The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Roger Q. Cracco - One of the best experts on this subject based on the ideXlab platform.
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The polarity of the induced electric field influences Magnetic Coil inhibition of human visual cortex: implications for the site of excitation.
Electroencephalography and Clinical Neurophysiology, 1994Co-Authors: Vahe E. Amassian, P.j. Maccabee, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Mahendra Somasundaram, Jc Rothwell, K. Henry, Alan P RudellAbstract:Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a Magnetic Coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.
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Unmasking human visual perception with the Magnetic Coil and its relationship to hemispheric asymmetry
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Visual suppression by a Magnetic Coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80–100 ms earlier 1 has been used to suppress the representation of a ‘masking’ visual stimulus and thus to unmask a ‘target’ visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.
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measurement of information processing delays in human visual cortex with repetitive Magnetic Coil stimulation
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Previous work disclosed that single Magnetic Coil (MC) pulses applied over human calcarine cortex could suppress perception of letter briefly presented, e.g. 80–100 ms earlier1. Although individual MC stimuli presented 0–60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.
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Magnetic Coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro locus of excitation
The Journal of Physiology, 1993Co-Authors: Paul J. Maccabee, V.e. Amassian, L Eberle, Roger Q. CraccoAbstract:1. According to classical cable theory, a Magnetic Coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative-going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic- or polyphasic-induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative-going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the 'figure of eight' MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative-going first phase at one location, and approximately 150 microseconds later, by the reversed negative-going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non-coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)
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Modelling Magnetic Coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation.
Electroencephalography and Clinical Neurophysiology, 1992Co-Authors: Vahe E. Amassian, Paul J. Maccabee, L Eberle, Roger Q. CraccoAbstract:Abstract To help elucidate some basic principles of Magnetic Coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the Coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially oriented, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially oriented MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.
Paul J. Maccabee - One of the best experts on this subject based on the ideXlab platform.
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Unmasking human visual perception with the Magnetic Coil and its relationship to hemispheric asymmetry
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Visual suppression by a Magnetic Coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80–100 ms earlier 1 has been used to suppress the representation of a ‘masking’ visual stimulus and thus to unmask a ‘target’ visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.
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measurement of information processing delays in human visual cortex with repetitive Magnetic Coil stimulation
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Previous work disclosed that single Magnetic Coil (MC) pulses applied over human calcarine cortex could suppress perception of letter briefly presented, e.g. 80–100 ms earlier1. Although individual MC stimuli presented 0–60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.
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Magnetic Coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro locus of excitation
The Journal of Physiology, 1993Co-Authors: Paul J. Maccabee, V.e. Amassian, L Eberle, Roger Q. CraccoAbstract:1. According to classical cable theory, a Magnetic Coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative-going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic- or polyphasic-induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative-going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the 'figure of eight' MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative-going first phase at one location, and approximately 150 microseconds later, by the reversed negative-going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non-coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)
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Modelling Magnetic Coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation.
Electroencephalography and Clinical Neurophysiology, 1992Co-Authors: Vahe E. Amassian, Paul J. Maccabee, L Eberle, Roger Q. CraccoAbstract:Abstract To help elucidate some basic principles of Magnetic Coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the Coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially oriented, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially oriented MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.
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Paraesthesias are elicited by single pulse, Magnetic Coil stimulation of motor cortex in susceptible humans.
Brain, 1991Co-Authors: V.e. Amassian, Roger Q. Cracco, Joan B. Cracco, Mahendra Somasundaram, Jc Rothwell, T. C. Britton, Paul J. MaccabeeAbstract:A minority of normal humans experience paraesthesias (usually tingling) projected to the contralateral hand in response to individual transcranial Magnetic Coil (MC) pulses. The cortical source of the paraesthesias was sought by comparing their incidence with that of muscle responses to focal MC stimulation with either a figure 8 MC or with edge stimulation of a tilted round MC in 4 susceptible subjects. In all 4, paraesthesias were best felt with MC stimulation either at, or anterior to sites yielding movement, implying an initial source in precentral gyrus (and possible premotor cortex), rather than parietal cortex. In the two subjects exhibiting the strongest paraesthesias, the threshold for the paraesthesias was less than that for movement in the relaxed arm. The optimal site of the paraesthesias within the hand was usually in the digits, but differed among subjects. Motor responses and paraesthesias following a given stimulus occurred at different sites in the hand. implying that excitation of differing sets of motor cortical neurons subserved sensory and motor responses.In only one subject were the paraesthesias sufficiently reproducible to warrant interacting electrical digital and transcranial MC pulses. The data suggested that central processing of the response to the MC pulse is slowed by an antecedent digital stimulus, but the delay for perception of each type of stimulus does not greatly differ.The central sense of movement (Amassian et al., 1989a) elicited by MC stimulation of motor cortex is compared with the paraesthesias. Both are attributed to brief, high frequency discharge by motor cortical neurons accessing the perceptual system more readily than after excitation of post-central gyrus. which requires prolonged repetitive stimulation (Libet et al., 1964). Given also the normal pattern of muscle responses in the 4 subjects, their paraesthesias are best explained by a heightened sensitivity of the perceptual system to the motor cortical response to MC stimulation.
