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Denis G. Pelli, Dennis M. Levi, Susana T. L. Chung; Using visual noise to characterize amblyopic letter identification. Journal of Vision 2004;4(10):6. doi: 10.1167/4.10.6.
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Amblyopia is a much-studied but poorly understood developmental visual disorder that reduces acuity, profoundly reducing contrast sensitivity for small targets. Here we use visual noise to probe the letter identification process and characterize its impairment by amblyopia. We apply five levels of analysis — threshold, threshold in noise, equivalent noise, optical MTF, and noise modeling — to obtain a two-factor model of the amblyopic deficit: substantially reduced efficiency for small letters and negligibly increased cortical noise. Cortical noise, expressed as an equivalent input noise, varies among amblyopes but is roughly 1.4× normal, as though only 1/1.4 the normal number of cortical spikes are devoted to the amblyopic eye. This raises threshold contrast for large letters by a factor of √1.4 = 1.2×, a negligible effect. All 16 amblyopic observers showed near-normal efficiency for large letters (> 4× acuity) and greatly reduced efficiency for small letters: 1/4 normal at 2× acuity and approaching 1/16 normal at acuity. Finding that the acuity loss represents a loss of efficiency rules out all models of amblyopia except those that predict the same sensitivity loss on blank and noisy backgrounds. One such model is the last-channel hypothesis, which supposes that the highest-spatial-frequency channels are missing, leaving the remaining highest-frequency channel struggling to identify the smallest letters. However, this hypothesis is rejected by critical band masking of letter identification, which shows that the channels used by the amblyopic eye have normal tuning for even the smallest letters. Finally, based on these results, we introduce a new “Dual Acuity” chart that promises to be a quick diagnostic test for amblyopia.
Theory of Pelli-Levi Dual Acuity Chart
The goal is to detect observers with lower-than-normal efficiency for small letters. The right side of the chart is at a fixed signal-to-noise ratio E/N = R that was chosen to be just above normal threshold, just barely readable by an observer with normal efficiency. It is below threshold (i.e., unreadable) for an observer whose efficiency is much lower than normal. In general, the observer’s efficiency and equivalent noise are both letter-size-dependent. The chart noise is high at the top and drops from line to line. For optical reasons discussed in the text, the observer’s equivalent noise increases for the smallest letters. The observer’s equivalent noise adds to the chart, and at some line is equal, in effect, to the noise on the chart. That dividing line is the key to understanding this chart. The triangular diagram above shows this horizontal dividing line and specifies the chart’s effective signal-to-noise ratio above and below it. Our analysis is all relative to the location of this dividing line, whose location on the chart depends on the observer and viewing distance, but the observer’s final score does not depend on where the dividing line is because, in the end, we consider only the difference between left and right acuities. In the upper part of the chart, above the dividing line, the chart noise is absent on the left and dominates on the right, producing an effective signal-to-noise ratio of R on the right, and greater than R on the left; in the lower part, below that line, the observer’s equivalent noise dominates both left and right sides, producing practically the same effective signal-to-noise ratio, less than R, on both sides. Thus the left and right sides are functionally different in the upper part and functionally identical in the lower part. One measures acuity by finding the smallest letter size at which the letters can be read (i.e., at which the chart’s effective signal-to-noise ratio is above the observer’s threshold). Any observer with normal efficiency for large letters will be able to read the top line, and thus will have a measurable acuity on both sides of the chart. Observers with normal efficiency for small letters will progress further down in noise (right side) than less efficient observers. If the in-noise acuity is below the line, then the no-noise acuity will be the same, because there’s effectively the same noise level on left and right. If the observer’s in-noise (right side) acuity is above the dividing line, then the no-noise (left side) acuity will be better, because there is less noise on the left. Thus the chart noise (right side) worsens the acuity of observers with low efficiency for small letters, but has no effect on the acuity of observers with normal efficiency for small letters. This acuity difference, occurring only for observers with low efficiency for small letters, is proposed as a diagnostic test for amblyopia. The acuity difference is relatively immune to optical blur, as increasing the blur simply increases the equivalent noise, shifting the dividing line up, with little effect on the left-right acuity difference.
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