Download Lab #8: The Special Senses

Lab #8: The Special Senses: Hearing, Vision, and Orientation
Background
The special senses (vision, hearing, equilibrium,
gustation, and olfaction) differ from the
somatesthetic senses in two fundamental ways.
First, the receptors for the special senses are all
found within specific locations the head, and
often within complex organs designed to modify
the environmental change in a way that focuses
and amplifies its effect on the receptor cells.
Second, all of the sensory neurons associated
with the special senses are found within cranial
nerves, and therefore sensory information from
these receptors travel directly into the brain
without the involvement of the spinal cord. In
this lab exercise, we will examine aspects of two
of the special senses—vision and hearing. We
will also examine how different sensory inputs
contribute to a person’s ability to orient
themselves in their surroundings.
Vision
The eyes consist of structures that are designed
to refract (bend) light from objects in a person’s
surroundings. There are four structures that
refract light as it travels through the eye: the
cornea, the aqueous body, the lens, and the
vitreous body (Fig. 8.1). As light passes into
each structure, the change in density from the
previous medium causes the angle that the light
is traveling to change. Ultimately, the refraction
of light by these various structures leads to a
focused image being projected on sensory cells
Cornea
Aqueous body
(anterior chamber)
Pupil
Iris
Suspensory
ligaments
within the retina. A focused image enables
specific
photoreceptors
or
groups
of
photoreceptors in the retina to be stimulated by
light eminating from a particular point on the
object being viewed, and thus enables maximal
visual acuity.
The degree to which light from an object
needs to be bent in order to focus an image
depends on the distance of an object from the
eye (Fig 8.2). Light eminating from a point on a
distant object enters the eye at a relatively
narrow range of angles. Thus, that light does not
need to be bent extensively to bring light from
that point into focus when projected on the
retina. In contrast, light fron a point on a near
object enters the eye at a much wider range of
angles. Therefore, light must be bent more by
the refractive structures of the eye to bring that
point into focus.
The degree to which light is bent by a lens
(any lens, not just the anatomical structure
within the eye) is referred to as its refractive
power. Refractive power is quantified in units
called diopters, which are calculated as
Ciliary muscle
Lens
Retina
Vitreous body
(posterior chamber)
Macula lutea
Fig 8.2. Effect of distance on light dispersion. Light
from objects at a distance enter the eye at a narrow range
of angles, whereas light from near objects enters the eye
at a wider ranges of angles.
Refractive power (diopters) =
1
Focal length (m)
Optic disk
Optic nerve
Fovea centralis
Fig 8.1. Diagram of major structures of the eye.
where the focal length is the distance that light
passing through a lens must travel before
converging on a single focal point. For convex
lenses (lenses that bend light inward, Fig 8.3),
the refractive power is a positive value based on
Convex Lens
Fig 8.5. Accomodation. In order for light from a distant
object to be brought into focus on the retina, the ciliary
muscles relax, tightening the suspensory ligaments and
stretching out the lens and reducing its refractive power.
To view near objects, the ciliary muscle contracts, thus
slackening the suspensory ligaments and allowing the lens
to recoil into a thicker shape with greater refractive
power.
Concave Lens
Fig 8.3. A comparison of convex and concave lenses.
Convex lenses converge light toward a single focal point,
whereas concave lenses disperse light away from a single
focal point.
the thickness of the lens. Thicker lenses bend
light more (i.e., have greater refractive power),
thus light that passes through the lens converges
into a single point in a shorter distance (i.e.,
shorter focal length) than for a thinner lens (Fig
8.4). Concave lenses (Fig 8.3), in contrast
disperse light passing through them rather than
converge that light, and thus have a negative
refractive power based on the degree to which
they disperse light.
Focal Length
Focal Length
Fig 8.4. Effect of lens thickness on refractive power in
convex lenses. Thicker lenses bend light more, thus the
focal length for the lens is less and the refractive power
(in diopters) is greater
Most lenses have a fixed refractive power,
and thus an object must be a specific distance
from the lens in order for light from that object
to be focused by that lens. However, the
refractive power of the lens of the human eye is
adjustable. The circumference of the lens is
attached to the ciliary muscle of the eye through
a series of ligaments called suspensory
ligaments (Fig 8.1). By contracting the ciliary
muscle to different degrees, the tension exerted
by the suspensory ligaments on the edges of the
lens can be altered, allowing the lens to be
stretched thin or allowed to elastically recoil into
a thicker shape. Thus, as the thickness of the
lens is altered, its refractive power is altered.
