IIRC, when the authors were doing this research I raised the objection that Epun was a language that nobody could think could be a human-speakable (let alone natural) language, whereas their research programme really needed a language that Principles & Parameters theory predicted was impossible but that others would predict to at least be human-speakable. But the authors' focus was on providing putative evidence in support of P&P theory rather than investigating Christopher's gift (and then perhaps using that gift to test the limits of human-speakability). I felt that the research opportunity that Christopher provided was squandered because of the researchers' theoretical agenda. 

--And.

John H. Chalmers, On 04/02/2009 20:41:
> Lingua 91 (1993) 279-347. North-Holland 279
> Learning the impossible :
> The acquisition of possible and
> impossible languages by a polyglot savant
> Neil V. Smith,” Ianthi-Maria Tsimpli,b and Jamal Ouhalla”
> ’ Department of Phonetics and Linguistics. University College London, 
> Gower Street, London
> WClE 6BT, UK
> b Department of English Language, University of Newcastle-upon-Tyne, 
> Newcastle-upon-Tyne
> NEI 7RU, UK
> ’ Department of Hispanic Studies, Queen Mary & Westfield College, Mile 
> End Road, London El
> 4NS. UK
> Received March 1993
> 
> We report on the case of a polyglot savant (Christopher) who has a 
> remarkable talent for
> learning and translating languages. Building on previous work which had 
> established both the
> range of languages at Christopher’s command and the extent to which his 
> linguistic knowledge
> was integrated into his cognitive ability, we taught him two new 
> languages for which we
> controlled the input. We had two main aims: the first was to test the 
> hypothesis (within one
> version of the Principles and Parameters framework) that parameter 
> resetting is not an option
> available to the second language learner; the second was to accrue 
> further evidence for or against
> Fodor’s modularity hypothesis and cast light on the possible range of 
> interactions between
> linguistic and ‘central’ cognitive processes. The languages chosen were 
> Berber, an Afro-Asiatic
> language spoken in North Africa, and Epun, an invented language 
> deliberately devised to contain
> constructions which violated universal grammatical principles. In 
> Christopher’s acquisition of
> Berber we gleaned evidence from a variety of phenomena, including word 
> order, null subjects,
> f/rat-trace effects, wh-island violations and cliticisation, that his 
> learning was conditioned by a
> combination of transfer effects from English and principles of UG, 
> rather than by the effect of
> parameter resetting. In Christopher’s acquisition of Epun we began with 
> a core of ‘normal’
> constructions, designed to make him feel at home in the new language, 
> and then proceeded to
> investigate a range of impossible constructions, both 
> structure-dependent and structure-independent.
> In the former case, we concentrated on negative sentences, constructed 
> with no overt
> negative morpheme, and past-tense sentences which involve unattested and 
> putatively impossible
> word-order differences, In the latter case, we concentrated on a rule of 
> emphasis that involved
> counting words, and a form of agreement which again violated putatively 
> universal generahsations.
> In each case we compared Christopher’s performance with that of a small 
> group of
> controls. The results were complex, but we think we can justify an 
> interpretation which lends
> support to both the main hypotheses being tested.
> 0024-3841/
>