Primary Questions in the Study of Perception

• The physiological question: How are the properties of objects in the environment represented by the activity in the nervous system?

• The stimulus question: How do we use information from the environment to create perceptions?

• The cognitive question: How do experiences and prior knowledge influence perceptions?

Distance Cues

• Monocular cues: Relative size of images on retina, motion parallax, linear perspective, aerial perspective, relative size, occlusion, shadowing, texture gradient, height in plane.

• Binocular cues: Retinal disparity.

Relative Size of Retinal Images

Which one is closer?

Why do you think so?

Occlusion

Texture gradient

Relative size

Height in plane or position relative to horizon

Linear perspective

Aerial perspective

Shadowing

Monocular Depth Cues

Relative Size

Motion Parallax

Size Perception

• Our perception of size depends on our perception of the distance of the object—we judge the size based on how far away we think the object is—the size of the image on the retina.

• Because of this the eye can be fooled into seeing things differently that they really are.

These are identical twins.

Why do they appear so different?

They are standing in an Ames room.

An Ames room is designed to produce this illusion. One views the objects or people through a peep hole monocularly, so that the room appears normal and we thus judge that the objects must be of different sizes.

The Ames Room

Ames was an optometrist who designed this room to create perceptual distortion.

Another Size IllusionMueller- Lyer Illusion

Which line appears longer?

Another Size IllusionMueller- Lyer Illusion

They are the same length. The brain is fooled by the arrows at the ends of the lines.

Motion Parallax

• Think of looking out the window of a moving automobile.

• Are different parts of the scene moving in different directions?

• Relative to the motion of the car, what direction are the nearest objects moving?

• What direction are the distant objects moving, again relative to the movement of the car?

Binocular Depth Cues

• Retinal Disparity: Each eye has a slightly different image of the world, although both are in the same plane. There are disparate images on the two retinas.

• The process of putting these images together in the brain is called stereopsis.

• Stereograms are flat two-dimensional images designed in such a way that if we can feed each eye a slightly different perspective of this image the result will be one three-dimensional image.

• Optical devices can be used to produce this effect. When the two-dimensional picture is designed to do this when view correctly we call it an autostereogram.

Autostereograms• The images that follow are autostereograms. There are

two possible ways that you might view these images to create a three-dimensional effect.– Parallel vision.– Cross-eyed vision.

• Parallel vision requires that each eye lock on the image separately. Each eye is staring straight ahead.

• Cross-eyed vision requires that each eye lock on the opposite sight, but again each is viewing the image separately.

• One procedure may work for some stereograms and not for others. You need to try both.

• Often there are two little dots or crosses at the bottom. Trying to make these converge can sometimes help. Trying to look behind the picture can also help.

Viewing Autostereograms

Try Parallel Vision

What You Might Have Seen Is a Cube

You might have seen a tea cup floating in this picure.

Did you see anything else?

Some people see a field of mushrooms.

Motion Perception

• How do we know that the person walking toward us is the one moving?

• How do we know that when we move our eyes and the scene changes, that the scene has not moved?

• How do we explain motion after-effects such as this: http://dogfeathers.com/java/spirals.html. Please be cautious in observing this—some people have trouble with this kind of stimulation.

• How is the illusion of movement created on movie marquees, or road signs?

• How do we know that the person walking toward us is the one moving?

• How do we know that when we move our eyes and the scene changes, that the scene has not moved?

• How do we explain motion after-effects such as this: http://dogfeathers.com/java/spirals.html. Please be cautious in observing this—some people have trouble with this kind of stimulation.

• How is the illusion of movement created on movie marquees, or road signs?

Motion Perception(Phi Phenomenon)

Stroboscopic Motion

• The foregoing illusion tells us that slightly different images presented in rapid succession can be perceived as a moving object if the rate of presentation is rapid enough.

• The object will appear to move smoothly at faster rates, and will appear to hop at slower rates.

• This is called stroboscopic movement.

Explaining Motion Perception

• There are neurons in the brain that respond only when neurons on the retina fire in sequence as an image moves across the retina.

• Different sets of neurons are specialized to respond to only one direction of movement.

• The interpretation of that experience depends on things like motion after-effects and the monitoring of our head and eye movements.

