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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

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2
Q

why is the visual field asymmetrical

A

angle of orbit + nose

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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)

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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

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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

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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)

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7
Q

tanx=

sinx=

cosx=

A

opp/adj

opp/hyp

adj/hyp

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8
Q

How to find the size of the blind spot

A

17tan(angle that subtends blind spot)

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9
Q

What is the direction of light in a histological section of the retina

A

from vitreous to choroid

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10
Q

What do the pigment epithelium do

A

regenerate rhodopsin

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11
Q

What is in the innermost axon layer of the retina

A

axons that head to the blind spot

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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

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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

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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

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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.

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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.

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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).

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18
Q

What should you do if the eye fixation quality is poor (below 90%) when testing visual field

A

repeat measurements

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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

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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

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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

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22
Q

What does binocular overlap allow

A

stereopsis

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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

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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

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25
Q

What is the optic disc

A

the optic nerve head / blind spot

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26
Q

Where is the blind spot usually on the retina

A

16 degrees from fovea along the horizontal meridian

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27
Q

If the blind spot is ‘blind’ why is there any sensitivity recorded

A

due to effects of small eye movements

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28
Q

Which type of animal tends to have binocular overlap in front and a posterior blind area

A

predators

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29
Q

How do the visual fields of prey differ from predators

A

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

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30
Q

Do rabbits have any visual field overlap

how does this differ in horses

A

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

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31
Q

True or false

a horse cannot see in front of it when its head is pulled back to vertical

A

true

because of the profound downward tilt of its eyes

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32
Q

What is visual acuity

A

how good the visual system is at resolving fine spatial detail

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33
Q

How is the critical size of an image specified

A

in terms of the size of its retinal image or the angle that the object subtends at the front of the eye

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34
Q

Give 3 things which affect the visibility of an object

A

depends largely upon the size of its image on the retina (it also depends on such things as its brightness and contrast)

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35
Q

How do you calculate retinal size

A

Retinal size = 17000.tan(acuity)

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36
Q

How far is the lens from the optics of the eye

A

17mm

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37
Q

What is the diameter of the eye

A

20 mm

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38
Q

Describe the structure of the eye ball

A

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.

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39
Q

Between which layers of the eye does the neural retina lie

A

between pigment epithelium and the vitreous

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40
Q

How does the retina change in size across the eye

what does retinal thickness reflect

A

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

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41
Q

What is the diameter of the fovea

A

1.5mm

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42
Q

What is the centre of the fovea called

How big is it

what is the purpose

A

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

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43
Q

Why does the retina change in thickness

A

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

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44
Q

Why does the fovea seem v thin

A

nearly all cell layers are displaced to the side

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45
Q

True or false

there are no cones in the foveola

A

false

there are no rods, only cones

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46
Q

What is the macula

A

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.

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47
Q

Are there 3 layers in the peripheral retina?

A

Yes

but the innermost ganglion cell layer is sparse

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48
Q

How can the retina become detached in life

how is it treated

A

bang to head leading to localised blindness

remedied by “gluing” the retina back onto the epithelium.

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49
Q

What forms the outer nuclear layer of the retina

what does this lead to

A

nuclei of the receptors (cone/rod)

receptors synapse onto bipolar and horizontal cell dendrites in the outer plexiform layer

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50
Q

What forms the inner nuclear layer of the retina

A

nuclei of interneuron cells and amacrine cells

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51
Q

What forms the inner plexiform layer

A

where the bipolar and amacrine cells synapse with each other and ganglion cells

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52
Q

What are Muller’s cells

A

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

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53
Q

What is the inner limiting membrane of the retina

A

a layer made of the dilated ends of Müller’s cells which separates the ganglion layer from the vitreous humour

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54
Q

Where are the blood vessels in the eye

A

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!

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55
Q

Can we see the blood vessels in the eyeballs

A

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.

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56
Q

describe the thickness of the innermost layer of ganglion cell axons right next to the vitreous

A

thicker than periphery because all the 1.5 million ganglion cell axons converge on the optic disk.

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57
Q

If the blind spot in the visual field is 4-6 degrees, how can we calculate the size of the optic nerve head

A

retinal mm = 17 tan(blind spot degrees)
=17tan(4)
=~1.19mm

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58
Q

If the optic disc is 1.45mm on the retina, what is the size of the blind spot

A

blind spot degrees = inv.tan(optic disk mm/17)

= inv.tan(1.45/17) = 4.88

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59
Q

How is visual acuity determined in the fovea

does this apply to the whole retina

A

determined largely by the size and spacing of the cone tips

no cones are almost exclusively in the central retina

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60
Q

What is human visual acuity in ideal conditions (give in minutes and then the equivalent in microns)

A

0.5 minutes (30 seconds)

In a 17mm diameter eyeball (corresponding to the primate eye), this translates to 2.47 microns.

