One of Chomsky’s more charming qualities is his way of making important conceptual points using simple linguistic examples. Who will ever forget that rowdy pair colorless green ideas sleep furiously and furiously sleep ideas green colorless and their pivotal roles in divorcing the notions ‘grammatical’ from ‘significant’ or ‘meaningful’ and in questioning the utility of bigram frequency in understanding the notion ‘grammaticality.’ Similarly, kudos goes to Chomsky’s argument for structure dependence as a defining property of UG using Yes/No question formation. Simple examples, deep point. However, this talent has led to serious widespread misunderstandings. Indeed, very soon “proofs” appeared “showing” that Chomsky’s argument did not establish that structure dependence was a built in feature of FL for it could have been learned from the statistical linear patterns available to the child (see here for a well-known recent effort). The idea is that one can compare the probabilities of bi/tri-gram sequences in a corpus of simple sentences and see if these suffice to distinguish the fine (1a) from the from the not so-fine (1b).
(1) a. Is the man who is in the corner smoking
b. *Is the man who in the corner is sleeping
It appears possible to do this, as the Reali and Christiensan (R&C) paper shows. However, the results, it seems, (see here p 26) are entirely driven by the greater frequency of sequences like who is over who crying in their corpus, which in turn derives from the very high frequency of simple who is questions (e.g. who is in the room?) in the chosen corpus.
There has been lots of discussion of these attempts to evade the consequences of Chomsky’s original examples (the best one is here). However, true to form, Chomsky has found a way to illustrate the pointlessness of these efforts in a simple and elegant way. He has found a way of making the same point with examples don’t affect the linear order of any of the relevant expressions (i.e. there are no bi/tri-gram differences in the relevant data). Here’s the example:
(2) Instinctively, eagles that fly swim
The relevant observation is that instinctively in (1) can only modify swim. It cannot be understood as modifying fly. This despite the fact that whereas it is true that eagles instinctively fly, they don’t swim. The point can be made yet more robustly if we substitute eat egg rolls with their sushi for swim. Regardless of how silly the delivered meaning, instinctively is limited to modifying matrix predicate.
This example has several pleasant features. First, there is no string linear difference to piggyback on as there was in Chomsky’s Y/N question example in (1). There is only one string under discussion, albeit with only one of two “possible” interpretations. Moreover, the fact that instinctively can only modify the matrix predicate has nothing to do with delivering a true or even sensible interpretation. In fact, what’s clear is that the is fronting facts above and the adverb modification facts are exactly the same. Just as there is no possible Aux movement from the relative clause there is no possible modification of the predicate within the relative clause by a sentence initial adverb. Thus, whatever is going on in the classical examples has nothing to do with differences in their string properties, as the simple contrast in (2) demonstrates.
Berwick et. al. emphasize that the Poverty of Stimulus Problem has always aimed to explain “constrained homophony,” a fact about absent meanings for given word strings. Structure has always been in service of explaining not only what sound-meaning pairs are available, but just as important which aren’t. The nice feature of Chomsky’s recent example is that it neutralizes (neuters?) a red herring, one that the technically sophisticated seem to be endlessly hooking on their statistical lines. It is hoped that clarifying the logic of the POS in terms of “absent possible interpretations,” as Berwick et. al. have done, will stop the diminishing school of red herrings from replenishing itself.
 See Syntactic Structures p. 15-16. Chomsky’s main point here, that we will need more than linear order properties of strings to understand how to differentiate the first sentence from the second has often been misunderstood. He is clearly pointing out here that we need higher order notion parts-of-speech categories to begin to unravel the difference. This “discovery” is remade every so often with the implication that it eluded Chomsky. See here for discussion.
 See Berwick et. al. here for a long and thorough discussion of the R&C paper here. Note, that the homophony between the relative pronoun and the question word appears to be entirely adventitious and so any theory that derives its results by generalizing from questions to relative clauses on the basis of lexical similarities is bound to be questionable.