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161 Cards in this Set
- Front
- Back
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Method of accommodation for fish?
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move lens closer to cornea for near vision; draw lens back toward retina for distant vision
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Method of accommodation for birds and reptiles?
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squeeze lens into pear shape with iris for near vision. In birds, cornea also bulges forward
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Method of accommodation for primates, humans and mammals?
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alter curvature of lens by a passive mechanism
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Method that obviates accommodation for deep-sea fish?
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tubular eyes with a double retinae - one set for distance and one for near vision
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Method that obviates accommodation for squids, cuttlefish, octopi?
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elongated rods and cones which increase depth of focus
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Method that obvitaes accommodation for bats?
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conical elevations of the choroid and retina allowing them to choose which visual cells will be used to view an image
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Method that obviates accommodation for horses, rays and skates?
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have RAMP retinae; the distance from the cornea to the retina varies regularly, alter position of eye/head
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Method that obviates accommodation for cats and seals?
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irides have stenopeic slits which increae depth of focus
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Hitorical Perspective: Scheiner?
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demonstrates existance of accommodation using pinholes
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Historical Perspective: Young?
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Shoes accomm does NOT result from change in cornea or length of eyebal
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Historical Perspective: Weber?
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Suggests that tonic accommodation (resting state of eye) does NOT coincide with infinity but with some near position
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Historical Perspective: Helmholtz?
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using Purkinje images proves accomm occurs via change in shape of lens
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Hofstetter's Formula for Max expected amplitude?
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25-0.4(age)
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Hofstetter's Formula for Average expected amp?
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18.5-0.3(age)
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Hofstetter's Formula for Min expected amplitude?
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15-0.25(age)
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Which accommodation measurement procedures are subjective?
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(5) Donders Push-Ups, Minus Lens, Accomm Flippers Facility, Scheiner Principle, badal System, Laser Optometer
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What is state of lag/lead for a target at optical infinity?
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lead 0.5-0.75 D
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What is state of lag/lead for a target beyond tonic/resting level?
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lead
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What is state of lag/lead for a target closer than tonic level?
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lag
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What is state of lag/lead for a target between 25-40 cm?
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lag 0.5-0.75
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What is state of lag/lead as attention/cognitive demand increases?
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decreased lag, more lead
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How is reading rate effected by an increased lag?
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reading rate decreases
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The point where an individual's response exactly equals is stimulus is called:
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the tonic level (because lead and lag are accounted for by the eye's depth of focus)
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Clinicials measure ____ whereas autorefractos measure _____
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Clinicians measure stimuls to accom, autorefractors measure the actual accomm response
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What is hysteresis?
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The constant stimulation of the accomm system reduces the ability to disaccommodate, thereby mimicking a myopic RE
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What is instrument myopia?
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myopia associated with the use of an optical instrument due to decreases field of view and small exit pupil
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What is night myopia?
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myopia due to limited illumination; contrast sensitivity decreases and photoreceptors shift from photopic to mesopic and scotopic lelvels.
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What is the correction for night myopia?
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Rx half the tonic level (if tonic level is measured at 1.00 D, Rx -0.50 D over distance Rx
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What is empty field myopia?
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myopia due to total darkness or Ganzfeld (a light field with NO variations in color, contrast or illumination) causes the accomm system to move to the resting level
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What is Blurout myopia?
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excessive fogging (too much plus) during the eye exam that causes the accomm system to move to resting level
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What happens to the accomm response for targets closer than near point of eye?
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accomm remains at a maximum or may decreases
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What happens to the accomm response for virtual targets beyond infinity?
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accomm response goes to tonic resting level as image blur increases
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What does dynamic ret evaluate?
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Accomm response with respect to accomm demand of a target
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How do you conduct dynamic ret?
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only need to perform in one meridian since accomm changes the power of all meridians equally
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What is the conjugate focus equation?