This alteration in lens thickness can be used to
focus on objects at different distances from the
eye through a process called accomodation (Fig
8.5). To view distant objects, from which light
enters the eye at a narrow range of angles, the
ciliary muscles are relaxed. This causes this ring
of muscle to become thin, which in turn pulls
outward, increasing the tension on the
suspensory ligaments. As these ligaments are
pulled taut, they stretch out the lens, causing it to
assume a thin shape with a lower refractive
power. In contract, to view close objects, the
ciliary muscles contract. As this ring of muscle
thickens it releases tension on the suspensory
ligaments, and the lens elastically recoils back to
a thicker shape with a higher refractive power.
Near
Myopia
Normal Cornea, Frontal View
Far
Astigmatism, Frontal View
Lateral
View
Hyperopia
Near
Dorsal
View
Far
Fig 8.6. Myopia and hyperopia. In myopia, they eye has
abnormally high refractive power, thus whereas near
objects can be seen in focus (since refractive power would
need to be increased anyway), the refractive power of the
eye cannot be lowered enough to focus on distant objects,
as light passing through the eye reaches its focal point in
front of the retina. In hyperopia, the eye has abnormally
lower refractive power. In this case, distant objects can
bee seen clearly, but the refractive power of the eye
cannot be increased enough to see near objects, as the
focal point for light passing through the eye would be
behind the retina.
Many visual disorders are associated with
abnormal refraction of light due to mishaped
structures in the eye.
Myopia, or
“nearsightedness”, is a condition where an
individual has trouble seeing distant objects,
although near objects are seen clearly (Fig 8.6).
Myopia results from having an unusually
elongated eyeball or an unusually thick cornea
or lens. In effect, the eye cannot reduce
refractive power enough to view distant objects,
so light from these objects comes into focus at a
point in front of the retina and is back out of
focus by the time it reaches the retina. Concave
lenses, which spread out light into a wider array
of angles before it enters the eyes, are prescribed
to correct this visual disorder.
Individuals
with
hyperopia,
or
“farsightedness” have the opposite problem of
those with myopia. Having a relatively short
eyeball, shallow cornea, or thin lens, individuals
with hyperopia cannot focus on nearby objects
(but can focus on distant objects) because the
eye does not have enough refractive power to
bring the image for close objects into focus by
the time it reaches the retina, and instead the
focal point is located somewhere behind the eye.
Corrective convex lenses, which bend light
Fig 8.7. Astigmatism, as illustrated with a mishapen
cornea. Notice that the cornea on the right is wider
than it is high. This means that light in the vertical
plane will be refracted more than light in the horizontal
plane. As a result, if the lens is adjusted to bring light
in the vertical plane into focus, light in the horizontal
plane will go out of focus.
inward before it enters the eye, are used to
correct for this visual disorder.
Astigmatisms are visual disorders causes by
a mishaping of some refractive structure in the
eye such that the focal length for light entering
in the eye is not the same at all angles (Fig 8.7).
as a result, the lens cannot be adjusted to bring
light at all angles into focus simultaneously.
Corrective lenses for astigmatisms have an
angular alteration in refractive power that
corrects for the angle of mishapening in the eye
(Fig 8.8).
Fig 8.8. Optometry prescription form. Sphere indicates
the overall refractive power adjustment (in diopters). The
cylinder indicates refractive power change for an
astigmatism, and the axis indicates the angle of the
astigmantism
correction.
From
http://www.pearlevision.com/veex/ve_page17.html
The ability of the eye to adjust its refractive
power can change with age. As a person gets
older, the lens becomes less flexible and
attachments for the the suspensory ligaments are
moved forward on the lens. As a result, the
lense remains in a stretched state even when the
ciliary muscles constrict. This reduces the
ability of the eye to increase its refractive power
to see near objects. This form of farsightedness,
called presbyopia, is nearly ubiquitous among
people over the age of 45 years, and is part of
the overall aging process.
Once light is bent correctly by the refractive
structures of the eye, a clearly focused image of
objects in the visual field is projected on the
retina of the eye. The ability to see objects in
the visual field and determine their spatial
relationship to one another depends upon what
part of the retina light from a point in the visual
field is projected. Light that passes through the
axis extending through the centers of the cornea
and lens is directed toward a structure in the
retina called the macula lutea (Figs 8.1 and 8.9),
where most of the cone cells of the retina (those
that permit color vison and high visula acuity)
are located. At the center of the macula lutea is
a pit-like region called the fovea centralis. This
particular location has an extremely high density
of cone cells, each connected with the peripheral
nervous system in such a way that each cone cell
stimulates its own ganglion cell (sensory
neuron). This means that the receptive field for
each ganglion cell is very small, and thus visual
Fig 8.9. A view of the fundus of the eye, showing
structures on the internal surface of the retina.
