Apparent Motion
The visual illusion called apparent motion occurs when two separate stimuli are presented close together in succession but at slightly different times and therefore appear to be one stimulus that is moving. This lab activity will introduce you to apparent motion and will teach you about the key variables involved in the generation of this illusion. The lab is broken into three blocks each of which is described below.
Moving Dots
In the first part of the activity, you will be presented with a number of trials. Each trial will contain a series of moving dots. To begin a trial, you will press the spacebar and you will then be shown a dot on the left followed by a dot on the right. If you experience the two dots as separate presentations then you will press the ‘D’ key, but if you experience one dot as moving across the screen (i.e., apparent motion) you will press the ‘K’ key. There are 30 trials to complete.
Motion and Distance
In the second part of the activity, you will be presented with a number of trials. Each trial will contain a series of moving dots similar to the first block but with different spacing between the two dots. To begin a trial, you will press the spacebar and you will then be shown a dot on the left followed by a dot on the right. If you experience the two dots as separate presentations, then you will press the ‘D’ key, but if you experience one dot as moving across the screen you will press the ‘K’ key. There are 48 trials to complete in this block.
Motion and Shapes
In the third part of this activity, you will be presented with two shapes on each trial. To begin a trial, you will press the spacebar and you will then be shown two shapes in succession. If you experience the triangle shape curving around the polygon shape, you will press the ‘D’ key. If you experience the triangle moving through the polygon, you will press the ‘K’ key. There are 30 trials to complete in this block.
Interpreting the Results
Before you interpret the results, you must understand what the inter-stimulus interval (ISI) is. The ISI is the time from the offset of the dot on the left to the onset of the dot on the right. In this lab activity, the ISI was varied from trial to trial. On some trials the ISI was very short but on other trials it was made longer. In the first block of trials, when ISI is very short or very long, two separate dots are perceived. When the ISI is in between (i.e., moderate length), apparent motion is perceived. Is this what your results show?
In the second block of trials, the distance between the dots was varied along with the ISI. The ISI needed for apparent motion to be perceived increases as the distance between the dots increases. The height of each bar in the graph shows the ISI that produced the most apparent motion at that distance. As distance increases (to the right on the graph), the ISI needed to perceive apparent motion also increases. Is this what your results show?
In the third and final block of trials you were presented with two shapes (a polygon and triangle) separated by different ISIs. At short ISIs, the triangle likely appeared to move through the polygon but at longer ISIs the triangle likely appeared to curve around the polygon. Does the proportion of trials on which you indicated you perceived apparent motion match this pattern?
Real-World Implications of Apparent Motion
Long ISIs allow the visual system to see dots or shapes as being separate. Shorter ISIs make the visual system see dots or shapes as apparently moving. But too short ISIs do not allow you to perceive movement. One would expect the ISI needed to experience apparent motion to be similar across people who have typically developed visual systems. The phenomenon of apparent motion might help survival because it could signal an approaching predator. In the world of entertainment, animators of cartoons take advantage of the apparent motion illusion by making still objects (i.e., individual drawings) appear to move. Flashing pictures of a still image in different locations quickly makes it appear that the image moves from one location to another location. Take for example a drawing of a stick person throwing a ball with separate drawings of the ball in different locations along its path until the last picture when the ball is on the ground. If you quickly flip through the different pictures, it will appear that the ball is actually being thrown. Animators take advantage of this to give the impression that inanimate objects move. The same principal can be applied to sounds. By making the ISI between successive sounds short but not too short it can be made to feel like the sound is moving through space.