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T=RE+A+L
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The illumination system of the retinsocpe deals with light ____
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light that enters the patient's eye
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The observation system of the retinscope deasls with light ____
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light that emerges from the patient's eye
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The reflex observed with ret is due to reflections off which retinal layer?
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at the interface of the pigmented epithelium and Bruch's membrane
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Static ret vs. Dynamic Ret?
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Static: assesses RE, Dynamic: accommodative response with respect to the accommodative demand
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2 types of ret?
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Spot and Streak
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Which type of ret is easier to learn?
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Streak
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Which type of ret is easier to perform, especially when time is short?
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Spot
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In a concave mirror, where is the focal length in relation to the source?
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The source is more than 1 focal length from the condensing lens
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When re-imaged by the mirror, where does the source 'appear' to be in concave mode?
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The source appears to be between the patient and examiner (hence why the motions are 'opposite the plane mirror mode')
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IN what situations is the concave mirror position helpful?
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to distinguish very high myopia from very high hyperopia
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In plane mirror mode, where is the focal length in relation to the source
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The source is just within 1 focal length of the condensing lens
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when re-imaged by the mirror, where does the source 'appear' to be in plane mirror mode?
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the source appears to be 1 m or more behind the examiner
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With no accommodation or lenses used, where is the conjugate focus of the patient's eye?
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the farpoint
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The apparent motion (in ret) depends on 2 things?
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1. conjugate focus of patients eye 2. nodal point of examiner's eye
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Perceived motion when the conjugate focus is in between the pt. and examiner?
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against motion
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Perceived motion when conj. Focus is behind the examiner?
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With motion
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Perceived motion when conjugate focus is at nodal point of examiner?
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neutrality, maximum brightness, reflex moving with INFINITE speed
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In moderate-high myopia, what are 2 ways in which neutrality can be achieved?
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1. examiner changes working distance until points coincide, 2. via a minus lens, which moves the conjugate focus point toward examiner.
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In low myopia, emmetropia, hyperopia, how is neutrality achieved?
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1. plus lenses to move conj focus toward examiner, 2. patient can accommodate to move conj focus
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Is the working distance a negative or positive value?
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Always negative, since the examiner is always IN FRONT OF the patient.
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What 2 factors are depth of focus determined by?
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1. pupil size, 2. blur tolerance (which is dependent on the eye's VA's and the target's characteristics- such as size, brightness and contracst)
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What is the advantage of a larger working distance?
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The error in calculating RE is very minimal, compared to if you were at a closer distance.
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What is disaccommodation?
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the act of decreasing total of RE of the eye to focus distant objects on the retina
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Is disaccommodation an active or passive process?
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Active, and not merely the relaxation of accommodation (even though the anatomical mechanism is passive)
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What is tonic accomm?
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the refractive state when there is no visual input or feedback. Average values between 1-1.5 D
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Tonic accommodation is implicated in the causes of 2 types of myopia?
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1. night myopia 2. instrument myopia
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What is the accommodation requirement in an uncorrected myope compared to an emmetrope?
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The accommodative requirement will be less then for an emmetrope
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What is the accommodation requirement in an uncorrected hyperope compared to an emmetrope?
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The accommodative requirement will be greater then for an emmetrope
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What is the definition of the range of clear vision?
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the LINEAR distance between object positions conjugate to retina under max accomm. And no accomm.
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Does this range of vision ever change, regardless of an accessory lenses?
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No, the DIOPTRIC difference is constant and is equal to the accommodative amplitude.
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When deciding how much reading add to give a presbyope, what rule of thumb should you use?
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Only 1/2 the accommodative amp should be used while the patient performs a near task as their customary distance (so when solving T=REAL problems, make sure that you use 1/2 of the A value when solving for L)
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What does the value T, in T=REAL stand for?
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this is the spectacle accommodative demand, and is the amount of accommodation required in the plane of any correcting lens.
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When can the equation T = REAL not be used
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When the correcting lens is significantly removed from the cornea, because the distances from the near object to the cornea and lens to cornea must be considered.