Vahe E. Amassian - One of the best experts on this subject based on the ideXlab platform.
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The polarity of the induced electric field influences Magnetic Coil inhibition of human visual cortex: implications for the site of excitation.
Electroencephalography and Clinical Neurophysiology, 1994Co-Authors: Vahe E. Amassian, P.j. Maccabee, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Mahendra Somasundaram, Jc Rothwell, K. Henry, Alan P RudellAbstract:Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a Magnetic Coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.
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Unmasking human visual perception with the Magnetic Coil and its relationship to hemispheric asymmetry
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Visual suppression by a Magnetic Coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80–100 ms earlier 1 has been used to suppress the representation of a ‘masking’ visual stimulus and thus to unmask a ‘target’ visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.
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measurement of information processing delays in human visual cortex with repetitive Magnetic Coil stimulation
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Previous work disclosed that single Magnetic Coil (MC) pulses applied over human calcarine cortex could suppress perception of letter briefly presented, e.g. 80–100 ms earlier1. Although individual MC stimuli presented 0–60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.
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Modelling Magnetic Coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation.
Electroencephalography and Clinical Neurophysiology, 1992Co-Authors: Vahe E. Amassian, Paul J. Maccabee, L Eberle, Roger Q. CraccoAbstract:Abstract To help elucidate some basic principles of Magnetic Coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the Coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially oriented, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially oriented MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.
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Cerebello-frontal cortical projections in humans studied with the Magnetic Coil.
Electroencephalography and Clinical Neurophysiology, 1992Co-Authors: Vahe E. Amassian, P.j. Maccabee, Roger Q. Cracco, Joan B. CraccoAbstract:Abstract Focal stimulation over human cerebellum with a figure 8 Magnetic Coil (MC) results in an evoked wave recorded from bipolar scalp electrodes on the interaural line and more anteriorly. In 3 subjects, the wave responses along the interaural line had latencies of 8.8–13.8 msec, lasted 17.4–29.0 msec and had a maximum amplitude of 14.4–26.8 μV. The responses were recorded more anteriorly from leads midway between the interaural line and frontal leads; responses recorded from frontal leads were up to 3.5 msec later. The evoked wave was preceded by a diphasic EMG response with a latency of 1.2–2.0 msec. Analysis of the averaged responses recorded by adjoining bipolar leads indicated that the response was predominantly surface positive and crossed. Control experiments eliminated eye movement and somatosensory input as explanations of the evoked response, thereby identifying it as a cortical response. The surface positive wave in humans was compared with the responses recorded in cat and monkey to cerebellar stimulation. The responses in humans could reflect dysfacilitation through MC activation of Purkinje cells, or feed-forward facilitation through transsynaptic or antidromic activation of dentate neurons. The latency of the surface positive wave exceeds that of cerebellar inhibition of MC elicited hand muscle responses, but the discrepancy is at least partly accounted for by the extra delay required to set up the indirect cortico-spinal component required for motoneuron discharge. Estimates made of the cerebello-frontal cortical and peripheral feedback loop times suggest that the central has less than one quarter the delay of the peripheral loop, which would be especially advantageous during fast skilled movements of the fingers.
Larry Eberle - One of the best experts on this subject based on the ideXlab platform.
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The polarity of the induced electric field influences Magnetic Coil inhibition of human visual cortex: implications for the site of excitation.
