SENSATION & PERCEPTION Sensation and perception result from a symphony of sensory receptors and the neurons those receptors communicate with. The receptors and neurons fire in different combinations and at different rates, the sum of which is consciousness, the huge range of perceptions that make up our experience of the world (Gazzaniga et al, Psychological Science [4 th ed.], p. 134) Psychophysics is the study of transduction, which is a process whereby stimuli are translated into neural activity in the brain. Transduction summarizes the parallel processes of sensation, which might be seen as the biological work of the sense organs, and perception, which might be seen as the psychological process of interpreting this activity 1. Early work in the history of psychology focused on the processes of sensation and perception. Some of this work was intended to discover the absolute threshold, which is the minimum intensity of stimulation that must occur before you experience a sensation, and the justnoticeable difference (or difference threshold), which is the minimum amount of change required for a person to detect a difference. 2 One of the findings in this area shows that people can become less sensitive to stimuli over extended periods of time. This is known as sensory adaptation. One important implication of this is that sensation and perception is not simply a mechanical process, but involves psychological experiences and interpretation that is influenced by past events. For each of the senses, there is a basic series of steps in the transduction process. Vision Sense organ: Eye Sensation: Light Waves Light passes across the cornea, the eye s thick, transparent outer layer Pupil regulates the amount of light entering the eye Lens focuses light on the back of the eye the retina Retina is at the back of the eye and houses the receptors, which are known as rods that are specialized in night and peripheral vision and cones that are specialized in daylight vision and color vision 3 The signal is carried by the optic nerve to the brain The optic nerve reaches the optic chiasm, where the visual information from the left half of the field sends information to the right-side of the brain and vice versa 1 Weiten, Psychology: Themes and Variations (9 th ed), p. 130 & Gazzaniga et al. Psychological Science (4 th ed), pp. 132-133 2 Gazzaniga et al. Psychological Science (4 th ed), p. 135 3 Quotes from Weiten, Psychology: Themes and Variations (9 th ed), p. 138 Page 1 of 5
The signal then reaches the thalamus (the central relay station of the brain so to speak, where most incoming information enters and is then directed to specialty areas of the brain) The thalamus sends the signal to the primary visual cortex, where the perception process primarily takes place and is housed in the occipital lobe of the brain The primary visual cortex then sends the information along two pathways: the dorsal stream (known as the where pathway, where the perception of motion and depth take place) and the ventral stream (known as the what pathway, where the perception of form and color take place) 4 Audition Sense organ: Ear Sensation: Sound Waves Sound waves are captured by the Pinna (or outer ear) The vibration passes and activates the Tympanic Membrane (or eardrum), which passes the vibration into the middle ear, made up of three bones the maleus (hammer), incus (anvil), and stapes (stirrup) This vibration is then passed into the inner ear the cochlea through its opening of the oval window inside the cochlea is the organ of corti, which is a fluid filled chamber in the cochlea where transduction takes place o The process is a bit complex, but involves wave action that bends hair cells (known as cilia) attached to the tectorial membrane at the top and the basilar membrane at the bottom. The wave action activates (polarize) or deactivates (hyperpolarize) the signal that is then sent to the electrical signal sent to the brain. The thalamus receives this information and passes it to the primary auditory cortex, housed in the temporal lobe of the brain 4 Quotes from Weiten, Psychology: Themes and Variations (9 th ed), p. 141-142 Page 2 of 5
Tectorial Membrane Basilar Membrane Cilia Depolarization Hyperpolarization There are two theories about how the sound is actually coded: Frequency theory asserts that perception of pitch corresponds to the rate, or frequency, at which the entire basilar membrane vibrates. 5 Place Theory asserts that specific sound frequencies vibrate specific portions of the basilar membrane, producing distinct pitches, so that perception of pitch corresponds to the vibration of different portions, or places, along the basilar membrane 6. As hearing is understood today, both of these theories complement each other to explain hearing at different frequencies Determining the location of sound also depends on the frequency of the sound. For lowfrequency sounds, the arrival of the sound wave determines the location (phase differences) and for high-frequency sounds, the location is influenced by a sound shadow, such that the ear closest to the sound determines the location (intensity differences) 5 Weiten, Psychology: Themes and Variations (9 th ed), p. 162 6 Weiten, Psychology: Themes and Variations (9 th ed), p. 162 Page 3 of 5
Taste Sense organ: Tongue Sensation: Chemicals The stimuli is made of several chemicals that are absorbed by the receptors in and around the taste buds (the tiny bumps on your tongue) that turn the chemicals into electrical/neural signals o Receptors, relatively evenly spread on the tongue, specialize in five different tastes: sweet, sour, salty, bitter, and savory The information is then sent to the thalamus, which sends the signal to various areas in the cortex Smell Sense organ: Nose Sensation: Chemicals The stimulus enters the nose and is processed in the olfactory bulb, where the olfactory cilia (hair-like structures that are the receptors for smell, which convert the physical stimulus into electrical/neural activity) The signal is sent to various parts of the cortex; this does not pass through the thalamus o The frontal cortex is involved in determining the pleasantness of the smell Touch Sense organ: Skin Sensation: pressure, heat, cold, etc. for a total of six different categories of sensations The skin has various touch receptors that specialize in different types of sensation; however, it is not as clean-cut in that each of the various skin receptors only responds to that particular signal The receptors send information through the spinal cord The thalamus receives the information and passes it onto the primary somatosensory cortex located in the parietal lobe of the brain Page 4 of 5
Transduction of Pain: Pain information is transmitted via a fast pathway that registers localized pain and relays it to the cortex in a fraction of a second; it primarily deals with the sharp pain. 7 Pain is also transmitted via a slow pathway that lags a second or two behind the fast system. This pathway conveys the less localized, longer-lasting, aching or burning pain that comes after the initial injury 8. o both of these pathways go to the thalamus (although different areas of the thalamus) Gate Control Theory helps to explain the subjective nature of pain. This theory asserts that neural gates in the spinal cord allow signals through. Those gates can be closed when information about touch is being transmitted (e.g., by rubbing a sore arm) or by distraction 9. The gate is not an actual gate but another signal at the of the spinal cord that sends signals to block the pain message. One example of this is endorphins the body s natural pain killer 7 Weiten, Psychology: Themes and Variations (9 th ed), p. 168 8 Weiten, Psychology: Themes and Variations (9 th ed), p. 168 9 Gazzaniga et al., Psychological Science (4 th ed), p. 160 Page 5 of 5