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We review a body of theoretical and experimental research on Hebbian and homeostatic plasticity, starting from a puzzling observation: while homeostasis of synapses found in experiments is a slow compensatory process, most mathematical models of synaptic plasticity use rapid compensatory processes (RCPs). Even worse, with the slow homeostatic plasticity reported in experiments, simulations of existing plasticity models cannot maintain network stability unless further control mechanisms are implemented. To solve this paradox, we suggest that in addition to slow forms of homeostatic plasticity there are RCPs which stabilize synaptic plasticity on short timescales. These rapid processes may include heterosynaptic depression triggered by episodes of high postsynaptic firing rate. While slower forms of homeostatic plasticity are not sufficient to stabilize Hebbian plasticity, they are important for fine-tuning neural circuits. Taken together we suggest that learning and memory rely on an intricate interplay of diverse plasticity mechanisms on different timescales which jointly ensure stability and plasticity of neural circuits. This article is part of the themed issue ‘Integrating Hebbian and homeostatic plasticity’.

In a longitudinal quasi-field setting, we develop and test a compensatory process model of social network closure over time on the development of a transactive memory system (TMS) in groups. Although a great deal of research examines the effects of closure on organizational outcomes, that research does not describe the microprocesses that explain when, and under what conditions, closure is beneficial or detrimental. Results from our analysis of the microprocesses associated with TMSs revealed a negative direct effect of closure over time on TMS development and a simultaneous positive indirect effect of closure over time on TMS development driven by a transitive triadic social network structure. It is important to note that the mediating effect of the number of transitive triads on the relationship between closure and a TMS was predictive of subsequent group performance. Results from our study suggest that closure may be a double-edged sword and that the microprocesses associated with TMS development can explain closure’s disparate performance consequences.

In addition to direct tissue consumption, herbivory may affect other important plant processes. Here, we evaluated the effects of short-time leaf feeding by Spodoptera exigua larvae on the photosynthetic efficiency of tomato plants, using chlorophyll a fluorescence imaging analysis. After 15 min of feeding, the light used for photochemistry at photosystem II (PSII) (ΦPSII), and the regulated heat loss at PSII (ΦNPQ) decreased locally at the feeding zones, accompanied by increased non-regulated energy losses (ΦNO) that indicated increased singlet oxygen (1O2) formation. In contrast, in zones neighboring the feeding zones and in the rest of the leaf, ΦPSII increased due to a decreased ΦNPQ. This suggests that leaf areas not directly affected by herbivory compensate for the photosynthetic losses by increasing the fraction of open PSII reaction centers (qp) and the efficiency of these centers (Fv’/Fm’), because of decreased non-photochemical quenching (NPQ). This compensatory reaction mechanism may be signaled by singlet oxygen formed at the feeding zone. PSII functionality at the feeding zones began to balance with the rest of the leaf 3 h after feeding, in parallel with decreased compensatory responses. Thus, 3 h after feeding, PSII efficiency at the whole-leaf level was the same as before feeding, indicating that the plant managed to overcome the feeding effects with no or minor photosynthetic costs.

A reserve in the motor domain may underlie the capacity exhibited by some patients to maintain motor functionality in the face of a certain level of disease. This form of “motor reserve” (MR) could include cortical, cerebellar, and muscular processes. However, a systematic definition has not been provided yet. Clarifying this concept in healthy individuals and patients would be crucial for implementing prevention strategies and rehabilitation protocols. Due to its wide application in the assessment of motor system functioning, non-invasive brain stimulation (NIBS) may support such definition. Here, studies focusing on reserve in the motor domain and studies using NIBS were revised. Current literature highlights the ability of the motor system to create a reserve and a possible role for NIBS. MR could include several mechanisms occurring in the brain, cerebellum, and muscles, and NIBS may support the understanding of such mechanisms.

Is it possible to carry out complex multi-attribute decisions (which require an estimation of the weighted average) intuitively, without resorting to simplifying heuristics? Over the course of 600 trials, 26 participants had to choose the better-suiting job-candidate, a task requiring comparison of two alternatives over three/four/five dimensions with specified importance weights, with a time constraint forcing intuitive decisions. Participants performed the task fast (mean reaction time (RT) ~ 1.5 s) and with high accuracy (~86%). The participants were classified as users of one of three strategies: Weighted Additive Utility (WADD), Equal Weight rule and Take-The-Best heuristic (TTB). Fifty-nine percent of the participants were classified as users of the compensatory WADD strategy and 29% as users of the non-compensatory TTB. Moreover, the WADD users achieved higher task accuracy without showing time costs. The results provide support for the existence of an automatic compensatory mechanism in weighted average estimations.

