Practicals Flashcards
1
Q
Give 3 of the instructions in the experimental protocol for finding visual fields and explain why they are important
A
darken room - dilates pupil so pupil size doesn’t restrict visual field (also allows screen/ flashes to be seen and limits any distractions in the room)
eye perpendicular to screen - prevents angles skewing the visual field
don’t screw up other eye - so this doesn’t restrict the size of the seeing eye
2
Q
why is the visual field asymmetrical
A
angle of orbit + nose
3
Q
What is the limitation of presenting the visual field on a 2D piece of paper
A
field is 3D IRL - visual field extends behind line of the eye (>90 degrees)
4
Q
Where is the blind spot on the retina and where is it in the visual field
A
in the temporal visual field so is on the nasal retina
5
Q
Why is the blind spot blind
A
where all the axons of the ganglion cells leave the eye so there is no space for rods/cones here -> it is where the optic nerve is
6
Q
How does CN II change in diameter as it leaves the eye
A
gets thicker as neurons become myelinated (no myelination in the retina as this would distort light entering)
7
Q
tanx=
sinx=
cosx=
A
opp/adj
opp/hyp
adj/hyp
8
Q
How to find the size of the blind spot
A
17tan(angle that subtends blind spot)
9
Q
What is the direction of light in a histological section of the retina
A
from vitreous to choroid
10
Q
What do the pigment epithelium do
A
regenerate rhodopsin
11
Q
What is in the innermost axon layer of the retina
A
axons that head to the blind spot
12
Q
How can you identify a section is of the fovea
A
fovea has a dip in the middle to allow more light to reach receptors in outer nuclear layer
13
Q
Give 1 way to tell the periphery from the centre of the retina
A
central retina has a dense ganglionic layer
periphery has a sparse ganglionic layer
you can also compare the number of cells in different layers within the same column to see if there is convergence or divergence
14
Q
How was the perimeter of the visual field usually assessed
A
manually using a flashing target light placed at different locations within the visual field. The subject would report whether or not the light was visible at any particular location
15
Q
Give the instructions before mapping the visual field
A
darken room
cover other eye (DO NOT screw it up)
It is important that the non-tested eye is completely covered and cannot detect anything, otherwise it would compensate for any blind area of the tested eye
Make sure that the uncovered eye is looking directly at the center of the screen and is perpendicular to the screen.
16
Q
How does the map of the visual field represent features of the retina
A
features on the subject’s retina will be accordingly reversed and inverted.
17
Q
What will a macular defect look like when looking at the visual field analysis
what will the blind spot look like
A
an area of slightly depressed sensitivity (orange)
an area of low sensitivity (blue; expressed in dB relative to a reference value).
18
Q
What should you do if the eye fixation quality is poor (below 90%) when testing visual field
A
repeat measurements
19
Q
As human eyes are forward facing, what does this mean for our visual fields?
A
overlap frontally (perhaps to allow good distance judgement using triangulation on objects and stereopsis )
blind area blind head and visual field is tilted down
20
Q
How can you test your field of vision beyond the edges of a screen
A
Using traditional perimetry with a flashing test light mounted on a rotating track which extends much further temporally than the margins of your computer screen it is possible to map the visual field to its boundaries in each eye
21
Q
How is the right eye’s visual field limited
A
limited on the left by the nose: the nasal field of the right eye. In contrast, the right eye’s view is not limited to the same degree on the right: the temporal field of the right eye - thus is lopsided
22
Q
What does binocular overlap allow
A
stereopsis
23
Q
the left eye’s temporal retina and right eye’s nasal retina point at the same area of visual space. What would we thus expect from the retinal ganglion cells from these areas?
A
ganglion cells from left temporal and right nasal retinae project to same part of the brain
one side of brain encodes vision for contralateral side
nasal right and temporal left retinae project to left visual cortex which encodes right visual field
24
Q
What should be noted about representation in the retinotopic map in the visual cortex
A
an enormous proportion of visual cortex is devoted to the fovea
25
What is the optic disc
the optic nerve head / blind spot
26
Where is the blind spot usually on the retina
16 degrees from fovea along the horizontal meridian
27
If the blind spot is 'blind' why is there any sensitivity recorded
due to effects of small eye movements
28
Which type of animal tends to have binocular overlap in front and a posterior blind area
predators
29
How do the visual fields of prey differ from predators
predators have binocular overlap in front and a posterior blind area
prey have eyes on the side of the skull with little frontal overlap - near panoramic view
30
Do rabbits have any visual field overlap
how does this differ in horses
some frontally
some above and behind
horses also have a little frontal overlap but there is a small blind area behind head
horse eyes tilt downwards to warn of attack from below and is blind from above
31
True or false
| a horse cannot see in front of it when its head is pulled back to vertical
true
| because of the profound downward tilt of its eyes
32
What is visual acuity
how good the visual system is at resolving fine spatial detail
33
How is the critical size of an image specified
in terms of the size of its retinal image or the angle that the object subtends at the front of the eye
34
Give 3 things which affect the visibility of an object
depends largely upon the size of its image on the retina (it also depends on such things as its brightness and contrast)
35
How do you calculate retinal size
Retinal size = 17000.tan(acuity)
36
How far is the lens from the optics of the eye
17mm
37
What is the diameter of the eye
20 mm
38
Describe the structure of the eye ball
has a tough outer collagen coat (the sclera) and filled with gelatinous vitreous humour.