• The combination of all these neural signals is interpreted to provide us with the perceptual experience that we have.

Object Perception

• How do we separate an object from the background? How do we know which is the object and which is the background?

• It seems to depend on recognition of form.

• How do you know there is a dog in this picture?

Object Perception

• Do we build our perception of objects and the environment piecemeal, collecting together all the parts to form a whole? This is called bottom-up or feature driven perception.

• We have neural pathways that are specialized to respond to certain features, colour, shapes, motion that are interconnected in the association areas. Early processing seems to be feature driven.

• Do we look at the world as wholes, and then break what we perceive into its component parts? This is called top-down processing.

• Evidence says we do both.

Object and Form PerceptionPop-out Evidence

• A: A search for a green line is easy—it pops out. Only one feature is different.

• B: A search for a horizontal line is easy—it pops out. Only one feature is different.

• C: A search for a horizontal red line is much more difficult—two features are different. The search becomes methodological, plodding analyzing feature after feature, feature driven, or bottom-up.

The pop-out evidence is often taken to indicate that the simpler scenes are processed all at once—a gestalt.

Object and Form PerceptionGestalt Principles

• Gestalt psychology: A school of pyschology that focused on the ‘whole’ perception, arguing that the whole is greater (different) than the sum of its parts.

• Developed a set of principles suggesting how we group items together when perceiving them.

• This argues for top down processing in perceiving form.

Gestalt Principles

• Law of Good Form: We will perceive objects in the simplest way possible. Is the following a multi-sided irregular figure or an oblique triangle on top of a rectangle?

Gestalt Principles• Law of Closure: We will fill in

missing information so as to create fully-formed figures. We perceive “illusory contours”—they aren’t really there.

Law of Good Continuation: The eye naturally follows a line or curve, and will not break the flow even if there is an obvious interruption.

Images from: http://graphicdesign.spokanefalls.edu/tutorials/process/gestaltprinciples/gestaltprinc.htm

Gestalt Principles• Law of Proximity: Items

placed close together tend to be perceived as a group. Twelve happy faces, or three rows of four, or four columns of three?

Law of Similarity: Objects that are similar tend to be perceived as a group. Twelve happy faces, or four columns of happy faces, alternating colours and direction?

Perceptual Constancies

Describe what you see here.

The tendency is to see a series of circles whose angles are changing, rather than four rows of different shapes. It seems as if the three black circles at the bottom are repeated into depth, as if printed on a cylinder.

This is referred to as shape constancy—we tend to perceive the shapes as constant and moving away from us.

Perceptual Constancies

As it opens, the image of the door changes shape on our retina but we do not perceive that its shape changes—we know it is still a rectangle—shape constancy.

http://www.aber.ac.uk/media/Modules/MC10220/visper02.html

Perceptual Constancies• Size constancy

– As objects move away from us their image on the retina becomes smaller, but we perceive that their size remains constant. Because of this we can be tricked, e.g., the Ames room.

• Colour and lightness constancy– The wave lengths and amount of light that reach our eye

from a given object vary with the lighting. In spite of this, the object is always perceived to be its original colour and lightness.

– White snow actually reflects much less light at dusk, so that the neural activity from the retina is similar to that of a grey object. In spite of that, we perceive the snow as white—lightness constancy.

– A red circle that is half in shadow is not perceived as two shades of red. We know it is still all red—colour constancy.

Perceptual Constancies• Our perception of an object’s size, shape, colour,

or lightness does not vary with changing the angle, distance, or illumination on that object.

• Perception is based on understanding the relationship between objects and these factors. The brain computes a ratio of one sensation to another, e.g., the illumination from the object to the background illumination, or the appearance of the object to the angle of viewing.

• Gibson’s direct perception theory: The visual system is designed to interpret the world.– Visual processing performed by the brain is prewired.– These innate processes occur at the same time (in

parallel) with learned processes.

Two Stage Model of Perception• Stage 1: Preattentive or automatic

processing: Gestalt laws apply, as do binocular and monocular cues to depth and other automatic perceptual processes.

• Selector or Filter: Cocktail party phenomenon—selective listening—some things get through. The capacity is limited.

• Stage 2: Selective attention.Physically orient to stimulus.Cognitively orient to stimulus.