4.7 SUMMARY
Sensation and Perception Are Distinct Activities
- Sensation is the simple stimulation of a sense organ, whereas perception organizes, identifies, and interprets sensation at the level of the brain.
- Transduction is the process which converts physical signals from the environment into neural signals carried by sensory neurons into the central nervous system.
- Psychophysics is an approach to studying perception that measures the strength of a stimulus and an observer’s sensitivity to that stimulus. The just noticeable difference (JND) is the smallest change in a stimulus that can be detected.
- Sensory adaptation occurs because sensitivity to lengthy stimulation tends to decline over time.
Vision I: How the Eyes and the Brain Convert Light Waves to Neural Signals
- Two types of photoreceptor cells in the retina transduce light into neural impulses: cones, which operate under normal daylight conditions and sense color; and rods, which are active under low-light conditions for night vision.
- Light striking the retina causes a specific pattern of response in each of the three cone types that are critical to color perception: short-wavelength (bluish) light, medium-wavelength (greenish) light, and long-wavelength (reddish) light. The overall pattern of response across the three cone types results in a unique code for each color.
- Information encoded by the retina travels to the brain along the optic nerve, to the thalamus and then to the primary visual cortex (area V1) in the occipital lobe.
- Two pathways project from the occipital lobe to visual areas in other parts of the brain. The ventral stream projects to areas of the temporal lobes that represent an object’s shape and identity. The dorsal stream projects to the parietal lobes that identify the location and motion of an object.
Vision II: Recognizing What We Perceive
- According to feature-integration theory, attention provides the glue necessary to bind features together. The parietal lobe is important for attention and contributes to feature binding.
- Some regions in the occipital and temporal lobes respond selectively to specific object categories, supporting the modular view that specialized brain areas represent particular classes of objects such as faces or houses or body parts.
- Gestalt principles of perceptual grouping, such as simplicity, closure, and continuity, govern how the features and regions of things fit together.
- Depth perception depends on monocular cues such as familiar size and linear perspective, binocular cues such as retinal disparity, and motion-based cues, which are based on the movement of the head over time.
- Change blindness and inattentional blindness occur when we fail to notice visible and even salient features of our environment, emphasizing that our conscious visual experience depends on focused attention.
Audition: More Than Meets the Ear
- Perceiving sound depends on three physical dimensions of a sound wave – frequency, amplitude, and complexity or mix of frequencies; respectively, these features determine our perception of pitch, loudness, and sound quality (timbre).
- Auditory pitch perception begins in the outer ear, which funnels sound waves toward the middle ear, which in turn sends the vibrations to the inner ear, which contains the cochlea.
- Action potentials from the inner ear travel along an auditory pathway through the thalamus to the primary auditory cortex (area A1) in the temporal lobe.
- Auditory perception depends on both a place code and a temporal code. Our ability to localize sound sources depends critically on the placement of our ears on opposite sides of the head.
The Body Senses: More Than Skin Deep
- Sensory receptors on the body send neural signals to locations in the somatosensory cortex, a part of the parietal lobe, which the brain translates as the sensation of touch.
- The experience of pain depends on signals that travel to the somatosensory cortex, which indicates the location and type of pain, and to the emotional centers of the brain, which result in unpleasant feelings.
- Balance and acceleration depend primarily on the vestibular system, but they are also influenced by vision.
The Chemical Senses: Adding Flavor
- Our experience of smell, or olfaction, is associated with odorant molecules binding to sites on specialized olfactory receptors, which send axons to the olfactory bulb. The olfactory bulb in turn sends signals to parts of the brain that control drives, emotions, and memories, which helps to explain why smells can have immediate and powerful effects on us.
- Smell is also involved in social behavior, as illustrated by pheromones, which are related to reproductive behavior and sexual responses in several species.
- Sensations of taste depend on taste buds, which are distributed across the tongue, roof of the mouth, and upper throat, and on taste receptors that correspond to the five primary taste sensations of salty, sour, bitter, sweet, and umami.