Sensation & Perception, 4e

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Visual Search

Introduction

Visual search is one of the most venerable paradigms in cognitive psychology. It is the laboratory equivalent of tasks like finding your car in a crowded parking lot, locating your favorite novel on a full bookshelf, or searching for the last red jellybean in a candy bowl. Search experiments can help us determine how the visual system allocates attention to individual items in multi-element scenes.

In the visual search tasks you’ll do here, your job will always be to determine whether the display at left contains a red vertical line or a blue vertical line (the sample display you see now contains a red vertical line).

Click one of the links at left to start doing a feature, conjunction, serial, or dynamic search task. You’ll get more instructions after you’ve chosen a task. It takes about 20–30 trials for the data to start showing the trends we will be discussing, so make sure to do each task for a minute or two before moving on to an explanation of the results.

Instructions

Click the START button to begin a trial. You’ll see a fixation cross, then a display with 5 or 17 items. Your job is always to determine whether there is a red vertical line or a blue vertical line in the display. (There will always be one or the other of these two stimuli, never both.) The distractors will change in different search tasks, but you’re always looking for a red or blue vertical line. As soon as you’ve determined whether the target is red or blue, click the appropriate button at the bottom of the screen. You’ll then see the results of that trial as well as the cumulative results of all the trials you’ve done so far.

When the results come up, you’ll be given an opportunity to disregard the last trial you did, in case you made a mistake. Only response times for correct responses will be taken into account in the results.

If this is the first time you have done this module, it will work best if you go through the search tasks in the order listed below.

Feature Search

This is the easiest type of search task you’ll do, because the distractors (the items in the display other than the target) are distinguished from the target by a primitive feature, in this case color. That is, the target will be red or blue (in the sample screen you see here, it is red), while all the distractors will be green.

Click the START button to start a trial. The first thing you'll see is a cross in the center of the screen. Fix your eyes on this cross. Then an array of elements will appear, and you should click either the RED or BLUE button as soon as you determine whether there is a red or blue vertical line in the display. Your response will be timed, so click one of the buttons as quickly as you can!

Click the START button now to begin a trial. After each trial you'll see a summary of your results so far.

Typical Feature Search Results

You probably found that it was just as easy to do this task when there were 17 items in the display as when there were five items. This is known as a parallel search, because you seem to be able to examine all items in the display at the same time (i.e., in parallel). Another reason we call it a parallel search is that if you were to draw a line graph of reaction time (RT) plotted against the number of items in the display, the line would be parallel to the x-axis. That is, the line would be flat, with a slope of 0 (if you’ve done at least 20 trials or so, the slope that you see for the feature search in the results screen should be something close to 0).

Click the START button to begin another trial, or click one of the links at left.

Conjunction Search

You should find this task to be more difficult than the feature search because now most of the distractors will share one feature in common with the target. For example, in the sample screen you see here, the target is red and vertical (remember that you are also searching for blue vertical lines). Some of the distractors are also red (or blue) but horizontal, while other distractors are vertical but green. Therefore, you must search for the conjunction of “redness” (or “blueness”) and “verticalness.”

Click the START button to start a trial. The first thing you'll see is a cross in the center of the screen. Fix your eyes on this cross. Then an array of elements will appear, and you should click either the RED or BLUE button as soon as you determine whether there is a red or blue vertical line in the display. Your response will be timed, so click one of the buttons as quickly as you can!

Click the START button now to begin a trial. After each trial you'll see a summary of your results so far.

Typical Conjunction Search Results

Searches for most conjunction targets cannot be done in a parallel fashion. For every extra distractor that is added to the display, you need a little more time (on average) to find the target. Therefore, the slope for this task should be significantly greater than 0, assuming you’ve done at least 20 trials or so. Slope values for this particular conjunction task typically range from about 10 to 30 ms/item, depending on the individual (and this isn’t an intelligence test—low slopes on visual search tasks won’t get you into medical school!).

