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Sensation and Perception • Now that we have looked at what a stimuli is, we need to understand what a sensation is, and how our brain perceives it.
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Sensation and Perception
• Now that we have looked at what a stimuli is, we need to understand what a sensation is, and how our brain perceives it.
Sensation vs. perception
• Sensation is the stimulation of sense organs.• Perception is the selection, organization, and
interpretation of sensory input• It is the organization of sensory input into
something meaningful.
Psychophysics
• Psychophysics – the study of how physical stimuli are translated into psychological experience.
A new way of looking at threshold…
• Threshold can also be explained as the dividing point between energy levels that do and do not have a detectible effect.
• Absolute threshold – the minimum amount of stimulation that an organism can detect.
JND’s• JND – Just Noticeable Difference –
the smallest difference in the amount of stimuli that an organism can detect. The size of the JND is constantly proportional to the initial stimuli. – Weber’s Law
• The perceived magnitude of the experience is proportional to the number of JND’s that the originating experience is above the absolute threshold.
Signal detection
• Signal Detection Theory states that the detection of a stimuli involves decision making processes as well sensory processes.
• These can be conscious or unconscious• This can be affected by a variety of factors. Eg.
Noise.• This is all important for perception without
awareness. – Subliminal advertising.
Sensory Adaptation
• Sensory adaptation – the gradual decline in sensitivity to a prolonged stimulus.
• Possibly an evolutionary development: we need to know about the changes rather than the constants in our environment.
Subliminal advertising
One to One Fallacy
• Sensory adaptation, Weber’s Law, Signal-Detection Theory, and JND’s all show that there is no one to one correspondence between sensory input and sensory experience.
The 5 Senses – The Eye
• The most sophisticated sensory organ that an organism can possess is the eye:
• It can see over vast distances in almost real time.
• It can detect minute changes and motion and can self-adjust quickly to varying levels of JND’s.
The stimulus: Light• Light is a form of electromagnetic radiation that has
three varying properties:– Wavelength– Amplitude– Purity
• These properties are all sensed in different ways by the eye:– Hue– Brightness– Saturation
Light interacts with the eye• Light enters the eye through
the cornea and is modified by the lens.
• The lens focuses light on the receptor surface at the back of the eye called the retina.
• The pupil is the opening in the iris, or the colored muscle surrounding it. It closes and opens to regulate the amount of light entering the eye.
Rods and cones• Light passes through the cornea,
pupil, and lens to fall on the retina – the light sensitive layer of receptors that line the back of the eye cavity.
• Light is detected by two different kinds of cells – rods and cones
• Rods – more sensitive to light and dark and peripheral vision
• Cones – more sensitive to daylight vision and color. Cones are concentrated in the center of the retina - fovea
Eye as a processing organ
• The layer of intervening cells between the light source and the rods and cones actually processes the image as inverted before sending the information through the optic disk to the brain.
Color vision
Color is a psychological interpretation, not a property of light itself.
Humans can distinguish roughly 1 million different colors created by subtractive or additive mixing.
Green exhibits the most variation in perception
Color vision
• Light perception is a mix of two different theories:
• Trichromatic theory – there are three different types of cones each excited to a different wavelength. – red, green, blue.
• Opponent-Process theory – receptors in the eye have antagonistic responses to three PAIRS of colors. – red/green, blue/yellow, black/white.
Opponent-process Theory
• When the three types of cones are stimulated, it has an inhibitory effect on the opposite color on the spectrum. This is what produces an afterimage – a visual image that persists after the stimuli is removed.
Color processing in the retina• Cells in the retina
respond in opposite ways to the same wavelength.
• Eg. The same cell will be excited by green and inhibited by red, and there are other cells that are excited by red and inhibited by green.
Perception
• As the eye sends signals to the brain, these are processed with implicit assumptions about the reality presented that go BEYOND what is only seen
• Impossible figures – objects that can be represented in two dimensional pictures but cannot exist in three dimensional space.
• Perceptual constancy – the tendency to experience a stable perception in the face of continually changing sensory input.
• Size, shape, and brightness
Depth perception• Involves interpretation of cues that indicate
how near or far away objects are.• There are two different sets of cues that can
help us judge distance.– Monocular cues– Binocular cues
Binocular cues
Clues about distance based on the differing view from the two eyes.–Retinal disparity – objects within 25 ft. of the viewer project images to slightly different locations on the right and left retina. The brain will interpret these two differing images as depth. –Convergence – sensing the eyes converging toward each other as they focus on closer objects.
Monocular cues
• Cues about distance based on the image in each eye alone.
• Motion parallax – images of objects at different distances move across the retina at different rates.
Monocular cues - Pictorial depth
• Cues about distance that can be given in a flat picture frame.
