Why do the eyes move?

Vision is an active process • Eye movements compensate for narrow field of vision Image stabilisation • If a visual target is moving Information acquisition • Humans move their eyes while examining objects and scenes

Perceiving the environment: Selective attention

Some aspects of environment are more important and interesting than others. • Visual system has evolved to operate in this fashion • Too much incoming stimulation at retina to process everything. • Selection is achieved partially through use of fovea

How is Selective Attention Achieved?

Scanning a scene - eye movements can take in different parts of a scene Measuring eye movements with eye trackers • Saccades - small, rapid eye movements • Fixations - pauses in eye movements that indicate where a person is attending • Approximately three fixations per second

What Determines Location of Fixations / our selective attention

Characteristics of the scene Knowledge about scenes Stimulus salience - areas of stimuli that attract attention due to their properties • Colour, brightness, contrast, orientation Observer interests/goals Scene schemas Demands of the observer’s task

Knowledge about scenes

Stimulus salience - areas of stimuli that attract attention due to their properties

Observer interests/goals

Yarbus (1967)

Scene schemas

Experiment by Shinoda et al. • Observers’ fixations were measured during computer simulated driving. • More likely to detect stop signs when they were at junctions. • Learned that this is where stop signs are typically placed. • Võ and Henderson (2009) • Less common/expected objects looked at more

Demands of the observer’s task

Looking at location before movement, Land and Hayhoe (2001)

Benefits of Attention

Experiment by Posner et al. • Observers looked at fixation point. • Arrow indicated on which side stimulus was likely to appear. • Stimuli appeared that were consistent (valid trial) or inconsistent (invalid trial) with cue. • Task: push button when target square was seen. information processing is most efficient where attention is directed.

Attention Speeds Responding

Egly et al. (1994) • Observer views two rectangles. • Cue signals where target may appear. • Task: press button when target appeared. • Fastest reaction time was at targeted position. • “Enhancement” effect for non-target was within target rectangle.

Attention Can Influence Appearance

Carrasco et al. • Task: fixate between gratings and indicate orientation of bars with higher contrast. • Small dot was flashed very quickly on one side before gratings appeared Large difference in contrast: no effect of dot. Same contrast: observers more likely to report grating preceded by dot had higher contrast. shift of attention affected perception.

Attention Can Influence Physiological Responses

O’Craven (1999): attending to moving or stationary face enhanced activity in FFA and attending to moving or stationary house enhanced activity in PPA Datta and DeYoe (2009) Directing attention to a specific area of space activates a specific brain area. Womelsdorf et al. (2006) Attention can cause MT receptive fields to shift toward place where attention is directed

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 brain areas

Feature Integration Theory - how does binding occur?

Treisman and Gelade Preattentive stage - features of objects are separated Focused attention stage - features are bound into a coherent perception In 18% of trials participants reported illusory conjunctions, e.g. reporting a small green triangle for display shown here • →Evidence for problem with/limitation of binding

Inattentional blindness

Subjects can be unaware of clearly visible stimuli if they aren’t directing their attention to them.

Change Blindness

Observers were shown picture with and without a missing element in an alternating fashion with blank screen. Pictures had to alternate a number of times before change was detected. When cue is added to show where to attend, observers noticed change more quickly.

Distraction

Task-irrelevant stimuli are stimuli that do not provide information relevant to task. Forster and Lavie (2008) • Load theory of attention • Perceptual capacity • Perceptual load • Low-load tasks

Distracted Driving

Driving requires constant attention. Strayer and Johnston (2001) on cell phone use and driving Anything that distracts attention can degrade driving performance.