Spatial resolution of epicranial somatosensory evoked potential mapping in mice
Spatial resolution of epicranial somatosensory evoked potential (SEP) mapping in mice.
A. Overview of the recording system. Adult male C57BL/6 mice were anesthetized with isoflurane and placed in a stereotaxic frame. The scalp was additionally anesthetized with bupivacaine and then incised to expose the skull. An array of 32 stainless steel electrodes was lowered onto the skull bones (epicranial electrodes).
B. Localization of the electrodes with respect to the brain, especially the somatosensory representation of the facial whiskers (posteromedial barrel subfield of the barrel cortex). Somatosensory stimuli were delivered by brief, sudden mechanical deflections of the whiskers on one side of the snout through an electromechanical device.
C. SEP waveforms to deflection of all the left-sided whiskers. Average responses to 200 stimuli, 2011-ms inter-stimulus interval.
D. In order to probe the spatial resolution of epicranial SEP mapping, we took advantage of the well-established one-to-one somatotopic relationship between a given whisker and its corresponding barrel in the contralateral barrel cortex. Two groups of 3 whiskers each were stimulated separately (red: whiskers alpha, A1 and B1; black: whiskers delta, D1 and E1).
E. The cortical representations of the 2 whisker groups were separated by a minimum of 300 micrometers, and the distance between their centers was about 1 mm. Scale bar: 300 micrometers.
F. SEP maps to stimulation of the delta, D1 and E1 whiskers. A k-means clustering algorithm was applied to the sequence of instantaneous grand average SEP maps between 5 and 40 ms after stimulation to optimally summarize that sequence in a small number of component maps. Top row: the colored segments on the global field power trace (a measure of the intensity of the brain response) depict the temporal extent of each component map. Bottom row: the topography of the 6 component maps is color-coded (red, positive voltage; blue, negative voltage) over an MRI brain surface. Note how the initial component map has a comparatively anterior topography, reflecting the initial activation of the delta, D1 and E1 barrels.
G. SEP maps to stimulation of the alpha, A1 and B1 whiskers. Note how the initial component map has a comparatively posterior topography, reflecting the initial activation of the alpha, A1 and B1 barrels. The global explained variance (a measure of the topographical similarity between one component map and all instantaneous maps attributed to it) of component maps 1 and 4 on the SEP map series of individual mice was significantly higher in the delta-D1-E1 group than in the alpha-A1-B1 group, while the reverse was true for maps 2 and 5 (all p values < 0.01, Bonferroni-corrected significance level = 0.01).
Conclusions: epicranial SEP mapping in mice is at least able to resolve the activation of cerebral areas that are separated by a minimum of 300 micrometers whose centers are distant by about 1 mm.
The recording system and data analysis are described in detail in: Mégevand P, Quairiaux C, Lascano AM, Kiss JZ, Michel CM. A mouse model for studying large-scale neuronal networks using EEG mapping techniques. Neuroimage. 2008 Aug 15;42(2):591-602.