• Linear perspective – parallel lines converge as they move away from the viewer
• Texture gradients – details become clearer as objects near, become smoother as objects are far away.
• Interposition – an object that comes between the viewer and another object, it must be closer.
• Relative size – closer objects appear larger.
• Height in plane – distant objects appear higher in picture.
Perception of form and shape
• The same visual input can produce radically different perceptions.
• Perceptual set – a readiness to perceive a stimuli in a particular way.
• This readiness causes us to assemble specific elements in an image into a more complex form – feature analysis.
Feature Analysis
• When we are faced with a stimuli, our brain keys on certain feature detectors to analyze it’s content and make a decision about what that object is and means.
Gestalt Analysis
Gestalt Analysis – the presentation of the entire set of stimuli as a whole leads to the organization of its individual parts. •There are several principles of Gestalt psychology and perception:
Figure and ground: What part of the image is grouped into the foreground or the background will determine how an image is perceived.
Proximity: Things that are nearer to each other are perceived to be grouped.
Similarity: People tend to group stimuli that are similar.
Continuity: People tend to follow whatever direction their eye is led. People connect points that form a line or a smooth curve.
Simplicity: People tend to organize complex visual images in the simplest way possible.
Closure: People tend to group elements in a way that creates a sense of closure or completeness.
Which principle applies?
Your homework• 1) find an image on the internet• 2) print it• 3) explain the Gestalt perceptive elements at
work in it• 4)write those explanations down• 5) staple the picture to the explanation, put
your name on it and hand it in FOR TOMORROW• 6) it will be marked out of 5 based on your
accuracy and explanation
Experiment
• What makes a symbol into a letter?• Define a particular letter.
Kinesthesis and Vestibular Senses
• The sensation that allows an organism to orient itself in space and informs about its own movement.
Skeletal movement and orientation
• Skeletal movement is sensed through kinesthesis, the feedback we get from the muscles, tendons, and joints as they move.
• Orientation is sensed through receptors in the inner ear of each side of the head called the semicircular canals.
• Three canals contain a fluid that rotates as the head moves, this motion causes tiny hairs in the vestibules to move.
• This provides information about the extent of the head’s rotation.
Vision stabilization
• This sense plays a vital role in stabilizing vision.
• As the head and body move through space, the vestibular senses from each side of the head relay their information directly to the muscles that control each eye.
• The motion of the head is cancelled by an equal and opposite motion of the eye.
False Motion
• The reverse effect can take place as motion of the eyes can affect the perceived motion of the vestibular senses.
The Skin
• Researchers believe that there are four distinct skin sensations: pressure, warmth, cold, and pain.
• In this case, there are different receptors for each kind of sense quality.
Multiple specialized receptors
• Variations in pressure sensations are produced by different receptors in the skin.
• Some are wrapped around the base of hair follicles and sense movements of the hair
• Others are capsules that are easily bent by slight deformations of the skin.
• Other capsules respond to vibrations, steady indentation of the skin, and others to sudden movement across the skin.
Mystery of pain
• Less is known about the sensation of temperature and pain.
• Some of these experiences are triggered by free nerve endings with no specialized structures attached to them.
• There is little known about the nature of pain. Research has shown that it results from an intense triggering of a given receptor.
Taste
• Taste is vital to the organism for providing information about substances that may or may not be ingested.
• In humans, taste receptors sense chemicals dissolved in water on the tongue. These receptors are grouped into taste buds on the tongue and elsewhere in the mouth.
• Taste sensations can be divided into four basic qualities: bitter, sour, sweet, and salty.
• All other tastes are a combination of these four.
Specific receptors
• Nerve fibers are specifically targeted to respond to similar chemical compounds. Some fibers respond best to salts, others to sugars, etc.
• Relatively few substances stimulate one type of nerve only so most taste sensations are a pattern of varying levels of each taste specific fiber.
Smell
• Smell is one of the three distant senses (hearing, sight)
Olfaction
• Smell (olfaction) occurs when chemicals in the environment excite receptors located at the top of the nasal cavity – olfactory epitheluim
The nature of smell
• Several sensory categories exist for smell: fragrant, spicy, and putrid.
• However, it remains unknown how chemicals can trigger these specific sensations.
• It is believed that it is a pattern sensation rather than sense-specific nerves that is responsible.
Smell + Taste
• Smell occurs within the mouth as well – flavor• Flavor depends largely on smell rather than
taste. • When our sense of smell is temporarily
impaired, our sense of taste goes with it.
Smell as a distant sense
• Studies have shown that smell has an important effect on behaviour.
• It can be used to warn an organism of danger• It can be used to sell things to consumers• It can be used to identify people.
Pheromones• Pheromones are chemical substances released by
organisms that communicate information and influence behaviour of other members of the same species.
