Does number interference occur during sentence processing?

Models of interference in sentence processing claim that object relative clauses are harder to process than subject relatives due to interference between the subject and object noun phrase. The interference effect for object relatives at the verb should be more pronounced when the two noun phrases retrieved from memory are similar. To test this, two eye tracking experiments manipulated whether the number feature of the noun phrases (singular or plural) was either the same or different. Both experiments showed the well-known relative clause effect. However, in Experiment 1 the effect of number congruency was in the opposite direction from that predicted by interference. Experiment 2 showed the interaction predicted by similarity based interference at sentence wrap-up, but because this interaction was observed later than the relative clause effect and only occurred in Experiment 2, it suggests that retrieval interference due to cue overlap is a weak effect that might be the result of a checking procedure in syntactically complex sentences.