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Integration of diverse synaptic inputs is a basic neuronal operation that relies on many neurocomputational principles, one of which is neural summation. However, we lack empirical understanding of neuronal summation in the human brains in vivo. Here, we explored the effect of neural summation on the motor cortex using two subthreshold pulses of transcranial magnetic stimulation (TMS), each with intensities ranging from 60 to 95 % of the resting motor threshold (RMT) and interstimulus interval (ISI) varying from 1 to 25 ms. We found that two subthreshold TMS pulses can produce suprathreshold motor response when ISIs were less than 10 ms, most prominent at 1, 1.5 and 3 ms. This facilitatory, above-threshold response was evident when the intensity of the subthreshold pulses was above 80 % of RMT but was absent as the intensity was 70 % or below. Modeling of the summation data across intensity suggested that they followed an exponential function with excellent model fitting. Understanding the constraints for inducing summation of subthreshold stimulations to generate above-threshold response may have implications in modeling neural operations and potential clinical applications. [ABSTRACT FROM AUTHOR]

Background Youths with a family history of alcohol and other drug use disorders ( FH+) are at a greater risk of developing substance use disorders than their peers with no such family histories ( FH−), and this increased risk may be related to impaired maturation of forebrain circuitry. FH+ individuals have shown altered forebrain activity at rest and while performing cognitive tasks. However, it is not fully understood how forebrain activity is altered in FH+ individuals, and ultimately how these alterations may contribute to substance use disorder risk. Methods In this study, we tested 72 FH+ and 32 FH− youths performing a go/no-go task and examined activations in blocks with only go trials (Go Only), blocks with 50% go and 50% no-go trials (Go/NoGo), and a contrast of those 2 blocks. Results FH+ youths had significantly greater cerebral activations in both the Go and Go/NoGo blocks than FH− youths in regions including the posterior cingulate/precuneus, bilateral middle/superior temporal gyrus, and medial superior frontal gyrus with no significant group differences in the subtraction between Go Only and Go/NoGo blocks. Additionally, FH+ youths had moderately slower reaction times on go trials in the Go Only blocks. Conclusions Our findings suggest that global activation increase in FH+ youths are modulated by FH density and are not specific to the inhibitory components of the task. This pattern of increased activations in FH+ youths may be at least partially due to impaired forebrain white matter development leading to greater activations/less efficient neural communication during task performance. [ABSTRACT FROM AUTHOR]

Abstract: Individuals learn to read by gradually recognizing repeated letter combinations. However, it is unclear how or when neural mechanisms associated with repetition of basic stimuli (i.e., strings of letters) shift to involvement of higher-order language networks. The present study investigated this question by repeatedly presenting unfamiliar letter strings in a one-back matching task during an hour-long period. Activation patterns indicated that only brain areas associated with visual processing were activated during the early period, but additional regions that are usually associated with semantic and phonological processing in inferior frontal gyrus were recruited after stimuli became more familiar. Changes in activation were also observed in bilateral superior temporal cortex, also suggestive of a shift toward a more language-based processing strategy. Connectivity analyses reveal two distinct networks that correspond to phonological and visual processing, which may reflect the indirect and direct routes of reading. The phonological route maintained a similar degree of connectivity throughout the experiment, whereas visual areas increased connectivity with language areas as stimuli became more familiar, suggesting early recruitment of the direct route. This study provides insight about plasticity of the brain as individuals become familiar with unfamiliar combinations of letters (i.e., words in a new language, new acronyms) and has implications for engaging these linguistic networks during development of language remediation therapies. [Copyright &y& Elsevier]