Prior knowledge the visual system constructs a

• Prior knowledge = the visual system constructs a clearer experience that one that is provided by the
stimulus alone.
• The context in which objects appear is important.
Context effects: The influence that the situation plays on the perception of a stimulus.
Moon illusion : The tendency for the moon to appear larger when on the horizon than when high in the sky.
Apparent distance theory : an explanation for the moon illusion; It posits that the moon on the horizon appears
larger because “distance” cues lead the observer to perceive it as being nearer that the zenith moon.
Letters in context
Jumbled word effect: The ability to read words in sentences despite having mixed-up letters in the middle of
some of the words (see definition p. 63)
• expectations for what the words in the sentence will be help determine what the words actually are.
• position of jumbled letters matters: middle = reading time not affected
beginning= ↑ reading time

Word superiority effect : It is easier to identify a letter if it appears in a word that if it appears alone
Parallel distributed processing (PDP): a model of perception that proposes that different features are processed
at the same time by different units connected together in a network.
• Attempts to specify the microstructure of cognition
 McClelland and Rumelhart (1981): model of context effects in letter perception, tries to
explain effects such as word superiority.
 units corresponding to basic features of letters
 a unit is activated if it present in letter being perceived
 Excitatory and inhibitory connections between units determine what we end up seeing.
Colours in context
• Top-down processes have an impact on colour vision
• The perception of colour is influenced by the perceptual context in which an object appears
 Purves and Lotto (2003): provided examples of contextual effects on colour vision
Empirical theory of colour vision : Colour perception involves not only the processing of
wavelengths of light, but also the influence of prior experiences about how different lighting
conditions affect the appearance of the colours of objects.
Cross-modal context
• Contextual influences can cross modalities (e.g. ventriloquism).
• We perceive something different from what it actually is because of prior
expectations/knowledge.
 McGurk and Macdonald (1976): Extent to which auditory info can be affected by visual processing
 2 participants stand one behind the other
 The person in front makes mouth movement: syllable “ga” but no sound
 The person behinds says “ba”
 Result : you hear “da”
• Mouth movement inconsistent with what you hear, so the perceptual system creates the experience of
something in between.
Mcgurk effect : The auditory experience of the syllable “da” when seeing a mouth silently saying “ga” and
hearing a voice say “ba”.
The Grand illusion
Change detection
 Rensink et al (1997): our experience of a picture-like visual word may be an illusion.