Agnosia is usually due to problems with high-level information processing rather than the basic visual system. However, a team at the University of Bologna in Italy has demonstrated that this isn't always the case.
Its study, published in the journal Cortex, focussed on the case of a 44-year-old man, referred to as SDV, who sustained an anoxic brain injury due to electrocution three years previously. As a result, his visual cortex was damaged and he was blind for almost 10 months.
The blindness gradually passed and he began to show apparently normal visual function. He was able to navigate, interact with others and was completely independent in daily life. The one obvious remaining deficit was his inability to recognise objects from drawings or figures. He could recognise them by touch, but not by looking at them. He also couldn't read or recognise familiar faces, but could recognise people by their voices and movements.
SDV appeared to show classic signs of visual agnosia. However, the researchers wondered whether some aspect of cortical blindness may remain that could explain the problem rather than the higher level processing areas. Sure enough, they discovered blindness in a small area of the central visual field, which SDV was unaware of.
Further tests revealed that the affected area was responsible for processing the orientation of lines. This is a very specific function of certain brain cells and is vital for the ability to detect and discriminate the edges of objects. Without these cells the world can
instead become a blur of indistinguishable parts.
These findings indicate that loss of basic visual functions can sometimes be responsible for object agnosia. The researchers recommend that a complete investigation of the primary visual system, as well as the lateral occipital and temporal regions, is necessary in some cases of apparent agnosia.
Reference
Serino, A. et al (2013) When apperceptive agnosia is explained by a deficit of primary visual processing. Cortex, (52), 12-27.
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