1 Perception and VR MONT 104S, Fall 2008 Session 13 Visual Attention

2 Visual Attention We don't see the entire scene we are looking at in detail. Only the region that is imaged in the fovea can be seen clearly and in detail. Peripheral regions are blurry and indistinct. (Try reading with words without looking directly at them). We only process in detail regions we are paying attention to. We don't have a detailed representation of other regions of the scene. Attention is a way for the visual system to apply detailed processing to select regions of the scene.

3 Where's Waldo? If we are looking for something in the scene, we must shift our attention around the scene to find it.

4 Visual Search Experiments We can study how people move their attention around using visual search experiments. Subjects are asked to look for a target figure among distractors. They must respond as quickly as possible saying whether or not the target is present. Their reaction time is measured. The number of items in the scene is varied.

5 Example 1: Find the Blue Square

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7 Example 2: Find the B

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9 Example 3: Find the O

10 Example 4: Find the Orange Square

11 Two types of search Efficient Search: As the number of distractors increases, the reaction time stays the same. (e.g. blue square among red squares). Inefficient Search: As the number of distractors increases, the reaction time increases. Reaction time Number of items Efficient Inefficient

12 What's happening Some features are processed efficiently, possibly in parallel, across the visual field. These basic features include color, orientation, shapes, motion. Search for items based on single features leads to efficient search. The items "pop out" of the surrounding distractors. Search for conjunction of features is inefficient. An example would be searching for a red square among blue squares and red triangles. We must examine each item individually to find the target.

13 What this tells us Inefficient searches show that we are not aware of everything in the entire scene at the same time. We constantly shift our gaze and our attention to look at different parts of the scene and examine them in detail. We think we see the scene in detail, but we don't.

14 Limits of attention Task: Count the number of passes made by the black team: While paying attention to a sustained task, we may be unaware of other events. When we do not see something in the scene because our attention is directed elsewhere, this is known as inattentional blindness. Discussion question: What are the practical effects of this in daily life?

15 A short film clip Watch the clip: Describe what you saw.

16 Change Blindness during Eye movements We are constantly making rapid eye movements, known as saccades, as we scan a scene. Vision is suppressed during saccades. People fail to notice large changes in the scene if the change occurs during a saccade. (McConkie, Grimes, Ballard and others). People also fail to notice large changes in the scene if they occur during a brief disruption (e.g. short blank period). This is known as change blindness. (Rensink et al., 1996) Demo:

17 Movie Cuts can also cause Change Blindness A cut between scenes, with a change in camera angle, can also induce change blindness. Simons and Levin showed this in a series of experiments. Can you detect the changes? Only 1 in 10 people detected a change. Change blindness occurs even for objects that are the center of attention: Only 33% of 40 people noticed the main change. Disruptions in real life can also lead to change blindness:

18 What's Going On? What causes change blindness? 1)We don't see the entire scene in detail. a)Only the region attended to is seen in detail. b)We constantly shift our eyes to see other parts of the scene in detail. 2)Only attended regions get into short term memory. a)Briefly presented pictures are quickly forgotten (Intraub, 1981; Potter, 1976) b)We must serially scan the picture, item by item, to find the one that is changing. 3)Attention is not enough. a)Change in actors show that attention is not enough. b)We must intentionally process the details in order to detect the changes.