An adequate stimulus is the type of stimulus for which a given sensory organ is particularly adapted. For example, the adequate stimulus for the eye is photic (light) energy. Furthermore, an electrical shock on the eye can create an illusory sensation of light.

Johns Hopkins University

A stimulus (plural: stimuli) is an event that affects the sensory organ. Stimuli can be sound waves, light entering the eye, or food touching the tongue.
- Adequate stimulus
- Labeled Lines
- Stimulus localization
- Selection and analysis along sensory pathways
- Selectivity and Range of Response to Stimuli

Stimuli

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Adequate Stimulus

Labeled lines is the concept that particular neurons are established at the outset for distinctive sensory experiences. For example, neural activity in one line signals a sound, whereas activity in another line signals a smell. The "lines" can become even more specific within modalities. 
This theory of stimulus type identification is correct, unlike the previous theory of specific nerve energies (check child nodes). 

Labeled Lines

Sensory organs and peripheral sensory pathways convey limited and, sometimes, distorted information to the brain. This is because a great deal of selection and analysis takes place along sensory pathways before conscious perception of the stimulus.
- Coding

Selection and Analysis Along Sensory Pathways

For the somatosensory and visual systems, cells are arranged in an orderly fashion, much like a map. This map reflects receptor position and density, and is not one-to-one exact (see somatosensory homunculus and higher foveal cortical representation). Densely innervated sites, like the fingers and lips, have higher cortical representation.
 
In bilateral receptor systems (two ears and two nostrils), localization is different. It is determined by general time of arrival at the two receptors, and/or stimulus intensity. For example, if a soundwave hits your left ear sooner than your right ear, the source of the sound is probably to your left. If this soundwave is loud, it’s probably close. 


Stimulus localization

The sensory systems of a particular animal are very selective. In other words, sensory organs in each organism are sensitive to a specific range of the stimuli the organ is specialized for.

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