No abstract

SummaryTo better understand how a stream of sensory data is transformed into a percept, we examined neuronal activity in vibrissal sensory cortex, vS1, together with vibrissal motor cortex, vM1 (a frontal cortex target of vS1), while rats compared the intensity of two vibrations separated by an interstimulus delay. Vibrations were “noisy,” constructed by stringing together over time a sequence of velocity values sampled from a normal distribution; each vibration’s mean speed was proportional to the width of the normal distribution. Durations of both stimulus 1 and stimulus 2 could vary from 100 to 600 ms. Psychometric curves reveal that rats overestimated the longer-duration stimulus—thus, perceived intensity of a vibration grew over the course of hundreds of milliseconds even while the sensory input remained, on average, stationary. Human subjects demonstrated the identical perceptual phenomenon, indicating that the underlying mechanisms of temporal integration generalize across species. The time dependence of the percept allowed us to ask to what extent neurons encoded the ongoing stimulus stream versus the animal’s percept. We demonstrate that vS1 firing correlated with the local features of the vibration, whereas vM1 firing correlated with the percept: the final vM1 population state varied, as did the rat’s behavior, according to both stimulus speed and stimulus duration. Moreover, vM1 populations appeared to participate in the trace of the percept of stimulus 1 as the rat awaited stimulus 2. In conclusion, the transformation of sensory data into the percept appears to involve the integration and storage of vS1 signals by vM1.

Non-technical summary Stimulation of the human brain with direct current is a simple but effective neuromodulation technique that is becoming increasingly popular due to its potentiality for non-invasively treating a variety of neurological and neuropsychiatric disorders. Recently, this neuromodulation technique has been extended to the stimulation of the human spinal cord. Here we investigated the mechanisms of action of spinal direct current stimulation (sDCS) in anaesthetized rats. We found that sDCS can selectively modulate the spontaneous activity entering the brain through the spinal cord via the somatosensory system, consequently modulating both the internal state of the brain and its responsiveness to external somatosensory stimuli. These findings have at least two levels of significance: from a physiological perspective, they remark on the importance of the spinal cord in regulating the state of the brain; from a clinical perspective, they offer a mechanistic rationale for the development of sDCS as an effective bottom-up neuromodulation technique.Abstract Afferent somatosensory activity from the spinal cord has a profound impact on the activity of the brain. Here we investigated the effects of spinal stimulation using direct current, delivered at the thoracic level, on the spontaneous activity and on the somatosensory evoked potentials of the gracile nucleus, which is the main entry point for hindpaw somatosensory signals reaching the brain from the dorsal columns, and of the primary somatosensory cortex in anaesthetized rats. Anodal spinal direct current stimulation (sDCS) increased the spontaneous activity and decreased the amplitude of evoked responses in the gracile nucleus, whereas cathodal sDCS produced the opposite effects. At the level of the primary somatosensory cortex, the changes in spontaneous activity induced by sDCS were consistent with the effects observed in the gracile nucleus, but the changes in cortical evoked responses were more variable and state dependent. Therefore, sDCS can modulate in a polarity-specific manner the supraspinal activity of the somatosensory system, offering a versatile bottom-up neuromodulation technique that could potentially be useful in a number of clinical applications.