Chapter 9 Visual search

From time to time, we all need to find things. Rummaging through our closet for a particular shirt, or wandering about the house trying to find our keys, or for some of us, groping about for our spectacles that we know we put down somewhere around here.

Our search performance in some tasks can reveal aspects of the bottlenecks in mental processing. Slow search can suggest a bottleneck is affecting processing. However, sometimes search is slow because the basic sensory signals are not good. For example, when I lose my spectacles, my vision is so poor that I have to bring my face close to each location in the room to check whether my glasses are there. Similarly, wandering about the house looking for one’s keys, to evaluate all the rooms of the house, one has to visit each room.

Sometimes, even though something is right in front of our face, the sensory signals aren’t good enough for it to be possible to know that the object is there. For example, try searching for the word “wilt” in the below image, which shows the first two pages of Shakespeare’s Romeo and Juliet.

The first two pages of Romeo and Juliet, by William Shakespeare.

The first two pages of Romeo and Juliet, by William Shakespeare.

Did you find “wilt” yet? To find it , your eyes have to move back and forth (it’s about 3/4 of the way down the left page). The task is impossible to do without moving your eyes. The main reason for this is that the sensory signals provided by your retinas are only good enough to read small words near the center of your vision. So, you have to move your eyes.

To see this sad fact about your vision more directly, try the following. Stare directly at the black cross and, while keeping your eyes fixed on the cross, try reading any of the words on the bottom of the page. You can’t do it. Not because of any bottleneck, or problem with selection, but simply because your photoreceptors are too widely spaced in the periphery. That is, outside of a central region, the spatial resolution of your vision is too low to see many details. The sensory signals from the periphery are coarse.

9.1 Information overload

A good way to assess whether there is a bottleneck in a system is to give it more and more things to process and see whether this degrades performance or whether the system can process each just as quickly as when it is given just one. Psychologists did this for visual processing by giving people many stimuli to process, by adding more and more to a display. In doing this, however, they had to be careful to make sure that the brain had a chance, by making sure that a person could see each individual stimulus even when it wasn’t in the center of their vision (unlike in the Romeo & Juliet demonstration above). If the person couldn’t even see the stimuli, then of course the brain wouldn’t process it well even if it didn’t have a bottleneck.

One of the tasks psychologists have used for this is called “visual search”. In a visual search experiment, people are shown a display with a particular number of stimuli and asked to find a target. This is discussed this in first-year psychology. The next section is, in part, a review of that.

9.3 Processing one thing at a time

But parallel search doesn’t happen in most cases for combinations of individual features. Instead, there is a bottleneck. To put that in context, I will remind you of aspects of parallel versus serial processing.

Imagine you were in an art installation where the artist had hung many speakers from the ceiling, and each speaker played a different person’s voice, each telling a different story Well this is precisely the situation I was in one day when I visited a museum in Havana, Cuba. What I heard sounded like an incoherent jumble. I didn’t hear any of the actual stories being told by the voices until I moved my ear up against an individual speaker. In other words, I could only process a single auditory stimulus at a time, and to do so, I had to select it.

A forest of speakers is not a situation you are likely to encounter. It does illustrate, however, one possibility for sensory processing - for certain things, you may be unable to process multiple signals at once. In that case, you need to select one stimulus to concentrate on it.

An art installation in Havana, Cuba

Fortunately, our visual brain can process certain aspects of the visual scene in parallel. But for combinations of features, you are in much the same boat as I was in that day in Havana, having to select individual locations to evaluate an aspect of what is present - specifically, the combination of features there.

9.6 Visual search and blank-screen sandwiches

Recall the blank-screen sandwich change detection animations of Chapter 6. In a typical blank-screen sandwich experiment, people are timed for how long they take to find the change happening in a photo of a scene. To better assess what is happening with attention, one can use a carefully crafted visual search display instead of a natural scene.

Rensink (2000) developed this technique.

As schematized above, the participant was shown blank screen sandwiches with one object changing, and how long it took them to indicate the location of the changing object was recorded. The displays were shown for 800 ms and the blank screen was shown for 120 ms.

Rensink’s hypothesis was that evaluating whether a change is present can only be done for one or a few items at a time, by attentionally selecting that location.

  • What do you predict should be the effect on number of items in the display on response time?

Here are the results:

On some trials, the target was absent (the unfilled triangles), and participants likely did not respond until they had evaluated every object in the display so they could be sure nothing was changing.

More important is the results for the trials when the target was present. The filled triangles show a steep increase in search time with number of objects in the display.

  • Was this predicted by the hypothesis?

9.7 Exercises

  • Why do people need to move their eyes for many searches?
  • What factors can make visual search slow?
  • Describe how the kinds of selection connect to visual search performance for different types of display - learning outcome #5 (2).
  • How does the finding for visual search performance for feature conjunctions relate to the rate limit found for pairing simultaneous features in the previous chapter?


Rensink, Ronald. 2000. “Visual Search for Change: A Probe into the Nature of Attentional Processing.” Visual Cognition 7 (1): 345–76.