The answer usually given to this question is, "not very". The hedgehog is a primitive animal; its brain and the rest of its anatomy have remained largely unchanged since the first hedgehogs rolled off the production line some 15 million years ago. This is because the hedgehog's way of life - for example, having a food supply which is abundant and readily available - does not demand the sophisticated mental and physical skills that are required by, for example, a leopard.
People who have a tame hedgehog often report that their pet has acquired a number of basic skills, such as responding to its name and using a litter tray. Many hedgehogs can distinguish between their carer and other people. Researchers have taught hedgehogs simple tasks, such as choosing between a black trapdoor and a white trapdoor to reach food regularly placed behind one but not the other.
One researcher even taught his tame hedgehog to roll and unroll in response to the relevant commands!
Hedgehogs seems to have quite a good memory, especially for places, and, remarkably, this memory is not affected by hibernation - during the winter, the hedgehog brain shuts down almost completely, yet when the animal emerges in the spring, its memory is 'switched on' again, unimpaired; it will head without hesitation for a place where food has been regularly put down for it.
As human beings, we tend to think of sight as the most important of the senses, but hedgehogs are chiefly active during the hours of darkness, so good eyesight is not particularly important. Also, as the hedgehog's eye-view is only a few inches above the ground and is often obscured by vegetation, it relies heavily on other senses.
But they can distinguish between shapes - particularly silhouettes against the sky - and moving objects. They may have a limited degree of colour vision, but as they're usually asleep during the day, they don't often have the chance to use it.
The sense of
smell is the one on which the hedgehog chiefly relies. It is mainly by smell that it finds its food (even under about three centimeters of soil), detects the approach of danger and recognizes other hedgehogs.
The hedgehogs external ears are small and inconspicuous, but its
hearing is very sensitive and is important in locating food and recognizing danger. An earthworm moving gently in the soil of a beetle rustling in leaf litter is making really loud noises to hedgehog ears. Its hearing is particularly sensitive to high frequencies; clicks, squeaks and hand-claps will cause a hedgehog to instantly crouch down, bristling its spines.
Info from "Everything You Want To Know about Hedgehogs - Dilys Breese"
