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 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.

Every visual field has a blind spot, where there are no rods and cones (known as photoreceptors) to detect external images. Our brain uses context clues from the environment to help fill in this blind spot to make a complete picture. The blind spot is located on the optic disk, which is the location where the optic nerve exits the back of the eye. Due to the nerve tissue in this spot, there are no photoreceptors to detect input.

In contrast, the fovea centralis is the region of the retina with the highest concentration of cones.

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 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 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.

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.

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.