248 B.J.Casey et al./Biological Psychology 54(2000)241-257 TTETAT7 0-Back(control) 1-Back TTT77777 2-Back 5 3-Back Fig.2.Illustration of the n-back task with memory load varying from 0to 3. children.Fig.I depicts the location of activation and the change in MR signal intensity as a function of scans across time.The increases in activity nicely map onto the experimental manipulation.Reliable activity was observed also in the anterior cingulate cortex.These results replicate an earlier fMRI study with adults showing dorsolateral prefrontal activity using the same paradigm (Cohen et al., 1994)and a more recent event-related fMRI study of working memory (Cohen et al.,1997)shov ral activation dur m rmation wit interferenc fre interveni The simil tributior activity across frontal gyn for children and adults taken from the Casey et a (1995)and Cohen et al.(1994)studies are striking.Taken together,these two initia studies suggest a similar distribution of prefrontal cortical activity in children and adults during performance of a working memory task.However,the percent change in signal observed for the children was on average two to three times that observed for the adults in the cohen et al study.Based on the behavioral data available.the children had more difficult with the task.Ona children 70-75 in the performed at y et al.study while or above 90% accuracy i Cohen et al.experiment This demonstrates the e importance of collecting behavioral responses in the scanner,but also raises concerns with regard to the interpretation of the findings given the behavioral differences.Are the differences maturational or strategic in nature or both? A study that may help address this question is one presented by Braver et al. 1997,and others (Jonides et al..1997;Rypma et al.,1999),which examined prefrontal cortical activity as a function of increasing memory load in adults.They ed the d emory task as aried the tored uentiaiyot snle leters nd respondedon memory 1 as can be seen in The the same as the letter n trials before it (e.g.if n=2,then A-F-A or G-B-G,but not A-F-G-A or A-A).Subjects practiced until they reached 90%accuracy on the highest memory load of n=3.The results revealed monotonic increases in percent change in MR signal intensity in prefrontal cortex as depicted in Fig.3.We 248 B.J. Casey et al. / Biological Psychology 54 (2000) 241–257 Fig. 2. Illustration of the n-back task with memory load varying from 0 to 3. The results demonstrated reliable activity in the dorsolateral prefrontal cortex in children. Fig. 1 depicts the location of activation and the change in MR signal intensity as a function of scans across time. The increases in activity nicely map onto the experimental manipulation. Reliable activity was observed also in the anterior cingulate cortex. These results replicate an earlier fMRI study with adults showing dorsolateral prefrontal activity using the same paradigm (Cohen et al., 1994) and a more recent event-related fMRI study of working memory (Cohen et al., 1997) showing dorsolateral activation during active maintenance of stimulus information with interference from intervening trials. The similar distribution of activity across frontal gyri for children and adults taken from the Casey et al. (1995) and Cohen et al. (1994) studies are striking. Taken together, these two initial studies suggest a similar distribution of prefrontal cortical activity in children and adults during performance of a working memory task. However, the percent change in signal observed for the children was on average two to three times that observed for the adults in the Cohen et al. study. Based on the behavioral data available, the children had more difficulty with the task. On average, children performed at 70–75% accuracy in the Casey et al. study while adults performed at or above 90% accuracy in the Cohen et al. experiment. This demonstrates the importance of collecting behavioral responses in the scanner, but also raises concerns with regard to the interpretation of the findings given the behavioral differences. Are the differences maturational or strategic in nature or both? A study that may help address this question is one presented by Braver et al., 1997, and others (Jonides et al., 1997; Rypma et al., 1999), which examined prefrontal cortical activity as a function of increasing memory load in adults. They performed the same sort of memory task as described previously, but varied the memory load from 0 to 3 as can be seen in Fig. 2. The subject monitored a sequential display of single letters and responded only when the current letter was the same as the letter n trials before it (e.g. if n=2, then A-F-A or G-B-G, but not A-F-G-A or A-A). Subjects practiced until they reached 90% accuracy on the highest memory load of n=3. The results revealed monotonic increases in percent change in MR signal intensity in prefrontal cortex as depicted in Fig. 3. We