Overview
Source: Laboratory of Jonathan Flombaum—Johns Hopkins University
In a staggeringly complex and engaging world, it is crucial to selectively process some stimuli at the expense of others. Experimental psychologists call this ability attention. Specifically, visual attention refers to the ability to selectively process aspects of a visual scene.
Many paradigms used to study visual attention involve brief, punctuated, and repeated trials. However, everyday situations often place sustained demands on attention, as opposed to requiring only brief focus. For example, compare driving through busy city streets, which demands sustained attention, with crossing a busy street, which demands just a few moments of caution. To investigate sustained visual attention, experimental psychologists typically rely on a paradigm called multiple object tracking.
This video demonstrates standard procedures for investigating sustained visual attention through multiple object tracking.
Procedure
1. Stimulus design (Figure 1).
Figure 1: Schematic depiction of a typical multiple object tracking trial. A participant tracks a subset of discs that move randomly among a group of identical nontargets.
- The basic design of a multiple object tracking (MOT) trial is relatively straightforward: begin the trial with a number of identical objects (such as blue discs) in a display. For the typical trial, include 8 discs in total, but varying the number is a crucial manipulation.
- Randomly select half of the discs to turn yellow. Turning yellow indicates to the participant which discs are the targets.
- After they turn yellow, have all the discs become an identical blue color again, and move all, including the non-targets, around the display randomly for about 10 s.
- Instruct the participant to mentally track the target discs.
- After the motion period is complete, stop all the discs from moving, and tell the participant to use the mouse to click once on each disc they believe is a target.
2. Procedure.
- In order to determine how many objects people can track on average, use the basic design described in Step 1 and translate it into a procedure for a whole experiment.
- For this demonstration, find how many objects a group of college-aged males can track.
- Recruit at least 10 participants within the age group (ages 18-22) to participate in the study.
- For students in this age group, researchers on college campuses usually have mechanisms in place by which students can be recruited from Psychology courses.
- When the participants arrive, ask them to consent in writing to participate in the experiment.
- Ask the participants if they have any known eyesight impairments or if eyesight is close to 20-20 once any necessary glasses or contacts are worn. If the participants have close to 20-20 vision, either with or without correction, they are qualified to run in the experiment.
- Seat a participant in front of a computer that runs the experimental program. Ensure that the participant sits approximately 60 cm away from the monitor (a piece of tape on the floor in front of the computer marks the position of the leading edge of the participant’s seat).
- Explain the instructions to the participant, emphasizing that some trials may seem difficult, but they should always do their best, and if they find themselves uncertain about the identity of some targets in some of the trials, then they should just guess.
- Have the participant complete 5 practice trials with only 2 targets in each. Remain in the room, using the practice trials to ensure that the participant has understood the instructions.
- Leave the testing room and have the participant complete 60 trials in a random order with 10 trials each including 2, 3, 4, 5, 6, and 7 targets (and equal numbers of non-targets).
3. Analysis.
- Compute the number of targets correctly identified in each trial (i.e., the number of items selected as targets that were actually targets).
- On each trial, the result is an accuracy score between 0 and 100%. Average these scores together for all trials with the same tracking load. This gives the experimenter a sense of the number of targets the individual can track.
Multiple object tracking is a common experimental psychology paradigm for investigating sustained visual attention.
Sustained visual attention refers to the prolonged ability to selectively process aspects of a visual scene.
For example, everyday situations, like driving a car through busy city streets, often demand sustained attention, as opposed to brief focus when crossing a busy street.
In an exceptionally complex and engaging world, paying more attention to certain stimuli rather than others is critical for safety and survival.
This video demonstrates methods for investigating sustained visual attention through multiple object tracking, including how to design the stimulus, perform the experiment, and how to analyze and interpret data.
In this straightforward experiment, all participants are asked to mentally track specific discs on a computer screen.
In this case, multiple discs are initially presented as identical objects of the same color, like blue. Half of them are then altered to appear as a different color, such as yellow. These yellow discs become the targets, which participants are asked to track.
