This worksheet is provided as a webpage. You can Save as / Print it to PDF if you like. To write on it, you could then open the PDF in Word.

Regardless of what you are paying attention to in a scene, the entire retinal image is processed a lot prior to your conscious experience of the associated visual information. This form of unconscious processing is sometimes called pre-attentive processing.

Some processing of visual information occurs only via selective attention to that information. This is the reason for the different effects of number of distractors on different visual search tasks, as explained in the visual search chapter of the mini-text.

To bolster your understanding, today you will:

  1. Participate in a mock visual search experiment
  2. Examine the data from the experiment.
  3. Consider a real-world case study where attention fell short.

Part 1: Visual search and limited processing capacity

LEARNING OUTCOMES:

  1. Know what sort of shape differences between target and distractors cause visual search to be rapid or slow.
  2. Know how feature selection’s ability and limitation affect visual search rate.

See the “Summing up” section at the end of this page for more things you need to know.

2. Click on “Start Visual Search Task” and complete the practice trials.

Do you have any questions about the task? If so, ask your tutor.

3. Complete only the first block of non-practice trials.



In that first block of trials you should now have completed, the target was the letter ‘O’.

In some trials, there were lots of distractor letters, and other trials had very few distractor letters. Do you think the number of distractor letters had much influence on how long it took you to find the target?





4. The search target in the second block is a green N. Do that second block of trials.



Was that easier or harder than the previous task? _____________________

Write one or two sentences explaining why this task should be easy or hard.

(The feature selection and visual search chapters are relevant.)






5. The search target in the third block is anything blue (it’s a very dark blue).



Was that easier or harder than the previous task? _____________________

Write one or two sentences explaining why this task should be easy or hard.





The data from the visual search task could be graphed on a plot like this:

On some trials, there were only a couple distractors that you had to hunt through to find the target. On other trials, there were a lot of distractors you had to hunt through. Imagine calculating the mean response time for each kind of trial. Add dots to the plot to show what you suspect the data would look like. If you don’t know how to draw on your computer, draw the plot on a piece of paper.

6. Imagine that in the search for the green N from step 4, all the distractors were red. How would this affect search performance?







If more distractors increases response time by a lot (a steep slope), that is the sign of capacity-limited processing being required, while a flat slope indicates pre-attentive parallel processing is sufficient to find the target (see the visual search chapter of the mini-text).

7. Change blindness

As the visual search experiment illustrated, only a limited number of regions of the visual scene can be processed extensively at the same time, and a consequence of that is that for some tasks, a time-consuming serial search is required. By “serial search”, we mean that not all objects could be evaluated simultaneously for whether they are the target.

These issues also manifest in change blindness with, for example, the blank screen sandwich videos discussed in Chapter 6, and shown here:

(If you don’t see an animation, try http://bit.ly/sphinx2016)



7A. How does this change blindness phenomenon illustrate limited processing capacity?




7B. You probably moved your eyes around a lot as you looked for the change. Why is that?




7C. Thought question for discussion: Imagine that your vision was equally good throughout the visual field, rather than being best in the center. In that scenario, what would it mean to “look at” something?






Part 2 - What are some implications for real-world situations?

Real-world case study: Professor Alex Holcombe’s driving test

A long time ago, Alex moved from the US to the UK (this was before he realized that Australia was the best place to live!). In the UK, he was told that his American driver’s license wasn’t good enough, and to get a UK driver’s licence, he needed to take driving lessons plus a road test.

Alex dutifully took a bunch of driving lessons, and then took a driving test, where the test examiner had him drive around the city for a while. The picture shows a small part of the road test score sheet used by the examiner. Failing the test occurs if a driver commits one “serious fault” and/or one “dangerous fault” (minor faults are also a thing, but it takes 15 of them to fail).

A few minutes after the test drive, Alex received his results:

Let’s zoom in on the bottom portion:

Twelve faults were recorded. Eleven of them are considered minor, including four for having a following distance of less than two seconds’ time behind another car. The tick mark in the “traffic lights” row is in the “S” column (column label not shown), which means “serious” fault. The examiner ticked that column because Alex had not braked for a red light they were approaching, until the examiner felt he had to intervene and press on the brake. Oh no! This means that Alex failed the test!

Alex felt humiliated.




Here is a photograph of a road scene similar to the one where Alex failed to respond quickly enough to the traffic light:

A temporary traffic light.

A temporary traffic light.





Why might Alex have made the mistake that meant he failed the test?

Was it because he was:

  • Just plain stupid? (Sydney Uni may have made a big mistake when they hired him!)
  • An inexperienced driver?
    • No - he’d been driving in the U.S. for 15 years, driving over three thousand kilometers most years, with no significant accidents, including on two 5,000 km cross-country trips.
  • An absent-minded professor?



1. What do you think?





2. What are some differences between the change blindness animations and the situation where Alex failed to respond to a red light?





3. Why do you usually notice traffic lights when you’re driving?







4. How does the scene with the temporary traffic light above differ from a typical intersection?







5. What reasons can you think of for why you might not notice a traffic light?







6. Watch the Vision in Driving video made by a driving instructor.

Only the first 3min 17s are important.

What points does he make about eye movements?

By the way, when he uses the word “blurred”, he may not know it but he’s referring to the poor spatial resolution of the periphery (discussed in this chapter), not the periphery being literally optically blurred.





7. Where do you think people point their eyes when approaching an intersection?







8. Watch this driver eyetracking video. What does the driver look at?

The red dot in the movie shows where the eyes are looking, as registered by an eyetracker the driver is wearing. The movie shows where he looks as he drives. Unlike the previous video, the creator does not blur the parts of the scene far from the centre of gaze, but please don’t forget that the driver has good resolution only for the parts of the scene close to the centre of gaze. Where does the driver look when approaching a traffic light?







Summing up

  1. For the three searches, involving the green N, the O, and the blue targets, how did the results differ from each other and why?





  1. How are eye movements important in visual search?





  1. Why might eye movements be important in traffic light detection?





  1. How is Alex’s UK driving test relevant to this unit of study?