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What is the accommodative demand for a spectacle corrected myope in comparison to an emmetrope or CL corrected myope?
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The Ad is LESS.
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What is the accommodative demand for a spectacle corrected hyperope in comparison to an emmetrope or CL corrected hyperope?
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Greater
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Depth of field vs. Depth of focus?
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Depth of field = range of distance from the eye through which an OBJECT appears clear w/o change in A. Depth of focus = range at retina through which an IMAGE can move w/o change in clarity.
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DOF depends on multiple factors?
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1. Pupil size along with other optical factors, like CA, astigmatism 2. Neural factors, like photoreceptor size and function and blur processing
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For a pupil diameter of 3-4 mm, the DOF is usually about _____ D?
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.25 D, smaller pupils have a larger DOF and vice cersa
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At what diameter pupil do diffraction effects predominate, such that any benefit of the larger DOF is eliminated?
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2.4 mm
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At what size pupil diameter is the DOF eliminated?
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7-8 mm
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What is the hyperfocal distance?
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One end of the DOF is at infinity and the other is at some finite distance. This varies with pupil diameter and assumes a static eye.
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In what situations is the hyperfocal distance used?
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IN exam lanes using the snellen chart. The chart is typically at 6 m (not infinity), but the .25 DOF be able to compensate for the undercorrection of the negative vergence at the patients eye.
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What is the accommodative response for targets beyond the tonic or resting level?
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Lead, .5- .75 D
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What is the accommodative response for targets closer then the target level?
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Response lags the stimulus. Between 25-40 cm, lag is .5-.75 D
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IN what situation is lag increased?
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When reading rate decreases
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In what situations is lag decreased?
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When attentional/cognitive demand increases.
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When do the accommodative response and stimulus exactly equal each other?
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At the tonic level.
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What is the accommodative response for tagets closer than the near point of the eye?
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Response remains at a maximum or may even decrease.
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What is the accommodative response for virtual targets; or ones placed beyond infinity by an additional opitcal system?
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Response tends to approach the tonic level as the image blur increases.
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In what cases may a person become a 'pseudo-myope'?
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Emmetropic individuals who spend a ton of time doing near work (hysteresis effects)
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Patients with pseudo-myopia complain of what sx?
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eye strain, HA, asthenopia, distance blur, decreased VA's
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Does ret. And an autorefractor show consistency in a pseudo-myope?
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No, because the eye undergoes accommodative spasm
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Does hysteresis also affect uncorrected low to moderate hyperopes?
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Yes, as they must continually accommodate to see clearly at a distance.
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What 3 situations will reveal an uncorrected hyperopes rx?
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1. cycloplegic 2. reduced accommodative amp or facility 3. onset of presbyopia.
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Is push-up Amplitute (donders) patient subjective or objective? Does it overestimate or underestimate AA?
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Patient subjective, overestimates AA
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Is the minus lens method patient subjective or objective? Does it over or underestimate AA?
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Patinet subjective, overestimates AA
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Is the accommodative facility/flippers patient subjective or objective?
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Patient subjective
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For the flipper technique of measuring A, what should the minus and plus lenses be?
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Minus lens can be any power up to the patients AA, plus lens cannot exceed the vergence demand of the target.
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For the flipper technique, what is the expected value of cycles/min?
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10-12 cycles/min, or 2.5-3 seconds to clear an image.
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For a laboratory method of measuring A, what is photorefraction, and is it patient subjective or objective?
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Patient objective, taking a photo of retina and measuring amount of blur
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For a laboratory method of measuring A, what is the scheiner principle, and is it patient subjective or objective?
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Patient subjective, uses two separate images of different colors and judges whether the person can fuse them and see them at the plane of regard.
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What are the 4 variables in the Badal system and which variable changes to measure a pateints RE?
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1. Brightness 2. Blur 3. Image Location --> all are constants and do not change, 4. image size, changes
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Is badal patient objective or subjective?
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patient subjective
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Is the laser optometer patient subjective or objective, and how does it work?