Within the sclera is the choroid which contains blood vessels for the energy-hungry neural retina, and then a single layered pigment epithelium with melanin-containing cells.
39
Between which layers of the eye does the neural retina lie
between pigment epithelium and the vitreous
40
How does the retina change in size across the eye
what does retinal thickness reflect
thinnest nearest to the entrance pupil, and is fattest more or less diametrically opposite the pupil. However, in the middle of the fat bulge is a pit – the fovea
density of cells in that location
41
What is the diameter of the fovea
1.5mm
42
What is the centre of the fovea called
How big is it
what is the purpose
foveola
300μm in diameter
part of the eye giving the most acute vision and is the part which is aimed at objects of interest during fixation.
no rods here, only cone
43
Why does the retina change in thickness
thin in the periphery as receptors are less densely packed and there are few bipolar cells and v few ganglion cells
thick near fovea as receptors are small and densely packed with lots of bipolar and ganglion cells
44
Why does the fovea seem v thin
nearly all cell layers are displaced to the side
45
True or false
| there are no cones in the foveola
false
| there are no rods, only cones
46
What is the macula
term used by ophthalmologists. It refers to the part of the retina covered by a yellow pigment that can be seen with an ophthalmoscope
It is 6mm diameter with the fovea roughly in the middle.
47
Are there 3 layers in the peripheral retina?
Yes
| but the innermost ganglion cell layer is sparse
48
How can the retina become detached in life
how is it treated
bang to head leading to localised blindness
remedied by “gluing” the retina back onto the epithelium.
49
What forms the outer nuclear layer of the retina
what does this lead to
nuclei of the receptors (cone/rod)
receptors synapse onto bipolar and horizontal cell dendrites in the outer plexiform layer
50
What forms the inner nuclear layer of the retina
nuclei of interneuron cells and amacrine cells
51
What forms the inner plexiform layer
where the bipolar and amacrine cells synapse with each other and ganglion cells
52
What are Muller's cells
long, branched glial cells which run radially through the retina (with nuclei in the inner nuclear layer)
the dilated ends of these make the inner limiting membrane of the retina
53
What is the inner limiting membrane of the retina
a layer made of the dilated ends of Müller's cells which separates the ganglion layer from the vitreous humour
54
Where are the blood vessels in the eye
present in the inner layers of the retina (e.g. amongst the ganglion cell nuclei).
Diffusion of oxygen and glucose from the choroid is not adequate to supply the innermost layers of the retina.
NB the blood and the blood vessels impede the light on its passage to the receptors!
55
Can we see the blood vessels in the eyeballs
as they lie superficially to the photoreceptor layer, they cast shadows on the receptors, but these shadows are normally stationary on the retina and, like all stabilised retinal images, are not perceived.
However, if the shadows are made to move over the photoreceptors by illuminating the vessels obliquely with a moving light, they can be visualised.
56
describe the thickness of the innermost layer of ganglion cell axons right next to the vitreous
thicker than periphery because all the 1.5 million ganglion cell axons converge on the optic disk.
57
If the blind spot in the visual field is 4-6 degrees, how can we calculate the size of the optic nerve head
retinal mm = 17 tan(blind spot degrees)
=17tan(4)
=~1.19mm
58
If the optic disc is 1.45mm on the retina, what is the size of the blind spot
blind spot degrees = inv.tan(optic disk mm/17)
| = inv.tan(1.45/17) = 4.88
59
How is visual acuity determined in the fovea
does this apply to the whole retina
determined largely by the size and spacing of the cone tips
no cones are almost exclusively in the central retina
60
What is human visual acuity in ideal conditions (give in minutes and then the equivalent in microns)
0.5 minutes (30 seconds)
In a 17mm diameter eyeball (corresponding to the primate eye), this translates to 2.47 microns.
61
Can we estimate the width of a cone tip in the class by just counting cone nuclei in a slide?
