Sensory perception

Cognition is based on the way the brain information enters the organisms. Even bacteria need to sense food (or danger) in their environment to survive. In humans, the somatosensory system processes externally received information in the brain to create perceptions such as smell, taste, touch, and vision. 

1 - An incoming stimulus (such as an odor in smell or the electromagnetic field in vision). This causes a firing of a neuron, which ignites an adjacent one and on and on up to the brain.

2 - A filtering of the noise. Neurons are firing all the time, randomly. Stimuli cause them to do it in an organized, synchronized way. 

3 - A form of expectation to confront from his previous knowledge. In fact, training occurs by repeated contact  with a particular stimulus, which  self-organizes neurons in the brain.

The molecular biology of perception signaling pathways is advancing tremendously. It is really an exciting time to use computational methods based on the ever-increasing amount of biological data to start uncovering the mechanisms underlying our senses. 

The smell is the simplest perception, the only direct route of our somatosensory system, without any mediation: it is very fast and it has no filtering. The signal provided by all other senses, including taste and vision pass thru the thalamus, which filter them.