| ISLE | Chapter 7
| Experiment: Size Constancy
Size constancy is the perception of an object as having a fixed size, despite the change in the size of the visual angle that accompanies changes in distance. That is, we have a tendency to see an object as the same size regardless of the size of its image on our retinae. As we have seen, there are limits to size constancy. Cars look like small toys as we get high enough during take off so tall when we see him from far away. However, at the range of normal viewing, size constancy allows us to see objects as the “right” size even as they move away from or toward us. For example, consider talking to a friend. You see her as the same size if she is standing 6 feet away from you or if she is standing 3 feet away from you. If she is standing 6 feet away and suddenly takes a step closer to you, you do not see her grow, even though her visual angle is now twice as large on your retina. Thus, size constancy usually allows us to see objects as not changing in size.
In this experiment, you can see how depth cues contribute to size constancy. You can add, singly or in combination, different depth cues. In addition, you can change the amount of different depth cues to examine the impact of these on the perception of the size of a circle.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On this tab you can adjust the type of stimulus and the various parameters of the motion of the stimulus to see how this might effect the aftereffect. To adjust the depth cue, you first must check the box before the depth cue name. You will be adjusing the size of the right hand circle to match the left hand circle. The settings include the following:
Relative Height: The height on the screen of the right hand circle. The 0.5 value
is the middle of the screen. Less than 0.5 is above and greater than 0.5 is below the middle.
Lindear Perspective: Add the linear perspective depth cue. The larger the value, the greater the depth
indicated.
Texture Gradient: Add texture gradient. The larger the value
the greater the depth indicated.
Atmospheric Persp.: Add atmospheric perspective. The larger the value
the greater the depth indicated by shading and tinting the right hand circle.
Shadow: Add shadows to the left hand circle. The larger the value
the greater the depth indicated by making the shadow move farther from under the right
hand circle.
Use Binocular Disparity: Add binocular disparity as a depth cue. Control
the disparity of each eye separately with the sliders below. The unit is in pixels and positive
values refer to crossed disparity and negative values to uncrossed disparity.
Anaglyph Type: If using binocular disparity, this selector controls the
type of anaglyph image being created. You will need to have matching glasses. The text images are
Red-Cyan anaglyph images.
Reset At the top of the settings page is a Reset button. Pressing this button restores the method settings to their default values.
On this tab you can adjust how the method will work. The settings include the following:
Number of Trials in the Experiment: How many times you will make your adjustment.
Range of Variation of Relative Dot Luminance: Over what range will the starting value of the stimulus
change. The range is a percentage of the total range of values the stimulus can have.
Minimum Value of Relative Circle Size: What is the smallest that the right hand circle can be displayed.
Maximum Value of Relative Dot Luminance: What is the largest that the right hand circle can be displayed.
Reset At the top of the settings page is a Reset button. Pressing this button restores the stimulus settings to their default values.
On the Experiment tab, press the space bar or the button on the screen to start the experiment.
Soon two circles will be presented.
Your task is to adjust the size of the right hand circle till it looks exactly the same
size as the left hand circle.
When you have accomplished this goal, click the Done button to the right of the slider to proceed to the next trial.
You can also use the arrow keys to adjust the slider. Up and right arrow keys will increase the size
of the right hand circle and
down and left arrow keys will decrease the size of the right hand circle.
The enter key or space bar will act like the Done button.
When you have finished all of your trials, you will instructed that you can view your results.
You data will presented on this tab. On the x-axis the trials. On the y-axis will be the value of your dependent measure that you set when you pressed the Match button on that trial. To see your Point of Subjective Equality where the two circles look to be the same size, click the button that says Show PSE. To display the data for all of the trials, click the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.