Simple counting might give the wrong answer – the count will depend on such factors as how big a tip is with respect to the thickness of the histological specimen
62
How can you estimate the width of a cone tip in the retina
count the number of nuclei in the ganglion cell layer, compare to umber of cone nuclei and calculate the degree of cone convergence or divergence onto ganglion cells. The “professionals” would say that, in the centre of the fovea, there are 2-2.5 ganglion cells per cone
63
What does the eye need to do to focus light on the retina
refract light at curved borders between 2 media with different refractive indices
64
What is responsible for refraction of light in the eye
```
cornea = 2/3
lens = 1/3
```
65
Why is the cornea responsible for most of the light refraction
it is highly curved
big difference in refractive index between air and cornea (1:1.33)
66
What happens to the lens when you accommodate a near object
bulges to give extra power
67
Why can humans not see properly underwater (without goggles)
water and cornea have same refractive index so light is not refracted
68
Is the human eye diurnal or nocturnal
it is a typical diurnal eye with some nocturnal ability
69
How does a diurnal eye differ from a nocturnal one
works with luxury of lots of photons so can select which light rays to exclude from the eye, leaving only those which form a sharp image
closely packed cones with little/no pooling can thus send a fine grained spatial 'neural image'
70
What does pupil area determine
what is the usual size
how much light can enter the eye
2-8mm diameter (
71
What happens to the photons that miss the cones/rods
absorbed by the black pigment epithelium
their scatter back to the wrong receptors would degrade the sharpness of the image
72
How does convergence differ between cats and humans
cats: millions of rods/cones onto 200,000 ganglion cells
human: converge onto 1.6 million ganglion cells
73
What is the Stiles Crawford effect
the ellipsoids of cones act as a wave guide, reducing cone absorption of oblique rays allowing cones to see a sharper retinal image
74
How does the distribution of cones in the fovea vary in the human eye
cones are more common than rods in the fovea (but rods are still present)
in the foveola, small cones are so densely packed there are no rods at all - here cones actually diverge onto ganglion cells
75
How can visual acuity be measured
the highest number of cycles in a pattern of fine alternating black and white stripes
expressed as cycles per degree
where a cycle is one bright and one black stripe
76
Why can you use cycles of black and white stripes to test visual acuity
good acuity requires retinal image to be sharp so stripes don't blur into each other
needs a small pupil
77
Which cells are used for visual acuity tests
cones
they connect to ganglion with little/no convergence
requires 1 message to the brain per stripe to show one has a different brightness to the next
78
When is acuity poor
why
in dim light
wide pupil to capture as many photons as possible but leads to image degradation due to spherical and chromatic aberration
blurred image is acptured by rods with lots of convergence so neural smapling is too coarse
79
Why is acuity better in bright light
pupil is small, reducing spherical and chromatic aberration
if the pupil gets too small diffraction around the edge becomes noticeable, degrading the image
80
What is the best compromise with pupil size
diameter of 2mm (compromise as decreasing size decreases aberration but increases diffraction)
81
What is the max acuity a human can image
60c/deg
82
What does 1 deg occupy on the human retina
how many cone tips are there in this area
how many P ganglion cells are connected to each cone? what does this mean?
270 micrometres
120
1
the foveola can send 120 samples to the brain per deg
83
How can we measure eye movements?
* Indirect method: optical transducer attached to the head
* Measures eye position relative to head position
* Subject’s head must stay still (When the head moves, the eyes move as well!)
* Use infrared emission/ detection
84
How does your eye move when reading
is this feedforward or feedback
fixated on 1 space briefly then jump to the next until you reach the end of the line when you jump back to the left (saccade)
feedforward
85
How do eye movements change when the writing is smaller/ denser
more jumps/ more individual fixation points
86
How does the shape of a saccade change as the size of the movement changes
the shape of the saccade is the same for small and large movements
87
If you were comparing the latencies for different amplitude saccades, what statistical test would you use
use a t-test. This is continuous data (each saccade can
| take any value of latency along a continuous axis)
88
How does the shape and latency change for different sized saccades
what does this suggest
Saccades are fixed shape and latency for different sized saccades
- therefore similar motoneuron activity patterns must occur for different saccades
89
How does the latency variation change for different sized saccades
the latency varies as much for small as for large saccades
- so the same pattern of motorneuron activity is generated at different times for different similar saccades
90
Are saccades feedback movements?
Saccades are generated by a programmed FEEDFORWARD MECHANISM.
91
Which brain structure is important for foveation
superior colliculus
Through the superior colliculus, visual stimuli can produce saccades that orient the eyes to foveate the stimulus.
92
What is the basal ganglia's role in oculomotor saccades
* Output of the basal ganglia ‘brakes’ movement until a decision is made.
* The long and variable latency of saccades represents "decision" time on whether or not to make a movement
* when a movement is made it involves the same motor program, regardless of size or time.
* Evidence for a role in selection and timing of movements
93
Which part of the substantia nigra is important for eye movements
pars reticulata (non dopaminergic neurons)
94
What is smooth pursuit
when ou want to keep your eye on one moving objec
95
What would a trace of a subject's eye movements be as they follow an LED to test smooth pursuit
quite smooth with a few small saccades
96
Is smooth pursuit feedforward or feedback
feedforward
Even for slow movements the eyes are too slow to respond to allow negative feedback mechanisms to be useful
97
How does smooth pursuit change for objects moving at different speeds
* Even at modest speeds and with predictable motion the ability to keep the eye on a moving object is poor. This limits vision to identify moving objects
* For faster movements the eye uses saccades as well as smooth pursuit
98
Which part of the brain is important for smooth pursuit
cerebellum to learn a model system for prediction
99
What kind of movement is used when looking out the window
what pattern of movement is seen
OPTOKINETIC MOVEMENTS
sawtooth -> nystagmus
the eyes drift to follow the target then rest using saccades
100
Is nystagmus when looking out a train window conscious?
no and we cannot suppress it
101
What happens to the sawtooth motion of the eyes when looking out the window if the train speeds up
slope of slow movements would increase
102
What explains the shape of the optokinetic eye movements?