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Patient subjective, patient reports the movement of the speckle pattern toward or away from the eye.
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What did the laser optometer establish in the 1970s?
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Established that the resting level of accommodation for most ppl is at a near distance, not infinity.
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What occurs during Accommodation, according to the Helmholtz-Fincham Capsular relaxation theory?
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1. Ciliary muscle contracts, ciliary body shifts forward. 2. Zonules relax 3. Lens circ decreases and axial length increases 4. anterior lens pole moves anterior and becomes steeper 5. posterior lens essentially stays in place 6. due to decrease in zonular tension, the lens moves slightly downward due to gravity.
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What is the effecct on the ciliary body when the ciliary muscle contracts?
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Ciliary muscle contraction --> CB moves anterior, along with choroid and retina --> accounts for about .05 D of A
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What is the Stiles-Crawford effect?
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Has to do with Accommodation changes, when retina moves anteriorly, and due to this, photoreceptors are oriented optimally with respect to imcoming light.
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During accommodation, when ciliary muscle contracts, what happens to scleral spur and TM?
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Tension of scleral spur and TM increases, which reduces total IOP by increasing aqueous outflow.
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Is the iris displaced by the change in shape of the lens?
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Yes, the iris is displaced anteriorly.
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What is the effect of parasympathetic innervation?
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Causes the ciliary muscle to contract, hence increasing A.
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Accommodation is stimulated via?
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Foveal blur
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What is the effect of sympathetic innervation?
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Causes the ciliary muscle to relaz, thus decreasing A.
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Which pathway, Parasym (PNS) or SNS involves the EW nucleus?
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PNS
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Which pathway, Parasym (PNS) or SNS involves the ciliary ganglion?
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PNS
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Which pathway, Parasym (PNS) or SNS involves the ciliospinal center, carotid plexus?
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SNS
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What 2 pharmacological agents increase accommodation?
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1. parasympathomimetics 2. sympatholytics
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What 2 pharmacological agents decrease accommodation?
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1. parasympatholytics 2. sympathomimetics
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What 2 things lead to reduced accommodative response?
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1. decreased target contrast 2. lowered target brightness
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Other then blur, what is another stimulus to accommodation?
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proximity (or knowledge of nearness)
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Is there a psychic, or voluntary component to accommodation?
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Yes.
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What is the most likely candidate that explains the ease at which eyes can alter accommodation in the right direction?
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Chromatic aberration in the human eye!!
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The cornea can transmit wavelengths between ______ and ________?
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310- 2500 nm
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Retinal photoreceptors are insensitive to the IR wavelengths longer then?
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760 nm
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Light with shorter wavelengths will focus light in front of or behind the retina?
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In front of the retina
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Does the eye, therefore, become more myopic or hyperopic when light becomes shorter? Longer?
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More myopic when light wavelength becomes shorter, my hyperopic when wavelength becomes longer.
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If chromatic abberation is reversed, can ppl alter their accommodation in the proper direction?
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NOPE!
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What is implied by the reaction time, or latency, of accommodation?
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represents the time between the presentation of an accommodative stimulus and the start of the accommodative response.
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What are the reaction times, far to near and near to far?
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about 290-400 ms
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When are these reaction times shorter? For what circumstances?
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1. contraction of the pupil due to light 2. saccadic eye mov't 3. convergene of eye mov'ts
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What is implied by the movement time, or implicit time, of accommodation?
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Measures how long the accommodative response takes after the initial latency period.
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What is the total time, or elapsed time, or accommodation?
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is the sum of the reaction and movement times. When accommodation is performed in a single step, total time can be as long as 1 sec, for multiple steps, as much as 3 sec.
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What is the typical speed of A?
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between 2-5 D/sec, with max speed about 10 D/sec
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Is accommodation ballistic, like saccadic eye movements?
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No, thus the response can be altered before it is complete.
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During steady state accommodation on a fixed stimulus, what affects the amplitude and frequency of the fluctuations?
