Ch 3. Perception

Perception

Perception

Assigning meaning to sensory information we take in.

Distal Stimulus

The thing to be perceived.

Proximal Stimulus

The reception of information and its registration by a sense organ.

Retinal Image

When an object is perceived by the retina, a two dimensional image is formed. Its size depends on the distance to the object (the closer you are the larger the image), the image is also upside down and left and right are reversed.

Percept

The meaningful interpretation of the proximal stimulus.

Size constancy

This is an example showing the difference between a percept and a proximal stimulus. Ex: Extend your arm out in front of you with your hand straight up. Now move your hand towards and away from you. Note that your hand doesn't appear to be getting larger and smaller. Your retinal image is changing size but your brain is interpreting that it isn't changing size. Therefore perception is different from retinal formation.

Pattern recognition

• the recognition of a particular object, event, so on belonging to a class of object, event, so on.

• most percept creations use pattern recognition.

Form Perception

The segregation of the whole display into objects (called the figure) and the background (called the ground).

Bottom-up Processing

Starting with small bits of information about the environment that are combined in various ways to make a percept.

Top-down Processing

The perceiver's expectations, theories or concepts guide the selection and combination of the information in the pattern recognition process.

Template matching

Every object, event or stimulus that we encounter and want to derive meaning from is compared to some previously stored pattern or template. If a number of templates come close, a different type of processing is required. Limitations/reasons template matching cannot explain perception on its own:

• we would need impossibly large number of templates.

• as technology develops we are able to recognize new objects

• people tend to recognize many patterns as more or less the same thing even when stimulus patterns differ (e.g. handwriting)

Feature Analysis

Instead of processing stimuli as a whole unit, we might instead break it down into its components using our recognition of those parts to infer what the whole represents. The parts recognized and searched for are called features.

Prototype Matching

When sensory information is observed, it is compared with previously stored prototypes. An exact match is not required; only an approximate match is expected. An object is perceived when a match is found. Evaluation: -More flexible than template matching. Ex: dog is a prototype, where as a specific bread of dog is a template.

• do not need to contain specific features or sets of features like template matching or feature analysis -takes into account relationship between features in addition to the features themselves whereas feature analysis and template matching only take into account the features.

Prototype

idealized representation of some class of objects or events.

Top-down processing

process that is directed by expectations derived from context or past learning or both.

Context effects

people recognize objects better when they are in a matching context. E.g. people recognize food easier when in a kitchen as opposed to jumbled up.

Primal sketch

consists of a set of blobs oriented in various directions. These reductions are conceived as mental representations or symbolic depictions of the "raw information" transmitted by the light.

Change blindness

the inability to detect changes or scene especially when given different views of that object.

THis suggests that perception is based on expectation about meaning. Instead of keeping track of every single detail of a scene, we seem to represent the overall meaning of the scene.

Word superiority effect

Letters are easier to perceive in context (in a word) than the same letters presented alone.

Direct perception

All of the previous models rely on the fact that the user must do something to the proximal stimulus. Direct perception describes the act of perception as the construction of mental representation of objects. From the information, we somehow construct a depiction that may or may not physically resemble the object or event being perceived but that our cognitive and physiological processes can recognize as corresponding to the information perceived. We use both proximal stimulus and long term memory to construct these mental representations.

Schema

a mental representation

visual agnosia

impairments in the ability to interpret visual information. E.g a person may not be able to recognize animals by sight but they can through sound or touch or smell. THe problem is in creating a percept from a proximal stimulus

apperceptive agnosia

Patients can process very little visual information. They can see contours, or outlines of drawings or objects but have a very difficult time matching objects to each other or categorizing objects. Associated with the right hemisphere of the brain.

associative agnosia

Patients can match objects but they tend to do so very slowly and very carefully. These patients cannot readily name the objects they have seen or drawn. Associated with both the right and left cerebral hemispheres.

Prosopagnosia

May have intact object recognition but are unable to recognize faces. They see details but cannot put them together. This is associated with damage to a particular region of the right hemisphere

unilateral neglect

Damage to the parietal cortex. Causes patients to ignore stimuli on the opposite side. E.g damage to the right hemisphere may make a person not wash the left side of their body.

Synesthesia

ordinary stimuli lead to experiences that are anything but ordinary. E.g seeing colour when you hear, see or think about a number

Gestalt approaches

Aims to explain organization of objects. Selects the simplest organization.

Capgras

• Opposite of prosopagnosia, face recognition abilities are intact. However they fail to show GSR response that separates familiar faces and unfamiliar faces.

• One explanation is that there is no unconscious feeling of familiarity and they agree that the person looks identical to someone they know.