The eyes repetitively track the movement of a bar (slow phases) and then make a saccade to fix on a new bar (fast phases)
103
What would happen if the bars moved
| faster when you are focussing on them
The slow phases become faster (and
| the number of saccades also increases)
104
Describe eye movements when testing VOR after putting a subject on a rotating chair with a bag on their head (3)
Chair starts rotating: vestibular signals generate nystagmus with the eyes counter rotating
After about 40 seconds of continuous rotation the vestibular system adapts and the nystagmus eventually disappears
When the chair stops rotating a powerful nystagmus in the opposite direction
appears, and the subject feels as though they are spinning in the opposite direction
105
Why do vestibular generated nystagmus gets weaker over time as the chair continues to spin
the relative difference between the movement of the canals (and the receptors) and the endolymph fluid within them disappears over time as the fluid starts to move at the same speed as the head.
When the chair stops spinning the opposite occurs: the head stay still but the fluid continues spinning
106
When does vergence occur
occurs during accommodation near and far focus
Vergence movements ensure that both eyes foveate the same target, which is essential for binocular vision
107
True or false
| most humans have a dominant eye
true
| right is usually dominant
108
How do you calculate which is your dominant eye
* Sit about 1 m from the screen with spot at its centre
* fix your eyes on the red circle
* hold up a finger about 20 cm in front of your eyes
* Close one eye, then the other.
When you close one of your eyes
the image should remain stationary. When you close the other the image should jump:
this is the dominant eye
109
is vergence feedforward or feedback
explain
negative feedback
* Vergence is ERROR DRIVEN: the image is not fused and so the eyes are moved until they are
* Initial movement is fast because there is a large error
* As error decreases, movement is slower
* When the error is zero, movement stops
* Vergence is therefore a NEGATIVE FEEDBACK SYSTEM
110
What are vergence movements
aimed to maintain an image at the same location on the retina of both eyes, and are driven by retinal disparity, which represents an error. Large disparity drives faster movement, which becomes smaller as the disparity declines and disappears when the images on the 2 ret any become fused
111
What are smooth pursuit movements
s aimed to move the eye to foveate a predicted current location of a moving target, based on the "historical" information on where it has just moved. These use a feedforward mechanism based upon an internal "simulation" or model of target movement.
learning is very important.
112
Describe Vestibular ocular movements
controlled via a feedforward mechanism: movements of the head detected by the vestibular system I used to generate predictive equal and opposite movements of the eyes maintain fixation of objects in the world.
learning is very important for this form of movement
113
Why do you put a bag on the subject's head when testing VOR
removes external stimuli and prevents gaze fixing
114
How is amplitude of sound waves usually expressed
in terms of the ratio of the pressure of the source in question to that of another reference source.
20 log10 (Pressure of A in Pa /2x10-5 Pa)
115
What is the formula for dB SPL
20log(A/B)
A is in P
B is the reference so usually = 20micropascals
116
Why is 2e10^-5 P taken as the reference
In studies of hearing the reference level is taken at frequencies around 1 kHz - that can just be detected by the human ear and corresponds to a root mean square pressure fluctuation of 2 x 10-5 Pascals, so a common way of expressing the level of a sound is as a sound pressure level.
117
What does 1 Pascal=?
1 Pascal = 1 Newton / sq. m.
118
For a pure tone, what does the threshold intensity depend on
frequency
119
How do you obtain monaural hearing threshold
obtained while listening through headphones and in a sound-attenuating chamber.
120
```
monaural (one ear) thresholds were
obtained while listening through
headphones and in a sound-attenuating
chamber
What are the pros and cons of using headphones?
```
```
Advantage:
Ease of control of stimuli,
can examine one ear,
reduction in background noise,
eliminates the effects of reverberation
```
Disadvantage:
reduces the amplification produced by the external ear,
eliminates the slight increase in sensitivity from use of both ears (binaural)
121
How can you reduce bias when finding MAP to test someone's hearing threshold (2)
You could randomize the presentation of stimulus level.
You could also present no-stimulus to the listener and recording their proportion of yes-no responses would indicate any bias
122
Describe the minimum audible pressure (MAP) graph as a function of frequency (how does MAP change as frequency changes)?
(i) the lowest thresholds (best sensitivity) lie between 500 and 5000 Hz. In fact, thresholds only vary by ~10 dB between 200 and 15000 Hz.
(ii) thresholds rise rapidly beyond 15 kHz as they approach the upper limit of 20 kHz
(iii) thresholds also rise quickly as
frequency declines between 200 and 20Hz.
