[visionlist] Teller Acuity Cards

John Robson jgr11 at cam.ac.uk
Wed Feb 19 08:10:49 -04 2020

Ruth et al,

Teller and other similar acuity tests.

Unless I have missed something, all those who have contributed so far to 
this thread seem to have assumed implicitly, or in some cases explicitly, 
that the patterned area will be indistinguishable from the uniform area 
when the average luminance of the two areas is the same. While there has, 
very reasonably, been some discussion about the conditions necessary to 
prevent the contour between the two areas being independently visible, the 
basic requirement of equal mean luminance has not been questioned.

If the spatial contrast in the retinal image were to be reduced to below 
visual threshold by a linear process (e.g. simple optical blurring) I am 
sure that this would be a correct assumption. But this is not normally the 
case even in adults. Even in adults the retinal image of a grating at the 
acuity limit will still have some, albeit much reduced, spatial contrast so 
that some cones will be more stimulated than the average and some less than 
the average. As was shown long ago by Donald Macleod (see Macleod, Williams 
and Makous 1992 Vis Res 32: 347-363 and other papers quoting this one) 
cones are non-linear so that the average signal from an array of cones 
which are non-uniformly illuminated will not be the same as the average 
signals from the same cones when illuminated uniformly. In general the 
brightness of the patterned and non-patterned areas will only be equal when 
the luminance of the uniform area is slightly different from that of the 
average luminance of the patterned area.

In fact it is not only the cones that are non-linear; there are intensive 
non-linearities at every stage in the visual system which will all no doubt 
contribute to distortion of the visual signal to an extent that depends 
upon how spatially non-uniform the neural signal is at that stage. While 
for adults it may well be that the non-linearity of the cones is the most 
important factor in affecting the apparent brightness of invisibly 
patterned areas with spatial frequencies close to the resolution limit of 
the eye's optics, this may well not be the case for visible patterns of 
lower spatial frequency whose apparent brightness is probably dependent on 
later non-linearities. It may well also not be true in infants in whom 
development of acuity lags behind development of their eyes' optics so that 
the retinal contrast of patterns with elements close to their acuity limit 
may have quite high retinal contrast.

I am not aware of any published report of the experimental determination of 
the apparent brightness of grating patterns with spatial frequencies above 
the acuity limit but I wonder if Ruth may not just have reported such 
observations. It is my understanding that Teller charts made by Precision 
Vision have the reflectance of the uniform area matched to that of the 
grating by visual inspection of the charts at a distance at which the 
grating pattern is invisible to an adult observer. I believe that they do 
this because they found that instrumentally matched luminances did not 
result in perceptual matches. I should be very interested to hear whether 
the charts on which Ruth made luminance measurements provided perceptual 
brightness matches. I have no idea whether any of the other similar tests 
use instrumental or subjective matching but for the sake of uniformity I 
think the matches should be made instrumentally. It is not at all certain 
that an adult match would be the same as that of an infant.

Does any of this matter? I am with Christopher on this. The test is what it 
is. Does it matter if the patterned patch is seen by virtue of its 
different apparent brightness, by having an illusional contour or because 
the pattern itself is visible. My answer would be no. We are discussing a 
clinical test here and what is important is whether the test is practical, 
reliable and useful. It is always important to remember that no two tests 
that are not physically identical can be expected to give exactly the same 
numerical answer and the existence of norms for each particular test is 
obviously an important consideration.

As to Ruth's observation that some subjects have anomalously high acuity 
with charts of this kind, it is not generally appreciated that the acuity 
limit for a grating may be determined by two different criteria: 1) can the 
bars of the grating be resolved well enough for the subject to determine 
their direction? or 2) can the subject distinguish a grating from a 
non-patterned area? It is particularly obvious when interference fringes 
are viewed that when the spatial frequency is raised above the level at 
which the direction of the bars can be clearly seen and reported the 
grating is still easily distinguishable from a uniform area. This is a 
result of the irregular moire pattern that is set up by interaction of the 
grating with the retinal cone mosaic (see p723 of Byram 1944 JOSA 34:718 
and Campbell & Green 1965). In subjects with particularly good optics this 
can be seen with regular imaging.