For each trial, the number of discs to track, referred to as the tracking load, is manipulated and randomly presented to contain 2, 3, 4, 5, 6, or 7 targets with an equal number of non-targets.
Once the targets are identified, all discs return to their blue state and move around a computer screen for 10 seconds.
When the motion period elapses, participants are asked to identify which ones they believe are targets. Here, the dependent variable is the number of targets correctly identified in each trial, also referred to as performance accuracy.
As more targets are tracked, participants are expected to show decreased accuracy. Thus, sustained attention becomes more difficult when the scene is more complex.
To create the stimuli for this study, program identical discs to randomly display on a computer screen. Note that the number will range from four to 14 in total during each trial and the color varies, depending on the trial phase.
To begin the study, greet the participant and ask for consent in writing to participate in the experiment. Make sure that participants do not have any known vision impairments and report 20-20 vision, with or without correction.
After the participant consents to the experiment, have them sit approximately 60 cm away from the monitor. To ensure this distance, use a piece of tape on the floor to mark the position of the leading edge of the seat.
Before starting the program, explain the instructions to the participant, emphasizing that some trials may seem difficult, but they should always do their best and guess if they are uncertain about how to respond.
To make sure each participant understands the directions, load the program and watch them complete five practice trials with only two targets.
As the final step, leave the testing room and allow each participant to complete 60 trials with variable difficulty. Note that the task is programmed to randomize the tracking load during each trial.
To analyze the data, compute the performance accuracy by dividing the number of targets correctly identified by the number of targets assigned for each trial.
To visualize tracking performance, graph the mean accuracy values as a function of tracking load. Notice that participants perform relatively accurately with only 2-4 targets to track.
While performance decreased as the number of targets increased, also notice that performance is closer to chance when participants are asked to track the maximum of 7 targets.
Now that you are familiar with designing a multiple object tracking paradigm, you can apply this approach to answer specific questions about sustained visual attention.
For example, researchers interested in the limits of sustained attention engaged participants in conversation while performing the task. Perhaps unsurprisingly, individuals made significantly more tracking errors when they were distracted-which explains why drivers cannot remain attentive if they multi-task while driving down the street.
In addition, the paradigm has been used to investigate differences in attentional abilities between individuals with Attention Deficit Hyperactivity Disorder, those given therapeutic interventions such as Ritalin, and age-matched controls. In this case, participants given Ritalin showed improved sustained attention, such that fewer tracking errors were made as the tracking loads increased.
You've just watched JoVE's introduction to multiple object tracking. Now you should have a good understanding of how to design and perform the experiment, as well as how to analyze results and apply the phenomenon of sustained visual attention.
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Results
The results of the experiment include several subjects, and the average performance for the group of participants is reported as a function of tracking load (Figure 2).
Figure 2: A depiction of typical tracking performance as a function of target load. Researchers often find that participants perform relatively accurately with only 2-4 targets to track, then suffer large costs when asked to track more than 4 or 5.
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Applications and Summary
For the last 25 years, multiple object tracking has been one of the primary methods for investigating the limits of human sustained attention and the causes of those limits. It can be used to investigate differences in attentional abilities between individuals in different populations, such as those with ADHD compared to age-matched controls. And it can also be used to investigate the efficacy of interventions for improving sustained attention, for instance, the effects of drugs, such as Ritalin or Adderall.
The paradigm has been used productively and influentially in investigating the human ability to multitask. When a person is driving a car down the highway—a task that clearly requires sustained attention—what will the impact be of talking on a cell phone, for example? Asking people to do this for the purpose of an experiment is ethically questionable. Instead, researchers have used multiple object tracking to investigate the impacts of engaging secondary tasks on the performance of sustained attention, including the secondary task of talking on a cell phone. The result, perhaps unsurprisingly, is that participants make significantly more tracking errors when asked to engage in a conversation over the phone with an experimenter in another room.
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