Hippocampal Volume, Neurite Density and Trace Eye-Blink Conditioning in Young Children

The current study examined the relations between hippocampal structure (e.g., volume and neurite density) and performance on a trace eye blink conditioning (EBC) task in young children. Our first aim assessed whether individual differences in hippocampal volume were associated with trace EBC performance, using both percent Conditioned Responses (% CR) and CR onset latency or the average latency (ms) at which the child started their blink, as measures of hippocampal-dependent associative learning. Our second aim evaluated whether individual differences in hippocampal neurite density were associated with EBC performance using the same outcome measures. Typically developing 4- to 6-year-olds (N = 31; 14 girls; Mage = 5.67; SDage = 0.89) completed T1 and diffusion-weighted MRI scans and a 15-minute trace eyeblink conditioning task outside of the scanner. % CR and CR onset latency were computed across all tone-puff and tone-alone trials. While hippocampal volume was not associated with any of our EBC measures, greater hippocampal neurite density bilaterally, was associated with later CR onset. In other words, children with greater left and right hippocampal neurite density blinked closer to the US (i.e., air puff) than children with less hippocampal neurite density, indicating that structural changes in the hippocampus assisted in the accurate timing of conditioned responses.

Measuring human trace fear conditioning

Trace fear conditioning is an important research paradigm to model aversive learning in biological or clinical scenarios, where predictors (conditioned stimuli, CS) and aversive outcomes (unconditioned stimuli, US) are separated in time. The optimal measurement of human trace fear conditioning, and in particular of memory recall after consolidation, is currently unclear. We conducted two identical experiments with a 15-s trace interval and a recall test 1 week after acquisition, while recording several psychophysiological observables. We explored learning and memory measures in the first experiment and confirmed the most sensitive measures in the second experiment. Retrodictive validity was used as a metric to estimate measurement error. We found that in the recall test without reinforcement, only fear-potentiated startle but not skin conductance, pupil size, heart period, or respiration amplitude, differentiated CS+ and CS-. During acquisition without startle probes, only skin conductance responses and pupil size responses but none of the other measures differentiated CS+ and CS-. We establish the optimal way of quantifying these conditioned responses. As a side finding, there was no evidence for extinction of fear-potentiated startle over 30 trials without reinforcement. These results may be useful to inform future substantive research using human trace fear conditioning protocols.

Startle responses in Duchenne muscular dystrophy: a novel biomarker of brain dystrophin deficiency

Duchenne muscular dystrophy (DMD) is characterised by loss of dystrophin in muscle. Patients affected by DMD also have variable degree of intellectual disability and neurobehavioural co-morbidities. In contrast to muscle, in which a single full-length isoform (Dp427) is produced, multiple dystrophin isoforms are produced in the brain, and their deficiency accounts for the variability of CNS manifestations, with increased risk of comorbidities in patients carrying mutations affecting the 3’ end of gene, disrupting the shorter Dp140 and Dp71 isoforms. The mdx mouse model of DMD lacks Dp427 in muscle and CNS and exhibits exaggerated startle responses to threat, linked to the deficiency of dystrophin in limbic structures such as the amygdala, which normalise with postnatal brain dystrophin-restoration therapies. A pathological startle response is not a recognised feature of DMD, and its characterisation has implications for improved clinical management and translational research.To investigate startle responses in DMD, we used a novel fear-conditioning task in an observational study of 56 males aged 7-12 years (31 DMD, mean age 9.7±1.8 years; 25 Controls, mean age 9.6±1.4 years). Trials of two neutral visual stimuli were presented to participants: one ‘safe’ cue presented alone; one ‘threat’ cue paired with an aversive noise to enable conditioning of physiological startle responses (skin conductance response, SCR; heart rate, HR). Retention of conditioned physiological responses was subsequently tested with presentation of both cues without the aversive noise in an ‘extinction’ phase. Primary outcomes were the magnitude of the initial unconditioned SCR and HR change responses to the aversive ‘threat’ and acquisition and retention of conditioned responses after conditioning. Secondary outcomes were neuropsychological measures and genotype associations.The initial (unconditioned) mean SCR to threat was greater in DMD than Controls (Mean difference 3.0 µS (95% CI 1.0, 5.1), P=.004), associated with a significant threat-induced bradycardia only in the DMD group (mean difference -5.6 bpm (95% CI 0.51, 16.9); P=.04). DMD participants found the task more aversive than Controls, consequently early termination during the extinction phase occurred in 26% of the DMD group (vs. 0% Controls; P=.007).This study provides the first evidence that boys with DMD show increased unconditioned startle responses to threat, similar to the mdx mouse phenotype that also responds to brain dystrophin restoration. Our study provides new insights into the neurobiology underlying the complex neuropsychiatric co-morbidities in DMD and defines an objective measure of this CNS phenotype, which will be valuable for future CNS-targeted dystrophin-restoration studies.

