Your brain looks for all the parts - all present - all in the right general area - everything must be fine...
Tahts why it so esay to raed wrods eevn wehn the lteetrs are mdduled up? it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteer is in the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit graet porbelm.