Visual Perception - Revision

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Sensation

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Sensation is the process in which our sense organs and receptors detect and respond to sensory information that stimulates them.

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Perception

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Perception is the process in which we give meaning to the sensory information, resulting in our personal interpretation of that information.

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Visual Perception System

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Consists of the complete network of physiological structures involved in vision. Includes all parts of the eyes, nervous system pathways that connect the eyes and brain and the areas of the brain that process visual information.

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Cornea

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A transparent, convex shaped covering which protects the front of the eye. The curve of the cornea also helps it to focus light onto the retina.

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Aqueous Humour

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A watery fluid that helps maintain the shape of the eyeball and provides nutrients and oxygen to the eye and takes away waste products.

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Pupil

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A black circle in the centre of the eye that is an opening in the iris that helps control the amount of light entering the eye. It expands to let in more light in dim-lit areas and contracts in brightly-lit places.

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Iris

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Surrounds the pupil and is the coloured part of the eye. It is a ring of muscles that expand or contract to change the size of the pupil.

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Lens

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A transparent, convex shaped structure located immediately behind the pupil. The lens plays a major role in focusing light onto the retina. It adjusts its shape according to the distance of the object being viewed. It flattens out when the object is in the distance and bulges when the object is closer. This change in size is controlled by the ciliary muscles.

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Ciliary Muscles

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These muscles expand or contract in order for the lens to flatten and bulge according to the object being viewed.

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Vitreous Humour

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A jelly like substance that also helps maintain the shape of they eyeball and helps focus light onto the retina.

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Retina

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This reflective structure receives and absorbs light and also processes images. The retina is connected to the brain by the optic nerve. The retina contains specialised nerve cells called Photoreceptors.

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Optic Nerve

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The optic nerve is a tube like structure that transmits visual information to the brain from the retina.

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Photorecptors

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The very back layer of the retina contains these special nerve cells known as rods and cones.

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Rods

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Rods are special nerve cells responsible for your vision in low lit and night vision areas.

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Cones

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Cones are responsible for your vision is well lit areas and they detect finer levels of detail.

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Process of Light Travelling Through Eye - In Order

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Cornea

Aqueous Humour

Pupil (Iris Helps)

Lens (Ciliary Muscles Help)

Vitrous Humour

Retina

Optic Nerve

Brain

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Reception

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The process by which the eye receives incoming light from the external environment and focuses it onto the retina where an image of the visual stimulus is captured.

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Transduction

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The process by which the photoreceptors change electromagnetic energy (‘light’) into electrical impulses (‘signals’) which can travel along the optic nerve to the brain.

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Transmission

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Involves sending information in the form of electrical impulses along the optic nerve to the brain. These impulses are sent to the visual cortex part of the brain which specialises in receiving and processing visual information.

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Organisation

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Organisation involves assembling or arranging the features of a visual image in a meaningful way.

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Interpretation

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The process of assigning (or ‘giving’) meaning to visual information so that we can understand what we are looking at.

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Visual Perception Principles

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‘Rules’ that we apply to visual information to assist our organisation and interpretation of the information in consistent and meaningful ways.

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Gestalt Principles

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Refer to the ways in which we organise the features of a visual scene by grouping them to perceive a whole, complete form.

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Four Gestalt Principles

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Figure Ground

Proximity

Closure

Similarity

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Figure Ground Organisation

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This principle involves organising visual information by dividing a scene into a figure which stands out from the ground.

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Countour

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This line is the line that seperates between the figure and the ground in the figure ground organisation principle.

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Camouflage

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This occurs when the figure and the ground are not easily seperated and blend together.

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Closure

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Refers to the perceptual tendency to mentally ‘close up’, fill in or ignore gaps in a visual image and to perceive objects as complete (‘whole’).

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Similarity

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Involves the tendency to perceive parts of a visual image that have similar features — such as size, shape, texture or colour — as belonging together in a unit, group or ‘whole’.

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Proximity

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The tendency to perceive parts of a visual image which are positioned close together as belonging together in a group.

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Depth Perception

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The ability to accurately estimate the distance of objects and therefore perceive the world in three dimensions.