Evaluating an internet-delivered fear conditioning and extinction protocol using response times and affective ratings.

Pavlovian fear conditioning is widely used to study mechanisms of fear learning, but high-throughput studies are hampered by the labor-intensive nature of examining participants in the lab. To circumvent this bottle-neck, fear conditioning tasks have been developed for remote delivery. Previous studies have examined remotely delivered fear conditioning protocols using expectancy ratings and affective ratings. Here we replicate and extend these findings using an internet-delivered version of the Screaming Lady paradigm, evaluating the effects on negative affective ratings and response time to an auditory probe during stimulus presentations. In a sample of 80 adults, we observed clear evidence of both fear acquisition and extinction using affective ratings. Response times were faster when probed early, but not later, during presentation of stimuli paired with an aversive scream. The response time findings are at odds with previous lab-based studies showing slower responses to threat-predicting cues. The findings underscore the feasibility of employing remotely delivered fear conditioning paradigms with affective ratings as outcome, and highlights the need for further research examining optimal parameters for concurrent response time measures or alternate modes of non-verbal estimation of conditioned responses in Pavlovian conditioning protocols.

Development of a novel startle response task in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), an X-linked childhood-onset muscular dystrophy caused by loss of the protein dystrophin, can be associated with neurodevelopmental, emotional and behavioural problems. A DMD mouse model also displays a neuropsychiatric phenotype, including increased startle responses to threat which normalise when dystrophin is restored in the brain.We hypothesised that startle responses may also be increased in humans with DMD, which would have potential translational therapeutic implications. To investigate this, we first designed a novel discrimination fear-conditioning task and tested it in six healthy volunteers, followed by male DMD (n=11) and Control (n=9) participants aged 7-12 years. The aims of this methodological task development study were to: i) confirm the task efficacy; ii) optimise data processing procedures; iii) determine the most appropriate outcome measures.In the task, two neutral visual stimuli were presented: one ‘safe’ cue presented alone; one ‘threat’ cue paired with a threat stimulus (aversive noise) to enable conditioning of physiological startle responses (skin conductance response, SCR, and heart rate). Outcomes were the unconditioned physiological startle responses to the initial threat, and retention of conditioned responses in the absence of the threat stimulus.We present the protocol development and optimisation of data processing methods based on empirical data. We found that the task was effective in producing significantly higher physiological startle SCR in reinforced ‘threat’ trials compared to ‘safe’ trials (P<.001). Different data extraction methods were compared and optimised, and the optimal sampling window was derived empirically. SCR amplitude was the most effective physiological outcome measure when compared to SCR area and change in heart rate, with the best profile on data processing, the least variance, successful conditioned response retention (P=.01) and reliability assessment in test-retest analysis (rho=.86). The definition of this novel outcome will allow us to study this response in a DMD population.

Higher-Order Conditioning: What Is Learnt and How it Is Expressed

Pairing a neutral conditioned stimulus (CS) with a motivationally significant unconditioned stimulus (US) results in the CS coming to elicit conditioned responses (CRs). The widespread significance and translational value of Pavlovian conditioning are increased by the fact that pairing two neutral CSs (A and X) enables conditioning with X to affect behavior to A. There are two traditional informal accounts of such higher-order conditioning, which build on more formal associative analyses of Pavlovian conditioning. But, higher-order conditioning and Pavlovian conditioning have characteristics that are beyond these accounts: Notably, the two are influenced in different ways by the same experimental manipulations, and both generate conditioned responses that do not reflect the US per se. Here, we present a formal analysis that sought to address these characteristics.

