Sentence:  In this model it is assumed that each hemiretina consists of thousands of hemicircular regions called <WSD>'hyperfields</WSD>', arranged in a sunflower seed growth pattern mosaic, which map topographically onto a corresponding square mosaic of regions in the striate cortex called hypercolumns. 
Meaning of the word marked with <WSD> tags:  A region of the visual field that maps to multiple hypercolumns.