Download 1. a) The auditory system is said to be organized in a tonotopic

MSP Neuro Week 13
1. An unfortunate soldier in the Vietnam War was severely injured when several pieces
of shrapnel from a nearby explosion were lodged into both of his feet. Since at the time
he was in the heat of battle, he was unable to receive immediate treatment for his wounds.
Eventually they became gangrenous and both feet had to be amputated from the knees
down. Upon returning home to the United States he noticed that whenever he was
engaging in sexual activities or even sexually aroused at all, he felt the sensation of
something tickling his feet. Describe the phenomena occurring and explain the possible
mechanism.
This is the phantom limb syndrome and it results due to cortical plasticity. "When one
area of the cortex is deprived of its normal input, neighboring areas may take over. Thus
the area of cortex previously activated by the feet may be activated by the genitalia.
Stimulation of the genitalia may create the feeling of foot stimulation." (pg.1 of notes)
2. a.) Several centuries ago there lived a pirate named James who decided to have a little
pirate child with his beloved wife Danka. James was so proud of his newborn son that he
decided to place a black patch over the boy's right eye so that he can grow up to look just
like his father. Several years later, James became very disappointed because his son
seamed to have great difficulty in learning how to sword fight. The boy felt uncoordinated
with the sword and frequently misjudged the distance of his opponents. Assuming the boy
was healthy at the time of delivery, explain what may have gone wrong during his
development.
The child did not receive any impulse stimulation to his right ocular dominance slabs
during the critical period of his development. This did not allow for the proper
connections to form for his right eye and he therefore his binocular vision suffered,
leading to has present condition.
b. Eventually Danka, who was at the time pregnant with a second child, grew tired of her
tyrant husband whom she claims was abusive, and left him. The next child, another boy,
was then raised by Danka for a year and a half until James finally gained custody due to a
court ruling. James, who was also proud of his second son, again raised the kid with a
patch over his right eye and eventually taught him how to sword fight. Assuming that the
second child became a great swordsman, and that he was born and raised in the same
manner as was the first child, what (relevant to this course) can account for the
differences in abilities between the two children?
The patch was placed on the eye of the second child after the time of the critical period in
development had past. Most if not all of the important connections were already formed
by this time and so the child did not suffer from disturbances in ocular dominance or
binocular vision.
3. Describe what changes in thalamocortical connections may occur upon completion of
the critical period of development which account for the decrease in cortical plasticity
after this point.
"It has been shown that the cortex undergoes LTP early in development but not later
suggesting that the end of the critical period is due to the loss of the ability of these
synapses to undergo LTP." (pg.7)
4. True and False
------ A cataract in one eye can result in monocular deprivation which may lead to shifts
in the ocular
dominance columns. (T)
------ Strabismus and anisometropia can cause ocular dominance shifts but have no effect
on binocular
vision. (F, binocular vision is also affected by both conditions)
------ More area of cortex is donated to the portions of the body which have the largest
surface area.
(F, the amount of cortex is determined by the number of receptors located in that
area)
------ Neurons rely on correlated activity between sensory inputs to guide the formation
of topographic
maps. (T)
5. a) The auditory system is said to be organized in a tonotopic fashion. Explain what this
means (comparing/contrasting with the somatotopic and retinotopic natures of the
somatosensory and visual systems.) Where does this tonotopic organization begin?
Different frequencies of sound are coded in different locations in the neural apparatus.
This is similar to somatotopicity and retinotopicity in that a map is formed on the cortex
with “adjacent stimuli” being encoded in adjacent parts of the cortex. In somatotopicity
and retinotopicity, however, spatial relations are being mapped spatially, while in
tonotopicity, tonal frequency relations are being mapped spatially.
b) What part of the cochlea begins the “separation of frequencies” to allow for tonotopic
mapping? Briefly describe the physical features of this part which enable it to separate
sound frequencies?
The basilar membrane separates frequencies of sound in that the base of the basilar
membrane is narrow and stiff, responding to higher frequency sounds while the apex of
the basilar membrane is wide and flexible, responding to lower frequency sounds.
c) Tonotopicity is maintained in the auditory cortex. Describe the organization of
stimulus dimensions in the auditory cortical map.
Just as adjacent areas of the visual field are mapped onto, adjacent areas of the primary
visual cortex, auditory stimuli of increasing “adjacent” frequencies are mapped onto
the primary auditory cortex in adjacent “isofrequency bands” (which also correspond
with the location of the cochlear basilar membrane – apex to base – sensitive to the
stimulus). Intensity of stimulus and mono/binaural interactions are also organized into
the cortical map.
6. You record, over time, the action potential generated by an auditory nerve in response
to a 500-Hz sound. Looking at the recording, you find 3 action potentials which are
equidistant from each other and are the “closest together” of any action potentials on the
recording. What is the likely amount of time that elapses between each of these action
potentials? What phenomenon accounts for this uniform spacing between the action
potentials? Why would you not expect to see it if you used a 3 kHz sound?
This demonstrates phase locking: action potentials occur in phase with the sound wave,
therefore the closest spaced ones would be 1/500 of a second apart. This would not be
seen with high frequency stimuli since it would require precision of 1/3000 of a second.
7. Sound localization along the horizontal axis is accomplished by two different
mechanisms which use two different parts of the superior olive in their mechanisms.
Diagram each mechanism and explain how the system arrives at a “final output” which
encodes the location of the sound source along the horizontal axis. Finally, what
anatomical part of the body allows for vertical sound localization?
The diagrams for sound localization via interaural time delays (medial superior olive –
the coincidence detectors) and interaural intensity differences (lateral superior olives)
are on pp 5 and 6 of the auditory handout. The captions in the diagrams are very good at
explaining the mechanism. Be sure the students understand the idea of the coincidence
detector sending out information about which ear the stimulus is closer to and how the
information leaving the left or the right lateral superior olive encodes that information as
well. The pinna enables vertical sound localization.
8. A brother and a sister are brought into your office by their mother. The mother tells you
that the brother is deaf in one ear and the sister has hearing loss in each ear. She then tells
you that both the brother and the sister each have one lesion, and that there lesions are in
successive “steps” in the auditory pathway. Where could the brother’s and the sister’s
lesions be so that they are in successive “steps” in the auditory pathway, yet one
produces unilateral hearing loss and the other produces bilateral loss?
The brother has a lesion in the cochlear nucleus while the sister has one in the
superior olive.
What kind of hearing loss do these two children have in common that differentiates them
from someone with hearing loss from otitis media ?
They have sensorineural hearing loss, versus the conduction hearing loss of someone
with a middle ear infection (otitis media).