The escalating food demand and loss to herbivores has led to increasing interest in using resistance-inducing microbes for pest control. Here, we evaluated whether root-inoculation with fungi that are otherwise known as entomopathogens improves tomato (Solanum lycopersicum) leaflets’ reaction to herbivory by Spodoptera exigua (beet armyworm) larvae using chlorophyll fluorescence imaging. Plants were inoculated with Metarhizium brunneum or Beauveria bassiana, and photosystem II reactions were evaluated before and after larval feeding. Before herbivory, the fraction of absorbed light energy used for photochemistry (ΦPSII) was lower in M. brunneum-inoculated than in control plants, but not in B. bassiana-inoculated plants. After herbivory, however, ΦPSII increased in the fungal-inoculated plants compared with that before herbivory, similar to the reaction of control plants. At the same time, the fraction of energy dissipated as heat (ΦNPQ) decreased in the inoculated plants, resulting in an increased fraction of nonregulated energy loss (ΦNO) in M. brunneum. This indicates an increased singlet oxygen (1O2) formation not detected in B. bassiana-inoculated plants, showing that the two entomopathogenic fungi differentially modulate the leaflets’ response to herbivory. Overall, our results show that M. brunneum inoculation had a negative effect on the photosynthetic efficiency before herbivory, while B. bassiana inoculation had no significant effect. However, S. exigua leaf biting activated the same compensatory PSII response mechanism in tomato plants of both fungal-inoculated treatments as in control plants.

The paper analyzes the main regulatory experiments for Privately Owned Public Spaces (POPS) in North America and Asia, comparing requirements for implementation and outcomes. It also underlines the need to make these spaces representative of the social and environmental complexity of a public place, presenting the original proposal to include in the exchange conditions regulations that guarantee ecosystem services equivalent to the loss for the community due to the bonus granted. The reflection leads to three main aspects for the edition of updated regulations: the construction of integrated networks of spaces, the consideration of the urban ecosystem and the meta-design of thematic cultural guidelines.

A seminal case study by Festinger found, paradoxically, that evidence that discontinued religious beliefs increased individuals' tendency to proselytize to others. Although this finding is renowned, surprisingly, it has never been subjected to experimental scrutiny and is open to multiple interpretations. We examined a general form of the question first posed by Festinger, namely, how does shaken confidence influence advocacy? Across three experiments, people whose confidence in closely held beliefs was undermined engaged in more advocacy of their beliefs (as measured by both advocacy effort and intention to advocate) than did people whose confidence was not undermined. The effect was attenuated when individuals affirmed their beliefs, and was moderated by both importance of the belief and open-mindedness of a message recipient. These findings not only have implications for the results of Festinger's seminal study, but also offer new insights into people's motives for advocating their beliefs.

It is well established that off-line sentence judgment tasks (oSJTs) typically rely on phonological working memory (WM), beyond specific linguistic processing. Nevertheless, empirical findings suggest that a juvenile level of performance in an oSJT could be associated with the recruitment of age-specific additional supportive neural network in healthy aging. In particular, in one of our previous study, healthy elderlies showed the additional activation of associative visual cortices when compared with young controls. We suggested that age-related hyperactivations, during an auditory sentence judgment task, might represent the neurofunctional correlate of the recruitment of compensatory strategies that are necessary to maintain a juvenile level of performance. To explicitly test this hypothesis we adopted repetitive transcranial magnetic stimulation (rTMS). Twelve healthy elderlies and 12 young participants were engaged in an off-line semantic plausibility judgment task while rTMS was delivered over: (1) the left inferior frontal gyrus (LIFG; i.e., a core region of the WM network); (2) the precuneus; and (3) a Control Site (vertex). Results showed a significant main effect of Stimulation Site and a significant Group-by-Stimulation Site interaction effect. In particular, the rTMS stimulation of the LIFG slowed down reaction times (RTs) both in young and healthy elderly participants, while only healthy elderlies showed an increment of RTs during the stimulation of the precuneus. Taken together our results further support the idea that the maintenance of a juvenile level of performance in graceful aging may be associated with task-specific compensatory processes that would manifest them-selves, from the neurofunctional point of view, by the recruitment of additional neural supportive regions.

Graceful aging has been associated with frontal hyperactivations in working- and episodic long-term memory tasks, a compensatory process, according to some, that allows the best normal elders to perform these tasks at a juvenile level, in spite of natural cortical impoverishment. In this study, 24 young and 24 healthy elderly participants were compared. Graceful aging was explored by investigating domains where most healthy elders perform like youngers (e.g. lexical-semantic knowledge) and tasks that are typically more challenging, like episodic long-term recognition memory tasks. With voxel-based morphometry, we also studied to what extent changes of fMRI activation were consistent with the pattern of brain atrophy. We found that hyperactivations and hypoactivations of the elders were not restricted to the frontal lobes, rather they presented with task-dependent patterns. Only hypoactivations and normal levels of activation systematically overlapped with regional atrophy. We conclude that compensatory processes associated with graceful aging may not necessarily be a sign of early saturation of executive resources, if this was to be represented by a systematic frontal hyperactivation, but rather they may represent the ability of recruiting new cognitive strategies. We discuss two possible approaches to further test this hypothesis.