• Females will secrete a substance that signals their sexual receptiveness• Rats who have been shocked in a cage will secrete a substance that will cause the next rat
placed in the same cage to respond with anxiety.
• Studies have also shown that menstrual synchrony is triggered by pheromones of females living together.
• Also, female sensitivity to pheomones fluctuates over the duration of the menstrual cycle and will peak during ovulation.
Hearing• Hearing is produced by variations in the air
pressure around the head. • These variations have two properties that are
sensed by the ear.• 1)Wavelength – the distance between two
incoming waves.• 2) Amplitude – the height of the incoming waves. • These two properties can be sensed by the brain
as: loudness and pitch.
Complex waves
• Most waves encountered in the evironment are a combination of several other waves.
• The brain is able to separate the various sound patterns into their component parts
Sound
• Sound waves are collected by the outer ear and funneled to a membrane called the ear drum.
• Waves are then transferred across a second chamber where they trigger movement in a series of small bones called the ossicles.
• Vibrations of this membrane cause changes in the fluid contained in the cochlea, a snail shaped chamber lined with tiny hairs.
• These fluid changes trigger movement in the hairs that are sensed by receptors and passed along the auditory nerve to the brain.
Sensory interaction
• This pattern of stimulation illustrates a principle of sensation in which the response by a sensory system to a stimulus rarely depends on that stimulus alone, but is also affected by other stimuli that are occurring or may have just occurred recently – Sensory interaction.
Sensory Coding
• With all the different senses that we have looked at, the final problem becomes
• how does the body encode neural impulses into complex sensations?
• There must be some property of the neural impulses that differentiate one sensation from another across all the different sense receptors.
Stimulus intensity
• There is one code for intensity. We have seen that this can be communicated in two ways:
• The number of neurons firing.• The rate of each neuron firing.
Stimulus quality
• There is another code for quality.• There are two theories that try to explain this:• Specificity theory – different sense qualities are
triggered by different nerves being stimulated, each one specific for a different quality.
• Across-fiber pattern theory – different sense qualities are triggered by a pattern of activation across numerous nerves. Different sensations are actually different patterns of nerves firing in specific ways.
Perceptual Cues
• One primary example of perceptual cues at work is depth perception - involves interpretation of cues that indicate how near or far away objects are.
There are two different sets of cues that can help us judge distance.
• Binocular cues – clues about distance based on the differing view from the two eyes.
• Retinal disparity – objects within 25 ft. of the viewer project images to slightly different locations on the right and left retina. The brain will interpret these two differing images as depth
• Convergence – sensing the eyes converging toward each other as they focus on closer objects.
• Monocular cues – cues about distance based on the image in each eye alone
• Motion parallax – images of objects at different distances move across the retina at different rates.
• Pictorial depth cues – cues about distance that can be given in a flat picture frame.
• Linear perspective – parallel lines converge as they move away from the viewer
• Texture gradients – details become clearer as objects near, become smoother as objects are far away.
• Interposition – an object that comes between the viewer and another object, it must be closer.
• Relative size – closer objects appear larger.• Height in plane – distant objects appear higher in
picture.
Misleading cues…
• Impossible figures – objects that can be represented in two dimensional pictures but cannot exist in three dimensional space.
• Perceptual constancy – the tendency to experience a stable perception in the face of continually changing sensory input.
Describe the difference between pre-Renaissance painters and post-Renaissance painters. How did
understanding of perception cues change?
• Pre-Renaissance painters did not have a grasp of pictorial depth cues. Their renderings are awkward and flat. Renaissance painters mastered these cues, especially linear perspective, height in plane, and interposition. Instead of just relating the story of an event, they sought to re-create the reality or the illusion of the event happening.
How did the Impressionists use principles of perception to go beyond reality?
• Impressionists used separate spots of pure colors that would blur together at a distance. He used complementary colors to create the impression of a scene after it has been experienced rather than recreating the scene exactly at it was.
Using what we have already learned about sight sensation, describe how Pointillism
operates.
• Seurat used small points of pure colors in an additive way. From a distance, these colours would be combines by the brain into stimuli of a single color. This shows an awareness of texture gradient. Also, continuity and closure Gestalt principles operate to create objects out of groupings of dots.
What perceptual principles are operating in Picasso’s work? How does this affect the
way you like it?
• Cubists applied the principles of feature analysis to their paintings, They reduced objects to their geometric component shapes and reassembled them on a flat plane. This triggers responses in the viewer using closure, continuity, similarity, and proximity. It is seen by the eye but understood by the mind – freaky!
How many different images can you find in Salvatore Dali’s The Hallucinogenic Toreador? Describe the effect this has on
you as the viewer.
• Bullfighter in the Venus de Milos, • a bull in the shapes below to the left, • a man waving a cape in the air. • This gives the painting a dream like quality.
