Thresholds: absolute threshold- minimum stimulation needed to detect a particular stimulus 50% of the time, difference threshold- minimum difference between two stimuli required for detection 50% of the time, also called just noticeable difference (jnd) Habituation: the tendency of the brain to stop attending to constant, unchanging information, the brain ignores stimuli that are being sensed, but do not changed. Sensory adaptation: the tendency of sensory receptors to become less responsive to a stimulus that is unchanging, receptors less responsive to the stimulus, and therefore no longer send signals to the brain. Habituation and vision: microsaccades: constant movement of the eyes; tine little vibrations that people do not notice consciously; prevents sensory adaptation to visual stimuli. Intensity (brightness)- intensity- amount of energy in a wave determined by amplitude; related to perceived brightness. Parts of the eye: cornea- transparent tissue where light enter the eye. Iris- muscle that expands or contracts to change the size of opening (pupil) for light.