Chapter 4 Sensation and Perception

The Man Who Mistook His Wife for a Hat By neurologist Oliver Sacks describing his most fascinating patients The case of Dr.P is most interesting for neuropsychology and the study of the effect of brain damage on behavior.. Dr.P’s condition would cause him to commit strange Mr. “Magoo-like” mistakes from which we may draw inferences about his rare disorder. For example he would often mistake inanimate objects for people, and he had a problem recognizing people by face. Dr.P appeared to have lead a normal, good natured and full life with profound musical talent.

Dr. P Unable to recognize a rose by sight, Dr. P could identify it by smell. He described it as “About six inches in length, a convoluted red form with a green linear attachment”

Sensation and Perception: The Distinction Sensation: stimulation of sense organs Perception: selection, organization, and interpretation of sensory input Psychophysics = the study of how physical stimuli are translated into psychological experience Sensation and perception are hard to separate, because people automatically start organizing incoming sensory stimulation the moment it arrives. For theory and research, however, the distinction between the two is useful. Sensation is the stimulation of sense organs…for example absorbing energy from light by the eyes. Perception is the selection, organization, and interpretation of sensory input…translating the sensory input into something meaningful. Look at a photo of a rose in your text, your eyes sense the light reflecting from the page; what you perceive, however, is the picture of the rose. Psychophysics is the study of how physical stimuli are translated into psychological experience, thus psychologists in this area are interested mainly in sensation and perception.

Figure 4.1 The distinction between sensation and perception

Psychophysics: Basic Concepts Sensation begins with a detectable stimulus Fechner: the concept of the threshold Absolute threshold: detected 50% of the time Just noticeable difference (JND): smallest difference detectable Weber’s law: size of JND proportional to size of initial stimulus (1/3 of original stimulus) Sensation begins with a detectable stimulus…but what counts as detectable depends on whom or what is doing the detecting. Gustav Fechner (1860) questioned, for any given sense, what is the weakest detectable stimulus? The concept of the threshold is implicit in Fechner’s question. A threshold is defined in the text as a dividing point between energy levels that do and do not have a detectable effect…example, automatic lights turn on when a threshold is reached. An absolute threshold is the minimal amount of stimulation that an organism can detect…depends on the boundaries of an organism’s sensory capabilities…researchers discovered, however, that there is no single stimulus intensity that results in a jump from no detection to 100% detection every time…thus researchers use the concept of the absolute threshold…the stimulus intensity that can be detected 50% of the time. Fechner was also interested in the smallest difference in the amount of stimulation that a specific sense can detect…the just noticeable difference (JND): smallest difference detectable. Research on the JND by Ernst Weber illustrated that the size of a JND is a constant proportion of the size of the initial stimulus…in general, as stimuli increase in magnitude, the JND becomes larger.

Psychophysics: Concepts and Issues Selective Attention Cocktail party effect – type of selective attention in which you can attend to only one voice at a time Cell phones and driving? Listening to music and studying? Card Trick Who Dunnit? Signal detection theory holds that the detection of sensory information is influenced by two things…1) noise in the system (irrelevant stimuli in the environment that elicit neural activity), and 2) decision making processes. Signal detection theory was important in that it emphasized factors other than stimulus intensity influencing detectability (in contrast to Fechner’s ideas). Many researchers, using very different methods, have demonstrated that perception can occur without awareness. Many people believe that advertisers attempt to place subliminal messages in ads, while others say that people are just reading things into ads, like seeing familiar shapes in the clouds. Regardless, research shows that the effects of subliminal perception are relatively weak and of little practical impact. Prolonged stimulation may lead to sensory adaptation, or a decline in sensitivity to the stimulus…you don’t smell the skunk that sprayed you yesterday, but everyone else does…the pool is only cold at first…etc.

Psychophysics: Concepts and Issues Signal-Detection Theory: Sensory processes + decision processes Depends on the criteion you set for how sure you must feel before you react Depends on “noise” in the background Listening for a doorbell at a party

Figure 4.3 Signal-detection theory

Psychophysics: Concepts and Issues Subliminal Perception: Existence vs. practical effects Sublimnal means “below threshold” 1957 “Eat Popcorn” messages in movies increased popcorn sales Jon Krosnick (1992) hidden messages to affect feeling about neutral situations – significant difference. Disney Movies Lyrics in Reverse

Psychophysics: Concepts and Issues Sensory Adaptation: Decline in sensitivity to a stimulus over time Jumping into a cold pool Stinky garbage in the kitchen Why would we adapt to sensations?

Sensory Transduction In physiology, transduction is the conversion of a stimulus from one form to another. “Transduction in the nervous system typically refers to stimulus alerting events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central nervous system where it is integrated” The process of converting a sensation into a perception

The Eye: Converting Light into Neural Impulses The eye: housing and channeling Components: Cornea: where light enters the eye Lens: focuses the light rays on the retina Iris: colored ring of muscle, constricts or dilates via amount of light Pupil: regulates amount of light The eye has two main purposes, providing a “house” for the neural tissue that receives light, the retina, and channeling light toward the retina. The eye is composed of the cornea, a transparent window where light enters the eye, the lens, which is a crystalline structure that lies right behind the cornea and focuses the light rays on the retina. The iris is the colored ring of muscle around the pupil (the black center of the eye), which constricts or dilates depending on the amount of light present in the environment, and changes the size of the pupil. The size of the pupil regulates the amount of light by constricting to let in less light and vice versa. Iris, Cornea, and Lens

