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Chapter 2

Perception

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Extramission:Extramission: seeing occurs by something issuing from the eye

Study found that 69% of college students held this misunderstanding.(Winer, Cottrell, Gref, Fournier & Bica, 2002)

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Assume a real world exists out there!!!

Distal Stimuli

How do we get information about it into our cognitive systems?

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The response of sensory receptors is referred to as the Proximal Stimuli.

Percept is information that is processed for meaning.

Perception involves reducing and categorizing information.

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• Vision - Light Waves• Hearing - Sound Waves• Taste & Smell - Chemicals• Touch & Pain - Direct Pressure

Information about Distal Stimuli is carried by Energy

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Visual PerceptionVisual PerceptionQualities of Light that make it ideal for

carrying information fast constant speed travels in straight lines abundant Predictably interacts with object surfaces

- absorbed (by pigments)- reflected

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Retina has three layers of neurons1) is a single layer of photo sensitive cells (rods & cones)2) Bipolar cells3) Ganglion cells

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Photoreceptors – Rods and Cones

Transduce light into neural messages.

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Cone system has 3 types ofCone shaped photoreceptorsEach with a different type ofPhotopigment.

The Rod system has 1 type of Rod shaped Photoreceptor with a photopigment called rhodopsin.

Rod vision responds to the full range of light wavelenghts.

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Visual Information is carried by the optic nerve which crosses hemispheres at the optic chiasm.

The optic tract then carries the neural messages tothe visual receiving area in the occipital lobe.

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Information from both eyesis carried to each the occipital lobe of both hemispheres of the brain.

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130 million photoreceptors per eye.1 million ganglion neurons leave each eye.

Conclusion: A lot of processing of visual information goes on in the nervecells of the eye.

Compression.

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Photoreceptors pass info to collector cells which pass it on to ganglion cells.

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Compression occurs through synapses made between receptors and connector cells (horizontal cells, Bipolar cells and amacrine cells) and ganglion cells

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Compression - reduction in neural messages

Between photoreceptors and Ganglion cells.

Rods Cones

120 million 7 million

Ganglion

1 Million

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Approx 120 rods compress onto a single ganglion.

Approx 6 cones compress onto a single ganglion.

Effects of Convergence.When a large number of photoreceptors are converged on a single Ganglion cell, acuity (detail) is lost, but sensitivity is gained.

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Why do Cones have greater acuity?

Less convergence. Maintains specific location.

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Fovea

• mostly cones

• densely packed

• provides most accurate precise vision

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A great deal of processing is done before the visual information leaves the eye.

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Mach Bands - Edge Enhancement Illusion

Between two regions of different intensity a thin bright band appears at the lighter side and a thin dark band appears on the darker side.

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Hermann Grid When the viewer looks at the grid, the white dots and the center of each 'corridor' seem to shift between white and gray. When the viewer focused his or her attention on a specific dot, it is obvious that it is white. But as soon as attention is shifted away, the dot shifts to a gray color.

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Processes (neural cross-talk) that occurs in the eye, detects and emphasize important aspects of the visual stimuli, while removing redundant information.

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Sensation vs. PerceptionSensation is the registration of sensory input atthe sensory receptor cells (proximal).

Perception is the process of interpreting and understanding sensory information (precept).

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SaccadesSaccades- - Eye movements from point to point.

FixationsFixations- pauses in eye movement.

During the saccade there is suppression of the normal visual processes.

Visual Information is taken in only during fixations.

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Your visual system is really just sampling the scene.

Your sensory processors are selective – maintaining attention to stimuli that are important – while tuning out those that are not.

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By looking at the grid you can see that your brain is continuously trying to figure out if there's anything meaningful hidden in the pattern.

First it tries one approach, then another, then another until it finds something it can recognize. Called perceptual organizationperceptual organization.

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Gestalt ApproachGestalt is a psychology term which means "unified whole". It refers to theories of visual perception developed by German psychologists in the 1920s. These theories attempt to describe how people tend to organize visual elements into groups or unified wholes when certain principles are applied.

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The Whole is different than the sum of its parts.

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Figure-GroundTendency to separate a visual display intofigure and ground. One part is depictedas Figure and the other as Ground.

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Gestalt Principles of Perceptual Organization

Law of Pragnanz (good form or simplicity)

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Objects with similar properties (e.g. shape, color)

Nearby objects

Objects that define smooth lines or curves

Objects that form symmetrical patterns

Objects that form periodic patterns

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The example above (containing 11 distinct objects) appears as single unit because all of the shapes have similarity. 

Similarity occurs when objects look similar to one another. People often perceive them as a group or pattern.

 

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Good Continuation:Group together items that follow in astraight or smooth line.

Common Fate

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ClosureClosure occurs when an object is incomplete or a space is not completely enclosed. If enough of the shape is indicated, people perceive the whole by filling in the missing information. 

Although the panda above is not complete, enough is present for the eye to complete the shape. When the viewer's perception completes a shape, closure occurs.

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Witness the Ontario launch of the "Flick Off" campaign, and Environment Minister Laurel Broten's desperate attempt to maintain her dignity in front of its official logo.