Intrinsic regulators of the action potential waveform control dopamine release to shape behavior

Despite the widely known role of dopamine in reinforcement learning, how the patterns of dopamine release that are critical to the acquisition, performance, and extinction of conditioned responses are generated is poorly resolved. Here, we demonstrate that the coordinated actions of two ion channels, Kv4.3 and BKCa1.1, control the pattern of dopamine release on different time scales to regulate separate phases of reinforced behavior in mice. Inactivation of Kv4.3 in VTA dopamine neurons increases pacemaker activity and excitability that is associated with increased ramping prior to lever press in a learned instrumental response paradigm. Loss of Kv4.3 enhanced performance of the learned response and facilitated extinction. In contrast, loss of BKCa1.1 increased burst firing and phasic dopamine release that enhanced learning of an instrumental response. Inactivation of BKCa1.1 increased the reward prediction error that was associated with an enhanced extinction burst in early extinction training. These data demonstrate that temporally distinct patterns of dopamine release are regulated by the intrinsic properties of the cell to shape behavior.

Role of the motor cortex in the generation of classically conditioned eyelid and vibrissae responses

The eyelid motor system has been used for years as an experimental model for studying the neuronal mechanisms underlying motor and cognitive learning, mainly with classical conditioning procedures. Nonetheless, it is not known yet which brain structures, or neuronal mechanisms, are responsible for the acquisition, storage, and expression of these motor responses. Here, we studied the temporal correlation between unitary activities of identified eyelid and vibrissae motor cortex neurons and the electromyographic activity of the orbicularis oculi and vibrissae muscles and magnetically recorded eyelid positions during classical conditioning of eyelid and vibrissae responses, using both delay and trace conditioning paradigms in behaving mice. We also studied the involvement of motor cortex neurons in reflexively evoked eyelid responses and the kinematics and oscillatory properties of eyelid movements evoked by motor cortex microstimulation. Results show the involvement of the motor cortex in the performance of conditioned responses elicited during the classical conditioning task. However, a timing correlation analysis showed that both electromyographic activities preceded the firing of motor cortex neurons, which must therefore be related more with the reinforcement and/or proper performance of the conditioned responses than with their acquisition and storage.

Estradiol during (analogue-) trauma: risk- or protective factor for intrusive re-experiencing?

Intrusions, a key symptom of posttraumatic stress disorder (PTSD), can occur as classically conditioned responses to trauma-related cues, both in the form of images and pain sensations. Women are more vulnerable to experiencing intrusions, and gonadal hormones may underlie this sex difference. Yet so far, particularly estradiol’s influence on intrusions is unclear, as PTSD-symptom studies suggesting a vulnerable window for intrusions during the high estradiol-progesterone phase diverge from fear-conditioning studies suggesting a protective role of estradiol. Here, we aim to address this discrepancy and examine the effects of estradiol on intrusions while also considering stress as potential moderator.Forty free-cycling women participated in an ecologically informed trauma-pain-conditioning (TPC) paradigm, using trauma-films and pain as unconditioned stimuli. Predictors were salivary estradiol and stress indexed by salivary cortisol and self-reported state-anxiety during TPC. Outcomes were film- and pain-intrusions occurring during daily-life in the week following TPC and a memory-triggering-task in response to conditioned stimuli 24h after TPC.Estradiol yielded time- and stress-dependent effects on film-intrusions during daily-life: women with higher estradiol showed initially greater probability of experiencing film-intrusions, switching to lower probability toward the end of the week. This late protective effect of estradiol on film-intrusions only held for higher state-anxious women. In contrast, estradiol showed consistent protective effects on pain-intrusions during daily-life and memory-triggering-task. Together, these data suggest that high estradiol during trauma may shield women from long-term audiovisual trauma intrusions, as well as from pain-intrusions, and thereby possibly constitute a protective factor for PTSD and potentially also for chronic pain.

Linking Homeostatically Protected Mood, Mindfulness, and Depression: A Conceptual Synthesis and Model of Moodfulness

Mindfulness is an ancient practice, derived from Buddhism and recently adapted for the treatment of depression and other psychological conditions. The mechanism of action is thought to involve the extinction of habitual or conditioned responses to internal cognitive and emotional content. In turn, this relies on mechanisms of attentional control and emotion regulation. The resulting state of consciousness is sometimes described as equanimity. This conceptual review paper explores the process of achieving equanimity within a homeostatic framework. The result is a model of moodfulness, which combines mindfulness with Homeostatically Protected Mood to provide a new theoretical view of recovery from symptoms of depression. This model presents a case for mindfulness restoration of mood homeostasis following homeostatic defeat.