The psychology of vision - a summary of chapter 8 of Psychology by Gray and Bjorklund (7th edition)

Psychology
Chapter 8 (in part)
The psychology of vision

Seeing forms, patterns and objects

The purpose of human vision is to identify meaningful objects and actions.
Your visual system has sorted all the points and graduations that are present in the reflected light into useful renditions of the objects. It has provided you’re with all the information you need to reach out and touch, or pick up, whichever object you want to use next.

Vision researchers generally conceive of object perception as a type of unconscious problem solving, in which sensory information provides clues that are analyzed using information that is already stored in the person’s head.

The detection and integration of stimulus features

Any object that we see can be thought of as consisting of a set of elementary stimulus features, including the various straight and curved lines that form the object’s contours, the brightness and color of the light that the object reflects and the object’s movement or lack of movements with respect to the background.

Feature detection in the visual cortex

Ganglion cells of the optic nerve run to the thalamus and form synapses with other neurons that carry their output to the primary visual area of the cerebral cortex.
Within the primary visual area, millions of neurons are involved in analyzing the sensory input.
Different neurons respond to different patterns.

Edge detectors: neurons that respond best to stimuli that contains a straight contour separating a black patch from a white patch.
Bar detectors: respond best to a narrow white bar against a black background, or a narrow black bar against a
white background.
Any edge detector or bar detector responds best to a particular orientation of the edge or bar.

Neurons in the primary visual cortex are sensitive not just to the orientation of visual stimuli, but also to other visual features, including color and rate of movement. (One neuron might respond best to a yellow bar on a blue background, tilted 15 degrees clockwise and moving slowly from left to right).
Taken as a whole, the neurons of the primary visual cortex and nearby areas seem to keep track of all the bits and pieces of visual information that would be available in a scene.
Because of their sensitivity to the elementary features of a scene, these neurons are referred to as feature detectors.

Treisman’s two-stage feature-integration theory of perception

The feature-integration theory.
Any perceived stimulus (even a simple one such as an X) consist of a number of distinct primitive sensory features, like color and the slant of its individual lines.
To perceive the stimulus as a unified entity, the perceptual system must detect these individual features and integrate them into a whole.
The detection and integration occur sequentially, in two fundamentally different steps or stages of information processing.

1. Detection of features
   Occurs instantaneously and involves parallel processing (this step operates simultaneously on all parts of the stimulus array). Our visual system picks up at once all the primitive features of all the objects whose light rays strike our retinals.
2. The integration of features
   Requires more time and leads eventually to our perception of a whole, spatially organized patterns and objects.
   This step involves serial processing (occurs sequentially, at one spatial location at a time).

Research support for Treisman’s theory

Distractors: nontarged stimuli

As long as the target differed from all the distractors in one or more of Treisman’s list of primitive features subjects detected it equally quickly no matter how many distractors were present.
This is indicative of parallel processing.

Illusory conjunctions: subjects who saw simple stimuli flashed briefly on a screen easily identified which primitive features were present but sometimes misperceived which features went together.

Parallel processing registers features independently of their spatial location.
Different features that coincide in space (such as the color and curvature of a given line) are joined perceptually only in serial processing.
Serial processing operates by assigning primitive features to specific spatial locations and then weaving the features together in patterns that reflect their locations.

Gestalt principles of perceptual grouping

Gestalt psychology.
Gestalt means whole form.
The whole is different from the sum of pairs. (Like a painting is not the sum of individual spots).
In our experience we do typically perceive wholes before we perceive parts. The building up of the wholes from the parts occurs through unconscious mental processes.
The parts are organized. So it is differed from the sum of parts.