Hope this helps rather than confuses.


On Feb 18 2020, Ruth Hamilton wrote:

> Dear Rowan Further to your point, our team made some systematic 
> measurements of target vs background luminance for TAC, as well as Keeler 
> Cards for Infants, Lea Paddles and Cardiff Acuity Cards, and also for a 
> digital test (ipad, Peekaboo). The data are currently being peer 
> reviewed, but we found luminance (and colour) mismatches of differing 
> degrees for all the card-based tests. We also found some improbably good 
> psychophysical acuities (better than -0.300 logMAR) from some adults 
> tested at 10 m with the card-based tests. Best wishes Ruth
>Dr R Hamilton PhD
>Consultant Clinical Scientist, Paediatric Physiological Measurement
>Royal Hospital for Children, Glasgow, UK
>+44 141 452 4217
>Secretary, BriSCEV
> President, ISCEV www.iscev.org<http://www.iscev.org/> 
> www.facebook.com/ISCEV.org<http://www.facebook.com/ISCEV.org> 
> www.linkedin.com/groups/ISCEV-4811521<http://www.linkedin.com/groups/ISCEV-4811521>
> From: visionlist [mailto:visionlist-bounces at visionscience.com] On Behalf 
> Of rcandy Sent: 14 February 2020 13:54 To: Meindert de Vries Cc: Margaret 
> Woodhouse; gislin at jhu.edu; visionlist at visionscience.com Subject: Re: 
> [visionlist] Teller Acuity Cards
> Davida Teller, Velma Dobson and their colleagues developed the Teller 
> Acuity Cards to test resolution acuity. To do that, the target must 
> 'disappear' when the white and black components of the stimulus can no 
> longer be resolved. This requires that the mean luminance of the target 
> area is perfectly matched to the mean luminance of the rest of the card. 
> This is hard to do and makes the printing process difficult and more 
> expensive (including visual inspection of each individual card from 
> extended distance to confirm that there are no visible artifacts beyond 
> the resolution limit). Vanishing optotype tests, such as the Cardiff 
> Cards, use the same principle. Even though the vanishing optotype has a 
> shape that can be recognized, the shape is defined by a single black & 
> white paired outline that 'disappears' to mean luminance when the 
> components can no longer be resolved.
> A test that is constructed to use the forced-choice preferential looking 
> principle (is the target on the left or right of the card?) is critically 
> dependent on having no visible artifact to indicate where the target is 
> once the resolution limit is passed. This new test appears to have a 
> difference in mean luminance between the target area and background, and 
> therefore it is not a test of resolution acuity. It then becomes a test 
> of detection acuity (can the target be detected using the difference in 
> luminance?), similar to asking whether we can detect a star against a 
> dark sky. Based on these photographs, it seems very likely that these 
> large targets can be detected easily at the viewing distance that this 
> test would be performed, making it a poor test of detection acuity.
> Any new acuity test should be provided with testing norms that have 
> demonstrated its validity for the population being tested. One easy 
> confirmation of validity in the absence of normative data is to determine 
> the viewing distance at which an observer can no longer perform the task. 
> If the observer has 20/20 (6/6) acuity, is the card labeled approximately 
> equivalent to 20/20 at the observer's performance limit at the 
> recommended testing distance? Is the card labeled approximately 
> equivalent to 20/40 (6/12) at the observer's performance limit at twice 
> that viewing distance etc. I am guessing that a typical observer will 
> still be able to do this test at the end of a long corridor and, 
> therefore, that the test is not valid. (Candy, Mishoulam, Nosovsky & 
> Dobson; IOVS, 2011)
>I am doing my best to channel Davida and Velma!
>   Rowan
> ......................................................................................................................... 
> T. Rowan Candy, Executive Associate Dean for Academic Affairs, Professor 
> of Optometry and Vision Science, Adjunct Professor of Psychological & 
> Brain Sciences Neuroscience and Cognitive Science Programs Indiana 
> University
>Phone: (812) 855-9340
> On Feb 13, 2020, at 7:52 PM, Gislin Dagnelie 
> <gislin at lions.med.jhu.edu<mailto:gislin at lions.med.jhu.edu>> wrote:
