The thalamus is much more than a simple sensory relay. High-order thalamic nuclei, such as the mediodorsal thalamus, exert a profound influence over animal cognition. However, given the difficulty of directly recording from the thalamus in humans, next-to-nothing is known about thalamic and thalamocortical contributions to human cognition. To address this, we analysed simultaneously-recorded thalamic iEEG and whole-head MEG in six patients (plus MEG recordings from twelve healthy controls) as they completed a visual detection task. We observed that the phase of both ongoing mediodorsal thalamic and prefrontal low-frequency activity was predictive of perceptual performance. Critically however, mediodorsal thalamic activity mediated prefrontal contributions to perceptual performance. These results suggest that it is thalamocortical interactions, rather than cortical activity alone, that is predictive of upcoming perceptual performance and, more generally, highlights the importance of accounting for the thalamus when theorising about cortical contributions to human cognition.
It has been demonstrated that, while otherwise detrimental, noise can improve sensory perception under optimal conditions. The mechanism underlying this improvement is stochastic resonance. An inverted U-shaped relationship between noise level and task performance is considered as the signature of stochastic resonance. Previous studies have proposed the existence of stochastic resonance also in the human auditory system. However, the reported beneficial effects of noise are small, based on a small sample, and do not confirm the proposed inverted U-shaped function. Here, we investigated in two separate studies whether stochastic resonance may be present in the human auditory system by applying noise of different levels, either acoustically or electrically via transcranial random noise stimulation, while participants had to detect acoustic stimuli adjusted to their individual hearing threshold. We find no evidence for behaviorally relevant effects of stochastic resonance. Although detection rate for near-threshold acoustic stimuli appears to vary in an inverted U-shaped manner for some subjects, it varies in a U-shaped manner or in other manners for other subjects. Our results show that subjects do not benefit from noise, irrespective of its modality. In conclusion, our results question the existence of stochastic resonance in the human auditory system. Noise is generally considered detrimental for signal processing. Under certain circumstances, however, noise can be beneficial. It has been demonstrated in a variety of micro-and macro-systems that applying noise to a weak signal can increase the probability that the signal crosses a threshold 1-3. Given such a threshold, the probability of detecting a subthreshold signal may therefore be higher in the presence of noise at a specific optimal level than in the absence of noise or in the presence of noise of suboptimal and supraoptimal levels. This results in a non-linear relationship between the probability of detecting a weak signal and the noise level. With regard to sensory perception, an inverted U-shaped function between signal and noise is typically considered as the signature of stochastic resonance (SR) 4-7. Animal data suggest that the responses of hair cells, auditory-nerve fibers, and brainstem neurons to weak acoustic stimuli are enhanced by low levels of acoustic noise in a manner consistent with stochastic resonance 7-10. Human electrophysiological data suggest that noise can increase neural synchrony in the human auditory cortex, which, in turn, might lead to improved hearing performance 11. Furthermore, a few studies claim improvements in perceptual detection thresholds due to the application of low-level noise. This has been reported for normally hearing subjects 9,12 and for subjects having cochlear and brainstem implants 9. However, although this previous research argues for a beneficial effect of noise on auditory perception, to the best of our knowledge, none of the available studies provide robust evidence for SR in the human auditory system, ei...
The thalamus is much more than a simple sensory relay. High-order thalamic nuclei, such as the mediodorsal thalamus, exert a profound influence over animal cognition. However, given the difficulty of directly recording from the thalamus in humans, next-to-nothing is known about thalamic and thalamocortical contributions to human cognition. To address this, we analysed simultaneously-recorded thalamic iEEG and whole-head MEG in six patients (four female, two male; plus MEG recordings from twelve healthy controls) as they completed a visual detection task. We observed that the phase of both ongoing mediodorsal thalamic and prefrontal low-frequency activity was predictive of perceptual performance. Critically however, mediodorsal thalamic activity mediated prefrontal contributions to perceptual performance. These results suggest that it is thalamocortical interactions, rather than cortical activity alone, that is predictive of upcoming perceptual performance and, more generally, highlights the importance of accounting for the thalamus when theorising about cortical contributions to human cognition.
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