Built-in rules for organizing stimulus elements into wholes

(According to Gestaltist) the nervous system is innately predisposed to respond to patterns in the stimulus world according to certain rules, or Gestalt principles of grouping. These principles are:

• Proximity
  We tent to see stimulus elements that are near each other as parts of the same object and those that are separated as parts of different objects.
• Similarity
  We tend to see stimulus elements that physically resemble each other as parts of the same object and those that do not resemble each other as parts of different objects.
• Closure
  We tend to see forms as completely enclosed by border and to ignore gaps in the border.
• Good continuation
  When lines intersect, we tend to group the line segments in such a way as to form continuous lines with minimal change in direction.
• Common movement
  When stimulus elements move in the same direction and at the same rate, we tend to see them as a part of a single object.
• Good form
  The perceptual system strives to produce percepts that are elegant. (Simple, uncluttered, symmetrical, regular and predictable).

Figure and ground

We have an automatic tendency to divide any visual scene into figure (the object that attracts attention) and ground (the background).

Circumscription: other thing being equal, we tend to see the circumscribing from as the ground and the circumscribed form as the figure.

Evidence that wholes can affect the perception of parts

When you look at a visual scene, the elementary stimulus features certainly affect your perception of the whole, but the converse is also true. The whole affects your perception of the features.

Unconscious inference: without your conscious awareness, at a speed measurable in milliseconds, your visual system uses the sensory input from a scene to draw inferences about what is actually present.

Once your visual system has hit upon a particular solution to the problem of what is there, it may actually create or distort features in ways that are consistent with that evidence.

Illusory contours

An illusory contour emerges from the brain’s attempt to make sense of the sensory input.
Illusory contours can not be explained in simple stimulus terms.

People are more likely to see illusory contours in cases where they are needed to make sense of the figure than in cases where they are not, even when the amount of actual dark-light border is constant.

Unconscious inference involves top-down control within the brain

Unconscious or conscious is a product of neural activity.
Unconscious-inference theories imply that the phenomena in question result from neural activity in higher brain areas, which are able to bring together the pieces of sensory information and make complex calculations concerning them.

Connections between the primary visual area and higher visual areas in the brain are not one-way.
Higher areas receive essential input from the primary visual area, but they also feed back to that area and influence neural activity there.

• Top-down control: control that comes from higher up in the brain
  Bring to bear the results of calculations based on that sensory information plus other information, such as that derived from previous experience and from the larger context in which the stimulus appears.
• Bottom-up control: control that comes more directly from the sensory input.
  Bring in the sensory information that is actually present in the stimulus.

Perception always involves interplay between bottom-up and top-down control in the brain.

Recognizing objects

To recognize an object is to categorize it.
To recognize an object visually, we must form a visual perception of it that we can match to our stored definition, or understanding, of the appropriate object category.

Biederman’s recognition-by-components theory

Recognition-by-components theory
To recognize an object our visual system first organizes the stimulus information into a set of basic, three-dimensional components, which are named geons. Then the brain uses the arrangement of those geons to recognize the object.
If we can see the geons and their arrangement, then we have the information necessary for identifying the object.

There are 36 different geons.
By smoothing the edges and ignoring the details, we can depict any object as a small subset of such geons organized in a certain way.
To see an object the visual system first integrates the elementary stimulus features in such a way as to detect the geons, then integrates the geons in such a way as to identify the object. This process occurs unconsciously.

Evidence from people who suffer from visual agnosias

Visual agnosia: after brain damage, when you can still see, but can no longer make sense of that you see.
This is classified into a number of general types.

• Visual form agnosia
  They can see that something is present and can identify some of its elements (like color and brightness) but cannot perceive its shape.
• Visual object agnosia
  They can draw the shapes of objects that they are shown, but still cannot identity the objects.

Recognition of an object occurs through:
Picking-up of sensory features → detection of geons → recognition of object

Two streams of visual processing in the brain

The visual areas beyond the primary area exist in two relatively distinct cortical pathways or ‘streams’, which serve different functions.

• The what pathway
  The lower, temporal stream.
  Specialized for identifying objects.
  Neurons in this pathway typically respond best to complex geometric shapes and to whole objects.
• The where pathway
  The upper, parietal stream.
  Specialized for maintaining a map of three-dimensional space and localizing object within that space.
  Crucial for the use of visual information to guide a person’s movements.
  Neurons in this pathway are concerned with where the object is located and with how the person must move in order to interact with it in some way.

