Sensation: detecting stimuli from the body or environment

• The immediate experience of basic properties of an object or event that occurs when a sensory receptor is stimulated
• Sensory receptors: detect stimuli & convert energy into neural impulses
• Are found in the back of eye, in the ear, on the skin, etc.

• The act of organizing and interpreting sensory input as signaling a particular object or event

• This relationship is not a simple one-to-one equation…although we can measure the physical stimulus precisely, its effect on the observer is not so simple

I. The Visual System

• The visible light that humans see occupies only a small portion of the electromagnetic radiation spectrum: all forms of waves produced by electrically charged particles.
• Long wavelengths include infrared rays, radiowaves, and AC circuit currents (refer to slide)
• Short wavelengths include ultraviolet rays (the ones that give you a suntan) and X rays
• Visible light spectrum refers to the only part of the electromagnetic spectrum that we can see
• Ranges from 400 (violet) to 700 (red) nanometers in wavelength
• The entire electromagnetic spectrum ranges from .001 nm (gamma rays) to 1,000,000,000,000,000 nm (AC circuits)

• Cornea: Light is initially focused by this transparent covering over the eye
• Pupil: light enters the eye through this opening
• Iris: Muscle connected to the pupil that changes its size to let in more or less light
• Lens: this flexible disk under the cornea focuses light onto the back of the eye; flexibility of lens allows eye muscles to adjust light from objects at various distances away from person
• With age, the lens thickens and becomes less flexible
• Retina: light reflected from the lens is received by this sheet of tissue at the back of the eye; contains the receptors that convert light to nerve impulses…
• Cones: retinal cells that respond to particular wavelengths of light, allowing us to see color
• Most of our cones are located on the fovea, which gives us the sharpest resolution of visual stimuli
• Rods: retinal cells that are very sensitive to light but only register shades of gray (i.e., no color)
• Rods are located everywhere in the retina except in the fovea
• Rods allow us to see at night without strong light – this is why we see less color at night
• Because of where the rods are on the retina, we see best at night without light in the periphery of our vision (i.e., not focusing our gaze straight at the object but to its side)
• Dark adaptation: it takes about 30 mins in darkness for your rods to kick in at full strength (e.g., in a movie theater)
• From the receptor cells in the retina, the converted impulse from light is directed to the optic nerve: the large bundle of nerve fibers carrying impulses from the retina to the brain
• The optic nerve sits on the retina, but contains no rods or cones, so this is where you experience a “blind spot”; we are not aware that we have a blind spot because our brain completes patterns that fall across this blind spot; try the activity in Figure 3.7 on p. 99 of your text for a demonstration of this blind spot

• Trichromatic theory: argues that eye has three kinds of color sensors (cones), with each sensor responding maximally to a distinct range of wavelength
• Color vision arises from the combinations of neural impulses from these three different kinds of sensors
• A lot of researchers believe that the corresponding wavelengths here map onto blue (short wavelength: 460 nm), green (medium: 530 nm), and red (long: 650 nm)
• Example: presenting a yellow (580nm) stimulus to a participant, you would most likely see a large amount firing of the green (530 nm) & red (650 nm) cones, with little to no firing of the blue (460 nm) cones
• Opponent-process theory
• Other researchers argued that some color combinations don’t make sense…red and green mixed together doesn’t look yellow; it looks like mud…but, in addition to fact that mixing physical colors before the wavelengths get to the retina is a different process than the cones firing when the resulting wavelength reaches the retina, sensing color on the retina from various wavelengths then gets sent to brain for perception via opponent-process cells in the brain…this is where the opponent-process theory of color perception is relevant...
• This complementary theory to the trichromatic theory of color states that if a color is present, it causes cells that register it to inhibit the perception of an opposing color
• Proposes six psychologically primary colors, which are assigned by pairs to three kinds of receptors…
• (1) White-black receptor: if white is present, the perception of black is inhibited
• (2) Red-green receptor: if red is present, green is inhibited
• (3) Yellow-blue receptor: if yellow is present, blue is inhibited
• Inhibitions, along with various mixtures of these primary light wavelengths, more readily explain the complex range of colors that we experience after sensations of colors are sent from retina to brain’s opponent-process receptor cells.
• To clarify some issues here…
• Trichromatic Theory and Opponent-Process Theory work at two different levels, specifying different receptor cells at different stages in the process
• When you are looking at black words on a white page, you see both black and white because you are stimulating many white-black receptors on your retina!  You see a black letter with one white-black receptor (and doing so inhibits seeing white in that particular receptor), and you see the background of white with other white-black receptors (and doing so inhibits seeing black in those particular receptors)

