Chapter 14 Divided attention and objects
You already know about selecting a location with attention. You can actually select multiple locations, and it seems you can select them simultaneously rather than your attention having to switch back and forth between them. However, as we’ll see in the next section, we can only select a very limited number of locations or stimuli at one time.
You’ll recall that higher-level processing is so limited that even identifying a simple feature of two objects yields much worse performance than identifying one (3). However, if all a person needs to do is keep your attention on multiple locations, without trying to identify things there simultaneously, a person can do pretty well with more than two objects. This is consistent with the idea that any capacity limit on selection is not as severe as that on processing things for identification.
14.1 Tracking moving objects
In the real world, objects of interest are often moving, or your eyes are moving, or both! When playing football, for example, while every point in the scene is processed by the first layers of neurons in cortex, certain parts are of special interest to continuously be aware of, like the location of the ball and of a defender. Thus, it pays to keep attention on those locations to ensure they are fully processed.
If all we had was feature selection and location selection, then if we were interested in monitoring a particular object, like the football, or a child playing in the ocean at a crowded beach, every time the object moved, we would have to find it all over again to attend to it. Finding it would likely require a visual search, which can be time-consuming (9). Even if it only took a third of a second to find the ball, for example, during that time a penalty kick traveling at 30 metres per second will have travelled a full ten metres.
Fortunately, attentional selection can easily follow a moving object. This is usually studied with the “multiple object tracking” procedure. First a bunch of identical objects appear on the screen.
After the objects appear, some of them are cued by briefly appearing in a different color or by flashing. The participant’s task is to keep track of the cued objects as they move around. Try it here.
The task feels pretty natural. Once one selects an object with attention, if it starts moving, your attentional focus will tend to move along with it. What we have previously been referring to as location selection (7) may best be thought of as object selection, because if an object in a selected location starts moving, your attention naturally follows along rather than sticking to its original location. Magicians like Apollo Robbins exploit this, moving their hand or another objects of interest with smooth gestures, knowing that if they do that, the viewer’s attention is likely to come along.
This fact that attention can select objects has important implications. For it to be possible for attention to select objects, some think the visual system must have first processed the image preattentively into objects, otherwise attention may not have been able to select an entire object. This is similar what Treisman used when arguing that processing of individual features occurred prior to the action of attention (preattentively) - without that pre-processing, attention could not go straight to a stimulus with a particular color.
But what does the preattentive visual system consider an object, that attention can then select? This was investigated by VanMarle and Scholl (2003). In what they called a “substance” condition, objects sort of poured from one location to another.
.
People performed much more poorly when trying to track the pouring “substances” than they did when tracking the normal objects. Performance was similarly poor in a related ‘slinky’ condition.
Experiments like that of VanMarle and Scholl (2003) give an indication of how our visual system carves up the world into objects prior to the action of our attention. Our visual system not only carves up the world into objects, it subsequently groups objects based on how they move in relation to one another or are connected to each other.
.](movies/MOTmovies/beesAndBombsHierarchicalMotion.gif)
Figure 14.1: An animation by beesandbombs.
The lines in the above animation fade in and out. When the lines are present, you may see the circles moving in individual revolving triplet groups. This is an instance of grouping by motion. When the large lines are present, the visual system unifies the discs of each color into a large circle structure.
14.2 The limits on tracking implicate slow, capacity-limited processing
People can only attend to a limited number of moving objects at a time. This is unsurprising, because the nature of attentional processing is that it is a limited resource for more extensive processing.
That limited resource also seems to be doing some of the work or computation required to keep attention on a moving object. One indication of this is that the maximum speed for tracking one target is faster than the maximum speed for tracking two targets. Measure your speed limit here for one target and here for two targets. What were the two speeds, and which one was faster?
The processes that allow attention to keep up with moving objects are still mysterious. But we do know something about the limits on those processes, as illustrated here. These limits are much slower than those of basic motion perception (Verstraten, Cavanagh, and Labianca 2000; Alex O. Holcombe and Chen 2012). This is probably because tracking is mediated by high-level processes rather than the specialized processes that mediate motion perception.
14.3 Test yourself
You can test your multiple object tracking ability more quantitatively at the following site:
https://www.testmybrain.org/classroom/
Username: education
For the password, see Alex’s Canvas module.
The test first tests your forward digit span, then your backwards digit span, tests that you may have heard about in memory and intelligence lectures, and then measures speed thresholds for tracking.
Is there any relation between tracking ability and intelligence? We don’t really know. But in the Testmybrain sample of thousands of people, those who did their degree in a STEM field (science, technology, engineering, or medicine) did better than those who did their degree in another field, such as arts or law (Treviño et al. 2021).
However, the difference is not large. If you took a random STEM person and a random non-STEM person, the overlap of the distributions above implies that a STEM person would have a 55% chance of having gotten a higher score on the MOT test. In other words, there are plenty of non-STEM people who have higher tracking performance than the average STEM person, but on average STEM people do slightly better.
14.4 Exercises
- In 7, you learned about three kinds of attentional selection. How does this chapter change your interpretation of location selection?
- What does the fact that attention can select objects have to do with magic tricks?
- What have you learned about what processing occurs prior to a bottleneck?