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61
Q

Can we estimate the width of a cone tip in the class by just counting cone nuclei in a slide?

A

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

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62
Q

How can you estimate the width of a cone tip in the retina

A

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

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63
Q

What does the eye need to do to focus light on the retina

A

refract light at curved borders between 2 media with different refractive indices

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64
Q

What is responsible for refraction of light in the eye

A 
cornea = 2/3
lens = 1/3
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65
Q

Why is the cornea responsible for most of the light refraction

A

it is highly curved

big difference in refractive index between air and cornea (1:1.33)

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66
Q

What happens to the lens when you accommodate a near object

A

bulges to give extra power

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67
Q

Why can humans not see properly underwater (without goggles)

A

water and cornea have same refractive index so light is not refracted

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68
Q

Is the human eye diurnal or nocturnal

A

it is a typical diurnal eye with some nocturnal ability

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69
Q

How does a diurnal eye differ from a nocturnal one

A

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’

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70
Q

What does pupil area determine

what is the usual size

A

how much light can enter the eye

2-8mm diameter (

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71
Q

What happens to the photons that miss the cones/rods

A

absorbed by the black pigment epithelium

their scatter back to the wrong receptors would degrade the sharpness of the image

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72
Q

How does convergence differ between cats and humans

A

cats: millions of rods/cones onto 200,000 ganglion cells
human: converge onto 1.6 million ganglion cells

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73
Q

What is the Stiles Crawford effect

A

the ellipsoids of cones act as a wave guide, reducing cone absorption of oblique rays allowing cones to see a sharper retinal image

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74
Q

How does the distribution of cones in the fovea vary in the human eye

A

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

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75
Q

How can visual acuity be measured

A

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

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76
Q

Why can you use cycles of black and white stripes to test visual acuity

A

good acuity requires retinal image to be sharp so stripes don’t blur into each other

needs a small pupil

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77
Q

Which cells are used for visual acuity tests

A

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

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78
Q

When is acuity poor

why

A

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

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79
Q

Why is acuity better in bright light

A

pupil is small, reducing spherical and chromatic aberration

if the pupil gets too small diffraction around the edge becomes noticeable, degrading the image

80
Q

What is the best compromise with pupil size

A

diameter of 2mm (compromise as decreasing size decreases aberration but increases diffraction)

81
Q

What is the max acuity a human can image

A

60c/deg

82
Q

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?

A

270 micrometres

120

1

the foveola can send 120 samples to the brain per deg

83
Q

How can we measure eye movements?

A
  • 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
Q

How does your eye move when reading

is this feedforward or feedback

A

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
Q

How do eye movements change when the writing is smaller/ denser

A

more jumps/ more individual fixation points

86
Q

How does the shape of a saccade change as the size of the movement changes

A

the shape of the saccade is the same for small and large movements

87
Q

If you were comparing the latencies for different amplitude saccades, what statistical test would you use

A

use a t-test. This is continuous data (each saccade can

take any value of latency along a continuous axis)

88
Q

How does the shape and latency change for different sized saccades

what does this suggest

A

Saccades are fixed shape and latency for different sized saccades

  • therefore similar motoneuron activity patterns must occur for different saccades
89
Q

How does the latency variation change for different sized saccades

A

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
Q

Are saccades feedback movements?

A

Saccades are generated by a programmed FEEDFORWARD MECHANISM.

91
Q

Which brain structure is important for foveation

A

superior colliculus

Through the superior colliculus, visual stimuli can produce saccades that orient the eyes to foveate the stimulus.

92
Q

What is the basal ganglia’s role in oculomotor saccades

A
  • 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
Q

Which part of the substantia nigra is important for eye movements

A

pars reticulata (non dopaminergic neurons)

94
Q

What is smooth pursuit

A

when ou want to keep your eye on one moving objec

95
Q

What would a trace of a subject’s eye movements be as they follow an LED to test smooth pursuit

A

quite smooth with a few small saccades

96
Q

Is smooth pursuit feedforward or feedback

A

feedforward

Even for slow movements the eyes are too slow to respond to allow negative feedback mechanisms to be useful

97
Q

How does smooth pursuit change for objects moving at different speeds

A
  • 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
Q

Which part of the brain is important for smooth pursuit

A

cerebellum to learn a model system for prediction

99
Q

What kind of movement is used when looking out the window

what pattern of movement is seen

A

OPTOKINETIC MOVEMENTS

sawtooth -> nystagmus
the eyes drift to follow the target then rest using saccades

100
Q

Is nystagmus when looking out a train window conscious?

A

no and we cannot suppress it

101
Q

What happens to the sawtooth motion of the eyes when looking out the window if the train speeds up

A

slope of slow movements would increase

102
Q

What explains the shape of the optokinetic eye movements?

