I saw frescos from Pompeii at an expo last week and I was stricken by their technique, particularly in terms of anatomy, lighting and perspective. For some reason I have an impression that European Early Middle Ages art has been nothing but a downgrade in comparison to Ancient (?) Roman art. Is it just a big misconception on my side or there was indeed some techniques and knowledge being lost throughout the ages?
Quality is different then conception. You have to remember that European Art of the Middle Ages was not concerned with reflecting the physical world. Instead, since most of it was focused around Christianity, it was meant to reflect a spiritual world. So, art historians often argue that artists were freed from having to reproduce the physical, much like artists of the 20th century were freed from having to reproduce what a camera could record more accurately. Compare the artwork of the Fauvists, in particular Matisse. They gave up on the physical world instead to paint the inner emotional world.
Additionally, art work also became smaller and more portable. While Romans had large artworks depicting their gods and goddess, Christianity specifically forbid the worship of images (images could be used as educational items to tell the stories of the bible). So, large public artworks were no where near as common. You do see smaller personal items, and the interiors of churches being decorated, but again the emphasis is on the spiritual. You will notice the enlarged eyes of most figures from this time period.
Neither the Romans nor early Christian artists understood linear perspective as we do today. So, in terms of quality, I think the two art periods are very similar. It's just that the artists had very different ideals.