. Human Auditory System

Some partial description is possible though. Sound waves travel down the ear canal in the form

of compression pressure waves. These pressure waves induce deflection of the eardrum, and the

various structures attached to it. Part of this structure is coupled to the inner ear in a way whichseems to produce an outward deflection in reaction to an inward pressure force. The inner ear is


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configured as a spiral with two chambers. One of the chambers is connected to the eardrum

through the mechanism described above, while the other is in acoustic contact with the eardrum.

The apparent result of all of this complexity is the development of an oscillating force betweenthe chambers of the inner ear.

The inner ear is a seashell-like spiral structure, in which the two chambers are separated by a thindiaphragm impregnated with rapid adapting mechanoreceptors. Acoustic signals produce

oscillations in this diaphragm. One effect of the shape of the spiral diaphragm is that the resonantfrequency is a function of position along the spiral. As a result, any particular acoustic signal

frequency will produce a mechanical oscillation in the inner ear at a particular physical location.

In this way, the inner ear acts as an acoustic spectrum analyzer, with individualmechanoreceptors configured for detection of particular audio frequencies. The central nervous

system receives all of these signals and processes them into recognizable patterns.

The semicircular canals are three fluid filled ring-like structures with hairs that are sensitive to

motion of the fluid. A rotational acceleration will be detected by the canals, making them useful

for maintaining balance. The three canals are oriented orthogonally to one another, providinginformation about all three axes. It is rather impressive to realize that humans are walking

around with a couple of 3-axis rotational accelerometers in their heads whose output isautomatically processed by the brain to provide information about any rotational accelerations

that are experienced.

This mechanical system is capable of reasonable resolution for audio signals. Experiments have

shown that particular mechanoreceptors are sensitive to audio frequencies in a 10-20% bandaround their primary peak. Clearly, the nervous system has acquired substantial post-processing

because humans are capable of resolving tones to a much higher degree of accuracy (


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Answer

Humans, and most mammals make use of the time delay between the arrival time

at the two ears to determine direction. There are also some other cues being made

use of - because the time delay only serves to constrain the source to the surface of

a cone. Also, the almost instinctual behavior of tilting the head in more than one

direction during attempts to locate the source serve to narrow the possible

locations.

Visual System

Finally, some time is spent looking at the human eye. It consists of a spherical organ with atransparent lens, and a photosensitive region filled with several different types of photoreceptor

cells. The different shapes of the photoreceptor cells serve to offer sensitivity to different colors

of light. Their locations are also highly specialized to optimize the absorptivity.


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Fig 3. Human Visual System

An interesting thing to consider is the appearance of animal eyes at night. When illuminated,

they actually appear to be reflective.

What advantage is there in this?

The human eye has a nerve bundle which runs from all of the photoreceptors to the nervous

system positioned on the back surface of the eye. This location does not have any

photoreceptors, and so there is a 'blind spot' in human vision. Have you ever noticed it?

Fig 4. Finding Your Blind Spot


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By looking at the picture of the rat above, it is possible to notice your blind spot. Cover your left

eye, and hold the drawing about 30 cm from your eye. While looking at the upper 'X'. you should

notice that there is a distance from your eye at which the spot disappears. On the other hand, ifyou look at the lower 'X', you should find that the rat disappears, but that the lines seem to stay.

Answer

It turns out that bright illumination at night is not a common evolutionary problem, so most

nocturnal animals with mammalian eyes have this reflectivity feature. The reflecting layer is

positioned below the photoreceptor cells. It turns out that the reflectivity offers an advantage atnight in that it causes photons which were not absorbed to be reflected back through the sensitive

part of the eye, thereby increasing the optical efficiency by a factor of 2.

What is the explanation for the above phenomenon?

Answer

In this case, your on-board image processing software is acting to fill in the

background pattern based on the information in the rest of the image. This filling-in

can compensate for the blind spot, as long as the pattern is continuous. Notice also

that use of both eyes results in a seamless filling of the pattern without any

thought. Based on the use of both eyes, and your on-board signal processing

software, it is conceivable that you've gone through your entire life so far withoutnoticing this substantial defect in your vision system.

Summary

All of this has served to indicate some of how a set of sensing hardware which has been the

result of evolutionary development can be highly optimized, and operate in spite of some designproblems. The mechanoreceptors have adapted to achieve excellent use of the hands, the hearing

system has developed excellent capabilities for resolving events in the frequency and time

domain, and the vision system uses some sophisticated image processing to overcome thepositioning of the connection of the optic nerve.

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Lecture 1-Human Senror

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