A

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
Q

What would happen if the bars moved

faster when you are focussing on them

A

The slow phases become faster (and

the number of saccades also increases)

104
Q

Describe eye movements when testing VOR after putting a subject on a rotating chair with a bag on their head (3)

A

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
Q

Why do vestibular generated nystagmus gets weaker over time as the chair continues to spin

A

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
Q

When does vergence occur

A

occurs during accommodation near and far focus

Vergence movements ensure that both eyes foveate the same target, which is essential for binocular vision

107
Q

True or false

most humans have a dominant eye

A

true

right is usually dominant

108
Q

How do you calculate which is your dominant eye

A
  • 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
Q

is vergence feedforward or feedback

explain

A

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
Q

What are vergence movements

A

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
Q

What are smooth pursuit movements

A

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
Q

Describe Vestibular ocular movements

A

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
Q

Why do you put a bag on the subject’s head when testing VOR

A

removes external stimuli and prevents gaze fixing

114
Q

How is amplitude of sound waves usually expressed

A

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
Q

What is the formula for dB SPL

A

20log(A/B)

A is in P
B is the reference so usually = 20micropascals

116
Q

Why is 2e10^-5 P taken as the reference

A

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
Q

What does 1 Pascal=?

A

1 Pascal = 1 Newton / sq. m.

118
Q

For a pure tone, what does the threshold intensity depend on

A

frequency

119
Q

How do you obtain monaural hearing threshold

A

obtained while listening through headphones and in a sound-attenuating chamber.

120
Q 
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?
A 
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
Q

How can you reduce bias when finding MAP to test someone’s hearing threshold (2)

A

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
Q

Describe the minimum audible pressure (MAP) graph as a function of frequency (how does MAP change as frequency changes)?

A

(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
Q

If you were testing your MAP in a practical class, how would the results differ from one in a sound attenuating chamber

A

your thresholds would be 20-30 dB higher.
This is presumably due to the level of background noise in the histology
classroom

124
Q

What is the method for testing MAP thresholds with respect to frequency in the classroom (6 steps)

what is this method called

A
  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
Q

What does the method of limits help to demonstrate

A

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
Q

What is a source of bias in the method of limits

A

the predictable

change in stimulus level (either an increase or decrease).

127
Q

How can you reduce bias in the method of limits

A

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
Q

What is a complex sound wave

A

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
Q

Why can the cochlear be considered an acoustic prism

A

the cochlea breaks down complex sounds into their constituent sinusoidal components

130
Q

Why do sinusoidal and square waves sound different?

when do they sound the same?

A

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
Q

What is the equation for the graph of a square sound wave

A

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
Q

If speech is arguably the most meaningful stimulus for us why do Audiologists persist in measuring pure-tone thresholds?

A

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
Q

Conventional pure tone audiometry estimates thresholds for frequencies between 125 Hz and 8000 Hz. Why?

A

This range is similar to the range of frequencies

important for speech (100 Hz – 6000 Hz)

134
Q

How are clinical thresholds plotted when measuring hearing in an audiology clinic?

A

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
Q

What is dB HL?

A

dB HL =

dB SPL threshold - frequency-specific audiometric

136
Q

What is the equation for magnification

A

Actual Length = length of the Image divided by the Magnification.

137
Q

What are the relationships found when studying sensitive spots per cm^2 for touch, pain, cold and warm (3)

A

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
Q

Is the distribution of sensory receptors evenly distributed throughout the body

A

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
Q

How are pain receptors distributed?

A

relatively evenly

140
Q

a) what is temporal dissociation

b) what is the point of this

A

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
Q

What is the advantage of better 2 point discrimination on fingertips but lower numbers of nociceptors

A

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
Q

How does the shoulder’s sensitivity to pain compare to that for touch

why is this

A

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
Q

There are fewer somatosensory nerve fibres in the fingertips than hand dorsum but the fingers are more sensitive. Why is this?

A

Cortical magnification of nociceptive signals in SI is the likely neuronal correlate of the high spatial resolution for pain on theglabrousskin of the hand. However, it remains unclear what transformation, in spinal or other areas is responsible for this.

144
Q

What are the limitations of testing where the sensitive spots on the body are ? (2)

A

not specific to different somatosensory modalities

assumes 1 spot= one mechanoreceptor

145
Q

How could you differentiate between 2 point discrimination for touch and pain?

