Premed · Premed · Introductory Psychology
Lecture 5: Sensation and Perception I: Vision and Hearing
Introductory Psychology
Learning Objectives
By the end of this lecture, students will be able to:
- Distinguish between sensation and perception and explain the process of transduction
- Define absolute threshold, difference threshold, and signal detection theory
- Describe the anatomy of the eye and the processes underlying vision
- Explain color vision theories (trichromatic and opponent-process)
- Describe the anatomy of the ear and the processes underlying hearing
Lecture Content
I. Basic Principles of Sensation and Perception
Sensation is the process by which sensory receptors detect stimuli and convert them into neural signals, while perception is the higher-level process of organizing and interpreting that sensory information to give it meaning. The conversion of physical energy — whether light, sound, or pressure — into neural impulses is called transduction. Two complementary modes of processing are at work in perception. Bottom-up processing begins with raw sensory data and works upward to the brain's interpretive centers, whereas top-down processing is guided by higher-level knowledge, expectations, and context. Reading messy handwriting, for instance, relies heavily on top-down processing, since context clues help fill in ambiguous letter forms.
The absolute threshold is the minimum stimulation needed to detect a stimulus 50% of the time. Classic examples include seeing a candle flame at 30 miles on a clear night or hearing a watch ticking at 20 feet in a quiet room. The difference threshold, also known as the just noticeable difference (JND), is the minimum difference between two stimuli that can be detected 50% of the time. Weber's law states that this threshold is a constant proportion of the original stimulus intensity — for weight, the ratio is approximately 1/50, meaning you would need to add about 1 pound to a 50-pound object to notice the change. The specific ratio varies by sensory modality.
Signal detection theory (SDT) recognizes that detecting a stimulus depends not only on its physical intensity but also on psychological factors such as motivation, expectations, fatigue, and alertness. SDT identifies four possible outcomes in any detection task: a hit, a miss, a false alarm, and a correct rejection. A person's response bias — their tendency to say "yes" or "no" — shifts depending on the consequences of each type of error. Finally, sensory adaptation describes the phenomenon whereby sensitivity decreases in response to constant, unchanging stimulation. You stop noticing the feeling of your clothing shortly after dressing because your sensory system has adapted, freeing attention for detecting new changes in the environment.
II. Vision: The Eye
Light is electromagnetic radiation, and human vision detects wavelengths ranging from approximately 380 to 740 nanometers. The wavelength of light determines hue (color), its amplitude determines brightness, and its purity determines saturation (the richness of the color).
Light enters the eye through the cornea, a transparent outer covering that bends (refracts) the incoming rays. It then passes through the pupil, an opening whose size is regulated by the iris, the colored muscle surrounding it. The iris dilates the pupil in low light and constricts it in bright conditions. Behind the pupil sits the lens, which fine-tunes focus by changing shape through a process called accommodation — thickening for near objects and flattening for distant ones. The focused light lands on the retina, the light-sensitive inner surface at the back of the eye.
The retina contains two types of photoreceptors. Rods, numbering approximately 120 million, are concentrated in the peripheral retina and are responsible for detecting shades of black, white, and gray. They are highly sensitive to low light levels, enabling night (scotopic) vision, but they provide poor visual acuity. Cones, numbering about 6 million, are concentrated in the fovea — the central point of sharpest vision — and are responsible for color and fine detail under daylight (photopic) conditions. Three types of cones are sensitive to short (blue), medium (green), and long (red) wavelengths. The blind spot, where the optic nerve exits the eye, contains no photoreceptors.
The visual pathway proceeds from photoreceptors to bipolar cells to ganglion cells, whose axons form the optic nerve. The nerve crosses at the optic chiasm, where fibers partially cross over, and signals are relayed through the lateral geniculate nucleus of the thalamus to the primary visual cortex in the occipital lobe. There, feature detectors — neurons identified through the Nobel Prize-winning work of Hubel and Wiesel — respond to specific features such as edges, bars, angles, and movement.
<image>A cross-sectional diagram of the human eye showing the path of light. Panel A: Light enters through the cornea, passes through the pupil (regulated by the iris), is focused by the lens, and projects onto the retina at the back of the eye. Key structures labeled include the cornea, iris, pupil, lens, vitreous humor, retina, fovea, optic nerve, and blind spot. Panel B: An enlarged view of the retina layers showing photoreceptors (rods and cones) at the back, bipolar cells in the middle, and ganglion cells at the front, with their axons forming the optic nerve. Light direction and signal direction arrows are shown going in opposite directions.</image>
III. Color Vision
Two complementary theories explain how humans perceive color, and both turn out to be correct at different levels of the visual system. The trichromatic theory, proposed by Young and Helmholtz, holds that color perception results from the combined activity of three types of cones — short-wave (blue), medium-wave (green), and long-wave (red). This theory elegantly explains color mixing and the various types of color blindness. The most common form is red-green color blindness (deuteranopia or protanopia), an X-linked recessive trait affecting roughly 8% of males and fewer than 1% of females.
