People often forget information because they fail to effectively encode it. Here, we test the hypothesis that targeted electrical stimulation can modulate neural encoding states and subsequent memory outcomes. Using recordings from neurosurgical epilepsy patients with intracranially implanted electrodes, we trained multivariate classifiers to discriminate spectral activity during learning that predicted remembering from forgetting, then decoded neural activity in later sessions in which we applied stimulation during learning. Stimulation increased encoding-state estimates and recall if delivered when the classifier indicated low encoding efficiency but had the reverse effect if stimulation was delivered when the classifier indicated high encoding efficiency. Higher encoding-state estimates from stimulation were associated with greater evidence of neural activity linked to contextual memory encoding. In identifying the conditions under which stimulation modulates memory, the data suggest strategies for therapeutically treating memory dysfunction.

Plant sap-feeding insects and blood-feeding parasites are frequently depleted in 15 N relative to their diet. Unfortunately, most fluid-feeder/host nitrogen stable-isotope studies simply report stable-isotope signatures, but few attempt to elucidate the mechanism of isotopic trophic depletion. Here we address this deficit by investigating the nitrogen stable-isotope dynamics of a fluidfeeding herbivore-host plant system: the green peach aphid, Myzus persicae, feeding on multiple brassicaceous host plants. M. persicae was consistently more than 6‰ depleted in 15 N relative to their hosts, although aphid colonized plants were 1.5‰ to 2.0‰ enriched in 15 N relative to uncolonized control plants. Isotopic depletion of aphids relative to hosts was strongly related to host nitrogen content. We tested whether the concomitant aphid 15 N depletion and host 15 N enrichment was coupled by isotopic mass balance and determined that aphid 15 N depletion and host 15 N enrichment are uncoupled processes. We hypothesized that colonized plants would have higher nitrate reductase activity than uncolonized plants because previous studies had demonstrated that high nitrate reductase activity under substrate-limiting conditions can result in increased plant δ 15 N values. Consistent with our hypothesis, nitrate reductase activity in colonized plants was twice that of uncolonized plants. This study offers two important insights that are likely applicable to understanding nitrogen dynamics in fluid-feeder/host systems. First, isotopic separation of aphid and host depends on nitrogen availability. Second, aphid colonization alters host nitrogen metabolism and subsequently host nitrogen stable-isotope signature. Notably, this work establishes a metabolic framework for future hypothesis-driven studies focused on aphid manipulation of host nitrogen metabolism.host-parasite interaction | nitrogen budget | plant-herbivore interaction | trophic enrichment N itrogen, required for the synthesis of nucleic and amino acids, is essential for development and growth. Found in the atmosphere, soil, and oceans in inorganic forms (1), animals depend on plants and microorganisms for nitrogen fixation and organic nitrogen supply in the form of proteins and amino acids. Universally, metabolic fractionation during nitrogen fixation, assimilation, respiration, and excretion results in differences in the ratio of nitrogen stable isotopes in different tissues within organisms (2) and at different levels within ecosystems (3). Typically, consumers are enriched in 15 N by ∼2‰ to 3‰ relative to their diet (2, 4, 5). However, important exceptions exist. Fluidfeeders, a group that includes blood-feeding parasites and hempiteran insects (2), frequently have been found to deviate from these typical patterns such that fluid-feeders are usually reported as showing no enrichment or even depletion in 15 N relative to their diet (2, 6-17). Although it is now commonly accepted that fluid-feeders deviate from typical patterns of enrichment, these observations have stimulat...

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