Sensation and Perception

Sensation versus Perception


Sense organs receive sensory stimulation, convert it into neural activity, and deliver it to the brain.
Sensation is the process of detecting an environmental stimulus and converting that stimulus into neural activity. The steps of sensation are similar for each of the five major senses (sight, hearing, touch, taste, and smell). First, a specific type of environmental stimulus (e.g., light) is received by a particular sense organ (e.g., the eye). Then a specialized sensory receptor cell (e.g., rods and cones) converts the environmental stimulus into a neural signal (transduction). For all senses except smell, these neural signals pass through the thalamus, a brain structure below the cortex that acts as a relay station for sensory systems. The thalamus routes different sensory information to different regions of the brain. The occipital lobe processes visual signals, the temporal lobe processes sounds, the parietal lobe processes touch and temperature, and the gustatory cortex processes taste. Neural signals for smell are processed through the amygdala (a brain area associated with emotion), the hippocampus (the brain's center for emotion, memory, and the autonomic nervous system), and the frontal lobe (which governs behavior, learning, personality, and voluntary movement).

The Five Major Senses

Sense Stimulus Energy Sense Organ(s) Sensory Receptor Cell(s) Brain Pathway
Vision Light Eye, specifically the retina Rods and cones Thalamus to occipital lobe
Hearing (audition) Sound Ear, specifically the cochlea Hair cells Thalamus to temporal lobe
Touch (haptic or tactile sense) Pressure, vibration, temperature, and pain Skin Pressure receptors, low-frequency vibration receptors, high-frequency vibration receptors, thermoreceptors, free nerve endings (nociceptors) Thalamus to parietal lobe
Taste (gustation) Chemical components in food/drink Tongue, specifically the taste buds Taste receptor cells Thalamus to gustatory cortex (in the frontal lobe)
Smell (olfaction) Chemicals in the air Nose, specifically the olfactory bulb Olfactory receptor neurons Amygdala, hippocampus, and frontal lobe

Each sense organ responds to a unique stimulus and converts that information into a message processed in a specific area of the brain.


The brain processes inputs from all senses simultaneously. Aspects of both the stimuli and the perceiver can affect how sensory information is interpreted.
Perception is the identification and/or interpretation of a stimulus. In other words, perception is what is done with sensory information to make sense of the world. Although the brain can process incoming sensory information from all senses simultaneously, not all aspects of the environment can be perceived at once. Whether a stimulus is perceived depends on a number of characteristics of the perceiver. The absolute threshold is the minimum amount of stimulus intensity needed for a receptor to react. For example, the absolute threshold for sound would be the lowest volume of a tone that can be perceived by normal hearing when there are no other sound distractions. The absolute threshold will vary, as a person can be more or less sensitive to detecting a particular stimulus. The just noticeable difference is the minimum change in stimulus intensity that can be detected. For example, it would be difficult to tell the difference between picking up a 50-pound box and picking up a 51-pound box (a 2% increase). However, it would be relatively easy to tell the difference between a one-pound dumbbell and a two-pound dumbbell (a 100% increase). Although the weight change is only one pound in each case, this example shows how the difference threshold depends on the proportion of the second stimulus to the first.

Absolute Thresholds for the Five Major Senses in Humans

Sense Approximate Absolute Threshold
Vision A candle flame on a clear night from 30 miles away
Hearing A ticking clock from 20 feet away in a quiet room
Touch The wing of a fly falling on the cheek from half an inch away
Smell One drop of perfume in the space of six rooms
Taste One teaspoon of sugar dissolved in two gallons of water

Each sense organ requires a minimal level of stimulation in order to send a signal to the brain, called the absolute threshold. The minimum threshold for detecting a sight, sound, smell, taste, or touch differs across species.

Perception of a stimulus can be influenced in other ways. Sensory adaptation occurs when a sensory receptor cell's response starts to decrease after continuous or repeated stimulation. This reduces the perceived intensity of a stimulus. For example, people may easily detect a foul odor when they walk into a room but quickly lose the ability to smell it. Sensory adaptation can occur in any of the five major senses. With respect to temperature sensation, different temperatures of water can feel different depending on which thermoreceptors have undergone adaptation. If a person places one hand in a bowl of cold water and the other in a bowl of hot water, that will cause sensory adaptation of cold and hot thermoreceptors, respectively. After a period of a few minutes, if the person puts both hands in a bowl of lukewarm water, the water temperature will feel different to each hand. The lukewarm water will feel quite warm on the hand adapted to cold water, but cool on the hand adapted to the hot water.
Placing one hand in cold water and the other in hot water will cause sensory adaptation of thermoreceptors. If both hands are placed in lukewarm water, the water will feel warm on the hand adapted to cold water and cool on the hand adapted to hot water.
Perceptual constancy is the human tendency to see familiar objects as unchanging, even if there are slight changes to the stimulus. This allows people to recognize objects as having a consistent shape, size, and color, regardless of viewing angle, lighting conditions, or distance from the object. Perceptual set is the human tendency to perceive some stimuli but not others, and to base expectations on past experiences. Perceptual set can influence how people interpret ambiguous information and lead to processing errors. Perceptual constancy and perceptual set are top-down processes that use what a person already knows to influence the interpretation of incoming sensory information.

Examples of Perceptual Set Errors

Stimulus Likely Response Perceptual Set Error
People are asked to repeat silk, bilk, pilk. Then they are asked what adult cows drink. Milk Milk Although adult cows do not drink milk, saying the words that rhyme with milk sets people up to say milk, rather than water.
People are asked to read the following text out loud:
Perceptual set can cause people to miss the
the error in this sentence.
People are likely to misread the text, saying, “Perceptual set can cause people to miss the error in this sentence.” Because people see what they expect to see, they miss the extra the in the text.

In perceptual set, people may see what they expect to see rather than what is actually there. Perceptual set can also influence the conclusions people draw.