The opponent-process theory, proposed by Hering, posits that color is processed in opposing pairs: red-green, blue-yellow, and black-white. In this framework, neurons are excited by one color and inhibited by its opponent. This theory explains the phenomenon of afterimages — staring at a green image for an extended period and then looking away produces a red afterimage. Research has shown that trichromatic processing occurs at the level of the retina (in the cones themselves), while opponent processing occurs at higher levels, specifically in retinal ganglion cells and in the lateral geniculate nucleus of the thalamus.
IV. Hearing: The Ear
Sound consists of pressure waves traveling through a medium such as air or water. The frequency of these waves, measured in hertz, determines pitch (higher frequency produces higher pitch), with the human audible range spanning approximately 20 to 20,000 Hz. The amplitude of sound waves, measured in decibels, determines loudness; prolonged exposure above 85 dB can cause permanent hearing damage. The complexity of a sound wave, or its timbre, determines the quality or richness that allows us to distinguish one instrument from another.
The ear is divided into three sections. The outer ear comprises the pinna, which collects sound waves, the auditory canal, and the tympanic membrane (eardrum). In the middle ear, three tiny bones called ossicles — the malleus (hammer), incus (anvil), and stapes (stirrup) — amplify sound vibrations by about 20 times. The stapes pushes against the oval window, transmitting vibrations to the inner ear.
The inner ear contains the cochlea, a fluid-filled, snail-shaped structure. Running along the cochlea is the basilar membrane, which vibrates in response to incoming sound. The hair cells of the organ of Corti, resting on the basilar membrane, are the actual sensory receptors that convert mechanical vibrations into neural signals. These signals travel via the auditory nerve to the medial geniculate nucleus of the thalamus and then to the auditory cortex in the temporal lobe.
<image>A diagram of the human ear in three sections. Panel A: Outer ear showing the pinna, ear canal, and tympanic membrane. Panel B: Middle ear showing the three ossicles (malleus, incus, stapes) connecting the eardrum to the oval window of the cochlea. Panel C: Inner ear with the cochlea unrolled to show the basilar membrane, hair cells (organ of Corti), and auditory nerve fibers. Arrows trace the path of sound from air vibration to neural signal. Panel D: A cross-section of the cochlea showing the three chambers (scala vestibuli, scala media, scala tympani), the basilar membrane, tectorial membrane, and hair cells.</image>
V. Theories of Pitch Perception
Place theory, proposed by Helmholtz, holds that different sound frequencies cause different locations along the basilar membrane to vibrate. High-frequency sounds activate the base of the cochlea near the oval window, while low-frequency sounds activate the apex. This theory best explains the perception of high-frequency sounds above approximately 1,000 Hz. Frequency theory, proposed by Rutherford, suggests that the basilar membrane vibrates at the same rate as the incoming sound wave, with the neural firing rate matching the sound's frequency. However, this mechanism is limited by the maximum firing rate of individual neurons, roughly 1,000 times per second. The volley principle extends the range of frequency theory by proposing that neurons can "take turns" firing to collectively encode frequencies up to about 5,000 Hz. In practice, place theory best accounts for high-frequency perception, frequency theory for low frequencies, and the volley principle for the mid-range.
Sound localization relies on two cues processed in the superior olivary nuclei of the brainstem. The interaural time difference refers to the fact that sound reaches the nearer ear slightly sooner, while the interaural intensity difference reflects the fact that sound is slightly louder in the nearer ear.
VI. Hearing Loss
Conductive hearing loss results from a problem in the outer or middle ear — a damaged eardrum, ossicle dysfunction, or fluid buildup — that prevents sound from being efficiently conducted to the inner ear. This type of loss is often treatable with hearing aids or surgery. Sensorineural hearing loss, sometimes called nerve deafness, involves damage to the hair cells in the cochlea or the auditory nerve itself. It can be caused by aging (presbycusis), prolonged noise exposure, genetics, or disease. Sensorineural hearing loss is usually permanent, although cochlear implants can provide some benefit by directly stimulating the auditory nerve.