Effects of damage in the what pathway

• Deficits in ability to make conscious sense of what they see, depending on just where the damage is.
  They do retain the ability to reach accurately for objects and act on them in coordinated ways, guided by vision, even if they can’t consciously see the objects

Effects of damage in the where-and-how pathway

• Interferes most strongly with people’s abilities to use vision to guide their actions.
  They have little or no difficulty identifying objects that they see, and often they can describe verbally where the object is located, but they have great difficulty using visual input to coordinate their movements.
  Even though they can consciously see and describe an object verbally and report its general location, they reach for it gropingly, much as a blind person does.

Seeing in three dimensions

Seeing is an active mental process.

Cues for depth perception

Depth perception works best when you use both eyes.

Binocular cues for depth

Binocular disparity
The slightly different views that the two eyes have of the same object of scene.
The degree of disparity between the two eyes’ views can serve as a cue to judge an object’s distance from the eyes. The less the disparity, the greater the distance.
In normal, binocular vision your brain fuses the two eyes’ views to give a perception of depth.
This ability is called stereopsis.

There are neurons in an area of the visual cortex close to the primary visual area that respond best to stimuli that are presented to both eyes at slightly disparate locations on the retina. These neurons appear to be ideally designed to permit depth perception.

Monocular cues for depth

One eye

• Motion parallax
  The changed view one had of a scene or object when one’s head moves sideways.
  The degree of change in either eye’s view at one moment compared with the next, as the head moves in space, can serve as a cue for assessing the object’s distance from the eyes. The smaller the change, the greater the distance.
• Pictorial cues of depth
  Provide a sense of depth in pictures as well as in the real three-dimensional world.
  These are:
  • Occlusion
  • Near objects occlude more distant ones.
  • Relative image size for familiar objects
  • Linear perspective
  • Texture gradient
    A gradual decrease in size and spacing of texture elements indicates depth.
  • Position relative to the horizon
  • Differential lightning of surfaces.

The role of depth cues in size perception

The ability to judge the size of an object is intimately tied to the ability to judge its distance.
Size constancy: the ability to see an object as unchanged in size, despite change in the image size as it moves farther away or closer.

Unconscious depth processing as a basis for size illusions

The depth-processing theory form Richard Gregory

The theory maintains that one object in each illusion appears larger than the other because of distance cues that, at some early stage of perceptual processing, lead it to be judged as farther away.
If one object is judged to be farther away than the other but the two produce the same-size retinal image, then the object judged as farther away will be judged larger.

The moon illusion

Our visual system did not evolve to judge such huge distances as that from the earth to the moon.
So we automatically assess its distance in relation to more familiar earthly objects.

Multisensory perception: combining senses

Our experiences are multisensory.
Multisensory integration: the integration of information from different senses by the nervous system.

Multisensory integration

The visual dominance effect: when sight and sound are put in conflict with one another, vision usually ‘wins’.

Neuroscience of multisensory integration

For someone to experience multisensory integration, the brain must somehow be able to respond appropriately to stimuli from two sensory modalities.

There are neurons in the superior colliculus of mammals that respond to information from more than one sensory stimulus.
Multisensory neurons are neurons that are influenced by stimuli from more than one sense modality.
Multisensory integration is most apt to be perceived when the individual sensory stimuli:

• Come from the same location
• Arise at approximately the same time
• Evoke relatively weak responses when presented in isolation

Multisensory neurons are found throughout the brain, but multisensory integration also occurs when the outputs of unimodal neurons are integrated.

The ‘bouba/kiki’ effect

The more rounded stimulus corresponds to the rounded way one must form one’s mouth when saying the word.
There is an implicit multisensory match between a visual stimulus and sound, or perhaps the muscle patterns we use to make those sounds.

The development of multisensory integration

Multisensory perception is well established by early childhood.
Multisensory integration is the norm rather than the exception and it is present in early live.

Synesthesia

Synesthesia: joined perception, a condition in which sensory stimulation in one modality induces sensation in a different modality. (Like seeing sounds).
It does not interfere with normal functioning and is not classified as a mental disorder.
It can come in many forms.