Halliday & Matthiessen (1999: 30):
This parallelism is even more foregrounded where the modality of expression is spatial, as in the Sign Languages of deaf communities (see Johnston, 1989; 1992 on AUSLAN). Here the domain of expression is a 4-dimensional signing space-time in a field of perception shared by signer and addressee (though clearly perceived from different angles). The spatial orientation and the shared perception increase the potential for iconicity in the expression; Johnston (1992) points out: "Despite an oral-aural language being suited to iconically encode sounds, the fact that our experience as a whole is visual, temporal and spatial means that a language which has itself visual and temporal resources for representation has greater means than an auditory one to map onto itself those very visual and spatial qualities of the world it wishes to represent."