Size Constancy Lab



  1. Purpose and Goals
    1. To determine the depth cues that provide the best size constancy.
    2. The develop an understanding of the difference between quantitative and qualitative predictions
    3. To compare data to hypothesis when the hypothesis is mathematically precise
    4. To learn more about designing your own experiment
  2. Size Constancy - Briefly, will discuss more in class next week of classes
    1. Observations
      1. Objects have smaller retinal image sizes the farther away they are from the image.
      2. But they do not appear to shrink as they recede into the distance.
      3. One key factors in creating size constancy is the perception of depth.
      4. But not all depth cues provide an equal perception of depth. 
    2. Stereopsis:
      1. The measure of the difference of position that images fall on the two eyes is disparity.
      2. The ability to see in depth using the depth cue binocular disparity is called stereopsis.
    3. Question:
      1. Stereopsis (binocular disparity) has a very precise relationship with depth, see below, can it support size constancy when all other depth cues are removed.
      2. How does size constancy change as we add more depth cues.
  3. Quantitative Predictions
    1. A qualitative prediction deals with trends.
      1. An example would be from the retinal acuity lab
      2. acuity gets worse farther out in the periphery. 
      3. This statement does not predict how worse.  Thus, it is qualitative.
    2. A quantitative prediction deals with both trends, but the size of the trends.
      1. To apply this to the last lab it might say, for each degree of visual angle out from the fovea acuity gets worse by 10%.
      2. This type of statement allows a much finer test of the idea.
      3. This type of statement can also be written as an equation

        Acuity = (distance from Fovea * .10*acuity at fovea)+

                         acuity at fovea.

        This is simply a translation of the statement at 3.b.i into a mathematical expression.

    3. Quantitative Elements of this situation.
      1. Size Constancy

        Sr α 1/dp
        Sp =  k

        Sr is the size of the retinal image

        Sp is the perceived size of the object

        dp is the perceived distance of the object (since in our case the distances will all be virtually created)

        α means proportional to.

        k is a constant.

        To translate all this into English: The retinal image size decrease as distance increases such that they are proportional.  That means as distances doubles, the height of the image in the eye is cut in half.  However, the perceive size stays the same regardless of the change in image distance.

      2. Perceived Distance

        Distance from Screen = Viewing dist(mm) * disp(mm)
                                                 Interpup dist (mm)

        Perceived Dist = Viewing Dist - Dist from Screen

      3. These will be brought together when the experiment is explained

The Experiment:

  1. Equipment
    1. Here is the link to the lab experiment: ISLE 7.4 (b.2) Size Constancy Experiment
    2. Special hardware
      1. Filter glasses, red filter over the left eye and the cyan filter of the right eye.
      2. Right eye see what is drawn in cyan, left eye sees what is drawn in red.
      3. Allows different images to be seen by each eye, which allows for disparity
  2. Design:
    1. Arrangement of Stimuli:

             Standard               Comparison
    2. Left circle will have a disparity of 0. 
    3. There will be no fixation so the horoptor will vary.  You will be measuring relative disparity from this point.
    4. Right circle will be a different disparities in a range of +/-15.
    5. IV1 is amount of depth (in this case, amount of binocular disparity).
    6. IV2 is the depth cue being tested: disparity and we will talk about the others next week.
    7. DV is size of comparison circle relative to the Standard (left) circle
  3. Method:
    1. Viewing Distance is 75 centimeters (be precise).
    2. Open the lab webpage.
    3. Stimulus Settings:
      1. Select: Use Binocular Disparity
        1. Disparity of Standard (left)Square = 0 pixels (measure in mm)
        2. Disparity of Comparison (right) Square (in pixels, measure in mm)

          pick your own values, use 0, some both + and - and in range +/- 15 pixels (measure in mm).

      2. Make sure all other depth cues are unchecked this week.
      3. Make sure Anaglyph Type is set to Red-Cyan.
      4. Next two weeks we will add to this experiment.
    4. Method Settings: Method of Adjustment
      1. We are Measuring a Point of Subject Equality (PSE). What is this?
      2. Choose Number of Trials: Should do several
        1. Set the parameters for the method as you think will give you sufficient data and clean results
        2. Again, try some of the settings out
        3. Messy results may be easy to collect but no fun to interpret
      3. Leave range of variation and maximum and minimum values unchanged
  4. Procedure:
    1. After setting up the condition, put on glasses
    2. On each trial respond as directed on the screen.
  5. Generating Predictions:
    1. Measure Perceived Distance for each disparity condition: e.g., viewing distance = 750 mm, disparity = 3.4 mm, interpupillary distance = 63 mm
      1. Distance from Screen = (750*3.4)/63 = 40.5 mm
      2. Perceived Distance = 750-40.5 = 709.5
    2. According to Size Constancy: when two objects appear the same size the retinal image sizes are inversely proportional to the two distance.
      1. in English, an object that is twice as far away has to be half as tall on the retina to appear the same size.
      2. So, the predicted relative size is 750/709.5 = 1.06
  6. Write-up: (Full report)
    1. Week 3: Do a graph of your predictions to hand in so I can inspect them.
    2. At end of lab, do all sections of the lab report specificed in the lab report format.
      1. This lab report has a particularly complicated stimulus section you need to measure the following elements:
        • The disparity in mm: from edge or color (red or cyan) to edge of white on same side
        • The size of the standard in mm
        • Will talk more measure ments later.