EXPLORING PSYCHOLOGY EIGHTH EDITION IN MODULES David Myers PowerPoint Slides Aneeq Ahmad Henderson State University Worth Publishers, © 2011

Sensation and Perception

Basic Concepts and Vision Module 14

Sensing the World: Some Basic Principles Vision Thresholds Sensory Adaptation Vision The Stimulus Input: Light Energy The Eye Visual Information Processing Color Vision

Sensing the World: Some Basic Principles How do we construct our representations of the external world? To represent the world, we must detect physical energy (a stimulus) from the environment and convert it into neural signals. This is a process called sensation. When we select, organize, and interpret our sensations, the process is called perception. Preview Question 1: What do we mean by bottom-up processing and top-down processing?

Sensing the World: Some Basic Principles Analysis of a stimulus that begins with the sense receptors and works up to the level of the brain and mind is referred to as bottom-up processing. Top-down processing is guided by higher-level mental processes, as when we construct perceptions, drawing on our experience and expectations.

What’s Going On Here? Our sensory and perceptual processes work together to help us sort out complex images. “The Forest Has Eyes,” Bev Doolittle

Psychophysics A study of the relationship between physical characteristics of stimuli and our psychological experience with them. Physical World Psychological World Light Brightness Sound Volume Pressure Weight Sugar Sweet

Thresholds Our awareness of faint stimuli illustrate our absolute threshold – the minimum stimulation needed to detect a particular stimulus 50% of the time. Preview Question 2: What are absolute and difference thresholds, and do stimuli below the absolute threshold have any influence?

Subliminal Threshold Subliminal Threshold: When stimuli are below one’s absolute threshold for conscious awareness.

Can we be affected by stimuli too weak to be noticed? Subliminal Threshold Can we be affected by stimuli too weak to be noticed? In some instances yes. An invisible word or image can prime your response to a later question by activating certain associations which predispose your memory, perception, or response. However, studies have shown that subliminal messages in advertising are ineffective, as the effect is subtle and fleeting.

Difference Thresholds The difference threshold is the minimum difference between two stimuli required for detection 50 percent of the time. Ernest Weber noted that two stimuli must differ by a constant minimum percentage (rather than a constant amount), to be perceived as different. Weber fraction: k = dI/I. This is known as Weber’s Law. Stimulus Constant (k) Light 8% Weight 2% Tone 3%

Sensory Adaptation The diminished sensitivity to an unchanging stimulus is known as sensory adaptation. This is why we seem to get used to an unpleasant smell in a room or an annoying background sound. After constant exposure to a stimulus, nerve cells fire less frequently. This allows us to focus on informative information rather than being distracted by constant background stimulation. Preview Question 3: What is the function of sensory adaptation?

Now you see, now you don’t Why, then, if we stare at an object without flinching, does it not vanish from sight? Because our eyes are constantly moving, enough that stimulation constantly changes. However, if you can stop the eyes from moving, images will begin to disappear bit by bit, reappear, and disappear again.

of electromagnetic energy. Vision What strikes our eyes is actually pulses of energy that our eyes perceive as color. It is a thin slice of the of the whole spectrum of electromagnetic energy. Visible Spectrum Preview Question 4: What is the energy that we see as visible light?

The Stimulus Input: Light Energy Two physical characteristics of light help determine our sensory experience of them. Wavelength: the distance from the peak of one light or sound wave to the peak of the next. Influences hue – the color we experience. Intensity: the amount of energy in a light or sound wave. Influences our perception of its brightness.

The Physical Properties of Waves

the rays by changing its curvature in a process called accommodation. The Eye Light enters the eye through the cornea, which protects the eye and bends light to provide focus, then passes through the pupil, a small adjustable opening surrounded by the iris, a colored muscle that adjusts light intake. The iris dilates or constricts in response to light intensity. Behind the pupil is a lens that focuses incoming light rays into an image on the retina, a multilayered tissue on the eyeball’s sensitive inner surface. The lens focuses the rays by changing its curvature in a process called accommodation. Preview Question 5: How does the eye transform light energy into neural messages?

The Eye

The Retina’s Reaction to Light Light makes its way through the retina’s outer layers to the rods – retinal receptors that detect black, white, and gray – and cones – retinal receptors that detect fine detail and give rise to color sensations.

Optic Nerve, Blind Spot & Fovea Blind Spot: Point where the optic nerve leaves the eye because there are no receptor cells located there. Close your left eye, look at the black dot, and hold the page about a foot from your face, at which point the car will disappear.

Receptors in the eye Rods and cones differ in their geography and in the tasks they handle. Cones cluster in and around the fovea, the retina’s area of central focus.

Visual Information Processing After processing by the retina, information travels to the bipolar cells, and then to the ganglion cells, through their axons making up the optic nerve and to the brain. Any retinal area relays information to the visual cortex. Preview Question 6: How does the brain process visual information?

Feature Detection Feature detectors are nerve cells in the brain that respond to specific features of a stimulus, such as shape, angle, or movement. Feature detectors in the visual cortex pass information to other cortical areas where supercell clusters respond to more complex patterns.

The Telltale Brain Looking at faces, houses, and chairs activates different brain areas in this right-facing brain.

Parallel Processing Processing of several aspects of the stimulus simultaneously is called parallel processing. The brain divides a visual scene into subdivisions such as color, depth, form, movement, etc.

Summary of Visual Information Processing

Color Vision We talk as if objects possess color, when in fact they do not. We perceive an object as certain color. How then, do we see the world in color? Knowing that any color can be created by combining the light waves of three primary colors—red, green, and blue—Young and von Helmholtz inferred that the eye must have three corresponding types of color receptors. This is now known as the Young-Helmholtz Trichromatic (three-color) theory and has been confirmed by further research. Preview Question 7: What theories help us understand color vision?

Color Blindness Genetic disorder in which people are blind to green or red colors. This supports the Trichromatic theory. People who suffer red – green deficiency have trouble perceiving the number within the design.

Color Vision The trichromatic theory can’t solve all of the mysteries of color vision. Ewald Hering studied the effect of afterimages and determined that that there must be two additional color processes, one responsible for red - versus - green perception, and one for blue - versus - yellow. This was later confirmed by the opponent-process theory - the theory that opposing retinal processes (red – green, yellow - blue, white - black) enable color vision.

Opponent Colors Gaze at the middle of the flag for about 30 Seconds. When it disappears, stare at the dot and report whether or not you see Britain's flag.