Dynamic Molecular Switches Drive Negative Memristance Mimicking Synaptic Behavior

To realize molecular scale electrical operations beyond the von Neumann bottleneck, new types of multi-functional switches are needed that mimic self-learning or neuromorphic computing by dynamically toggling between multiple operations that depend on their past. Here we report a molecule that switches from high to low conductance states with massive negative memristive behavior that depends on the drive speed and the number of past switching events. This dynamic molecular switch emulates synaptic behavior and Pavlovian learning and can provide all of the fundamental logic gates because of its time-domain and voltage-dependent plasticity. This multi-functional switch represents molecular scale hardware operable in solid-state devices opening a pathway to dynamic complex electrical operations encoded within a single ultra-compact component.

In search of the behavioural effects of fear: a paradigm to assess conditioned suppression in humans

Conditioned fear can substantially reduce the likelihood that an individual will engage in reward- related behaviour - a phenomenon coined conditioned suppression. Despite the unmistakable relevance of conditioned suppression for excessive fears and their adverse consequences, the phenomenon has primarily been observed in animal models and is not yet well understood. Here, we aimed to develop a conditioned suppression paradigm that enables a robust quantification of the effect of Pavlovian aversive stimuli on subsequent reward-related behaviour in humans and assess its potential relation to physiological measures of fear. In phase 1, an instrumental response was incentivized with monetary rewards. In phase 2, one of two conditioned stimuli (CS+) was reinforced with an aversive unconditioned stimulus (US, i.e., electric stimulus). During aversive Pavlovian learning we assessed differential skin conductance (SCR) and fear potentiated startle responses (FPS). Lastly, we tested the effect of the aversively conditioned CS+ on the response rate of the instrumental response in a transfer phase. Despite strong aversive Pavlovian conditioning, as indicated by large effect sizes in differential SCR and FPS, we did not find any evidence for conditioned suppression: i.e., there was no significant reduction of instrumental responding in the presence of the CS+ compared to a new control stimulus. This lack of conditioned suppression is in line with previous studies that reported difficulties inducing conditioned suppression and points towards a general challenge in investigating conditioned suppression in humans. Implications and directions for future research on the highly relevant behavioural effects of fear and anxiety are discussed.

The influence of associative reward learning on motor inhibition

Stimuli that predict a rewarding outcome can cause difficulties to inhibit unfavourable behaviour. Research suggests that this is also the case for stimuli with a history of reward extending these effects on action control to situations, where reward is no longer accessible. We expand this line of research by investigating if previously reward-predictive stimuli promote behavioural activation and impair motor inhibition in a second unrelated task. In two experiments participants were trained to associate colours with a monetary reward or neutral feedback. Afterwards participants performed a cued go/no-go task, where cues appeared in the colours previously associated with feedback during training. In both experiments training resulted in faster responses in rewarded trials providing evidence of a value-driven response bias as long as reward was accessible. However, stimuli with a history of reward did not interfere with goal-directed action and inhibition in a subsequent task after removal of the reward incentives. While the first experiment was not conclusive regarding an impact of reward-associated cues on response inhibition, the second experiment, validated by Bayesian statistics, clearly questioned an effect of reward history on inhibitory control. This stands in contrast to earlier findings suggesting that the effect of reward history on subsequent action control is not as consistent as previously assumed. Our results show that participants are able to overcome influences from Pavlovian learning in a simple inhibition task. We discuss our findings with respect to features of the experimental design which may help or complicate overcoming behavioural biases induced by reward history.

Ambiguity Drives Higher-Order Pavlovian Learning

In the natural world, stimulus-outcome associations are often noisy and ambiguous. Learning to disambiguate these associations to identify which specific outcomes will occur is critical for survival. Pavlovian occasion setters are stimuli that determine whether other stimuli that are ambiguous will result in a specific outcome. Occasion setting is a well-established field, but very little investigation has been conducted on how occasion setters are disambiguated when they themselves are ambiguous. We investigated the role of higher-order Pavlovian occasion setting in humans. We also developed and tested the first computational model predicting direct associations, traditional occasion setting, and 2nd-order occasion setting. Results showed that occasion setters affected ambiguous but not unambiguous lower-order stimuli and that 2nd-order occasion setting was indeed learned. Our computational model demonstrated excellent fit with the data, advancing our theoretical understanding of learning with ambiguity. These results may ultimately improve treatment of Pavlovian-based mental health disorders (e.g., anxiety).

Debunking a myth: plant consciousness

Claims that plants have conscious experiences have increased in recent years and have received wide coverage, from the popular media to scientific journals. Such claims are misleading and have the potential to misdirect funding and governmental policy decisions. After defining basic, primary consciousness, we provide new arguments against 12 core claims made by the proponents of plant consciousness. Three important new conclusions of our study are (1) plants have not been shown to perform the proactive, anticipatory behaviors associated with consciousness, but only to sense and follow stimulus trails reactively; (2) electrophysiological signaling in plants serves immediate physiological functions rather than integrative-information processing as in nervous systems of animals, giving no indication of plant consciousness; (3) the controversial claim of classical Pavlovian learning in plants, even if correct, is irrelevant because this type of learning does not require consciousness. Finally, we present our own hypothesis, based on two logical assumptions, concerning which organisms possess consciousness. Our first assumption is that affective (emotional) consciousness is marked by an advanced capacity for operant learning about rewards and punishments. Our second assumption is that image-based conscious experience is marked by demonstrably mapped representations of the external environment within the body. Certain animals fit both of these criteria, but plants fit neither. We conclude that claims for plant consciousness are highly speculative and lack sound scientific support.