Figure 4.7 The human eye

The Retina: An Extension of the CNS Retina: absorbs light, processes images Optic disk: optic nerve connection/blind spot Receptor cells: Rods: black and white/low light vision Cones: color and daylight vision Adaptation: becoming more or less sensitive to light as needed The retina is a piece of neural tissue that lines the back of the eye…it absorbs light, processes images, and sends information to the brain. Axons from the retina to the brain converge at the optic disk, a hole in the retina where the optic nerve leaves the eye. If an image falls on this hole, it can’t be seen…the blind spot. The visual receptor cells in the axon are the rods (for black and white and low light vision) and the cones (for color and daylight vision). Adaptation, or becoming more or less sensitive to light as needed, occurs in part due to chemical changes in the rods and cones. Receptive fields are the collection of rod and cone receptors that funnel signals to a particular visual cell in the retina. Lateral antagonism, or lateral inhibition, occurs when neural activity in a cell opposes activity in surrounding cells. Retina, Optic Nerve, & Brain

Figure 4.8 Nearsightedness and farsightedness

Figure 4.9 The retina

Figure 4.10 The process of dark adaptation

Visual Information Processing Light  rods and cones  neural signals  optic nerve  optic chiasm  lateral geniculate nucleus (thalamus)  opposite half brain  primary visual cortex (occipital lobe) The Retina and the Brain: Visual Information Processing Light striking the rods and cones triggers neural signals to move to bipolar cells then to ganglion cells, then along the optic nerve to the optic chiasm, where the optic nerves from the inside half of each eye cross over and project to the opposite half brain. This crossing ensures that signals from both eyes go to both hemispheres of the brain. After the crossing, 2 visual pathways exist. The main pathway goes through the lateral geniculate nucleus in the thalamus and on to the primary visual cortex in the occipital lobe. The other goes through the superior colliculus to the thalamus and on to the primary visual cortex. The main visual pathway is subdivided into two subspecialty pathways, the magnocellular channel and the parvocellular channel. These channels engage in parallel processing, which involves simultaneously extracting different kinds of information from the same input, with the parvocellular channel handling perception of color, for example, and the magnocellular channel handling brightness.

Figure 4.13 Visual pathways through the brain

Figure 4.15 The what and where pathways from the primary visual cortex

Hubel and Wiesel: Feature Detectors and the Nobel Prize Early 1960’s: Hubel and Wiesel Microelectrode recording of axons in primary visual cortex of animals Discovered feature detectors: neurons that respond selectively to lines, edges, etc. Groundbreaking research: Nobel Prize in 1981 Later research: cells specific to faces in the temporal lobes of monkeys and humans Hubel and Wiesel: Feature Detectors and the Nobel Prize In the early 1960’s H and W started research using microelectrode recording of axons in the primary visual cortex of animals…initially, they had little success getting neurons to fire by having the cats look at flashing spots of light. Accidentally, they introduced a straight line light…rapid firing occurred in the visual cortex. They went on to discover that the visual cortex has feature detectors in it, neurons that respond selectively to very specific features of complex stimuli…lines, edges, etc. This was groundbreaking research, which won them the Nobel Prize in 1981. Later research has demonstrated that there are cells in the temporal lobes of monkeys and humans (along the visual pathway) that specifically respond to pictures of faces…grandmother cells.

Theories of Color Vision Trichromatic theory - Young and Helmholtz Receptors for red, green, blue – color mixing (like old televisions) Colorblind = dichromatic Opponent Process theory – Hering 3 pairs of antagonistic colors red/green, blue/yellow, black/white Current perspective: both theories necessary Young and Helmholtz, in the mid 1800’s, came up with the first theory of color vision…trichromatic theory. This theory holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths…one for red, one for green, and one for blue. All colors can be seen, according to this theory, because of color mixing. But what about yellow? Is it just reddish-green? Edward Hering, in 1878, proposed opponent process theory…which holds that color perception depends on receptors that make antagonistic responses to three pairs of colors…red on, green off; yellow on, blue off; black on, white off. This not only takes care of yellow, but also explains the phenomenon of complimentary afterimages. While researchers argued about which was right for almost a century, most psychologists now agree that it takes both theories to explain color vision.

Figure 4.18 The color circle and complementary colors

The Afterimage Effect Castle Afterimage Effect A visual image the persists after a stimulus is removed Will be a compliment color to original stimulus Castle Afterimage Effect

Perceiving Forms, Patterns, and Objects Bottom-up processing Elements to the whole Top-down processing Whole to elements Top dwon prcossenig alolws you to raed snetneces like this. A reversible figure is a drawing that is compatible with two interpretations that can shift back and forth (see following slides for depiction). A perceptual set is a readiness to perceive a stimulus in a particular way. Inattentional blindness involves the failure to see fully visible objects or events in a visual display. According to feature detection theory, people detect specific elements in stimuli and build them up into recognizable forms…bottom-up processing. Subjective contours is a phenomenon whereby contours are perceived where none actually exist, attributed to top-down processing. Form perception involves top-down processing…clearly emphasized by the Gestalt psychologists, who demonstrated that the whole is more than the sum of its parts.

Bottom-up v. Top-down

Stroop Effect

Reversible figures

Reversible Image

Reversible Image: The Necker Cube

Figure and Ground First step in perceiving an image is determining figure and ground.