Two faces on one head
Where twinning of the head itself is only partial, it can result in the condition known as diprosopus—one head with two faces. Earliest known occurrence
The February 22, 2007, issue of the journal Biology Letters detailed the discovery of a 122 million-year-old fossil of a two-headed Hyphalosaurus lingyuanensis, marking the earliest known occurrence of axial bifurcation. List of notable occurrences
Humans
Dicephalic conjoined twins (dicephalus parapagus dipus)
Lycosthenes described a pair of adult female twins who had separate necks but one body. Both heads ate, drank, slept, and spoke. They had to beg from door to door, "everie one giveing (sic) to her freely". They were banished to Bavaria due to fears pregnant women who saw them would give birth to similar children; nothing else is known of them. In 1990 Abigail and Brittany Hensel were born in Minnesota, United States. In 2000 Ayse and Sema Tanrikulu were born in Kahramanmaraş, Turkey
In June 2000 Carmen and Lupita Andrade were born in Veracruz, Mexico. They later moved to the United States for healthcare with their parents. In 2003 Sohna and Mohna were born in India
On June 13, 2003, twin girls named Huda and Manal Abdel Nasser Mohammed Mahmoud, were born in Asyut, Egypt
In 2006 Syafitri was born in Indonesia
In 2007 Mary Grace and Mary Divine Asis were born in the Philippines with only one heart. They died on April 30, 2008. On August 25, 2008, a baby boy named Kiron was born with two heads in south-western Bangladesh.
Other potential adaptations to improve visual function in higher environmental complexity may include strategies like clearing away excess clutter from a work spaces, or providing a plain background to view objects against, reducing noise levels, or other methods of monitoring and adapting for environmental complexity. One eye may perform significantly worse than the other, and depth perception can be very limited (although not necessarily zero). The field of view may be severely limited. The best vision might be in the centre (like tunnel vision) but more often it is at some other point, and it is difficult to tell what the person is really looking at. Note that if the person also has a common ocular visual impairment such as nystagmus then this can also affect which part(s) of the visual field are best. (Sometimes there exists a certain gaze direction which minimises the nystagmus, called a "null point.") Even though the field of view may be very narrow indeed, it is often possible for the person to detect and track movement. Movement is handled by the V5 part of the visual cortex, which may have escaped the damage. Sometimes a moving object can be seen better than a stationary one; at other times the person can sense movement but cannot identify what is moving. (This can be annoying if the movement is prolonged, and to escape the annoyance the person may have to either gaze right at the movement or else obscure it.)