After the acquisition of sensory stimuli, the process of perception involves the subsequent encoding and transmission of the sensory signal to the central nervous system. Further processing or decoding yields biologically useful information.
Environmental stimuli that have been partially processed by a sensory receptor must be conveyed to the CNS in such a way that the complete range of the intensity of the stimulus is preserved.
Compression. The first step in the encoding process is compression. Even when the receptor sensitivity is modified by accessory structures and adaptation, the range of input intensities is quite large, as shown in Figure 4.5. At the left is a 100-fold range in the intensity of a stimulus. At the right is an intensity scale that results from events in the sensory receptor. In most receptors, the magnitude of the generator potential is not exactly proportional to the stimulus intensity,- it increases less and less as the stimulus intensity increases. The frequency of the action potentials produced in the impulse initiation region is also not proportional to the strength of the local excitatory currents,- there is an upper limit to the number of action potentials per second because of the refractory period of the nerve membrane. These factors are responsible for the process of compression,- changes in the intensity of a small stimulus cause a greater change in action potential frequency than the same change would cause if the stimulus intensity were high. As a result, the 100-fold variation in the stimulus is compressed into a threefold range after the receptor has processed the stimulus. Some information is necessarily lost in this process, but integrative processes in the CNS can restore the information or compensate for its absence. Physiological evidence for compression is based on the observed nonlinear (logarithmic or power function) relation between the actual intensity of a stimulus and its perceived intensity.
Information Transfer. The next step is to transfer the sensory information from the receptor to the CNS. The encoding processes in the receptors have already provided the basis for this transfer by producing a series of action potentials related to the stimulus intensity. A special process is necessary for the transfer because of the nature of the conduction of action potentials. As an action potential travels along a nerve fiber, it is sequentially recreated at a se-
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