11.1 the stimulus-response curve

Question 1

why do we measure accommodation ?

Answer 1

• important for studying the dynamics of accommodation

- important for studying the performance of presbyopic corrections that claim to produce accommodation

Question 2

what do auto refractors do ?

Answer 2

autorefractors measure the refractive status of the eye without trail case lens

Question 3

how can you tell you have free-space auto refractor ?

Answer 3

• at the top you have some kind of window to look through
• the window is an infrared mirror which allows the instrument to shine infrared light into the eye to measure the refractive error but allows the subject to look through this fairly clear window and look at different targets at different distances
• this allows the instrument measure the refractive state of the eye as target is moved to different distances and that allows us to measure the accommodative response of eye for targets at different distances

Question 4

explain the typical response curve for accommodation ?

Answer 4

• x-axis we plot the accommodative stimulus (D) and as we go from left to right the actual target is getting closer to the eye so the accommodative stimulus is increasing
• as number increases target is getting closer
• y-axis we plot the accommodative response (D) measured objectively using an auto refractor
• the solid curve is response of an eye as we change the target position from infinitely distant to closer

Question 5

explain the 1:1 response line in the stimulus-response curve for accommodation ?

Answer 5

• it is the response the eye would make if it exactly tracks the target
• provided the graph is square , this line is at 45deg
• this means that if target moved to a distance 1/2 m from the eye , the accommodation to completely focus that on retina is 2D
• if target moved to 1/4 of D the accommodation will be 4D

Question 6

explain the typical response curve in the stimulus-response curve for accommodation ?

Answer 6

• we can interpret this by comparing to 1:1 response
• accommodative lead or over accommodation can be seen for distant objects
• accommodative lag or under accommodation can be seen for near this means that we are not accommodating enough to track down the object-

Question 7

why can we get away with accommodative lead and/or accommodative lag ?

Answer 7

• our eyes have a depth of focus (i.e. range of positions for which targets appear to be clear )
• this depth of focus depends on what you’re looking at ( i.e.contrast of target )

Question 8

what is resting state ?

Answer 8

• we noticed from stimulus response curve cross-over between parasympathetic and sympathetic dominance is the resting state
• insufficient stimulus to drive accommodation in one direction or other whether it being near or far

Question 9

what is accommodation driven by ?

Answer 9

accommodation id driven by the parasympathetic innervation of the ciliary muscle which comes from the third nerve (cranial nerve |||)

Question 10

what happens when looking at near objects ?

Answer 10

we need to have parasympathetic innervation which causes ciliary muscle contraction and lens takes on a more convex form

Question 11

what indicates tonic accommodation ?

Answer 11

• curve cross-over between parasympathetic and sympathetic dominance is the resting state which traditionally indicates tonic accommodation ( not completely correct

Question 12

what is tonic accommodation ?

Answer 12

• accommodation you get where there is random firing of parasympathetic and sympathetic systems and therefore they balanced out each other

Question 13

what is inadequate stimulus myopia ?

Answer 13

• without sufficient stimulus to drive accommodation ( target lacks detail or sufficient contrast ) accommodation drifts towards a tonic level
• this is governed by low-level parasympathetic innervation

Question 14

what is the cross over point on curve for tonic accommodation (a normal stimulus response curve) ?

Answer 14

• cross-over point on the curve is around 1 to 2.5D - variable between people but tonic accommodation is less

Question 15

what is resting state theory ?

Answer 15

• accommodation acts from an intermediate resting-state
• there needs to be some disaccommodation for resting state if looking at distant object and there needs to be some accommodation starting from resting state when looking at near objects
• this suggests that accommodation us economical only departing from the resting-state just enough to give satisfactory vision

Question 16

what is the lazy theory of accommodation ?

Answer 16

• accommodation system just does enough for us to get satisfactory vision that depends the nature of target , contrast , detail
• system will not over - exert it self

Question 17

when is there a lack of complete disaccommodation ?

Answer 17

there is a lack of complete disaccommodation (reducing effect of accommodation) for distant vision this means that we don’t get to an accommodative response of 0

Question 18

what does lack of complete disaccommodation require ?

Answer 18

lack of complete disaccommodation requires sympathetic innervation

Question 19

what are the 4 inadequate stimulus myopia ?

Answer 19

• this is when eye drifts towards myopia under different conditions ( consequence of the stimulus response curve)
  1. night myopia - as light levels fall , our receptors switch from cones to using rods and cons and rods if light levels are low enough , we loose colour vision , contrast of object is less and pupil size is larger

2. dark-field myopia - myopic shift we get if we refracted somebody under normal photopic conditions compared to when we refracted them in the dark using infrared light we expect myopic shift - can be from -1 to -4D
3. ganzfeld ( empty-field ) myopia - effect where we just have no stimulus to accommodation , there is no detail or contrast that visual cortex can interpret and therefore use to drive accommodative system
4. instrument myopia - when we use instruments there tend to be a myopic shift and although this was initially thought to be proximal accommodation , there are some other factors
   - no adequate stimulus to accommodation

. all involve a drift to myopia cause by an inadequate stimulus to drive accommodation 
this can happen:
- at night 
- in fog 
- flying above clouds

Question 20

what does accommodation not do ?

Answer 20

• accommodation does not track the stimulus position

- doesn’t follow the 1:1 stimulus response line

Question 21

what does accommodation do ?

Answer 21

• exerts sufficient effort to achieve satisfactory vision - we get :
• we get over accommodation or accommodation lead for distance
• we get accommodative lag or under accommodation for near

Question 22

how does accommodation only exert sufficient effort to achieve satisfactory vision ?

Answer 22

• it does this by responding to the detail in the object and using the depth of focus of the eye-

Question 23

what can the eye do when there is inadequate stimulus to accommodate ?

Answer 23

• eye can become myopic with inadequate stimulus to accommodate