Layer-Specific Intracortical Amplification Shortens the Lifetime of Thalamocortical Repetition Suppression in Auditory Cortex

Neural adaptation in sensory cortex serves important sensory functions, and is altered by various neurophsychiatric diseases. Although adaptation is a widely studied phenomenon, much remains unknown about its underlying mechanisms on a cortical circuit level. Here, we investigated repetition suppression as fundamental aspect of adaptation by layer-specific current source density analyses of synaptic mass activities in primary auditory cortex of anesthetized Mongolian gerbils (Meriones unguiculatus). We disentangled different synaptic contributions to repetition suppression in different cortical layers, and separated thalamocortical from intracortical inputs by cortical silencing with GABAA-agonist muscimol. We systematically varied stimulus onset intervals and employed statistically robust model fitting based on bootstrapping to determine the full suppression kinetics of different synaptic responses in the steady state. Whereas thalamocortical input to granular and infragranular layers was governed by longer lasting repetition suppression, most likely reflecting depression of thalamocortical synapses, intracortical amplification in granular layers shortened the lifetime of suppression by re-enhancing granular responses mainly through synchronization of synaptic events. With increasing latency, the shorter lasting suppression kinetics observed in granular layers at early latencies (<100ms) passed on to deeper layers replacing the longer lasting infragranular suppression kinetics. Granular circuit dynamics can therefore actively shape neural adaptation across cortical layers.

Nurturing the reading brain: home literacy practices are associated with children’s neural response to printed words through vocabulary skills

Previous studies indicate that children are exposed to different literacy experiences at home. Although these disparities have been shown to affect children’s literacy skills, it remains unclear whether and how home literacy practices influence brain activity underlying word-level reading. In the present study, we asked parents of French children from various socioeconomic backgrounds (n = 66; 8.46 ± 0.36 years, range 7.52–9.22; 20 girls) to report the frequency of home literacy practices. Neural adaptation to the repetition of printed words was then measured using functional magnetic resonance imaging (fMRI) in a subset of these children (n = 44; 8.49 ± 0.33 years, range 8.02–9.14; 13 girls), thereby assessing how sensitive was the brain to the repeated presentation of these words. We found that more frequent home literacy practices were associated with enhanced word adaptation in the left posterior inferior frontal sulcus (r = 0.32). We also found that the frequency of home literacy practices was associated with children’s vocabulary skill (r = 0.25), which itself influenced the relation between home literacy practices and neural adaptation to words. Finally, none of these effects were observed in a digit adaptation task, highlighting their specificity to word recognition. These findings are consistent with a model positing that home literacy experiences may improve children’s vocabulary skill, which in turn may influence the neural mechanisms supporting word-level reading.

What Matters Most in Life? A German Cohort Study on the Sources of Meaning and Their Neurobiological Foundations in Four Age Groups

Existing work in the field of positive psychology suggests that people can draw meaning from a variety of sources. The present study aimed to identify the most important sources of meaning and to explore the role of age and neural adaptation processes in this context. As part of a large German cohort study, 1,587 individuals between 12 and 94 years were asked to provide a maximum of five responses to the question “What matters most to you in life?” We divided the study population into four age groups and analyzed the obtained answers qualitatively and quantitatively using (1) word clouds and (2) frequency comparisons based on a summarizing content analysis. A chi-squared test was used to test the observed differences between age groups. Identified sources of meaning could be clustered into 16 main and 76 subcategories, with relationships (by 90% of respondents) and health and well-being (by 65% of respondents) being the most frequently named main categories, followed by a good living environment (by 28%), (leisure) time (by 26%), and work (by 24%). The study revealed some remarkable age-related patterns. While the importance of partnership increased with age, social networks were less important to older individuals. We also found that, for example, the importance of self-realization, success and career decreased with age, while the opposite was true for life satisfaction and peace and harmony. Security was most important to individuals in the two middle age groups between 30 and 69 years. The study advances our understanding of meaning across various ages by showing that individuals of different ages perceive different things as meaningful to them. Interpreting our results in the light of a neurobiological model of motivation systems, we argue that neural adaptation processes may play an important role in the (changing) perceptions of meaning throughout life.

