their vision consists of two fields, rather than a binocular view like humans have. When porpoises surface, their lens and cornea correct the nearsightedness that results from the refraction of light; their eyes contain both rod and cone cells, meaning they can see in both dim and bright light. Porpoises do, however, lack short wavelength sensitive visual pigments in their cone cells indicating a more limited capacity for colour vision than most mammals. Most porpoises have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a tapetum lucidum; these adaptations allow for large amounts of light to pass through the eye and, therefore, they are able to form a very clear image of the surrounding area. The olfactory lobes are absent in porpoises, suggesting that they have no sense of smell. Porpoises are not thought to have a good sense of taste, as their taste buds are atrophied or missing altogether. However, some have preferences between different kinds of fish, indicating some sort of attachment to taste. Sleep Unlike most animals, porpoises are conscious breathers. All mammals sleep, but porpoises cannot afford to become unconscious for long because they may drown. While knowledge of sleep in wild cetaceans is limited, porpoises in captivity have been recorded to sleep with one side of their brain at a time, so that they may swim, breathe consciously, and avoid both predators and social contact during their period of rest. This means that the brain hemispheres take turns alternating between slow wave sleep and being awake. While one hemisphere displays slow waves, the other displays wake patterns on an electroencephalogram. It has been suggested that the brainstem controls this activity. All the while, porpoises also employ a minimal amount of suppressed REM sleep while swimming. The same neural systems that regulate sleep in bihemispheric mammals are used in harbor porpoises. Cetaceans use many of the same neurotransmitters that other mammals use, although they have a significantly higher number. Although the way porpoises sleep is different from other mammals, they use the same neural mechanisms and pathways. One key difference is that porpoises seem to inhibit REM sleep more than most mammals, likely to prevent losing muscle tone and thus decreasing the risk of drowning or suffering from hypothermia. Porpoises differ from other mammals in terms of the neurons that regulate their sleep-wake cycle, however, the sleep-wake