COGNITIVE NEUROSCIENCE NEUROREPORT 0959-4965 & Lippincott Williams & Wilkins Vol 11 No 10 14 July 2000 2145 Cognitive sequence processing and syntactic comprehension in schizophrenia T. Lelekov, 1 N. Franck, 1,2 P. F. Dominey 1,CA and N. Georgieff 1,2 1 Institut des Sciences Cognitives, CNRS UMR 5015, 67, Blvd Pinel, 69675 Bron, France; 2 Vinatier Psychiatric Hospital and EA 1943 Universite  Claude Bernard, Lyon, France CA Corresponding Author Received 24 March 2000; accepted 27 April 2000 It has been repeatedly demonstrated that schizophrenic pa- tients are impaired in the comprehension of sentences with complex syntax. We investigated the hypothesis that this syntactic comprehension impairment in schizophrenia is not a purely linguistic dysfunction, but rather the re¯ection of a cognitive sequence processing impairment that is revealed as task complexity increases. We tested 10 schizophrenic patients using a standard measure of syntactic comprehension, and a non-linguistic sequence processing task, both of which required simple and complex transformation processing. Patients' performance impairment on the two tasks was highly corre- lated (r 2  0.84), and there was a signi®cant effect for complex- ity, independent of the task. These results are quite similar to those of aphasic patients with left hemisphere lesions. This suggests that syntactic comprehension de®cits in schizophrenia reveal the dysfunction of cognitive sequence processing mechanisms that can be expressed both in linguistic and non- linguistic sequence tasks. NeuroReport 11:2145±2149 & 2000 Lippincott Williams & Wilkins. Key words: Abstract structure; language; schizophrenia; syntax INTRODUCTION Schizophrenic subjects display cognitive performance that is largely intact for implicit or automatic tasks, including word-stem completion, repetitive priming and procedural learning [1±6]. Their performance is more impaired, how- ever, on explicit or effortful tasks [7,8]. This behavioral dissociation has been linked to the hypoactivation of anterior cortex both at rest [9,10] and during effortful cognitive tasks [11±14]. The emerging picture is that explicit cognitive processing that relies on intact function- ing of the frontal cortex will be impaired in schizophrenia, while implicit processing will be more spared. It is interesting, from this perspective, to consider the impaired processing of language in schizophrenia. Several studies have now con®rmed the observation of impaired processing of complex syntax, both in terms of production and comprehension, in schizophrenic subjects [15±18]. Thus, schizophrenic subjects do well in understanding sentences with simple syntax such as `John gave the ball to Mary', but have trouble with more complex sentences involving syntactic movement such `The ball was given to Mary by John'. This pro®le appears similar to that ob- served in aphasic subjects with left hemisphere lesions in the peri-sylvian cortex. These agrammatic subjects have relatively intact performance for understanding simple syntax, and fail with more complex syntax [19,20]. It will thus be of great interest to make a detailed comparison of performance between these two groups. Regarding the underlying cause(s) of this syntactic dysfunction in schizophrenia, we can consider two possible explanations. The linguistic resources explanation is that the impairment in syntax processing results from damage to language-speci®c resources in schizophrenia. The shared resources explanation claims that the linguistic de®cit results from damage to cognitive-sequencing process that are used in language and other cognitive functions invol- ving the structural manipulation of sensorimotor se- quences. The shared resources explanation would thus predict that schizophrenic patients with syntactic compre- hension de®cits would display comparable impairments in a non-linguistic sequencing task that tap into these shared resources. From this perspective, we have recently demonstrated in a group of schizophrenic patients, an impaired capability to learn the abstract structure of sensorimotor sequences, while their capability to learn the surface structure re- mained intact [21]. We de®ne surface structure of a sequence as the serial order of sequence elements, and the abstract structure in terms of relations between repeating elements [22]. Thus the sequences ABCBAC and DEFEDF have different surface structures but identical abstract structure (123213), and are by our de®nition isomorphic. For any sequence based on the abstract structure 123213, we can see that the elements 213 are entirely predictable by the preceding elements 123, which are in contrast unpre- dictable. Given the fragment GHI of a new isomorphic sequence, based on the abstract structure we can predict the next three elements HGI. In other words, knowledge of