3 different ways

A
  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
Q

What allows the brain to be more sensitive to 2 point discrimination when there is temporal dissociation

A

receptor pooling

147
Q

Why is there a higher density of touch receptors in the fingertips (2)

A

small area so spacing has to be small

allows fine discrimination for touch

148
Q

How does the number of somatosensory fibres for the following relate to their cortical prependicular length:

a) trunk
b) fingers
c) palm

A

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
Q

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

A

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
Q

What is the hopping rabbit somatosensory illusion

explain it

A

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
Q

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

A

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
Q

What is the Aristotle Illusion

A

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
Q

Explain the physiology behind the Aristotle Illusion

A

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
Q

Why do some people not experience the Aristotle Illusion

A

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
Q

How do you test the effects of ischaemic nerve block experimentally

A

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
Q

What was the progression of somatosensory function loss during ischaemia

A
  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 stimulusincreased. 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
Q

Why does occlusion of the arterial blood supply to somatosensory nerves cause functional loss?

A

Nerves rely on oxygen to function. Local ischaemia from occluding blood flow leads to nerve dysfunction

158
Q

Explain the sequence of effects seen in ischaemic nerve block

A

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
Q

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?

A

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
Q

During ischaemic nerve block, the The latency to perceive the pin prick increased to a few seconds midway through the experiment. Whatmight account for this?

A

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
Q

During ischaemic nerve block, . Why might the sense of temperature become confused (loss of cold sense, or perceiving cold probes as hot)?

A

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
Q

Why is hyperalgesia observed in ischaemic nerve block

A

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
Q

What is pins and needles when you cross your legs?

A

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
Q

Which somatosensory modalities go first when anaesthetic is applied

A

narrower fibres (C fibres) go first as they require less anaesthetic to be numbed

165
Q

Give the most effective stimulus for each type of mechanoreceptor

A

Meissner’s corpuscles (RA I afferents): respond best to low frequency vibration

Merkel cells (SAI afferents): sensitive to points, edges and curvature 
and can resolve spatial detail of 0.5 mm

Ruffini endings (SAII afferents): stretching

Pacinian corpuscles: high frequency vibrations (~200Hz)

166
Q

Describe the set up for stimulating the ulnar nerve

A

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

4 electrodes:
-1 positive, 1 negative pole for
EMG
-1 positive, 1 negative pole for
stimulater 
• All need to conduct well with
skin
167
Q

What is important to remember about the inputs and outputs for the ulnar stimulation practical? (safety)

A

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
Q

How do you reduce pain in the ulnar nerve practical?

A

maximize skin contact and use a conducting gel

169
Q

Why can the electrodes on the abductor muscle not overlap in the ulnar nerve prac

A

so they don’t short circuit

170
Q

In the ulnar nerve prac, which electrode is the anode

A

indifferent electrode = anode

stimulating == cathode

171
Q

How do you perform the ulnar nerve prac

A

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
Q

abductor digiti minimi’s response to increasing amplitude stimulation of the ulnar nerve increases. why is this?

What eventually happens?

A

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
Q

How do you measure nerve conduction velocity from an EMG ?

A

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
Q

How do you record the Achilles tendon jerk

A

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
Q

What time base is used when recording a voluntary flexion EMG before testing the Achilles tendon jerk

why?

A

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
Q

How do you measure the latency of the Achilles tendon jerk

A

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
Q

What time sweep do you use when testing voluntary abduction of the little finger?

what about when it is stimulated?

A

record for 10s (v long)

25ms

178
Q

What is the Jendrassik Maneuver

A

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
Q

What did people used to believe explained the Jendrassik maneuver

how has recent evidence affected this

A

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
Q

What is an alternative theory for the Jendrassik maneuver

A

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
Q

How can you demonstrate the Hoffman reflex

A

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
Q

What is seen on the EMG for the Hoffman reflex

A

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
Q

What is the Hoffman reflex essentially

A

the electrically-evoked equivalent of the tendon jerk

184
Q

When is there a stretch reflex

A

Whenever a muscle is contracted/ shortened without our consent

185
Q

How does the H wave change as stimulus strength increases

why is this

A

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
Q

Explain the occlusion which causes h wave to decrease with increasing stimulus strength

A

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
Q

Why does occlusion not always stop the Hoffman reflex

A

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
Q

Why do human hand muscles exhibit a double reflex (spinal latency stretch reflex and a second reflex response to stretch after a greater delay)?

A

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
Q

How can you record long latency reflexes

A

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
Q

What must you remember about the EMG when demonstrating long latency reflexes

A

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
Q

What will the final EMG for testing long latency reflex responses look like

A

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
Q

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

A

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
Q

calculate the dB HL for a 500 Hz pure tone, whose threshold was measured at 33.5 dB SPL

A
  • n dB HL = pure-tone threshold –audiometric zero
  • n dB HL = 33.5 – 13.5
  • n dB HL = 20
194
Q

Why does the tendon reflex have a different latency than the Hoffman reflex

A

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
Q

How do we know the long latency reflex involves the cortex

A

studies on patients with Klippel Fiel syndrome who display bimanual synkinesia display the reflex in both hands

NHB (17 decks)

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