The Big Picture: Chapter Overview

Sensation is the process of detecting and encoding stimulus energy in the world by our sense organs. Through the process of transduction, the sensation is transformed into an action potential and a neural impulse or message is delivered to the brain. In the brain, the stimulated areas produce what we refer to as perception. Perception refers to the process of organizing and interpreting sensory information to give it meaning. The process of sensation starts with sensory receptors, which are cells found in your sensory organs that are dedicated to receiving the stimulation and transmitting the stimulus information to the afferent nerves that will then take the message to the brain. There are three categories of sensory receptors: photreception (sight), mechanoreception (touch, hearing, balance), and chemoreception (smell and taste). Psychophysics studies the links between the physical properties of stimuli and a person's experience of them. One important question in psychophysics is how much stimulus is needed for the person to sense and perceive a stimuli? The answer is in the concept of threshold. An absolute threshold is the minimum amount of energy that we can detect 50% of the time. In psychophysics, noise is the term used to refer to irrelevant and competing stimuli. When a person senses something without being aware of this sensation, he or she has experienced a subliminal perception. The difference threshold is the smallest difference in stimulation needed to recognize that two stimuli are different from each other 50% of the time. Signal detection theory holds that factors such as motivation, expectancy, and urgency of the moment influence sensitivity to sensory stimuli. Our perception of stimuli will be influenced by our selective and shiftable attention. The Stroop Effect and perceptual set illustrate some of the variables that shape the process of perception. Sensory adaptation happens because we tend to adapt to an average level of stimuli. If the level of stimili is changed, we will go through a process of re-adaptation.

The visual system is dedicated to photoreception. Light is a form of electromagnetic energy that travels in waves. Light has different wavelengths, the distance from the peak of one wave to the peak of the next wave. We can see light with wavelengths between 400 and 700 nanometers, which is also the range of wavelengths emitted by the sun. The eye has different parts, all specialized in the process of photoreception. The main external structures of the eye include the sclera, iris, pupil, and cornea. Within the eye, the retina records the light information and converts it to neural impulses for processing in the brain. The retina is made up of light-sensitive receptors called rods and cones, which perform transduction. The fovea is a small area in the center of the retina where vision is at its best. The blind spot lacks receptors since it is where neural impulses exit the eye on the optic nerve. The optic nerve fibers cross and divide at the optic chiasm, resulting in visual information received on the left eye to be processed on the right side of the brain and vice versa. In the visual cortex of the brain there are features detectors, which are neurons that process specific visual information. In the temporal lobe we process information about the color, form, and texture of the objects we see; in the parietal lobe we process information about the location, movement, and depth of the objects we see. All of this information is processed simultaneously; binding is the process through which it is all integrated into a complete perceptual experience. Objects have color because they reflect only certain wavelengths of light. Three important characteristics of color are hue, saturation, and brightness. Two major theories of color vision are the trichromatic theory and the opponent-process theory.

There are various dimensions to the process of perception. Some of these dimensions are shape, depth, motion, and constancy. The world is full of shapes and our perception of them is influenced by contour and figure-group relationship. Gestalt psychology describes how we tend to perceive according to certain patterns, and a main principle is that the whole is different than the sum of its parts. Depth perception allows us to perceive objects three-dimensionally and involves binocular and monocular cues. In studying motor perception, psychologists examine apparent movement, stroboscopic motion, and movement aftereffects. Perceptual constancy includes size constancy, shape constancy, and brightness constancy. Visual illusions occur when the same retinal image is produced by two objects, but are perceived as different images. Well-known visual illusions include the Muller-Lyer illusion, the Ponzo illusion, and the moon illusion.

The auditory system is dedicated to one type of mechanoreception, audition. Sound waves vary from each other in frequency and amplitude. Timbre is the perceptual quality of a sound. The ear has different parts, all working together to capture auditory information and send it to the brain for interpretation. The ear is made up of the outer ear, middle ear, and inner ear. The outer ear consists of the pinna and the external auditory canal. The middle ear consists of the eardrum, hammer, anvil, and stirrup. The inner ear is made up of the oval window, cochlea, and the basilar membrane. Three theories that explain auditory processing are place theory, frequency theory, and the volley principle. The auditory nerve carries the neural impulses from the auditory system to the brain. The timing and intensity of a sound are important in the process of localizing the source of the sound. Excessive noise has been associated with poor reading skills, high blood pressure, and easy distraction from tasks.

The skin senses are also dedicated to mechanoreception, specifically to the sensation of touch, temperature, and pain. Touch information goes to the thalamus, which relays information to the corresponding somatosensory areas of the parietal lobes. Females are more sensitive to touch than males. Thermoreceptors are located under the skin and are dedicated to the reception of temperature. There are warm and cold thermoreceptors. Pain is the sensation that warns us of damage to our body. The gate control theory has been suggested to explain pain perception and acupuncture. Some strategies used to reduce pain are distraction, focused breathing, and counterstimulation.

Taste and smell are sensations based on chemoreception, thus they are also referred to as the chemical senses. Taste buds, which are located on the tongue, respond to sweet, sour, bitter, and salty qualities. The olfactory epithelium contains the chemoreceptors for smell. The kinesthetic sense provides information to the brain about movement, posture, and orientation, while the vestibular system provides information about balance and movement.

Human factors psychology involves designing machines and arranging environments in which humans function. By considering perceptual factors, they can make the environments, usually the workplace, safer and more efficient.