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Depth Cues

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Sources of information from the environment (external cues) or from within our body (internal cues) that help us to perceive how far away objects are and therefore to perceive depth.

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Binocular Depth Cues

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Require the use of both eyes working together in order to provide information to the brain about depth and distance.

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List of Binocular Depth Cues

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Convergence

Retinal Disparity

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Convergence

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involves the brain detecting and interpreting depth or distance from changes in tension in the eye muscles that occur when the two eyes turn inwards to focus on objects that are close. The brain feels more tension when objects are closer and less when objects are further away.

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Retinal Disparity

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Refers to the very slight difference in the location of the visual images on the retinae which enables us to make judgements about the depth or distance of an object.

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Monocular Depth Cues

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Require the use of only one eye to provide information to the brain about depth and distance, but they also operate with both eyes.

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List of Monocular Depth Cues

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Accommodation

Linear Perspective

Interposition

Texture Gradient

Relative Size

Height in the visual Field

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Accommodation

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Involves the automatic adjustment of the shape of the lens to focus an object in response to changes in how far away the object is. The brain monitors the movement of the ciliary muscles which gives it information about depth

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Pictorial Cues

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Are so named because artists use them to create depth and distance on two-dimensional surfaces such as paper and canvas.

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Linear Perspective

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The apparent convergence of parallel lines as they recede (‘go back’) into the distance.

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Interposition

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Cccurs when one object partially blocks or covers another, and the partially blocked object is perceived as further away than the object that obscures it (and vice versa).

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Texture Gradient

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Refers to the gradual reduction of detail that occurs in a surface as it recedes into the distance, compared with a surface that is close and perceived in fine detail.

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Relative Size

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Refers to the tendency to visually perceive the object that produces the largest image on the retina as being closer, and the object that produces the smallest image on the retina as being further away.

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Height in the Visual Field

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Refers to the location of objects in our field of vision, whereby objects that are located closer to the horizon are perceived as being more distant than objects located further from the horizon.

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Perceptual Constancies

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Refers to the tendency to perceive an object as remaining stable and unchanging despite any changes that may occur to the image cast on the retina.

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Size Constancy

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Involves recognising that an object’s actual size remains the same, even though the size of the image it casts on each retina changes.

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Shape Constancy

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The tendency to perceive an object as maintaining its shape despite any change in shape of the image of the object on the retina.

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Brightness Constancy

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The tendency to perceive an object as maintaining its level of brightness in relation to its surroundings, despite changes in the amount of light being reflected from the object to the retina.

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Perceptual Set

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The predisposition, or ‘readiness’, to perceive something in accordance with what we expect it to be.

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Context

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Refers to the setting or environment in which a perception is made.

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Motivation

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Refers to processes within us which activate behaviour that we direct towards achieving a particular goal. Visual perception can be influenced by our motives when, for example, we see what we want to see, rather than what is actually there

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Emotional State

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Our emotional state (how we are feeling) can influence the way we see things. Different emotions can ‘set’ us to perceive information in a particular way which is consistent with the emotion being experienced.

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Past Experience

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Past experience refers to our personal experiences throughout our lives. Our unique combination of past experiences can lead us to many individual differences in perception.

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Cultural Factors

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Experience with or in a particular culture can influence the way we process and interpret visual information.

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Visual Illusion

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A misinterpretation (distortion or mistake) of real sensory information. It is an experience in which there is a mismatch between our perception and what we understand as physical reality.

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Muller Lyer Illusion

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A visual illusion in which one of two lines of equal length, each of which has opposite shaped ends, is incorrectly perceived as being longer than the other.

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Carpentered World Hypothesis for Muller Lyer Illusion

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Since we see corners everywhere, the arrow headed line looks like the nearer corner and the feather headed line looks like the furthest corner. Consequently, the two lines appear to be at different distances from the observer and the feather tailed line appears to be further away, and is therefore longer.

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Ames Room Illusion

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A visual illusion in which two equal sized people standing at the back of a trapezium shaped room appear to 'grow' when they walk from the left corner to the right corner.