• Gestalt principles
• Figure-ground: we recognize figures (objects) by distinguishing them from the ground (background)
• Proximity: Marks that are near one another tend to be grouped together
• Closure: We tend to fill in gaps in a figure
• Similarity: Marks that look alike tend to be grouped together
• Continuity: Marks that tend to fall along a smooth curve or a straight line tend to be grouped together
• Depth Perception: our brain uses different types of cues to perceive depth
• Binocular disparity: since we use both our eyes to focus on an image, the angles used by each eye to put the image on the fovea of our retina is used by the brain to perceive distance
• Monocular cues: our brain also uses information from the stimulus that does not involve our use of both eyes
• Motion: specifies distance of an object based on its movement
• Motion paralax: When you stare out the window of a moving car on the highway, you notice that closer objects that you are not focusing on whiz past you in the opposite direction, while farther objects that you focus on seem to move in the same direction as you; the objects also seem to shift at different speeds
• Texture gradient: progressive changes in texture that signal distance (e.g., you can make out less details of a path of grass/flowers the farther away the path is from you)
• Linear perspective: parallel objects seem to get closer together as they get farther away (e.g., railroad tracks)
• Perceptual Constancy: the image of an object on your retina can vary in size, shape, and brightness, but we will continue to perceive the object as stable in size, shape, and brightness
• Size constancy: if you see a car from a block away, you still perceive its size not to change even thought the image is smaller on your retina compared to a car parked right next to you
• Shape constancy: you perceive a book to maintain its rectangular shape even though at the different angles, your retinal image of the book is not a rectangle

Sound is the perception (i.e., transduction) of moving-air waves

• Pitch: frequency of air wave; loudness/volume: amplitude of air wave

• Tympanic membrane: the eardrum which is moved by air waves
• The eardrum then moves the hammer, anvil, and stirrup (the 3 smallest bones in our bodies), which all amplify the air wave and pass it on…
• …to the Basilar Membrane in the Cochlea: different freqs are transduced via hair cells (i.e., the “rods & cones” of the ear) into nerve impulses sent to the auditory cortex of the brain

• Frequency Theory: neural impulses are stimulated more with higher frequencies of air waves
• More plausible for small frequencies rather than high frequencies b/c we can hear freqs higher than the maximum rate of neural firing (1,000/sec)
• Place Theory: different frequencies of air waves activate different places along the basilar membrane
• Place Theory seems to win out compared to frequency theory

Touch

• Skin: the body’s largest sensory organ
• Millions of skin receptors mix and match to produce specific perception

• Paradoxical cold: even if stimulus is hot, if it touches a cold receptor, you will perceive coldness

• Endorphins: neurotransmitters in brain that have painkilling effects
• Gate-control theory: pain impulses can be inhibited by closing of neural gates in the spinal cord

• Provides info about position of joints, muscles, limbs
• Gives us control over body movements
• Example: when you close your eyes, you can touch your nose

• Provides info about body’s orientation relative to gravity and head’s position in space
• Helps us maintain balance
• Relies on semicircular canals in the inner ear!
• If you have an earache that affects these canals, you will have a tough time keeping your balance

Olfaction: sense of smell

• Lock-and-key: olfactory receptors (i.e., the “locks)” are built so that only molecules (the “keys”) with particular shapes will fit in particular receptors
• Receptors send the neural signals to the brain, passing the thalamus (memory) and the limbic system (emotions) along the way
• This is why odors often trigger emotional memories (e.g., smelling an ex-girl/boyfriend’s scent)

Gustation: sense of taste

• Taste buds: bumps on the tongue surface, back of throat, and inside the cheeks
• Detect molecules of substances that have dissolved in saliva
• Four types: sweet, sour, salty, and bitter
• The sense of taste combines with the sense of smell to produce perception of flavor of food
• Research suggests that neural impulses for both senses converge to some degree in brain area associated with perception of flavor

When smell of food is blocked, we have a harder time detecting most flavors