The Basic Principles of Sensory Transduction

     Each day, we experience countless sensations. A sensation is the process in which our sensory receptors and nervous system receive stimulus energies from our environment. The change of this stimulus energy into electrical energy that can be used in our body, specifically the brain, is known as signal transduction. However, the stimulus often doesn't have enough energy for us to be able to consciously recognize the stimulus. The minimum stimulation needed to detect a sensation 50% of the time is called the absolute threshold. This doesn't mean that if a stimulus is below your absolute threshold you won't detect it; it means that you are unlikely to detect it. The signal detection theory predicts when you will detect these weak signals, which is dependent upon experience, expectations, motivation, and alertness. This explains why each person responds to stimuli differently. A single person's response to the same stimuli can change as well, which is known as sensory adaption, the diminishing sensitivity to an unchanging stimulus. Due to constant exposure to a stimulus, our nerve cells fire less to the stimulus. However, our sight does not undergo sensory adaption because eyes are constantly moving. 

     Similar to the absolute threshold is the difference threshold, also known as the "just noticeable difference" (JND). The difference threshold is the minimum difference a person can detect between two different stimuli 50% of the time. This threshold increases as the size of the stimuli increases. For example, if one-ounce is added to five-ounces, you are likely to notice the difference. However, if one-ounce is added to fifty-ounces, you probably wouldn't be able to notice that one-ounce was added because the weight was already large. So, the difference threshold for the fifty-ounce example is large, meaning you would need to add a large amount, such as ten-ounces, in order to recognize that weight has changed. Weber's law explains this, stating that for their difference to be noticeable, the two stimuli must differ by a constant proportion, not amount.

 

 

This picture illustrates difference threshold and Weber's Law.