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Socioeconomic Context 375Two systems were the subject of a priori hypotheses, as described earlier. The“left perisylvian/language” system is a complex, distributed system encompassingsemantic, syntactic, and phonological aspects of language and dependent predominantlyon the temporal and frontal areas of the left hemisphere that surroundthe Sylvian fissure. The “prefrontal/executive” system enables flexible respondingin situations in which the appropriate response may not be the most routine orattractive one, or in which it requires maintenance or updating of informationconcerning recent events. It is dependent on prefrontal cortex, a late-maturing brainregion that is disproportionately developed in humans.In addition, we assessed three other neurocognitive systems that play importantroles in school and the real world. The “medial temporal/memory” system isresponsible for one-trial learning, the ability to retain a representation of a stimulusafter a single exposure to it (which contrasts with the ability to graduallystrengthen a representation through conditioning-like mechanisms), and is dependenton the hippocampus and related structures of the medial temporal lobe. The“parietal/spatial cognition” system underlies our ability to mentally represent andmanipulate the spatial relations among objects, and is primarily dependent onposterior parietal cortex. The “occipitotemporal/visual cognition” system is responsiblefor pattern recognition and visual mental imagery, translating imageformat visual representations into more abstract representations of object shapeand identity, and reciprocally translating visual memory knowledge into imageformat representations (mental images).The results of our first study replicated the well-known SES gap in cognitivetest performance in general, with the middle-SES children performing better thanthe low SES children on the battery of tasks as a whole. As predicted for the leftperisylvian/language system and the prefrontal/executive system, the disparitybetween low and middle SES kindergarteners was both large and statisticallysignificant. Indeed, the groups differed by over a standard deviation in their performancecomposite on language tests, and by over two thirds of a standard deviationin the executive function composite. The other systems did not differsignificantly between low and middle SES children, and differed significantlyless than the first two.In a subsequent study we attempted to replicate and extend these findings in alarger group of children, 150 first graders of varying ethnicities whose SES spanneda range from low through middle, as determined by parental education, job statusand, when available, income-to-needs ratio (Noble, McCandliss & Farah, 2006).These children completed a different set of tests designed to tap the same neurocognitivesystems as the previous study, with two main differences.The first difference was an improvement in the medial temporal/memory systemtests. In the previous study, the test phase had followed immediately afterthe initial exposure to the stimuli, making the tests more sensitive to immediatememory ability than the longer-term memory for which medial temporal structures