Just Noticeable Differences (JND) with Lenses or Prisms

Purpose: To develop visual awareness of spatial changes, from large differences to subtle differences.

JND with Yoked Prisms

Materials:15D Yoked Prisms

5D Yoked Prisms

Procedure:

  1. Have patient walk around room. Encourage them to look at walls, floor, their feet, etc. and discuss how things look (e.g, “floor is flat”)
  2. Have patient put on 15D yoked prisms. Repeat step #1, discuss how things look now.
  3. Repeat step #2 with each direction of the prisms. Goal is for patient to be able to immediately see and respond to the changes to their visual world.
  4. When patient can easily recognize all of the directions and what changes accompany each direction, decrease to 5D yoked prisms and repeat procedure.

JND with Loose Prisms

Materials:Patch

Loose/Stick prisms 1D to 12D

Fixation Target (e.g, a stuffed animal)

Procedure:

  1. Have patient put on patch. Place fixation object at 6-12 foot distance from patient.
  2. Standing behind patient so the patient cannot see which direction the prism is in, therapist put 12D prism in front of eye while patient looks at target. Ask patient to identify what spatial changes occur. Take prism on/off if patient has difficulty seeing the changes.
  3. Try prisms in all directions (up, down, left, right, diagonals). If patient can quickly and easily identify each change, decrease to lower diopter prism.
  4. Repeat procedure in descending order of diopters. Record what powers or directions were difficult for patient to assess. Repeat procedure with other eye. When easy to identify 12D – 1D prisms at distance, repeat procedure at near (16-20” distance from patient).

JND with Plus/Minus

Materials:Patch

Loose plus and minus lenses of different powers (minimum of 8 lenses)

Small fixation target (such as a toy soldier)

Procedure:

  1. The fixation target is put on the table at 16 – 20” distance from patient. Patient puts on patch.
  2. Give the patient a loose lens (start with lenses that are at least 0.50D power difference). Instruct patient to look thru the lens with the lens at nose or closer, and see what change they notice to the fixation target.
  3. Have patient take lens off to the side then back in front of eye to see changes. If patient cannot communicate what they see, help by asking leading questions (e.g., “does it get bigger or smaller, closer or farther?”)
  4. If patient reports that lens is blurry, have patient “trombone” the lens towards and away from fixation object until patient is able to clear target at nose or closer. Then ask patient to note the spatial changes.
  5. Instruct patient to put lenses in order from smallest image to largest image (or closest to farthest). Record accuracy and anything noteworthy about what powers they had difficulty judging.
  6. Repeat with other eye.
  7. When patient can easily identify lenses that have differences of 0.50D or greater, use lenses that are 0.25D difference.

*OVT

Category: Visual-Motor/Vestibular Integrationdate modified: 10/09