Optic disk
Fig 8.10. Projection of light from laterally-oriented
objects onto the retina. Note that more laterally
postioned objects project images on more medial
regions of the retina. Also note that images projected
onto the optic disk (e.g., the purple circle above) cannot
be seen, since therre are no photoreceptors at that
location on the retina.
acuity is very high in the fovea centralis. Acuity
tends to decrease somewhat for light projected
on more peripheral regions of the macula lutea,
where each ganglion cells receives signals from
multiple cone cells (creating a larger receptive
field for each ganglion cell), and even more so
for light falling outside of the macula lutea,
where each ganglion cell is stimulated by many
individual rod cells (those that have high light
sensitivity but no color discrimination and low
acuity).
Light from objects that are positioned in the
lateral parts of the visual field are projected onto
the media portions of the retina (Fig 8.10).
Interestingly, it is here in the media region of the
retina where the optic nerve joins with the eye,
and on the inner surface of the retina, a structure
called the optic disk can be seen that demarcates
this connection point (Figs 8.9 and 8.10). The
optic disk lacks photreceptor cells, so is light
from an object is projected directly on the optic
disk, no image will be perceived. This creates a
“blind spot” within the visual field, although the
brain perceives a continous visual field based on
sensory inputs from areas surrounding the blind
spot.
If a person directly faces an object, light
from that object (which would be medially
oriented) will tend to be projected somewhat
laterally on the inner surface of the retina (Fig.
8.11). The degree to which the image is
8.11. Effect of object distance on lateral projection of
images in stereoscopic vision. Light from near objects
tends to be projected onto more lateral regions of the
retina than does light from distant objects. Thus,
differential stimulation of photoreceptors in both eyes
simultaneously allows depth perception for objects that
are relatively near to the individual (< 100’).
projected laterally is related to the distance of
the object from the eyes. Recall that light from
points on distant objects enter the eye at a very
narrow range of angles. Light from these
objects would be projected more or less towards
the center of the retina for both eyes. However,
light from points on near objects enter the eye at
a broader range of angles, so light from those
objects would tend to be projected onto more
lateral regions fo the retina.
This lateral
projection onto the retinas of both eyes
stimultaneously, coupled with the relative size of
the object in relation to other objects appearing
in the visual field, enables depth perception in
human vision. In order for lateral projection of
images on the retina to be effective, however,
the image must fall into the visual field of both
eyes. Thus, the stereopsis (the viewing of the
same object from two slightly different angles
simultaneously) provided from having two eyes
postioned at the front of the head with
overlapping visual fields is important component
of our ability to see in three dimensions.
Hearing
The outer and middle regions of the ear act as a
conduction and amplification system that
collects sound waves in the air and amplifies
these vibrations enough to generate waves of
fluid pressure in the cochlea of the inner ear,
where the sensory receptors (hair cells) are
located. In order for hearing to take place,
sound must be effectively conducted into the
Fig. 8.12. Internal anatomy of the ear. Illustration from
http://www.vestibular.org/gallery.html
inner ear with enough strength to stimulate the
hair cells, the hair cells must be able to respond
to these vibrations by releasing enough
neurotransmitter to sensory neurons to trigger an
action potential, the action potentials must
propagate through the auditory nerve into the
central nervous system and through appropriate
second-order and third-order neurons to reach
the auditory cortex in the temporal lobe.
Hearing impairments can be caused by a
number of different conditions, but can be
categorized into two different types. The first,
conductive deafness, results from a condition
where the conduction and amplication of
vibrations between the external environment and
the fluid of the cochlea. As a result, the
vibrations that reach the hair cells are not strong
enough to lead to action potential generation in
the sensory neurons of the auditory nerve.
Examples of conditions that cause conductive
deafness include occlusion of the external
auditory meatus (e.g., excessive earwax
production), perforation of the tympanic
membrane, abnormal development or damage to
the audittory ossicles, or damage to the oval or
round windows. Conductive deafness can often
be corrected through the use of hearing aids that
amplify sound entering the ear so that the
vibrations reaching the inner ear are strong
enough to effectively stimulate the hair cells.
