Acuity and Retinal Location Lab



  1. Purpose and Goals
    1. to measure and see how acuity deteriorates as the stimulus is moved farther from the fovea
    2. to gain some experience in calibration or making sure the stimulus works the way your think it does
    3. to develop some understanding of what is an experiment
    4. To begin to learn how to design an experiment to answer a question
    5. to compare data to hypothesis
  2. Acuity and Retinal Physiology
    1. From class and chapter 3 of the text, we learn that the number of cones get fewer as the distance from the fovea increases.
    2. Observations:
      1. Since we will be testing in the day time, it is the cones that are the principal receptors involved in what we see
      2. The density of cones is by far the greatest in the fovea
      3. It has also been observed that receptive fields are smaller at the fovea than in the periphery
      4. Acuity and receptive fields are examples of spatial summation so should be linked
    3. Question: It would seem that if the density of the cones is greatest in the fovea and the receptive fields are smallest in the fovea that our acuity would be highest at the fovea and decrease in the periphery.
  3. Calibration
    1. Calibration is the process of checking out the equipment and the stimuli to make sure that they are doing what they ought to be doing. 
    2. In studying our sensation, sometimes the calibration has to include the participant.
    3. The stimulus being used in this experiment is a checkerboard.
    4. When the checks are too small, the checkerboard is supposed to blend in with the background.
    5. It should be easy to make this happen, have the white and black squares equally different from the background.  However, monitors do not make it easy.  Each monitor is different and monitors change over time.
    6. So, before the experiment, each participant will calibrate their monitor so that a checkerboard that is too small to see will blend into the background.
    7. This is a very simple and basic type of calibration.
  4. Experiments
    1. Scientist use many types of methods to collect their data.
    2. One type of research method unique to the sciences is the experiment.
    3. In simple terms that means the research changes something and sees what happens.  I will introduce a lot of formal language, but that is the basic idea, hold onto it.
    4. The thing that the experimenter changes is called the Independent Variable (IV).
    5. The thing that the experimenter measures to see what happened is the dependent variable (DV)
    6. See if you can figure out the IV and DV for this experiment.

The Experiment:

  1. Design:
    1. Stimulus Type: Your Pick of Either Grating or Checkerboard
    2. Number of Positions: in your groups you will pick
      1. Too few levels will not allow you to know what is going on in your data
      2. Too many will cause unnecessary wear and tear on your participants
      3. The goal is good clear data with as few measures as necessary
    3. Upper limit of staircase: leave as default but record (the unit is in pixels)
  2. Method (Method of Limits):
    1. Number of Levels to Test: You pick in your groups
      1. too few and you will not have a good measure of the threshold
      2. Too many and you bore and tire your participants which can lead to sloppy responses
      3. The goal is clear, accurate measures
    2. Number of staircases: 9 to 15
    3. Type of MOL: Traditional
  3. Calibration:
    1. Stare at the middle of the top of the screen area for the experiment
    2. In the upper right corner is a grating made up of alternating bright and dark lines.
    3. Using the slider at the bottom of the screen, adjust the brightness of the gray background until you cannot see the grating or at least it is minimally visible.
  4. Procedure:
    1. Place your head centered on the screen 24" from the monitor surface.
    2. Fixate on the red fixation mark.  Do not move your eyes from that spot.
    3. The stimulus will be presented at the position closest to the fovea that it will get. 
    4. If you see it, press the yes button or the z key.
    5. If you do not see it, press the no button or the / key.
    6. The computer will track the staircases and when each position is done it will move to the next position.
    7. The threshold are calculated and a summary of the results will be displayed at the end of the experiment.
    8. Press the Show Data button and record your results
    9. We will add a new wrinkle in the next lab.
    10. Here is the link to the lab.
  5. Write-up: (Method, Results, Summary)
    1. Week 1:
      1. Do a graph of your results to hand in. 
      2. Your graph should be and x-y scatter plot with points connected by straight line connectors
      3. Put error bars around mean values.
      1. Include a paragraph describing why you graphed the data the way you did.
    1. Week 2: Abbreviated Lab Report with the following sections:
      1. Method: Your method must include the following subsections:
        • Participants:  A brief description of who was in your experiment.  You need data from all members of your group
        • Stimulus: A description of all stimuli used and your calibration
        • Equipment: What equipment you used
        • Procedure: How you collected the data
      2. Results: a description of your findings
        • Use your data and the data from the other member(s) of your group.
      3. Summary: Do your results match your expectations, listed at the beginning of this page