123
If you were testing your MAP in a practical class, how would the results differ from one in a sound attenuating chamber
your thresholds would be 20-30 dB higher.
This is presumably due to the level of background noise in the histology
classroom
124
What is the method for testing MAP thresholds with respect to frequency in the classroom (6 steps)
what is this method called
1. Set the frequency to a suitable value, then
2. decrease the intensity until you can no longer hear the sound.
3. Record this sound level.
4. Now increase the intensity until you can hear the sound once more.
5. Record this sound level.
6. The mean of these two measurements may be taken as the
threshold level
method of limits
125
What does the method of limits help to demonstrate
that the ‘threshold’ is an ill-defined quantity and shows considerable random and other kinds of fluctuations from trial to trial. For example, sensitivity can depend upon the type of stimulus,
method used in testing and cognitive factors
126
What is a source of bias in the method of limits
the predictable
| change in stimulus level (either an increase or decrease).
127
How can you reduce bias in the method of limits
You could randomize the presentation of stimulus level. You could also present no-stimulus to the listener and recording their proportion of yes-no responses would indicate any bias
128
What is a complex sound wave
any shape that is not sinusoidal (or simple)
every complex wave can be broken down into its constituent sinusoidal components (using a Fourier transform).
129
Why can the cochlear be considered an acoustic prism
the cochlea breaks down complex sounds into their constituent sinusoidal components
130
Why do sinusoidal and square waves sound different?
when do they sound the same?
The presence of odd-integer harmonics in the square wave
When you can no longer hear the harmonics e.g. when the F0 is greater than 20/3 kHz.
This means that the next component in the square wave would be > 20 kHz (3 x (20/3) ),
i.e. above the upper limit of your audible frequency range.
131
What is the equation for the graph of a square sound wave
f+(A/3)3f+(A/5)5f+(A/7)7f ...
Where 𝒇 stands for the frequency of the lowest component (aka f0 or fundamental frequency or first harmonic) and A stands for the amplitude of the lowest‐frequency component.
132
If speech is arguably the most meaningful stimulus for us why do Audiologists persist in measuring pure-tone thresholds?
Damage to our hearing is often frequency specific. A frequency specific notch would be hard to detect using a broadband stimulus such as speech. Pure tone thresholds also provide baseline information for future comparison.
Pure-tone thresholds can also be useful for:
a) establishing the type of hearing loss
b) estimating the degree of handicap
c) determining candidacy for restorative treatments e.g. hearing aids or implants
d) selecting frequency-gain levels for hearing aids
e) providing a reference level for the selection of audiological test stimuli.
133
Conventional pure tone audiometry estimates thresholds for frequencies between 125 Hz and 8000 Hz. Why?
This range is similar to the range of frequencies
| important for speech (100 Hz – 6000 Hz)
134
How are clinical thresholds plotted when measuring hearing in an audiology clinic?
as dB HL relative to the median threshold, as a function of frequency, for young adults with no history of hearing problems. In contrast to dB SPL, the reference value for dB HL varies with frequency. Positive values of dB HL indicate a threshold that is higher than that found in the reference group.
It is convention to plot this scale downwards i.e. zero indicates no loss in sensitivity.
135
What is dB HL?
dB HL =
| dB SPL threshold - frequency-specific audiometric
136
What is the equation for magnification
Actual Length = length of the Image divided by the Magnification.
137
What are the relationships found when studying sensitive spots per cm^2 for touch, pain, cold and warm (3)
touch and pain sensitive spots have a negative correlation
warm and cold have a positive correlation
there are a lot more touch spots than cold or especially warm
138
Is the distribution of sensory receptors evenly distributed throughout the body
no - Areas where fine discrimination is needed (e.g., tips of fingers, lips) have a higher density than areas where fine discrimination is not necessary (e.g., back).
139
How are pain receptors distributed?
relatively evenly
140
a) what is temporal dissociation
| b) what is the point of this
a) when two point discrimination becomes more sensitive when the 2 points are spread out in time
Even small dissociations in time can enable the brain to localize in on the presence of a stimulus
b)Not only is there a lower density of peripheral touch receptors in these areas, but the brain devotes very little real estate to interpreting touch from these parts of the body compared to the fingers and face (particularly the lips and tongue) so the brain must compensate with temporal dissociation
141
What is the advantage of better 2 point discrimination on fingertips but lower numbers of nociceptors
we can do fine discrimination tasks (e.g., threading a needle!) but if we poke ourselves with the needle, we can withdraw quickly from the painful source without needing to know the precise location.
142
How does the shoulder's sensitivity to pain compare to that for touch
why is this
The shoulder shows better two discrimination for pain than touch. Again, it is advantageous on torso and limbs to be sensitive to pain or injury. However, it’s not necessary to have fine discrimination since we can use our fingers (and other senses) to explore further.
143
There are fewer somatosensory nerve fibres in the fingertips than hand dorsum but the fingers are more sensitive. Why is this?
