Sensation is the process by which physical energy from objects inthe world or in the body stimulates the sense organs. The brain interprets andorganizes this sensory information in a process called perception.

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Psychophysics is the study of how thephysical properties of stimuli relate to people’s experience of stimuli. Research inpsychophysics has revealed much information about the acuity of the senses.

Measuring the Senses

Psychologists assess the acuity of the senses in three ways:

Measuring theabsolute threshold Measuring thedifference threshold Applying signaldetection theory

The absolute threshold is the minimum amount of stimulationrequired for a person to detect the stimulus 50 percent of the time. The difference threshold is the smallest difference in stimulationthat can be detected 50 percent of the time. The difference threshold issometimes called the just noticeable difference (jnd), and it depends on thestrength of the stimulus.

Example: If someone were comparing two weak stimuli, such as twovery slightly sweet liquids, he’d be able to detect quite asmall difference in the amount of sweetness. However, if he werecomparing two intense stimuli, such as two extremely sweetliquids, he could detect only a much bigger difference in theamount of sweetness.


Weber’s Law

Nineteenth-century psychologist Ernst Weber proposed aprinciple demonstrating the fact that we can’t detect thedifference between two stimuli unless they differ by a certainproportion and that this proportion is constant. In other words,the just noticeable difference for a stimulus is in a fixedproportion to the magnitude of a stimulus. Weber’s Law holdstrue except in the most extreme kinds ofstimulation.


Researchers use signal detection theory to predict when aweak signal will be detected. This theory considers the fact that the ability todetect a signal depends not only on the strength of the signal but also on theperceiver’s experience, motivation, expectation, and degree of alertness.Different people respond differently to the same signal, and the same person maydetect a particular signal at one time but not another. Furthermore, people canoften detect one type of signal in a sensory modality such as hearing or visionbut be oblivious to other types of signals in the same sensory modality.

Sensory Adaptation

When people walk into a restaurant, they probably notice food smells rightaway. However, as they sit in the restaurant, the smells gradually become lessnoticeable. This phenomenon occurs because of sensory adaptation. Sensoryadaptation is the decrease in sensitivity to an unchanging stimulus.The smells don’t disappear—the people just become less sensitive to them.

Development of the Senses

Babies have all the basic sensory abilities and many perceptual skills,but these abilities develop and grow more sensitive over time. Babiescan recognize the difference between a human voice and othersounds, and they can locate a sound’s origin. They can recognize thedifference between smells and, very early on, can recognize their mother’sparticular smell. As for taste, they can differentiate between sweet andsalty. Babies also have fairly adept visual abilities. Soon after birth,they can distinguish objects of different colors and sizes. When they arejust a few weeks old, they begin to differentiate among contrasts, shadows,and patterns, and they can perceive depth after just a fewmonths.

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Sensitive Periods

Even innate perceptual skills need the right environment todevelop properly. A lack of certain experiences during sensitive periods ofdevelopment will impair a person’s ability to perceive the world.

Example: People who were born blind but regain their vision inadulthood usually find the visual world confusing. Since theseadults were blind in infancy, they missed the sensoryexperiences necessary for their visual system to developfully.