Everything we perceive is based on electrical signals in our nervous system. Part a the perceptual process: environmental stimulus, attended stimulus focus attention for additional processing, stimulus on the receptors light energy on back of retina becomes transformed. A retinal representation is transformed into electricity. Transformation of one form of energy into another. Occurs in the nervous system when energy in the environment is transformed into electrical energy by a receptor. Carrying out information through electrical signals to the brain. If these signals don"t reach the brain, there is no perception. Interaction between neurons in the nervous system so that perception can occur. Purpose of perception is to understand the world, not to make exact copies. Both types are involved in perception, but not always. Knowledge of both allows us to see both. Psychophysics: the study of the mathematical relationships between a stimulus (physics) and perception (psych) Psychometric function: the mathematical relationship between the stimulus and the perception.