Visual Angle

Visual Angle

Vision scientists usually express the size of an image on the retina in terms of visual angle. They use this measure instead of the size of the object or square millimeters of retina covered because it is consistent regardless of the distance from the viewer or the size of the eye. , shows how size, distance, and visual angle are related.

Before continuing, calibrate your monitor with . You will need a ruler to measure 2 inches or 5 cm on the screen and to measure your viewing distance. You only need to do it once for this computer/monitor combination. The results are saved and will be used whenever needed. Many PsyCog activities use this calibration to show stimuli at the proper size.

That’s all very well, but what does 1° of visual angle actually look like? It sounds like a rather small measurement. Use , to see for yourself.

Visual Field

The retina, which encompasses about ±90° of visual angle, varies across its surface area. Some areas have small densely packed cones (responsible for high-resolution color vision), while others have rods and larger cones specialized for low-light vision. Thus, the monocular visual field (the area you can see with one eye) varies in its sensitivity. shows parts of the visual field, corresponding to anatomical areas of the retina. You can also test sensitivity to shape, color, and motion across the visual field. Keep in mind that, because of the eye’s lens, the left half of the visual field corresponds to the right half of the retina.

Blind Spot

The optic disc, where blood vessels enter and axons leave the retina has no rods or cones. It corresponds to a blind spot in the retina. Measure your blind spot with , then use to explore how the brain “fills in” vision across this gap.

Sinusoidal Gratings

Many of the experiments discussed in textbooks use sinusoidal gratings to measure visual acuity, test the selectivity of cells in the visual system, or create moving patterns. The gratings are usually described in terms of the visual angle covered by one cycle of the grating. Use , to create your own sinusoidal gratings.

Further Exploration

Use and to form and test hypotheses about the visual field.

Questions

Visual Field

  1. In , how did central and peripheral vision differ in their sensitivity to shape, color, and motion? Can you explain these variations in terms of retinal anatomy?
  2. Some of the color options in the tests of shape, color, and motion made the tasks more difficult. These colors were chosen to be approximately isoluminant (equal brightness, contrasting only in color). Why would that make them hard to distinguish?

Blind Spot

  1. When you do , there is a delay between seeing the black dot disappear (or reappear) and pressing (or releasing) a key. How does this affect the accuracy of measurement? What aspect of the procedure might minimize that problem?
  2. Why are we not normally aware of having a blind spot in each eye?
  3. Would you expect someone who is missing an eye to be more aware of having a blind spot?
  4. The optic disc is on the nasal side of the retina, but the blind spot is on the temporal side of the visual field. Why?

Notes

The fovea is not actually on the optical axis of the eye, but is 4-8° to the temporal side. Most textbook illustrations ignore this, showing the fovea directly behind the lens, and I have followed that convention. For details, see this page and this one This does not affect measurement of the blind spot, which is still the correct offset from the fovea.

The Wikipedia macula entry has a good diagram of retina parts and locations and a good photograph of the retina. Wald (1967) says the central 0.12-0.13° of the retina lacks short-wavelength cones; Magnussen et al. (2001) behaviorally find the central 0.4-0.53° of the visual field to be insensitive to blue.

References