Cortical magnification of nociceptive signals in SI is the likely neuronal correlate of the high spatial resolution for pain on the glabrous skin of the hand. However, it remains unclear what transformation, in spinal or other areas is responsible for this.
144
What are the limitations of testing where the sensitive spots on the body are ? (2)
not specific to different somatosensory modalities
assumes 1 spot= one mechanoreceptor
145
How could you differentiate between 2 point discrimination for touch and pain?
3 different ways
1. use blood pressure cuff to cut off circulation to stop larger touch neurons but leave C fibres for pain
2. use compass for touch and laser for pain
3. use a subject who was born without touch or without pain fibres
146
What allows the brain to be more sensitive to 2 point discrimination when there is temporal dissociation
receptor pooling
147
Why is there a higher density of touch receptors in the fingertips (2)
small area so spacing has to be small
allows fine discrimination for touch
148
How does the number of somatosensory fibres for the following relate to their cortical prependicular length:
a) trunk
b) fingers
c) palm
a) number of fibres is much greater than cortical representation comparatively
b and c) cortical representation massively magnifies the amount of receptors in these areas
149
What is perceived and what are the aspects contributing to the following illusion:
get a room temp coin and 2 cold coins
place tip of your middle finger on the room temperature coin, and now place the tips of the index and ring finger of the same hand on the two cold coins
appears all 3 are cold
there are more cold receptors than warm in fingers, so dominant signal is cold
often brain recreates what we expect in the world so brain decides all 3 are cold (even ignoring warm peripheral signals)
150
What is the hopping rabbit somatosensory illusion
explain it
two regions (wrist and elbow) stimulated with a short latency between them can lead to the impression of stimulation in between, even though there is none.
we interpret this as a stimulus moving from wrist to elbow and our brain fills in the sensation in between.
151
In the somatosensory illusion with the 2 cold coins and 1 room temp one, what happens to the illusion when we use the middle finger of the other hand
what does this suggest
The illusion disappears, suggesting that the filling in occurs at an early stage of tactile information processing, not at the higher level of space representation in the brain. We know this occurs at an early stage because the sensory signals from two hands project to two separate hemispheres in the brain; information from them can be compared only at a relatively late stage of processing.
152
What is the Aristotle Illusion
Cross your left middle finger over your left index finger, making a small V at the end. close your eyes and place the V formed by the fingers on your nose - report feelings of having 2 noses
153
Explain the physiology behind the Aristotle Illusion
The brain learns to combine the information from multiple fingers with known spatial arrangement. Given the normal, habitual spatial arrangement of the fingers, the only way the left side of your left middle finger will be stimulated simultaneously with the right side of your left index finger is when they are touching two objects. So the brain interprets the tactile experience as “I must have two noses.”
The brain is used to combining the information from the two fingers in that arrangement, which may explain the illusion for people who do perceive two.
154
Why do some people not experience the Aristotle Illusion
top down processing that telling you that you do not have two noses dominates over any overlearned interpretation from the fingers as at the cortical level.
155
How do you test the effects of ischaemic nerve block experimentally
you occlude the blood flow to and from a subjects arm and hand for 30 minutes using a blood pressure cuff on the upper arm inflated above systolic pressure
You would have tested touch sensitivity, response to cold and hot (60°C) probes, muscle strength, and pain (pin-prick, or edge of a ruler). This typically resulted in a progressive loss of sensory (and motor) capability
156
What was the progression of somatosensory function loss during ischaemia
1. 1st lost was vibration, closely followed by touch and muscle strength.
2. People typically then report that pin-prick pain became more unpleasant, and the latency to perceiving the painful stimulus increased. They also found a loss of sensitivity to cold.
3. the last sensations before they removed the cuff were unpleasant. The pin-prick or simple touch began to hurt or “burn”.
many couldn’t tell the difference between the icy-cold and the 60°C probes; most people reported that both were unpleasant and felt “burning” or “hot”.
157
Why does occlusion of the arterial blood supply to somatosensory nerves cause functional loss?
Nerves rely on oxygen to function. Local ischaemia from occluding blood flow leads to nerve dysfunction
158
Explain the sequence of effects seen in ischaemic nerve block
Vibration and proprioception are carried by different nerves than pain and temperature. These fibers are different size and have different sensitivity to ischaemia (and other insults). The fibers that carry vibration are lost first because they are more susceptible.
Aα (proprioception) > Aβ (touch) > Aγ > Aδ (sharp pain) > (B) > C (dull pain)
159
During ischaemic nerve block Some report that a pin-prick, or even touch, became unpleasant and began to hurt or “burn”. How could you explain this effect?
As the nerves become ischaemic, they are not able to transmit information in the same way, making it more difficult for the brain to reconstruct and interpret the information from the periphery. Pain and temperature are carried by the same nerve fibers, likely contributing to the confusion.
160
During ischaemic nerve block, the The latency to perceive the pin prick increased to a few seconds midway through the experiment. What might account for this?
