Explain the process that links the physical sensory world and the brain for each of the senses (vision, hearing, taste, smell, and touch).
3.1: The Process of Sensation
The absolute threshold is the minimum amount of sensory stimulation that can be detected 50% of the time. The difference threshold is a measure of the smallest increase or decrease in a physical stimulus that can be detected 50% of the time.
For each of the senses, the body has sensory receptors that detect and respond to sensory stimuli. Through the process of transduction, the receptors change the sensory stimuli into neural impulses, which are then transmitted to precise locations in the brain.
The cornea bends light rays inward through the pupil—the small, dark opening in the eye. The iris dilates and contracts the pupil to regulate the amount of light entering the eye. The lens changes its shape as it focuses images of objects at varying distances on the retina, a thin layer of tissue that contains the sensory receptors for vision. The cones detect color and fine detail; they function best in adequate light. The rods are extremely sensitive and enable vision in dim light.
The rods and the cones transduce light waves into neural impulses that pass from the bipolar, amacrine, and horizontal cells to the ganglion cells, whose axons form the optic nerve beyond the retinal wall of each eye. At the optic chiasm, the two optic nerves come together, and some of the nerve fibers from each eye cross to the opposite side of the brain. They synapse with neurons in the thalamus, which transmit the neural impulses to the primary visual cortex.
The perception of color results from the reflection of particular wavelengths of the visual spectrum from the surfaces of objects. For example, an object that appears to be red reflects light of longer wavelengths than one that appears to be blue. Color blindness is the inability to distinguish certain colors from one another, rather than the total absence of color vision. Two major theories that attempt to explain color vision are the trichromatic theory and the opponent-process theory.
3.3: Hearing and Balance
The pitch of a sound is determined by the frequency of the sound waves, which is measured in hertz. The loudness of a sound is determined largely by the amplitude of the sound waves, which is measured in decibels.
Sound waves enter the pinna, the visible part of the outer ear, and travel to the end of the auditory canal, causing the eardrum to vibrate. This sets in motion the ossicles in the middle ear, which amplify the sound waves. The vibration of the oval window causes activity in the inner ear, setting in motion the fluid in the cochlea. The moving fluid pushes and pulls the hair cells attached to the thin basilar membrane, which transduces the vibrations into neural impulses. The auditory nerve then carries the neural impulses to the brain.
The kinesthetic sense provides information about the position of body parts in relation to one another and the movement of the entire body or its parts. This information is detected by sensory receptors in the joints, ligaments, and muscles. The vestibular sense detects movement and provides information about the body’s orientation in space. Sensory receptors in the semicircular canals and the vestibular sacs sense changes in motion and the orientation of the head.
3.4: Smell, Taste, and Touch
The act of smelling begins when odor molecules reach the smell receptors in the olfactory epithelium, at the top of the nasal cavity. The axons of these receptors relay the smell message to the olfactory bulbs. From there, the smell message travels to the thalamus and the orbitofrontal cortex, which distinguish the odor and relay that information to other parts of the brain.
The primary taste sensations are sweet, salty, sour, bitter, and umami. The receptor cells for taste are found in the taste buds on the tongue and in other parts of the mouth and throat.
Sensitive nerve endings in the skin convey tactile information to the brain when an object touches and depresses the skin. The neural impulses for touch sensations ultimately register in the brain’s somatosensory cortex. Pain can be a valuable warning and a protective mechanism, motivating people to tend to an injury, to restrict activity, and to seek medical help. Negative thinking can influence the perception of pain. Some cultures encourage individuals to suppress (or exaggerate) emotional reactions to pain. Endorphins are natural painkillers produced by the body, which block pain and produce a feeling of well-being.
3.5: Influences on Perception
Attention enables the brain to focus on some sensations while screening others out. Unattended stimuli may be missed altogether or incorrectly perceived. Inattentional blindness occurs when we try to keep track of several moving objects at the same time.
Individuals use bottom-up and top-down processing to apply their prior knowledge to perceptual problems. Expectations based on prior knowledge may predispose people to perceive sensations in a particular way.
Research suggests that the mirror neuron system processes information about others’ motor actions and emotional expressions. It is also active when we attempt to mimic these behaviors from models. The brain also possesses a network devoted to face processing. In addition, cross-modal perception of human actions varies from that which we use to perceive objects. When visual and auditory information from a human source is in conflict, we rely on visual cues. The opposite is true for object perception.
3.6: Principles of Perception
The Gestalt principles of a perceptual organization include figure-ground, similarity, proximity, continuity, and closure. Perceptual constancy is the tendency to perceive objects as maintaining the same size, shape, and brightness, despite changes in lighting conditions or changes in the retinal image that result when an object is viewed from different angles and distances.
The binocular depth cues include convergence and binocular disparity, which depend on both eyes working together for depth perception. The monocular depth cues, those that can be perceived by one eye, include interposition, linear perspective, relative size, texture gradient, atmospheric perspective, shadow or shading, and motion parallax.
The brain perceives real motion by comparing the movement of images across the retina to information derived from the spatial orientation senses. Apparent motion is the result of a psychological response to specific kinds of stimuli, such as flashing lights. The brain may also mistakenly perceive eye movement as object movement.
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