This was because averaging z-transformed values (and back transforming) tends to yield less biased estimates of the population value than averaging the raw coefficients (Silver and Dunlap, 1987).
the image-based model is built using a deep convolutional neural network approach, similar in nature to those recently used to study neural representations of visual stimuli (see Kriegeskorte (2015), although note this is the first application to study word elicited neural activation known to the authors).