Line Motion

(Editor:   Nancy Dess)


If people perceived the world exactly as it is, they would detect motion only when something really moved. But because perception is an active, constructive process, stationary objects can appear to move, a phenomenon called apparent motion. In addition, apparent motion can be induced independently of, and in a direction different from, real motion.

One way of inducing apparent motion is by presenting an attentional cue just before presenting a stationary line. For example, if the line is displayed by itself, it is correctly seen to have appeared in a single piece all at once. If, however, the line is preceded by a brief, attention-grabbing cue at one end, the line appears to grow from that point, creating the illusion of left to right or right to left motion. The entire line appears instantaneously, but one perceives motion due to the attentional cue presented just before the appearance of the stimulus.

This experiment tests the robustness of apparent motion by manipulating the speed with which a line moves onto the screen. The "fastest" speed is 0 msec, which means that the line appears instantaneously, without motion. This condition produces apparent motion from the side where the attentional cue appeared. But what will happen when the line does in fact move onto the screen, manifesting both real and apparent motion? Obviously, if an attentional cue is presented on the same side as the origin of line motion, the cue will only facilitate correct perception, because real and apparent motion are the same. But what will happen when they are opposite: a line moves from left to right but is preceded by an attentional cue on the right? This arrangement pits real motion against apparent motion and provides the test of the robustness of the apparent motion phenomenon.

One can reason that the answer to the question whether real or apparent motion will be perceived on trials that pit the two against each other will depend on the speed of the real line motion. When it is very fast, perception should be similar to the instantaneous condition. But as the speed gets slower, real motion should begin to be detected with greater accuracy. The empirical issue, then, is just how slowly the line has to move to offset the illusory motion induced by an attentional cue.


The two independent variables used in this experiment are both within subject variables. The first is line speed, which is 0 milliseconds for no-motion (instantaneous) trials and is either 15, 25, 35, 46, 55, 65, or 75 msec on real motion trials .There are 10 no-motion trials and 66 real motion trials. The second independent variable, with two levels, is whether the attentional cue and moving line are presented on the same or opposite sides of the screen. Of the 66 real-motion trials, 22 are of the same type and 44 of the opposite type. The left/right position of the attentional cue is balanced across all 76 trials. Because there is just one speed for the Instantaneous trials--by definition, the design is not a completely crossed (or factorial) design. The table below shows the design and the number of trials in each condition.

Sample data image from the Line Motion study
Table 1

Data Analysis

The data can be used to address two questions. First, was apparent motion induced? This question can be answered by determining the proportion of the 10 no-motion trials that produced correct judgements of no motion. For most participants, this value will be near 100%.

Second, which line speeds sustained the illusion? Data from the 15-75 msec trials address this question. Once again, the judgments were categorical-"Left to right," Right to left," or "No motion." Of interest is the proportion of correct judgments for each of the line speeds when the cue was on the opposite versus the same side as where the line begins. The data may look like this:

  Line Speed (msec)
Trial Type 15 25 35 45 55 65 75
Cue Opposite 5% 20% 45% 66% 85% 85% 100%
Cue Same 100% 100% 100% 100% 100% 100% 100%
Figure 1

These data indicate that the probability of making a correct judgment on cue-opposite trials gradually increased as the line moved slower. From this one can infer that the strength of the illusion diminished as the direction of real movement was easier to discern.

To obtain a single descriptive statistic for these data, one can estimate the line speed that would produce optimal conflict between true and illusory perceptions, which is the line speed estimated to produce 50% correct and 50% incorrect judgments. This value -- the "point of optimal conflict" (POC) -- can be estimated using an interpolation process. In the example, the POC falls between 35 msec (45% correct) and 45 msec (66% correct) and is a little closer to the former. How many msec should be added to 35 msec to estimate the POC? This value( Δt ) can be computed by finding the ratio of the difference between 50% and 45% (change in accuracy from the POC to 35 msec) to the difference between 66% and 45% (change in accuracy over the whole 10-msec increment):

Picture of calculation derived from Line Motion experiment
Figure 2

Thus, the POC would be estimated as 35 msec + 2.4 msec, or 37.4 msec. This solution assumes a monotonic, linear increase across line speeds in the probability that observers will experience an illusion.

Data Format and Download

Sample data image from the Line Motion study
Figure 3

The data file includes the following variables for each participant: ID, gender, handedness, age, followed by the proportion of correct motion judgments (left-to-right, right-to-left, or no motion) for each of the conditions in the experiment. The first data column is NoMotion, next are seven columns for Opposite trial data at each speed, then seven columns for Same trial data at each speed. An excerpt of the data is shown above.


The current explanation for the line motion illusion is neural. Attentional cueing leads to faster visual processing on the side of the cue. Faster processing translates into earlier perception. Thus when retinal receptors at and near the attentional focus are stimulated, the message from these receptors is processed faster than the message from receptors whose function has not been enhanced by attention. This general theoretical explanation is dealt with in detail in the references.

A more general lesson to be gleaned from this experiment concerns the nonveridicality of perception. Apparent line motion is experienced subjectively as quite real; it is "projected" in the sense that the motion is perceived as deriving from events in the environment, not in one's head, even when one "knows better." In what sense, and in what sort of circumstances, might "errors" such as apparent motion be problematic -- and when might they be useful?


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Downing, P. E., & Treisman, A. M. (1997). The line-motion illusion: Attention or impletion. 
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