Introduction to Psychology

Introduction to Psychology

© 2015 of 97 3.3 Vision The Visual Stimulus- The normal stimulus for vision is electromagnetic energy, or light waves, which are measured in nanometers (nm), or one billionth of a meter. Light-sensitive visual receptor cells are located in the retina. The rods are brightness receptors, and the less numerous cones are color receptors. Light energy striking the retina is converted into nerve impulses by chemical reactions in the photo pigments of the rods and cones. Visual stimuli are analyzed by feature detectors in the primary visual cortex, and the stimulus elements are restructured and interpreted in light of input from the visual association cortex. The Eye Structure and Function- Light waves enter the eye through cornea, a transparent protective structure at the front of the eye. Behind the cornea is the pupil, an adjustable opening that can dilate or constrict to control the amount of light that enters the eye. The pupil’s size is controlled by muscles in the colored iris that surrounds the pupil. Behind the pupil is the lens, an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects. The lens of the eye focuses the visual image on the retina, a multilayered light sensitive tissue at the rear of the fluid filled eyeball. The retina contains two types of light sensitive receptor cells called rods and cones. Rods are found throughout the retina except in the fovea, a small area in the center of the retina that contains no rods but many densely packed cones. Rods and cones send their messages to the brain via two additional layers of cells. The rods and cones have synaptic connections with a thin layer of ganglion cells, whose axons are collected into a bundle to form the optic nerve. Color Vision- Around 1800 it was discovered that any color in the visible spectrum can be produced by some combination of the wavelengths that correspond to the colors blue, green and red. This theory is referred to as the trichromatic theory. The second influential theory is the opponent-process theory which proposes that each of the three cone types responds to two different wavelengths. One type responds to blue or yellow, another to red or green, and a third to black or white. In essence, color vision is a two-stage process having both trichromatic and opponent-process components. 3.4 Hearing Auditory Stimulus- The stimuli for our sense of hearing are sound waves, a form of mechanical energy. What we call sound is actually pressure waves in the air, water, or some other conducting medium. Sound waves have two characteristics: frequency, measured in terms of cycles per second or hertz (Hz); and amplitude, measured in terms of decibels (dB). Frequency is related to pitch, amplitude to loudness. Loudness is coded in terms of the number and types of auditory nerve fibers that fire. Pitch is coded in two ways. According to the frequency of pitch perception, nerve impulses sent to the brain match the frequency of the sound wave. The other theory, known as the place theory of pitch perception, suggests that the specific point in the cochlea where the fluid wave peaks and most strongly bends the hair cells, serves as a frequency coding cue. Achieve Page 28