162
Brain Research, 124 (1977) 162-~167
(~©Elsevier/North-Holland Biomedical Press, Amsterdam
Printed in The Netherlands
Multiple loci of evoked potentials in somatosensory cortex
ROBERT W. DYKES, NELSON G. PUBLICOVER, D. GARY TANJI and JOHN t). DUDAR Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scoti. B3H 4H7 (Canada)
(Accepted December 10th, 1976)
For nearly 40 years, neurophysiologists have used the classical method o f evoked potentials to create topological maps of sensory projections on the cerebral cortex. This technique has shaped our concept of sensory projection systems and. in deeply anesthetized animals, it has provided a means of rapidly determining the functional connectivity of the most synaptically secure projections of the major sensory pathways. Evoked potentials were first recorded with wick electrodes applied to the cortical surface by Marshall et al. s in 1938 and soon thereafter by AdrianL The first maps, showing the topological arrangement of the somatosensory cortex, were available within the year 3. Subsequently, innumerable cortical maps of all major projection areas have been produced using this method. Generally these studies have implied the existence of only two representations of the body surface and that a cortical locus of activity was uniquely specified by the site of stimulation on the body surface ~7 However, concerning the first of these implications, several groups have identified portions of at least 3 somatosensory projections ~,5,6m,1° even though they agree neither on their location nor their presumed function. And, concerning the second. Whitsel et al. 16 have suggested that at least some neurons in primary somatosensory cortex may require both an appropriate locus and an appropriate direction to be activated. Our data suggest that there are (i) at least 3 clearly identifiable foci of activity for the vibrissae in the cat somatosensory cortex, and that (ii) the waveform and latency of the evoked potential at a given site depend on more characteristics of the stimulus than simply its location. Our experiments were performed on barbituate-anesthetized (sodium pentobarbital 35 mg/kg) mongrel cats using the evoked potential method of Marshall et al. a. The inter-electrode mapping distance was decreased so that maps were created with a 0.5 mm rather than a 1 mm separation of points. One vibrissa in the middle of the mystacial pad was attached to the end of a tactile stimulator 4 5 m m from the skin surface and was deflected 100 ~zm in a posterior direction at a velocity of 66 m/sec. Evoked potentials of 20-400 uV peak amplitudes were recorded from the arachnoid of the contralateral frontal cortex with a 250 #m diameter monopolar silver wire
163 electrode (the indifferent electrode was clipped to the scalp). After 10,000 × amplification the signals were recorded digitally, averaged and stored on digital magnetic tape. An experiment consisted of 200-400 such records obtained from points on the coronal, anterior suprasylvian and anterior ectosylvian gyri (Fig. I D). The re~ulting matrix of points was processed by computer to locate and display local maxima and minima of the cortical evoked potential, in an area of about 36 sq.mm, it was possible, after deflection of a vibrisssa, to define routinely at least 3 loci of large positive waveforms having a latency to onset of 5-8 reset. Examples of the potentials obtained by marching from anterior to posterior across the cortical surface are shown in Fig. 1.
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Brain Research, 124 (1977) 162-~167
(~©Elsevier/North-Holland Biomedical Press, Amsterdam
Printed in The Netherlands
Multiple loci of evoked potentials in somatosensory cortex
ROBERT W. DYKES, NELSON G. PUBLICOVER, D. GARY TANJI and JOHN t). DUDAR Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scoti. B3H 4H7 (Canada)
(Accepted December 10th, 1976)
For nearly 40 years, neurophysiologists have used the classical method o f evoked potentials to create topological maps of sensory projections on the cerebral cortex. This technique has shaped our concept of sensory projection systems and. in deeply anesthetized animals, it has provided a means of rapidly determining the functional connectivity of the most synaptically secure projections of the major sensory pathways. Evoked potentials were first recorded with wick electrodes applied to the cortical surface by Marshall et al. s in 1938 and soon thereafter by AdrianL The first maps, showing the topological arrangement of the somatosensory cortex, were available within the year 3. Subsequently, innumerable cortical maps of all major projection areas have been produced using this method. Generally these studies have implied the existence of only two representations of the body surface and that a cortical locus of activity was uniquely specified by the site of stimulation on the body surface ~7 However, concerning the first of these implications, several groups have identified portions of at least 3 somatosensory projections ~,5,6m,1° even though they agree neither on their location nor their presumed function. And, concerning the second. Whitsel et al. 16 have suggested that at least some neurons in primary somatosensory cortex may require both an appropriate locus and an appropriate direction to be activated. Our data suggest that there are (i) at least 3 clearly identifiable foci of activity for the vibrissae in the cat somatosensory cortex, and that (ii) the waveform and latency of the evoked potential at a given site depend on more characteristics of the stimulus than simply its location. Our experiments were performed on barbituate-anesthetized (sodium pentobarbital 35 mg/kg) mongrel cats using the evoked potential method of Marshall et al. a. The inter-electrode mapping distance was decreased so that maps were created with a 0.5 mm rather than a 1 mm separation of points. One vibrissa in the middle of the mystacial pad was attached to the end of a tactile stimulator 4 5 m m from the skin surface and was deflected 100 ~zm in a posterior direction at a velocity of 66 m/sec. Evoked potentials of 20-400 uV peak amplitudes were recorded from the arachnoid of the contralateral frontal cortex with a 250 #m diameter monopolar silver wire
163 electrode (the indifferent electrode was clipped to the scalp). After 10,000 × amplification the signals were recorded digitally, averaged and stored on digital magnetic tape. An experiment consisted of 200-400 such records obtained from points on the coronal, anterior suprasylvian and anterior ectosylvian gyri (Fig. I D). The re~ulting matrix of points was processed by computer to locate and display local maxima and minima of the cortical evoked potential, in an area of about 36 sq.mm, it was possible, after deflection of a vibrisssa, to define routinely at least 3 loci of large positive waveforms having a latency to onset of 5-8 reset. Examples of the potentials obtained by marching from anterior to posterior across the cortical surface are shown in Fig. 1.
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