Double dissociation in optokinetic stimulation sensitivity between omission and substitution reading errors in neglect dyslexia

Although omission and substitution errors in neglect dyslexia (ND) patients have always
been considered as different manifestations of the same acquired reading disorder,
recently, we proposed a new dual mechanism model. While omissions are related to the
exploratory disorder which characterizes unilateral spatial neglect (USN), substitutions are
due to a perceptual integration mechanism. A consequence of this hypothesis is that specific
training for omission-typeNDpatientswould aim at restoring the oculo-motor scanning
and should not improve reading in substitution-type ND.With this aim we administered an
optokinetic stimulation (OKS) to two brain-damaged patients with both USN and ND, MA
and EP, who showed ND mainly characterized by omissions and substitutions, respectively.
MA also showed an impairment in oculo-motor behavior with a non-reading task, while EP
did not.The two patients presented a dissociation with respect to their sensitivity to OKS,
so that, as expected, MA was positively affected, while EP was not. Our results confirm a
dissociation between the two mechanisms underlying omission and substitution reading
errors in ND patients. Moreover, they suggest that such a dissociation could possibly be
extended to the effectiveness of rehabilitative procedures, and that patients who mainly
omit contralesional-sided letters would benefit from OKS.