Sound level context modulates neural activity in the human brainstem

Optimal perception requires adaptation to sounds in the environment. Adaptation involves representing the acoustic stimulation history in neural response patterns, for example, by altering response magnitude or latency as sound-level context changes. Neurons in the auditory brainstem of rodents are sensitive to acoustic stimulation history and sound-level context (often referred to as sensitivity to stimulus statistics), but the degree to which the human brainstem exhibits such neural adaptation is unclear. In six electroencephalography experiments with over 125 participants, we demonstrate that the response latency of the human brainstem is sensitive to the history of acoustic stimulation over a few tens of milliseconds. We further show that human brainstem responses adapt to sound-level context in, at least, the last 44 ms, but that neural sensitivity to sound-level context decreases when the time window over which acoustic stimuli need to be integrated becomes wider. Our study thus provides evidence of adaptation to sound-level context in the human brainstem and of the timescale over which sound-level information affects neural responses to sound. The research delivers an important link to studies on neural adaptation in non-human animals.

Shifts in Estimated Preferred Directions During Simulated BMI Experiments With No Adaptation

Experiments with brain-machine interfaces (BMIs) reveal that the estimated preferred direction (EPD) of cortical motor units may shift following the transition to brain control. However, the cause of those shifts, and in particular, whether they imply neural adaptation, is an open issue. Here we address this question in simulations and theoretical analysis. Simulations are based on the assumption that the brain implements optimal state estimation and feedback control and that cortical motor neurons encode the estimated state and control vector. Our simulations successfully reproduce apparent shifts in EPDs observed in BMI experiments with different BMI filters, including linear, Kalman and re-calibrated Kalman filters, even with no neural adaptation. Theoretical analysis identifies the conditions for reducing those shifts. We demonstrate that simulations that better satisfy those conditions result in smaller shifts in EPDs. We conclude that the observed shifts in EPDs may result from experimental conditions, and in particular correlated velocities or tuning weights, even with no adaptation. Under the above assumptions, we show that if neurons are tuned differently to the estimated velocity, estimated position and control signal, the EPD with respect to actual velocity may not capture the real PD in which the neuron encodes the estimated velocity. Our investigation provides theoretical and simulation tools for better understanding shifts in EPD and BMI experiments.

Neural activity in human brainstem adapts to sound-level statistics

Optimal perception requires adaptation to sounds in the environment. Adaptation involves representing the acoustic stimulation history in neural response patterns, for example, by altering response magnitude or latency as sound-level statistics change. Neurons in the auditory brainstem of rodents are sensitive to acoustic stimulation history and sound-level statistics, but the degree to which the human brainstem exhibits such neural adaptation is unclear. In six electroencephalography experiments with over 125 participants, we demonstrate that acoustic stimuli within a time window of at least 40 ms are represented in response latency of the human brainstem. We further show that human brainstem responses adapt to sound-level statistical information, but that neural sensitivity to sound-level statistics is less reliable when acoustic stimuli need to be integrated over periods of ~40 ms. Our results provide evidence of adaptation to sound-level statistics in the human brainstem and of the timescale over which sound-level statistics affect neural responses to sound. The research delivers an important link to studies on neural adaptation in non-human animals.

Optics and neural adaptation jointly limit human stereovision

Stereovision is the ability to perceive fine depth variations from small differences in the two eyes’ images. Using adaptive optics, we show that even minute optical aberrations that are not clinically correctable, and go unnoticed in everyday vision, can affect stereo acuity. Hence, the human binocular system is capable of using fine details that are not experienced in everyday vision. Interestingly, stereo acuity varied considerably across individuals even when they were provided identical perfect optics. We also found that individuals’ stereo acuity is better when viewing with their habitual optics rather than someone else’s (better) optics. Together, these findings suggest that the visual system compensates for habitual optical aberrations through neural adaptation and thereby optimizes stereovision uniquely for each individual. Thus, stereovision is limited by small optical aberrations and by neural adaptation to one’s own optics.