Campaign is aimed at getting young people to cut energy use http://www.cbc.ca/canada/toronto/story/2007/04/26/flick-off-campaign.html#ixzz0fnapnrAP

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Bottom-Up ProcessingIn the bottom-up processing approach, perception starts at the sensory input of the stimulus. Thus, perception can be described as data-driven. For example, there is a flower at the center of a person's field. The sight of the flower and all the information about the stimulus are carried from the retina to the visual cortex in the brain. The signal travels in one direction.

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Examples of Bottom-Up Approaches

Template Theories Comparing the 2-D shape of the object

with stored shapes in Memory (template).Problems1)Too many templates would be needed. 2) Inflexible

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Same object at different viewing angles has very different shapes.

Rotational Invariance:  People are capable of recognizing objects from many different vantage points, even views that have never before been seen (Biederman & Gerhardstein, 1993).

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Many things we can readily distinguish have very similar shapes.

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Feature Analysis or Detection: break objects down into features and match against stored object features.

Features of Stimuli Letter A Letter H Letter TTwo vertical lines Yes Yes NoOne horizontal line Yes Yes YesDiagonal converge at top Yes No NoHorizontal joins vertical Yes Yes No

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Do Feature Recognition Models Make Sense?? Evidence:

Neurological: feature detectors in visual cortex Hubel & Wiesel (1965).- located feature neurons that respond best tospecific feature “line orientations, curves, corners ends”.

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Video

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Feature detectors are neurons that respond to specific features of the environment, such as lines, orientation, edges (end-stops), and movement.

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Where are the feature detectors? Retina or Visual Cortex of Brain?Motion Illusion

 If people CANNOT see the "wave/spiral/afterimage" effect using the eye that was shut, then the feature detectors must be in our retina.

 If people generally still have the "wave/spiral/afterimage" effect using the eye that was shut, then the feature detectors are in the visual cortex.

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Most Feature theories assume that perception involves processing of features followed by more global or general processing to integrate information from features.

Evidence shows however that GLOBAL processing can precede processing of features.

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Navon (1977) Participants shown series of stimuli in which a larger letter was made up of small letters.

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On some trials they were asked to identify whether the large letter (Global) was an H or an S. On other trials whether the small (feature) letters were an H or an S.

When identifying the small letters they were much slower when the features were inconsistent with the Global Letter.

Slower than

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Indicates that processing of the Global meaning interferes with the identification of the features.

But Identifying the Larger Letter was not effected by what type of feature letter it was made up by.

Indicates that we may identify global objects or scenes before we process features!!

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Object Superiority EffectWeisstein & Harris (1974)

Line drawings are seen as box-like three-dimensional figures if the lines are arranged properly. A flat two-dimensional pattern is seen when these same lines are disarranged. A target line contained within the three-dimensional figure (A) is identified more readily than is the same line contained within a two-dimensional figure (B).

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What is this word?

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Limitations of Feature Analysis Models

Bottom-up models do not account for the large effects of context (i.e., surrounding information, your knowledge and expectations).

Proof Reading Effects

Repetition Blindness

Top-Down Processes

Conceptually-driven processes provide expectations from the top-down and speed processing of features.

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Word Superiority Effect (Reicher, 1969)Letter strings flashed very briefly

ONFK

WORK

K

A target letter K is shownTask: Respond “Y” if target was present

Word Superiority Effect

• WORK vs. ORWK vs. K

• Surprisingly, a single letter (K) is recognized faster in the context of a whole word (WORK) than when presented as an isolated letter. A nonword (ORWK) doesn’t provide this top-down advantage.

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Parallel Distributed ProcessingParallel - Features, Letters, words, all processed at the same time.

Distributed - Processing at several levels

Fingerprints, Experts and Emotions

“At present many agencies give fingerprint experts contextual information about the crime, which may, clearly, bias their decisions in marginal cases. One simple recommendation would be to just provide experts with the fingerprints and no other contextual information.”

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Implicit and Analytic Reasoning

Experts often begin by useing more implicit reasoning (fast/top-down) and cross-check using more analytic processes.

Novices rely more on analytic reasoning.

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Visual Object Recognition

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Recognition-By-Components Theory

• Biederman (1987)– Describes how 3D

images are identified – Breaks objects down into

geons – Objects are identified by

geons and relationship between them

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View Invariant: • Look the same from most angles

• Spatial arrangement of geons is the same. Biederman & Gerhardstein

(1993) Priming Study

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Discriminable •Difficult to confuse one for another Noise Resistant

• Can be identified under less-than-ideal conditions

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True only if non-relational information is intact. Take away information about how the Geons go together (vertexes) recognition takes longer.

Blotted Letters

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•Parsimonious 36 Geons can define 150 million 3-geon objects.

• Allows novel stimuli to be identified by comparing Geon structures to similar stored stimuli.

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Is object recognition really view independent??

It depends! When making easy discriminations it is (comparing a Dog to a Car). When the task requires difficult within category discriminations(distinguishing on type of car from another?)

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Foster & Gilson (2002)Are the two objects the same?Differ on viewpoint-invariant feature (number of parts) or viewpoint-variant features (part length).

Participants showed evidence of using viewpoint-invariant and viewpoint variant processing depending on the demands of the task.

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Top Down Influences Allow us to Get meaning out of even minimal Perceptual information.

Biological Motion

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Perception improves with practice as long as practice has feedback! Practice increases Top-down factors.