LOS ANGELES -- Twin Guatemalan girls who spent their first year of life joined at the head remained in critical condition nearly a week after a 22-hour operation to separate them, but one twin is more alert than the other, hospital officials said late on Monday.

Maria de Jesus Quiej-Alvarez and her twin, Maria Teresa, remained in critical condition with stable vital signs after the surgery to divide them ended Tuesday morning, according to Mattel Children's Hospital at the University of California at Los Angeles.

"We have a mobile over her crib, and Maria de Jesus is following it with her eyes. Maria Teresa appeared to recognize the voice of a nurse who has been caring for her over the past two months," Reuters quoted Clarice Marsh, the hospital's director of pediatric intensive care nursing, as telling reporters.

The hospital said Maria de Jesus is much more alert than last week, but Maria Teresa's recovery continues to lag behind that of her sister.

The gap in their recovery progress has been attributed to the fact that Maria Teresa underwent a second surgical procedure to clear out a blood clot on the brain.

"We hope in the next several days to be able to take them completely off the sedatives and ventilators," said Dr. Irwin Weiss, pediatric intensive care specialist at UCLA.

By late last week, both of the 1-year-old twins had opened their eyes and were responding to physical stimulation.

The UCLA doctors said the twins still have many hurdles to overcome, but they remain cautiously optimistic about the sisters' long-term prospects for recovery.

The girls -- affectionately known as "Las Maritas" or "the little Marias" -- were born in rural Guatemala with the tops of their heads fused and their faces tilted in opposite directions.

Surgeons had to separate their skulls and untangle the interconnected blood vessels that drained blood from their brains back to their hearts.

The girls' parents, Leticia Alba and Wenceslao Quiej-Alvarez, are from Belen, a hamlet along Guatemala's southern coast where people eke out a living in the banana, sugar and coffee industries.

Conjoined twins occur once in every 200,000 live births, but twins who are fused at the tops of their heads, known as craniopagus twins, make up only about 2 percent of those.

They usually die early because organs such as the heart and kidneys of one twin are doing most of the work and once they start to fail both twins will die.

The nonprofit group healing the children arranged for the twins to be treated at UCLA, which puts the cost of their care at $1.5 million, not including the services of the doctors, nurses and other professionals who donated their time.