Animals receive much more sensory information than they could possibly register in their brain and respond to. Therefore brain has to be selective and filter out certain information that is not so necessary. Sensory filtering or stimulus filtering takes place at several levels, namely, at the level of sense organs, nerves or different parts of brain.
Sensation is the basic data sent by sense organ to brain, and sense organs have their limitation and hence filter out much of the information. For example, human eye filters out ultraviolet and infrared rays from the spectrum.
Peripheral filtering is done by receptors because of their mechanical ability to receive and transmit information. Receptors are often highly specialized and respond to a narrow range of stimulus. For example caloreceptors can perceive sense of heat but not cold and frigidireceptors can only transmit the sense of cold. Bats can perceive ultrasonic sounds for echolocation but sense organs of other mammals do not possess that ability.
CNS filtering is done by different parts of brain by selective attention or because the part is not well developed. Perception is the interpretation by brain of sensory information in the light of earlier experience. A lot of information is received by brain but is not perceived.
Reticular Activating System located inside the medulla oblongata if inactivated, stops lots of nerve impulses coming through the cranial nerves.
Stimuli that reach respective areas of brain such as optic lobes, auditory lobes etc. can get filtered out if not important. Epithalamus, which functions as the central switch board, selects and sends only necessary nerve impulses to cerebral hemispheres.
When the nerve impulses arrive in different areas of cerebral cortex, they are analysed and interpreted and if found worthless can be rejected without perception.
Only information that is considered important is selected by the areas of cerebral cortex and interpreted, and motor action travels through the nerves to muscles to act. Neurochemical information coming from cerebral cortex affects hypothalamus, which stimulates endocrine system to alter the behaviour of the animal.
Muller’s Law of specific nerve energies: Sensation perceived depends on the part of nervous system activated, and not on the sense organ stimulated.
Examples: The male of the South American tree frog (Eleutherodactylus coqui) produces co-qui call to attract female and also to repel other competing males. The tympanic membranes of male and female are adapted differently. Males can hear only the co note and get warned and repelled. Males cannot hear the qui part of the call, while females hear only the qui part of the call and therefore get attracted to males.
Olfactory cells located on the antennae of male moths (Lepidoptera) can perceive only specific pheromones which are released by the female of the same species. These pheromones cannot be perceived by the receptors of males of another species.
European Robin (Erithacus rubicula) attack red-breasted male robins. Only red feathers of the competitors are perceived and the other colours are filtered out for attack and for defending the territory.