Why is this task more difficult than the simple feature search? Cognitive resources (i.e., “attention”) are needed to combine multiple features (e.g., orientation and color) into a representation of a single object. Allocating these resources takes time, and the more distractors there are in the display, the more resources need to be allocated and the more time it will take.

Click the START button to begin another trial, or click one of the links at left.

Serial Search

This task is also a type of conjunction search, but it is so hard that you will probably have to perform a serial search to find the target. That is, you will need to examine each item one at a time to determine whether it is the target or a distractor. As you can see in the sample screen here, every item in the display has a vertical element, a horizontal element, a green element, and either a red or a blue element. But only one item has an element that is both vertical and red or blue (in this case, it’s red).

Click the START button to start a trial. The first thing you'll see is a cross in the center of the screen. Fix your eyes on this cross. Then an array of elements will appear, and you should click either the RED or BLUE button as soon as you determine whether there is a red or blue vertical line in the display. Your response will be timed, so click one of the buttons as quickly as you can!

Click the START button now to begin a trial. After each trial you'll see a summary of your results so far.

Typical Serial Search Results

This search task highlights the binding problem that the visual system faces when confronted with complex objects in a scene. Since each of the elements in the display contains “greenness,” “redness” or “blueness,” “horizontalness,” and “verticalness,” it is difficult to successfully bind the right colors with the right orientations. In fact, it is so hard that we seem to need to deploy attention in a serial fashion (that is, one element at a time) to successfully find the targets in these displays. Thus your slope for this task (assuming you’ve done enough trials) should be considerably higher than the slope for the feature and conjunction tasks.

Click the START button to begin another trial, or click one of the links at left.

Dynamic Search

In this task, the displays will be the same as in the serial search task, except with one major difference. All the items on the screen will switch positions every 300 milliseconds, as seen in the sample display here. Your intuition probably tells you that this will make the task much harder, and in one sense you are right. But in another important respect, this task turns out to be equally as difficult as the static serial search. These cryptic statements will be explained when you click the link to read about typical dynamic search results from the results screen. But first, do some trials to collect some data of your own so that you get a sense of how this task works.

Click the START button to start a trial. The first thing you'll see is a cross in the center of the screen. Fix your eyes on this cross. Then an array of elements will appear, and you should click either the RED or BLUE button as soon as you determine whether there is a red or blue vertical line in the display. Your response will be timed, so click one of the buttons as quickly as you can!

Click the START button now to begin a trial. After each trial you'll see a summary of your results so far.

Typical Dynamic Search Results

As you probably guessed before you started this task, it usually takes people considerably longer to find targets in a dynamic search display than in a static search display. In other words, you should find that the average RT for five items is longer for the dynamic than the serial search task (and likewise for 17 items). Surprisingly, however, researchers have found that the slopes for these tasks are actually very similar (is this what you found for your results?).

Most theories of visual search hold that difficult search tasks are done in a serial, self-terminating fashion. That is, attention is shifted from item to item until the target is found, at which point a response is made. For this strategy to work well, the searcher must keep track of which items they have already examined. Otherwise, the search will go on much longer than necessary, since many distractors will be re-examined over and over again.

In the dynamic search task, there’s no way that you could keep track of which items have been examined, since all items switch places every 300 ms. Therefore, the fact that searches in dynamic and static displays take the same amount of time per item (i.e., have the same slope) calls into question the idea that attention is deployed in a serial, self-terminating manner. That is, we may not actually keep track of all of the locations of items we have checked during a search.

Click the START button to begin another trial, or click one of the links at left.

Results So Far

Last RT:
Current task: search

Task 5 Items 17 Items Slope (ms/item)
Trials Avg. RT (ms) Trials Avg. RT (ms)
Feature Search
Conjunction Search
Serial Search
Dynamic Search

If you made a mistake while performing the last trial (e.g., you were distracted by something in the environment or your first click didn't register), click here to disregard the trial.

Click here to reset all results for this task.

Click the START button to begin another trial.

 
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