As the nerves become more ischaemic, their ability to transmit information as quickly becomes impaired. This is at least in part due to the drop in temperature in the limbs. Blood flow also brings heat. Thus, the ischaemic limb is cooler than the rest of the body.
161
During ischaemic nerve block, . Why might the sense of temperature become confused (loss of cold sense, or perceiving cold probes as hot)?
Starting from infancy and reinforced over your life, the brain learns to interpret the signals of the nerves with specific sensations. Once the nerve function and signaling is impaired, the brain has to use more top down processing (guessing) to interpret the signals coming from the periphery. In cases of extreme trauma, the brain can manufacture sensation even when the limb is not there any more! People who have had amputation can perceive pain or other sensations in the missing limb.
162
Why is hyperalgesia observed in ischaemic nerve block
as the limb becomes ischaemic there are a number of metabolic processes that change, for example pH, and activate pain fibers. It is also protective to feel pain when there is ischaemia
163
What is pins and needles when you cross your legs?
When you cross your legs, you compress the common peroneal nerve. This compression causes micro ischaemia (temporarily!) which impairs nerve function. The first sensation of nerve compression is pins and needles. If you do not uncross you legs, it can become painful or uncomfortable. This typically leads to uncrossing your legs, thus protecting the nerve.
164
Which somatosensory modalities go first when anaesthetic is applied
narrower fibres (C fibres) go first as they require less anaesthetic to be numbed
165
Give the most effective stimulus for each type of mechanoreceptor
Meissner's corpuscles (RA I afferents): respond best to low frequency vibration
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Merkel cells (SAI afferents): sensitive to points, edges and curvature
and can resolve spatial detail of 0.5 mm
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Ruffini endings (SAII afferents): stretching
Pacinian corpuscles: high frequency vibrations (~200Hz)
166
Describe the set up for stimulating the ulnar nerve
wo surface electrodes affixed to the skin over the belly of the muscle. The signal from these electrodes passes to the isolated amplifier (BioAmp) of the computer.
To stimulate the nerve, you require both a movable stimulating electrode, which can be placed at varying positions along the course of the nerve, and an indifferent electrode to complete the circuit for current flow. A variable stimulus current is provided by the isolated stimulator of the computer: this is adjusted until it is sufficient to raise all of the nerve fibres supplying motor units in the muscle above threshold
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4 electrodes:
-1 positive, 1 negative pole for
EMG
-1 positive, 1 negative pole for
stimulater
• All need to conduct well with
skin
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167
What is important to remember about the inputs and outputs for the ulnar stimulation practical? (safety)
It is important that these inputs and outputs are isolated electrically from the rest of the computer, so as to eliminate any possibility that dangerous ("mains") voltages might be conducted to (and through) the subject!
168
How do you reduce pain in the ulnar nerve practical?
maximize skin contact and use a conducting gel
169
Why can the electrodes on the abductor muscle not overlap in the ulnar nerve prac
so they don't short circuit
170
In the ulnar nerve prac, which electrode is the anode
indifferent electrode = anode
stimulating == cathode
171
How do you perform the ulnar nerve prac
find the ulnar nerve by stimulating different areas around the 'funny bone' until the largest response is found
stimulate nerve with progressively increasing amplitude (max ~20mA for good response) and record response of abductor digiti minimi
172
abductor digiti minimi's response to increasing amplitude stimulation of the ulnar nerve increases. why is this?
What eventually happens?
represents the progressive recruitment of additional motor nerve fibres and therefore motor units in the muscle.
Ultimately the EMG response saturates once all motor units have been recruited.
173
How do you measure nerve conduction velocity from an EMG ?
find the distance between 2 peaks on the EMG, this is time
divide distance between electrodes by this time to find velocity
(you cannot use a single measurement bc there is a fixed time delay for the AP and muscle activation etc)
174
How do you record the Achilles tendon jerk
subject stand on 1 leg and stretch other bare leg backwards, resting on a stool
place pair of recording electrodes over soleus/ gastrocnemius
record EMG for voluntary plantarflexion
strike tendon and record for 100ms
175
What time base is used when recording a voluntary flexion EMG before testing the Achilles tendon jerk
why?
To check the magnitude of the voluntary EMG when the subject pushes with their toes, a slow timebase is used together with a reduced input voltage range, since motor units will not be firing synchronously.
176
How do you measure the latency of the Achilles tendon jerk
by measuring the time delay between the stimulus and the initial rise of the EMG: remember that this time the stimulus is the blow of the tendon hammer at the start of the sweep.
177
What time sweep do you use when testing voluntary abduction of the little finger?
what about when it is stimulated?
record for 10s (v long)
25ms
178
What is the Jendrassik Maneuver
If the subject clasps their hands together and pulls hard, the magnitude of the Achilles tendon jerk is increased
Clenching the jaw can enhance the effect further. The effect is maximised if the subject pulls dynamically just before the hammer strikes the tendon
179
What did people used to believe explained the Jendrassik maneuver
how has recent evidence affected this
the enhancement represented the withdrawal of descending inhibition to the motor neuron pools mediating the reflex responses of the lower limb when performing a task with the upper limb to enhance stability.
rule out effects mediated by the muscle spindle gamma efferents, or changes in excitation or presynaptic inhibition to the motor neurons themselves.
