chapter6

Chapter 6: Visual Attention

Scanning a Scene • Visual scanning – looking from place to place – Fixation – Saccadic eye movement

• Overt attention involves looking directly at the attended object • Covert attention is attention without looking

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What Directs Our Attention? • Characteristics of the scene: – Stimulus salience - areas of stimuli that attract attention due to their properties • Color, contrast, and orientation are relevant properties. • Saliency maps show fixations are related to such properties in the initial scanning process.

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Selection Based on Cognitive Factors • Picture meaning and observer knowledge – Scene schema - prior knowledge about what is found in typical scenes • Fixations are influenced by this knowledge

Selection Based on Cognitive Factors - continued • Experiment by Shinoda et al. – Observers’ fixations were measured during computer simulated driving. – They were more likely to detect stop signs when they were at intersections. – People have learned that this is where stop signs are typically placed.

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Task Demands • Influence of the observer’s task – Task demands override stimulus saliency. – Eye movements and fixations are closely linked to the action the person is about to take. – Dynamic environment experiment, Jovancevic-Misic and Hayhoe (2009)

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What Happens When We Attend? • Attention to specific locations is called spatial attention • Experiment by Posner (1978) et al. – Observers looked at a fixation point. – Precueing with an arrow indicated on which side a stimulus was likely to appear. – Stimuli appeared that were consistent (valid trial) or inconsistent (invalid trial) with the cue. – Task was to push button when a target square was seen.

What Happens When We Attend? continued • Results showed that observers responded fastest on valid trials. • Posner believed these results showed that information processing is most efficient where attention is directed.

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What Happens When We Attend? continued • Experiment by Egly et al. – Observer views two rectangles. – Cue signals where target may appear. – Task was to press button when target appeared. – Results show: • Fastest reaction time at targeted position • “Enhancement” effect for non-target within the target rectangle

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Attention Can Influence Appearance • Experiment by Carrasco et al. – Observers saw two grating stimuli with either similar or different contrast between the bars.

– Task was to fixate on center point between gratings and indicate orientation of bars with higher contrast. – Small dot was flashed very quickly on one side before gratings appeared.

Attention Can Influence Appearance. continued • Experiment by Carrasco et al. • Results showed that: – when there was a large difference in contrast, the dot had no effect. – when the contrast was the same, observers were more likely to report that the grating preceded by the dot had higher contrast. – Thus the shift of attention led to an effect on perception

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Attention Can Influence Physiological Responding • O’Craven (1999) – Subject attended to the house or face show that attending to the moving or stationary face caused enhanced activity in the FFA and attending to the moving or stationary house caused enhanced activity in the PPA

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Attention Can Influence Physiological Responding - continued • Datta and DeYoe (2009) – Attention maps show directing attention to a specific area of space activates a specific area of the brain. • Womelsdorf (2006) – showed that attention can cause a monkey’s receptive field to shift toward the place where the attention is directed.

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What Happened When We Don’t Attend? • Inattentional blindness - a stimulus is not perceived even when the person is looking directly at it – Experiment by Simons and Chabris (1999) • Observers are shown short film of teams passing a basketball. • Task is to count number of passes. • Either a woman with an umbrella or a person in gorilla suit walks through the teams. • 46% of observers fail to report the woman or gorilla.

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Change Detection • Change blindness – Observers were shown a picture with and without a missing element in an alternating fashion with a blank screen. – Results showed that the pictures had to alternate a number of times before the change was detected. – When a cue is added to show where to attend, observers noticed change more quickly.

Change Detection - continued • Change blindness also occurs for film shots. • People are “blind” to the fact that they experience change blindness. • Real objects in the environment change with some type of movement, which is why we normally don’t experience change blindness.

Video: Change Blindness

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IS Attention Necessary for Perceiving Scenes? • Experiment by Li et al (2002) – Observers performed one of three tasks • Central task - determine whether letters flashed in the center of the screen are the same • Peripheral task - determine whether faces flashed to the side of the screen are male or female • Dual task - do the same as the peripheral task and determine the color of a disc

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The Distracting Effects of TaskIrrelevant Stimuli • Task-irrelevant stimuli are stimuli that do not provide information relevant to the task • Foster and Lavie (2008) – Load theory of attention

– Perceptual capacity – Perceptual load – Low-load tasks

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Attention and Experiencing a Coherent World • Binding - process by which features are combined to create perception of coherent objects • Binding problem - features of objects are processed separately in different areas of the brain • So, how does binding occur?

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Feature Integration Theory

– Preattentive stage - features of objects are separated – Focused attention stage - features are bound into a coherent perception

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Feature Integration Theory - continued • Illusory Conjunctions - features that should be associated with an object become incorrectly associated with another. • Experiment by Triesman & Schmidt

– Stimulus was four shapes flanked by two numbers. – Display flashed briefly, followed by a mask.

– Task was to report numbers first followed by shapes at four locations.

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Feature Integration Theory - continued • Triesman & Schmidt results showed that: – incorrect associations of features with objects occurred 18% of the time. – asking observers to focus on the target objects eliminated this effect. • Balint’s syndrome - patients with parietal lobe damage show lack of focused attention results in incorrect combinations of features

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Visual Search • Conjunction search - finding target with two or more features – Patients with parietal lobe damage cannot perform conjunction searches well compared to people without such damage. – Parietal lobe is the destination for the where stream.

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Attention and Autism • A major symptom of autism is withdrawal from contact with people. • People with autism can solve reasoning problems about social situations, but cannot function when placed in these situations. • Experiment by Klin et al. – Participants were autistic and nonautistic people. – Watched movie while eye fixations were tracked

Attention and Autism - continued • Experiment by Klin et al. results for nonautistic observers showed: • Looked at eyes of actors to determine emotional state • Looked in the direction a person pointed and then at the face of the person who should reply • Autistic observers look at socially irrelevant stimuli in these situations. • Thus, where autistic individuals pay attention in a social situation may lead to perceiving the world differently.

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Attention and Autism - continued • Experiment by Pelphrey et al. – Measured responses in the STS – Nonautistic and autistic participants watched an animated character that: • moved eyes toward a checkerboard (congruent condition). • moved eyes away from a checkerboard (incongruent condition).

Attention and Autism - continued • Observers pressed a button when they saw the character’s eyes move. – All participants performed at 99% accuracy. – But, activation of the STS for nonautistic people was higher in the incongruent condition. – Autistic people showed equal activation in both conditions. • Results suggest that autistic people cannot read intentions of others.

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Attention and Perceptual Completion • Perceptual completion is the perception of an object as extending behind occluding objects • Habituation – one stimuli is presented to an infant repeatedly and the infant’s looking time is measures – Dishabituation is an increase in looking time when a stimulus is changed

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