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.
Polar bear’s zoological name is Ursus maritimus, which means “sea bear”. Polar bears are restricted to icy Arctic region where temperature falls as low as -50°C. They are well adapted to the freezing temperatures, which restricts them to the circumpolar region where 19 populations exist. Over forty percent of polar bears live in the north of Canada.
Polar bears have the distinction of being the largest land carnivores. Adults are about 8 ft long and weigh 300-700 kg, larger than the fearsome grizzly bear. Fifty percent of body weight is due to fat and blubber that makes 10 cm thick layer under their blackish skin.
Polar bear’s whitish fur is extremely dense, oil coated and water repellent, so much so that water does not reach the skin even when they are swimming in sea. Each hair is transparent with a hollow core that refracts and reflects visible light to make the fur appear snow white.
Polar bear paws measure up to 12 inches and have fur and foot pads with papillae on the soles that help distribute weight of body over larger area and to get firm grip on the slippery ice surface. Polar bear claws are two inches long, curved, sharp and strong to prevent slipping on ice and to help in climbing over icy slopes.
Polar bears are strong swimmers and divers which enables them to swim from one iceberg to another and to hunt seals in sea. They can also swim underneath ice sheet in search of food. Polar bears can swim continuously for hundreds of kms at speeds of about 10 km/hr. They are known to be the only terrestrial animals to be very comfortable in the sea, often covering long distances with ease helped by their fatty body and waterproof fur. There are instances of polar bears covering marathon 400 km by swimming continuously for several days without finding a resting place. These long journeys enable bears to reach long distances via the sea route.
Polar bears do not hibernate in true sense in dens like the brown and black bears do. Instead they remain active throughout winter in spite of freezing cold. However, pregnant females dig a protective den in snow where they give birth to 2-3 cubs in November- December. During denning cubs subsist on mother’s milk and mother on her fat reserves. They remain in the den for about three months, safe and protected from the cold and wind. Mother bears do not enter deep hibernation because they need to maintain higher body temperature in order to meet the demands of pregnancy, birth, and nursing. Polar bear families generally emerge from their dens in March-April when the cubs are strong enough to survive outside.
Polar bears hunt their favourite prey, the seals while they are basking in the sun or through the breathing holes made by them in the ice sheet. They also eat plants and berries and also hunt fish and other animals. They also act as scavengers of dead whales, walrus or fish. Generally they consume skin and fat of the prey in order to accumulate reserve fat in the body. No other animal can eat and digest such fat-rich diet as do these bears.
The fact that is least realised by the students is that competitive examinations are considerably different from the house examinations of colleges and universities, where they manage to score heavily but fail to do so in the competitives. The primary reason for this happens to be that they continue with the same approach that they have been accustomed to during their academic carrier. The methods of preparation, making of notes, memorising material, writing of answers and handling the diagrams, essentially remains the same which is not adequate. A different approach is therefore needed to do well in these examinations.