Your patient has 10/400 vision at 10 feet. What is the JND?

Your patient has 10/400 vision at 10 feet. What is the JND?

• A. 0.75
• B. 2.00
• C. 4.00
• D. 1.50

Answer: (C)

Understanding JND in vision rehab means recognizing the smallest change in acuity a patient can detect. In Snellen terms, for someone with very poor acuity, a meaningful detectable improvement is often seen when the distance acuity improves by about a fourfold factor—that is, reading a line four times closer in distance terms. Here, the patient’s acuity is 10/400. The next reasonably readable line would be around 10/100, which is a fourfold improvement in distance acuity (400 divided by 100 equals 4). That fourfold change is what we’re using to represent the JND in this context, which is why 4.00 is the best answer. Smaller steps, like 0.75, 1.50, or 2.00, would reflect more modest improvements that typically aren’t the threshold for detecting a change on the Snellen chart at this level of vision.

Understanding JND in vision rehab means recognizing the smallest change in acuity a patient can detect. In Snellen terms, for someone with very poor acuity, a meaningful detectable improvement is often seen when the distance acuity improves by about a fourfold factor—that is, reading a line four times closer in distance terms.

Here, the patient’s acuity is 10/400. The next reasonably readable line would be around 10/100, which is a fourfold improvement in distance acuity (400 divided by 100 equals 4). That fourfold change is what we’re using to represent the JND in this context, which is why 4.00 is the best answer.

Smaller steps, like 0.75, 1.50, or 2.00, would reflect more modest improvements that typically aren’t the threshold for detecting a change on the Snellen chart at this level of vision.