180
What is an alternative theory for the Jendrassik maneuver
enhancement might result from modulation of oligosynaptic pathways, containing more than one synapse, that may contribute to the recruitment of motor neurons with higher thresholds in this largely monosynaptic reflex response.
181
How can you demonstrate the Hoffman reflex
evoke a reflex response similar to the tendon jerk by direct electrical stimulation of the muscle nerve. This can best be done by stimulation of the nerve to the medial head of the gastrocnemius muscle in the lower part of the popliteal fossa.
computer is set to immediate triggering and the isolated stimulator enabled, while retaining the long sweep duration
182
What is seen on the EMG for the Hoffman reflex
M wave: due to stimulation of efferent motor fibres, which directly innervate the muscle
H wave: at a greater latency following stimulation a reflex
caused by activation of afferent sensory fibres in the muscle nerve, which is conducted via the monosynaptic spinal reflex arc
183
What is the Hoffman reflex essentially
the electrically-evoked equivalent of the tendon jerk
184
When is there a stretch reflex
Whenever a muscle is contracted/ shortened without our consent
185
How does the H wave change as stimulus strength increases
why is this
although increasing the strength of the electrical stimulus increases the amplitude of the M wave, it paradoxically decreases the amplitude of the H wave
occlusion
186
Explain the occlusion which causes h wave to decrease with increasing stimulus strength
antidromic conduction along
motor fibre means efferent motor nerve is refractory
at point of being activated in monosynaptic reflex
arc
ecent evidence from a comparison of normal and spastic patients suggests that diffuse inhibition resulting from electrical stimulation of Group IB afferents (which will stimulate inhibitory interneurons) or from Renshaw cells relaying feedback inhibition from efferent axon collaterals may also play a role
187
Why does occlusion not always stop the Hoffman reflex
If the shock is strong enough then all the motor axons will be recruited (hence a large M wave) and all the alpha motor neurons will be refractory at the moment they need to relay the reflex (hence a small or absent H wave). For smaller stimuli, the largest axons will be the first to be excited (since they have a low threshold, so are easy to stimulate electrically), but will be the last to be recruited by the reflex according to the Size Principle (since their cell bodies have a low input impedance, so are hard to excite synaptically).
188
Why do human hand muscles exhibit a double reflex (spinal latency stretch reflex and a second reflex response to stretch after a greater delay)?
1st is a spinal reflex
2nd has long latency believed to arise from a reflex arc which ascends to the cerebral cortex, and then descends again via the pyramidal tract to excite the motor neurones supplying the stretched muscle
189
How can you record long latency reflexes
by rapidly stretching the first dorsal interosseus, while recording the EMG response
muscle is stretched using a pneumatic cylinder gripped between the thumb and forefinger. The computer opens a solenoid-operated valve controlling the supply of compressed air, causing the piston to extend and forcing the thumb and forefinger apart. The position of the piston is measured using a magnetic sensor attached to the side of the pneumatic cylinder, and is recorded by PowerLab.
190
What must you remember about the EMG when demonstrating long latency reflexes
As this response is relatively small, it is necessary to average the EMG signal over a number of presentations of the stimulus so as to obtain a reliable recording
peaks are both positive and negative so would cancel out - negatives must be inverted to positive
this is called full wave rectification
191
What will the final EMG for testing long latency reflex responses look like
The rectified averaged EMG response to stretch consists of two components rather than just one, the first, M1, at a spinal latency and the second, M2, after a longer latency resulting from the additional conduction delay to and from the cortex.
192
t was once believed that muscle spindles played no role in conscious proprioception, serving solely as feedback sensors involved in reflexes. Give evidence against this
Their role in proprioception can be demonstrated by stimulating selectively the spindle primary endings, which respond especially well to the 100 Hz vibration of a physiotherapy massager. A human subject can normally accurately bring their forefinger to the tip of their nose without touching it even with their eyes closed. But if the triceps is vibrated the the subject stops well short of their nose, indicating that the vibration makes the muscle spindles send signals which are interpreted as the triceps being more stretched (and so the elbow more flexed) than it really is.
193
calculate the dB HL for a 500 Hz pure tone, whose threshold was measured at 33.5 dB SPL
* n dB HL = pure-tone threshold –audiometric zero
* n dB HL = 33.5 – 13.5
* n dB HL = 20
194
Why does the tendon reflex have a different latency than the Hoffman reflex
tendon reflex will be shorter latency than electrically evoked "Hoffman" reflex as it directly triggers spindles and their afferents up the monosynaptic arc, whereas the electrically evoked reflex has the delay of an AP propagating along the motoneuron and triggering muscle contraction.
195
How do we know the long latency reflex involves the cortex
studies on patients with Klippel Fiel syndrome who display bimanual synkinesia display the reflex in both hands