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1. pupil diameter (fast responses reduced for small pupils) 2. mimimal at near and far points 3. target contrast 4. target luminance (especially when below photopic levels) 5. astigmatism present (larger fluctuations for astigmats). See pg. 105
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What causes fast fluctuations of the accommodative response?
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mechanical or elastic properties of the lens, zonules and CB.
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What causes the slow fluctuations (drifting) of the accommodative response?
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Change in a manner that enhances the DOF of the eye, trying to resolve image in a sense.
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Discuss the accommodative properties of an amblyope?
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1. Accommodate less accurately and with low amplitude. They have normal fast fluctuations, but huge slow fluctuations and drifts during MONOCULAR fixation
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What is consensual lag?
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difference in accommodation between the fixating and nonfixating eye, up to .5 D is normal.
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What is excessive consensual lag caused by?
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aniso, amblyopia, neurological damage
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What is the normal range of AC/A ratio?
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4/1 --> 6/1
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What is a low AC/A ratio indicative of? A high ratio?
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Low: CI High: CE
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Which people suffer more from presbyopia? Low myopes or low hyperopes?
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Low hyperopes who have not been corrected previously as their hyperopia becomes manifest with decreasing accommodative ability.
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What does the Hess-Gullstrand theory base the development of presbyopia on?
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Changes in the crystalline lens and capsule. The ciliary muscle remains strong, as do the zonules.
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What happens to the lens with age?
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Becomes thicker (from 3.5 --> 4.5 mm) due to the lens capsule becoming less stiff, thus losing its ability to alter lens shape (thick of a stretched out rubber band)
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What is the definition of a prism diopter at 1 m?
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At 1 m: a deviation of 1 cm for each meter.
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What is the definition of a prism diopter at 50 cm?
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At 50 cm: 1 pd equals a 5 mm deviation
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What is the definition of a prism diopter at 2 m?
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At 2 m: 1 pd equals a 2 cm deviation
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1 pd is equal to howm many degrees?
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1 pd = 0.573 degrees
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1 degree is equal to how many prism diopters?
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1 degree = 7/4 p.d.
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What type of prism contains two counter-rotatable prisms of equal power at is found in a phoropter?
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Risley Prism
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In a Risley Prism, what is the power of the two prisms?
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Both are 15 p.d.
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What type of prism is a fixed combo of 2 equal-powered prisms set base to base?
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Biprism
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What is the name of the device that consists of a series of 12-15 discrete prisms (oriented either horizontally or vertically) ranging in power from 1-30 or 1-40 pd?
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Prism Bar
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Prism power depends ONLY on what two factors?
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1. Prism apex angle, 2. Index of refraction
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Does the power of the prism depend on prism size?
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NO
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What is the name of a "press-on" prism?
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Fresnel Prism
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What type of prisms are made of soft, pliable plastic that can easily be cut with scissors and attached to a spectacle lens?
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Fresnel Prism
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In what type of prism is a large prism replaced by a series of smaller prisms that have the same apex angle
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Fresnel Prisms
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What is effective deviation (delta E)?
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The degrees that an eye must rotate in order to maintain an image on the fovea when a prism is worn
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What is the relationship between effective deviation (delta E) and deviating power (delta) for distant objects?
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They are equal
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What is the relationship between effective deviation (delta E) and deviating power (delta) for NEAR objects?
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Effective deviation (Delta E) is ALWAYS LESS THAN the deviating power (delta)
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From where do we measure prism vertex distance, d?
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From the prism plane to the center of rotation of the eye
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Does the eye rotate about a single point?
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NO, but we assume that it does
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What is the prism vertex distance (d) from the cornea? What is it from the spectacle plane?
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Cornea = 13.5 mm, Spectacles = 27 mm
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Deviation produced by a plus or minus lens depends on what two factors? Hint: what is Prentice's Rule?
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Decentration in centimeters and lens power. Prentice's: deviation = cm x Power
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