Abstract
We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.
Highlights
To understand the basic elements of cortical somatosensory processing, it is necessary to study the spatio-temporal dynamics of cortical activation in response to somatosensory stimuli
This study provides a complete description of the spatio-temporal dynamics of voltage-sensitive dye (VSD) activation of the rat supragranular somatosensory cortex in response to stimulation of the paws
VSD Imaging of Cortical Somatosensory Responses The VSD signal is proportional to the membrane potential changes and to the membrane area of the neuronal elements stained under each measured pixel [12,42]
Summary
To understand the basic elements of cortical somatosensory processing, it is necessary to study the spatio-temporal dynamics of cortical activation in response to somatosensory stimuli. Supragranular layers play a critical role in distributing the signals between cortical columns and to other regions involved in sensorimotor processing [8,9,10]. VSD imaging allows the simultaneous imaging of the activation of large cortical regions with excellent spatial and temporal resolution [11,12]. This resolution has made it possible to study in detail the spatio-temporal dynamics of spontaneous and evoked activation in the supragranular layers of the somatosensory cortex, especially in the barrel cortex [13,14,15,16,17,18,19,20]. The exact spatio-temporal dynamics of supragranular cortical activation in response to contralateral and ipsilateral stimulation of the forepaw and hindpaw in physiological conditions remain unclear
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