Visual attention- week 9

MBB1

What is Attention

• is the ability to preferentially process some parts of a stimulus at the expense of processing of other parts of the stimulus.

• For example, if you focus your attention on my face, you will preferentially process my face at the expense of processing other objects in the scene.

• Consequently, you will perceive my face more clearly than other objects in the scene

Why is attention needed

• Your perceptual system has a limited capacity.

• You can’t process everything in the visual scene simultaneously

• For example, in front of me now there are lots of people.

• I can’t look at everyone at the same time.

• To avoid being overwhelmed, I pay attention to only one person at a time and ignore the rest.

• Attention therefore helps us avoid becoming overwhemed

Overt attention

involves looking directly at an object

Covert attention

involves looking at one object but attending to another object

Monitoring Attention

• Unless you purposely try not to, generally you always look at (i.e. fixate) the object to which you attend

• Thus, we can generally tell where someone is attending just by tracking their eye movements.

• When a person looks at an object, they are said to fixate it.

• The eye movements between fixations are ballistic (i.e. very fast).

• These eye movements are called saccades. 16

Fixation

• The rests between the jumps, where the eyes stay looking directly at one part of the scene, are known as fixations.

• is determine by your goals and expectations.

saccade

• Your eyes do not scan over a visual scene smoothly.

• Instead, they jump from point to point.

• They jumps are known as

What directs our attention?

• Two processes:

• An initial involuntary process (mediated by attentional capture)

• A subsequent voluntary process (guided by your goals and expectations)

• Initially, when a scene is first presented, your fixations are captured by salient parts of the scene

• This phenomenon is known as “attentional capture” and is involuntary.

• After the first few fixations, you can then direct your fixations according to your goals

• This process is voluntary

Attentional capture

• Determined by salience of image/object

• Salience = the quality of being noticeable

• Here, attention is first directed to the red match-head as it is the most salient object in the scene

• The task was to report the orientation of the line in the square

• The red circle was irrelevant.

• However, because it was salient, attention was initially directed to it, and participants often reported the orientation of its line

What Captures Our Attention?

• Basically…contrast

  • Regions of colour contrast or luminance contrast

  • Regions of size contrast

  • Regions orientation contrast

  • Regions of motion/flicker contrast

What directs our attention?

• Saliency determines what we attend to first (i.e. the first few fixations)…

• …after that what we attend to is determined by cognitive factors such as the observer’s goals and expectations

• Summary

  • Initially, fixations are often involuntary

    • attentional capture

  • Then, the fixations are voluntary, goal directed and are influenced by our expectations

Expectations

• Fixations not only determined by goals

• Your expectations also determine your fixations

• If an object is unexpected, you will fixate on it for longer and fixate it more often

What Are the Effects of Attention?

• Attention speeds responses

• Attention can influence appearance

• Attention can influence physiological responding

Attention Can Change the Apparent Contrast of an Object

• Required to report orientation of higher contrast grating

• When both gratings were of equal contrast, typically reported orientation of cued grating…

• …suggesting that cued grating appeared to be higher contrast

Attention Can Change the Appearance of an Object

• So Carrasco et al. (2004) showed that attention can make objects appear to have a higher contrast.

• Other studies have shown that attention can make objects appear bigger, faster, and more richly coloured (Anton-Erxleben et al., 2007; Fuller & Carrasco, 2006; Turatto et al., 2007).

• Essentially, attention makes perception more vivid.

• Attention affects not only how quickly a person can respond to a stimulus but also the appearance of the stimulus.

Attention Can Influence Physiological Responding

• Attention can also affect the physiological response to a stimulus.

• Essentially, neurons in the brain respond more strongly to attended stimuli than to unattended stimuli

The Binding Problem

• Different aspects of a stimulus are processed independently, often in separate brain areas.

• For example, motion is processed by the dorsal stream and form is processed by the ventral stream

• The issue of how an object’s individual features are combined (i.e. bound) to create a coherent percept is known as the binding problem

• The problem becomes more difficult when there are multiple object

• How are you able to see a red vertical bar and a green horizontal bar?

• Why don’t you see a green vertical bar and red horizontal bar?

• How are you able to associate the correct colour with the correct object?

Feature Integration Theory

• Feature Integration Theory (FIT) suggests that the binding problem is solved by attending to only one location at a time.

• Crucially, only features associated with that location are processed, so only those features are bound together.

• This avoids binding features from different objects

Illusory Conjunctions

• A prediction of FIT is that if attention is inhibited, features from different objects will be incorrectly bound together.

• Treisman & Schmidt (1982) showed that such illusory conjunctions occur

• They presented character strings very briefly (95-168 ms) followed by noise mask.

• The primary task was to report the two numbers.

• Then O’s were asked to report coloured letters.

• O’s often associated the wrong colour with the wrong letter

• Such incorrect bindings are known as illusory conjunctions

Balint’s Syndrome

• RM is a patient who has parietal lobe damage.

• As a result, has a condition called Balint’s syndrome.

• When multiple objects are present, RM has difficulty focusing attention on a single object.

• When shown two letters, each with a different colour, reported the wrong letter colour combinations on 23% of the trials, even when allowed to view the letters for as long as 10 seconds!

• So, RM very prone to experiencing illusory conjunctions because he could not focus attention on just a single object.

Visual search - that requires binding

• Some forms of visual search require binding to occur.

• For example, binding is required if the target contains the same features as the distractors

• If the target differs from the distractors only by its particular conjunction of features then that is a conjunction search

• FIT predicts that in conjunction searches attention needs to be applied to each object in turn (i.e. one at a time) to determine whether or not the attended object is the target

• Thus, these searches are predicted to be very slow

Visual search that does not require binding

• Other types of visual search can be performed without solving the binding problem

• Here the target contains a feature (red) that the distractors do not contain

• This is known as a feature search

• FIT predicts that because binding does not need to occur, attention does not need to be applied to each item in turn.

• Thus, such searches are predicted to be fast.

Change Blindness

• Attention can also determine what we remember.

• If you don’t attend to it, chances are you won’t remember it.

• You are now going to be shown two images in succession.

• Your task is to spot the two differences between the two images

• You can remember only a few parts of a scene at one time.

• If one of those parts change, you notice the change

• If some other part of the scene changes, chances are you won’t notice the change – change blindness

Motion transients- Why doesn’t change blindness occur all the time?

Because changes usually generate motion transients that draw attention to the location change, thereby making it easy to spot the change.

Motion Transients

• In the first demonstration, a blank screen was inserted between images.

• This meant that when the second image was shown, motion transients occurred for every part of the image - not just the parts that changed.

• This meant that motion transients did not guide attention to the change.

Change blinds also occurs

• Change blindness doesn’t just happen in static pictures.

• It can also occur in the real world – as demonstrated by Simons and Levin (1998)