Sentence:  The yawning <WSD>gap</WSD> in neuroscientists' understanding of their topic is in the intermediate scale of the brain's anatomy. Science has a passable knowledge of how individual nerve cells, known as neurons, work. It also knows which visible lobes and ganglia of the brain do what. But how the neurons are organised in these lobes and ganglia remains obscure. Yet this is the level of organisation that does the actual thinking—and is, presumably, the seat of consciousness. 
Meaning of the word marked with <WSD> tags:  A vacancy, deficit, absence, or lack.