Sensory Organs and Mechanisms - AP Psychology

Humans have about 100 to 125 million rods in the retina, but only about 5 to 6.4 million cones. The fovea only contains cones—no rods. In the remainder of the retina, rods vastly outnumber cones.

The human retina has two types of cells that respond to light: cones and rods. Rods are more numerous, but can only detect light and dark shades. Cones respond to different wavelenghts of light, and can thus transmit color information. They are highly concentrated in the fovea of the retina.

Over time, a constant stimulus in the environment evokes less and less of a response from one's sensory system. Sensory neurons respond at first to these stimuli, but over time they stop responding in order to focus attention on other stimuli in the environment.

The stages of cognitive development, according to Jean Piaget, are the sensorimotor, preoperational, concrete operational, and formal operational. During the sensorimotor stage, the period between birth and age two, cognitive development is limited to motor responses caused by sensory stimuli.

Between ages two and six—the preoperational stage—the child learns to use language. Between ages seven and eleven—the concrete operational stage—children begin thinking logically. During the formal operational stage, age twelve through adulthood, people develop the ability to think about abstract concepts such as deductive reasoning.

The iris is a ring of muscle tissue that forms the colored part of the eye. Additionally, the iris controls the size of the pupil opening, and thus controls how much light enters the eye.

The retina contains the rods and the cones, which are sensitive to light that enters through the pupil. Rods are able to detect shades of dark and light, while cones are able to detect color. When light strikes these cells, it is converted into electrical neural impulses that are transmitted through the optic nerve to the brain.

The middle ear contains three tiny bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones concentrate the vibrations coming from the eardrum, and send these signals to the cochlea's oval window.

Each region of the ear uses a different medium to transmit vibrations. Remember that the outer ear uses air as the medium (sound waves impact the ear drum). The middle ear uses bones to transmit these vibrations to the oval window. Finally, the inner ear uses fluid found in the cochlea to transmit the vibrations to neural tissue.

Serotonin is the neurotransmitter that is most responsible for mood control. Therefore, a lack of serotonin would likely result in clinical depression, a mood disorder.

Olfactory cells are receptors involved with our sense of smell, while the other answer choices are all related to vision, and are located in the eye.

The trichromatic theory is one of the oldest theories about why and how humans see in color and it states that there are three types of cones in the retina that can detect the three colors (hence trichromatic) of blue, green, and red.

As for the other answers, dichromatic theory and trichronological theory don't exist; they just sound similar to the correct answer. Place theory and frequency theory are theories about audition rather than vision. Place theory states that ear hairs respond to different frequencies based on their different locations in the cochlea, and frequency theory states that pitch is perceived by the rate at which hair cells fire.

The area devoted to a particular region of the body in the sensory cortex is proportional to the region's sensitivity. Lips are known to contain more sensory receptors than any of the other answers. Thus making "lips" the correct answer.

It is true that visual stimulation is different from olfactory stimulation; however, the heart of the question lies in the matter of sensory receptors. It is not a matter of the receptors being different strengths, but of how they respond to a stimulus. Olfactory receptors undergo sensory adaptation. This occurs after continuous exposure to a stimulus when nerve cells will fire less. As a result, we become less aware of the smell. Visual receptors cannot undergo adaptation because our eyes are always moving. As a result, the stimulation is constantly changing for our eyes' receptors. Psychologists have devised an experiment to test sensory adaptation with visual receptors. By maintaining a constant image on the inner surface of the eye, the individual will eventually cease to see the image that was once there. The image will occasionally reappear and disappear again; therefore, it is possible to become sensory adapted visually—but not under normal circumstances.

This phenomenon is known as sensory adaptation, where sensitivity diminishes over time with a continuous stimulus. Over time, as stimulation continues, nerve cells will fire less and as a result we will no longer notice a stimulus—in this case, touch.

The cornea is the clear thin layer that covers the eye. Aside from the obvious task of protecting the eye from foreign invaders, it is responsible for a majority of the eye's ability to focus. Because it is the outermost layer of the eye, light will initially pass through this structure. As the cornea bends the light through the pupil, it will direct it to the lens. The lens then plays the role of refocusing the light and directing it to receptor cells. The iris is responsible for controlling the pupil's size; therefore, it would be an incorrect answer choice. The fovea—a structure residing at the back of the eye near the optic nerve—is part of the eye; however, it is not the initial point of central focus.

Light will initially pass through the cornea and bend through the pupil to reach the lens. The lens is responsible for taking the refracted light and refocusing it. In doing so, the refocused rays will create an inverted image on the retina. This is accomplished through a process known as accommodation. The retina does not receive a complete image, but instead particles of light energy. Its receptor cells will take the light, translate it into neural impulses, and forward them to the brain where they will be reassembled right side up.

Cones, rods, bipolar cells, and ganglion cells are receptor cells found in the outer layer of the retina. Energy is first transmitted to rods and cones, then to bipolar cells, and eventually to ganglion cells. Rods and cones differ in terms of their locations and purposes. Cones are found clustered around the fovea (area of central focus on the retina). Cones have a direct pathway to the brain. One cone will be connected to one bipolar cell. This ensures that each cone can have its individual message directed to the visual cortex. The specific connections allow for information to be preserved, also while providing a large input for the visual cortex from the fovea. As a result, cones are better capable of detecting detail. Rods, on the other hand, do not exhibit the same wiring as cones. Rods will share bipolar cells with other rods; therefore, their input reaches the visual cortex as shared information.

As information is translated into neural impulses in the retina, the information is passed from the rods and cones to bipolar cells, which transfer the impulse to ganglion cells. Feature detectors are specialized neurons in the visual cortex that receive information from retinal ganglion. In order to receive the information, the impulses must pass through the optic chiasm. This is the "X" created by the two optic nerves crossing below the brain. The optic nerves will then meet at occipital lobe's visual cortex to deliver the information. Feature detectors are self-explanatory by their name. Their job is to detect a scene's features—edges, lines, angles, and movement. This information is then passed to cell clusters in other cortical areas that respond to more complex patterns.

The Young-Helmholtz trichromatic (three color) theory states that the retina contains three color receptors. Thomas Young and Hermann von Helmholtz understood that any color could be created through combining the primary colors with varying wavelengths. They inferred that each receptor was especially sensitive to one of the primary colors. They deduced that one receptor was sensitive to red, one to green and one to blue. These receptors were later discovered to be cones. When different combinations of these cones are stimulated, then we are able to see different colors.

The ear is separated into three parts: the outer ear, the middle ear, and the inner ear. The outer ear is the visible part of the ear. It acts to funnel the vibrations into the middle ear. The middle ear only contains a piston made up of three tiny bones. These bones are the anvil, hammer, and stirrup. Their purpose is to amplify the vibrations so that they can continue into the inner ear and create the necessary ripples in the basilar membrane to bend hairs. The bending hairs will initiate neural impulses, sending messages to the brain. It is the inner ear that contains most of the structures required for sensing sounds.