The other type of deafness, sensorineural
deafness, results from damage to either the
sensors (hair cells) for hearing, to the nerve
pathways that conduct signals from the ear to the
auditory cortex, or to the auditory cortex itself.
In this case, the basic process of sensation
cannot take place because the sensory pathway
Fig. 8.13. Damage to stereocilia resulting from
excessive volume. Normal stereocilia are above, and
damaged
sterocilia
below.
Photos
from
http://www.vestibular.org/gallery.html
is compromised. Often, sensorineural deafness
affects the ability to hear specific pitches rather
than a reduction in hearing at all pitches (as
would happen in conductive deafness). A
common cause of sensorineural deafness is
trauma to the hair cells induced by exposure to
excessively loud sounds (>85 dB). At this
volume, vibrations in the cochlea are strong
enough to damage the stereocilia. Indeed,
sounds in in excess of 140 dB are strong enough
to kill the hair cells themselves, and hair cells
are not regenerated in humans or other mammals
(Fig 8.13). Sensorineural deafness also occurs
as part of the normal aging process. This agerelated loss of hearing, called presbycusis,
involves a loss in the ability to hear high
frequencies, and typically begins in the early
20’s. Treatments for sensorineural deafness
include hearing aids for some conditions as well
as cochlear implants, which directly stimulate
the auditory nerve electrically in response to
sound.
The positioning of the ears on the lateral
surfaces of the head enables binaural hearing,
where the central nervous system not only
detects
vibrations
in
the
surrounding
environment, but by comparing characteristics
of the sound detected by each ear, can perceive
the direction from where the sound originated.
To illustrate why, imaging that you have your
left ear oriented toward a stereo speaker, and
your right ear oriented away from the speaker.
Sound eminating from the speaker will reach the
left ear a fraction of a second before the right
ear. Thus there will be difference in stimulation
time (called the interaural time difference). In
addition, the right ear will be in an acoustic
“shadow” created by the head blocking the
movement of some of the sound through the air.
Thus the intensity of the soundwaves entering
the right ear will be will be less than those
entering the left ear. The resulting interaural
intensity difference, coupled with the interaural
time difference, enables directional perception
of sound.
Orientation and Balance
A person’s ability to orient themselves within
their surroundings involves the input of a
number of senses that enable us to ascertain the
direction of objects in our surroundings and the
distance between our bodies and those objects.
Although some senses can provide information
regarding the direction of objects in our
surroundings (e.g., hearing, smell), our
orientation within our surroundings and
subsequently aour ability to maintain posture
and balance is based primarily upon
proprioception, cutaneous mechanoreception
(touch and pressure), equilibrium (from the
vestibular apparatus), and vision.
While
proprioception
and
equilibrium
enable
orientation of the body relative to itself and to
the force of gravity, balance and coordination
also require at least two different points of
reference in the surrounding environment, which
are derived from cutaneous mechanoreception
and/or sight.
Experimental Procedures
Experiment I: Vision
A. Visual Acuity – Testing for Myopia with the Snellen Eye
Chart.
Posted on the cabinets at the back of the lab are Snellen eye charts
(Fig 8.14). There are strips of tape marking on the floor near the
front of the room that mark a 20’ distance from the charts. Standing
at the 20’, remove glasses if you are wearing them, cover your left
eye, and read the smallest line of text you can see. If you correctly
read all of the characters in the line (verified by your lab partners),
record the visual acuity value associated with that line of text.
Repeat with your left eye, being sure to cover your right eye in the
process. If you wear glasses, put them back on, and retest your
vision in each eye with your vision corrected.
Visual acuity (specifically in reference to myopia) is
typically expressed in values such as “20/20”, 20/40”, etc.
This expression gives your visual ability in comparison to
what the average person should be able to see. For example,
if you have 20/40 vision, you can see clearly at a maximum
of 20’ what the average person could see at a maximum of
40’. This indicates myopia (since you cannot see distant
objects as well as the average person). Often prescription
glasses enable better than average acuity, thus while wearing
prescription glasses it is not uncommon to have a vision of
20/15 or even 20/10.
Fig 8.14. The Snellen eye chart.
Fig 8.15. An astigmatism chart.
B. Visual Acuity – Testing for Astigmatism.
Adjacent to the Snellen eye charts are astigmatism charts (Fig 8.15), which depict a series of banded
blocks and a semicircle of radiating lines. Stand 20’ from the chart, remove your glasses, and cover one
eye. If you have astigmatism, one of the blocks will appear sharply black and white, whereas the others
will appear more grayish. Similarly, one or a few adjacent lines of the semicircle will appear particularly
dark and sharp, whereas the others will appear more faded and grey. If you do not have astigmatism, all
of the lines and blocks will look identically sharp.