>Chris and Meindert,
>I definitely agree with Meindert's objection to the cards he presents.
> One thing that is critically important about any card that intends to 
> assess visual acuity (high-spatial frequency resolution, if you will) 
> through high-resolution texture filling a contour is that: 1) the average 
> luminance inside the contour is equal to that outside it, and 2) the 
> edges of the contour are filtered, preferably with a raised cosine of a 
> spatial frequency equal to the fundamental of the texture inside the 
> contour.
> Unless Meindert's photographic rendition is grossly misrepresenting the 
> actual hues and greyscale levels on the new cards, the cards do not meet 
> the first requirement: In all cases the average luminance inside the 
> contour is lower than outside.
> The cards definitely do not meet the second requirement: there is no 
> filtering around the contour edges
>So I have to agree with Meindert that there are serious problems with these
> Note, BTW, that the Teller cards do not use filtering around the outside 
> of the contour either, but this may be less important because of the 
> square contour of each pattern: black and white bars are equal in area. 
> Still this may lead to an overestimation of acuity compared to an 
> unconstrained grating
> So while I agree with Chris that there is room for valid alternatives to 
> the Teller cards, the ones shown here do not appear to meet the minimum 
> requirements for such an alternative.
>But maybe I'm missing something?
> Gislin -- Gislin Dagnelie, Ph.D. Associate Professor of Ophthalmology JHU 
> Lions Vision Research & Rehab Center Johns Hopkins Hospital, Wilmer Woods 
> 358 1800 Orleans St Baltimore, MD 21287-0023 
> http://ultralowvisionlabjhu.net/ USA e-mail: 
> gislin at jhu.edu<mailto:gislin at jhu.edu>
> On 13 Feb 2020 at 13:24, Christopher Taylor 
> <christopher.taylor at gmail.com<mailto:christopher.taylor at gmail.com>> 
> wrote:
>Why do you believe this test has a fundamental error versus TAC testing? It
>is a different test, no more, no less.
>To play devil's advocate, one could claim that Teller Acuity Cards lack
>ecological validity because they do not present contours and objects, which
>are more important to the visual system during daily living than sinusoidal
>or square-wave gratings.  That said performance on this test and TAC ought
>to correlate and if this new test has other benefits (e.g, faster/easier to
>administer, cheaper and more available to purchase, and so on...) and has
>appropriate age-norms for the population being screened then might it not
>be an advance on traditional TAC testing?
> On Thu, Feb 13, 2020 at 9:36 AM Meindert de Vries 
> <meindertdevries at visio.org<mailto:meindertdevries at visio.org>> wrote:
>Dear members ,
>Since 1992 I work for Visio, an institution the helps visually impaired
>people. We have always been using the Teller Acuity Cards to determine
>the visual acuity in children.
>A new test has been presented on the scene, proposing to replace the
>Teller Acuity cards, because the TAC are expensive and sometime difficult
>to get. I enclose an example picture of the new cards.
>From my perspective and knowledge they have made some fundamental errors
>1.       The test seems  ambiguous to me, because both object recognition
>part of our visual system as well as the much `lower" detection part of
>visual system is triggered by this stimulus
>2.       The spatial frequency content of these stimuli (checkerboard
>patterns with a distinctive contour) is in the Fourier domain essentially
>different from the TAC bar patterns without a contour; nevertheless the
>same cycl/cm are used.
>3.       In addition to point 2: I think that the contour is a much
>stronger stimulus than the checkerboards.
>Could anybody reflect on this ?
>Most kindley,
>*drs. M.J. de Vries 69024716001*
>*Koninklijke Visio Noord-West Nederland Revalidatie & Advies*
>Hettenheuvelweg 41-43 1101 BM Amsterdam
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Professor John Robson  ScD FRS
Senior Research Professor in Vision Science
University of Houston College of Optometry, 
Houston,  TX 77204-2020
+1 (713) 743 1807
Fellow, Gonville & Caius College, Trinity St., Cambridge, CB2 1TA
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jgr11 at cam.ac.uk

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