An echoic memory is a momentary, sensory memory of auditory stimuli that lasts for only a couple of seconds. This form of sensory memory may be forgotten or sent to our long-term memory for further encoding. 

The just-noticeable difference is the minimal change that can occur in a stimulus to just barely be detected by the observer. In this case, the stimulus is the brightness of light. 

As for the other answer choices, the absolute threshold is the minimal intensity at which a stimulus can just barely be detected. This concept is similar to the just-noticeable difference but has to do with detection of existence, rather than detection of change. Sensory adaptation refers to how an observer becomes less sensitive to a stimulus over time. Signal detection states that an observer's response to a stimulus depends on both their sensitivity and their decision criterion. Transduction is the process of physical signals being converted to neural signals. 

Neurons are specialized cells that transmit nerve signals. These cells make up the nervous system, and consist of an axon (sends nerve impulses to other neurons), dendrites (receive nerve impulses from other neurons), and soma ( the cell body of the neuron).

Glial cells are considered part of the nervous system as well, but do not represent the fundamental functional unit. Instead, glial cells supply support and nourishment to neurons.

Top-down processing uses larger context to assume smaller bits of missing information. Bottom-up processing is the opposite - using small bits of information to understand a bigger picture. Signal detection theory is the ability to choose between and identify stimuli. Perceptual constancy refers to the tendency in humans to see familiar things as having similar shapes, colors, and sizes, in spite of changes that may have occurred in that object.

Rods and cones are the light-sensitive receptor cells that are responsible for transducing light waves into neural impulses. 

Autokinetic illusion is when a single unmoving light in a dark room appears to move when stared at for several seconds. It is not a monocular depth clue.

Interposition refers to the monocular depth clue indicating an objects relative closeness by seeing whether that object is stacked on top of an object, or whether an object is stacked on it.

Relative size refers to objects farther away appearing smaller.

Shadow (or shading) refers to closer objects reflecting more light, and thus appearing less dark, than objects further away.

Absolute threshold refers to the minimum amount of a given stimulus (light, odor, sound, pressure, or taste) that can be detected at least 50% of the time. 

The primary function of the ear canal is to facilitate the travel of auditory information into the ear, and to the parts of the ear used to process this information. While the ear canal, in helping the sensitive organs of the inner ear be less exposed to damage, may be said to protect these structures it is not the ear canal's primary purpose.

Olfactory neurons come in direct contact with the stimuli. They also are directly in the brain. Other sensory receptors transmit information to a separate neuron that relays the information to the brain. 

Glutamate is found in protein rich foods and the artificial flavoring MSG (Monosodium Glutamate). It triggers the taste sensation umami. Umami is the most recently defined taste-sensation, it is best described as a meaty taste. The other four basic taste sensations are: salty, bitter, sweet, and sour.