C. Visual Acuity – Measuring the Near Point of Vision.
Place the end of a meter stick on the bridge of your nose and
hold it so that it extends outward horizontally (Fig 8.16).
Close one eye, then take a pencil and hold it at arms length so
the point is against the edge of the meter stick. Focus on the
tip of the pencil with the open eye, and move the point along
the edge of the meter stick towards you. At the point where
the tip of the pencil becomes slightly blurry, stop moving the
pencil toward you, and note where the pencil is located along
the meter stick. The distance is your near point of vision.
Fig 8.16. Near point of vision measurement
D. Observing the Blind Spot.
Obtain a strip of paper with a ● and a + on the two ends. Hold the slip of
paper between your thumb and forefinger in your right hand in front of
you at arm’s length so that the ● is just above your thumb and the +
extends laterally off to the right. Cover your left eye, and with the right
eye look straight at the ●, noting that you can see the + “out of the corner
of your eye”. Start to move the piece of paper towards your face, always
focusing on the ● but keeping track of the + in the periphery. Eventually
you will move the paper to a point where the + appears to disappear from
the end of the paper strip, and all you will see is white. When you
continue to move it even closer to your face, the + should reappear.
Fig 8.17. Testing depth perception.
E. Stereopsis and Depth Perception.
Have one person in your lab group hold a test tube up for you. Stand three steps away facing that person
and hold a pencil up in an overhand fashion (Fig 8.17). With both eyes open, take three steps forward,
and place the pencil in the test tube in a single smooth downward movement of your arm. Step back three
steps, then cover one eye and repeat the action. Did you get the pencil into the test tube? If you did, was
it easy this time as it was when you had both eyes open?
Experiment II: Hearing
A. Identifying Types of Deafness – Rinne Test and Weber Test
Conduct a Rinne test for hearing in the following manner.
Strike a tuning fork and place the end of the stem on the
mastoid process located just behind the ear (Fig 8.18). The
subject should be able to hear the tuning fork through the
vibrations conducted into the skull. When the sound dies
down, move the prongs of the tuning fork over to the opening
of the ear canal. The tone should return. Have the subject then
simulate conductive deafness by pushing the tragus of their
earlobe over the opening of the ear canal. Repeat the exercise.
The subject should be able to hear the vibrations through the
mastoid process just as well (if not better) than before, even
though their ability to hear the vibrating prongs through the air
will be impeded. If a person had real conductive deafness,
they would be able to hear the sound through the mastoid
process clearly, even though hearing through the ear canal
would be impeded. In contrast, if sensorineural deafness
accounts for the hearing impairment, then the subject would
not be able to hear the tone through either means.
Conduct a Weber test by striking the tuning fork and
placing it on the mid-sagittal line of the skull, either directly at
the top of the head or on the forehead (Fig 18.19). A tone
should be heard from the vibrations being conducted through
the skull. Try simulating conductive deafness in one ear. The
sound should be clearer in the ear with conductive deafness. If
Fig 8.18. The Rinne test. Photos from
http://www.rajavithi.go.th/ent/Educational%20
Resource/ENT%20Exam/ENT%20Exam.htm
an individual had sensorineural deafness in the affected ear,
the tone would be clearer in the unaffected ear.
B – Binaural Sound and Directional Perception.
Pull a stool away from you lab bench so that you can easily
walk around it. Have someone in your group sit on the stool
and close his/her eyes. Strike a tuning fork and have the
subject point to where the sound is coming from. Move the
tuning fork to various positions around the subject, having
them point to the tuning fork at all times. Were they able to
locate the tuning fork accurately at all times? Have the subject
simulate conductive deafness in one ear by pushing down on
the tragus. Repeat the experiment. How did their accuracy in
locating the tuning fork differ from when they had both ears
open?
Fig 18.19. The Weber test. Photo from
http://medicine.ucsd.edu/clinicalmed/head.htm
Experiment III: Orientation and Balance – Sensory Integration
Time your lab partner as he/she stands on one foot with their eyes open. Then time the same individual as
he/she stands on one foot with their eyes closed. Finally time them one more time as they stand on one
foot with their eyes closed but are lightly touching one finger to the top of the lab bench.
Discussion Question: Would Ralph Machio have been able to pull off the crane kick so
effectively if he had had his eyes closed?