Electroencephalography and Clinical Neurophysiology, 1994Co-Authors: Vahe E. Amassian, P.j. Maccabee, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Mahendra Somasundaram, Jc Rothwell, K. Henry, Alan P RudellAbstract:Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a Magnetic Coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.
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measurement of information processing delays in human visual cortex with repetitive Magnetic Coil stimulation
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Previous work disclosed that single Magnetic Coil (MC) pulses applied over human calcarine cortex could suppress perception of letter briefly presented, e.g. 80–100 ms earlier1. Although individual MC stimuli presented 0–60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.
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Unmasking human visual perception with the Magnetic Coil and its relationship to hemispheric asymmetry
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Visual suppression by a Magnetic Coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80–100 ms earlier 1 has been used to suppress the representation of a ‘masking’ visual stimulus and thus to unmask a ‘target’ visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.
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Stimulation of the human nervous system using the Magnetic Coil.
Journal of Clinical Neurophysiology, 1991Co-Authors: Paul J. Maccabee, Vahe E. Amassian, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Alan P RudellAbstract:: The Magnetic Coil (MC) is a unique probe that can be used to elucidate basic neurophysiological mechanisms in humans. Either by excitation or inhibition of responding neural elements, we have been able to investigate: (1) the distribution of the electric field induced within isotropic and anisotropic volume conductors by round and figure-eight MCs; (2) the theoretical relationship between electric field distribution and excitation of distal peripheral nerve, nerve root, cranial nerve, and motor cortex; (3) the effect of focal MC stimulation of motor and visual systems; (4) perturbation of sequential digit movements by MC stimulation of human premotor cortex; (5) activation of frontal motor areas related to speech; (6) elicitation of a sense of movement in an ischemic paralyzed limb by focal MC cortical stimulation; and (7) the effect of stimulation of the human visual system to (a) suppress and unmask visual perception using single MC stimuli and (b) prolong visual suppression using short trains of MC stimuli. In the future, prolongation of MC action by using repetitive stimuli should be useful in further investigating functions concerned with language, speech, and cognition.
Alan P Rudell - One of the best experts on this subject based on the ideXlab platform.
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The polarity of the induced electric field influences Magnetic Coil inhibition of human visual cortex: implications for the site of excitation.
Electroencephalography and Clinical Neurophysiology, 1994Co-Authors: Vahe E. Amassian, P.j. Maccabee, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Mahendra Somasundaram, Jc Rothwell, K. Henry, Alan P RudellAbstract:Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a Magnetic Coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.
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Unmasking human visual perception with the Magnetic Coil and its relationship to hemispheric asymmetry
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Visual suppression by a Magnetic Coil (MC) pulse delivered over human calcarine cortex after a transient visual stimulus 80–100 ms earlier 1 has been used to suppress the representation of a ‘masking’ visual stimulus and thus to unmask a ‘target’ visual stimulus given, e.g., 100 ms before the mask. The resulting target unmasking as a function of the interval between mask and MC pulse is approximately the inverse of the visual suppression curve. Arbitrary visual linear patterns can similarly be unmasked. At the long target-mask interval used, the site of masking is deduced to lie beyond calcarine cortex. In several right-handed subjects tested, powerful MC stimulation of the left (but not right) temporo-parieto-occipital cortex also led to (weaker) unmasking.
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measurement of information processing delays in human visual cortex with repetitive Magnetic Coil stimulation
Brain Research, 1993Co-Authors: Vahe E. Amassian, Paul J. Maccabee, Roger Q. Cracco, Joan B. Cracco, Alan P Rudell, Larry EberleAbstract:Abstract Previous work disclosed that single Magnetic Coil (MC) pulses applied over human calcarine cortex could suppress perception of letter briefly presented, e.g. 80–100 ms earlier1. Although individual MC stimuli presented 0–60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.
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Stimulation of the human nervous system using the Magnetic Coil.
Journal of Clinical Neurophysiology, 1991Co-Authors: Paul J. Maccabee, Vahe E. Amassian, Roger Q. Cracco, Joan B. Cracco, Larry Eberle, Alan P RudellAbstract:: The Magnetic Coil (MC) is a unique probe that can be used to elucidate basic neurophysiological mechanisms in humans. Either by excitation or inhibition of responding neural elements, we have been able to investigate: (1) the distribution of the electric field induced within isotropic and anisotropic volume conductors by round and figure-eight MCs; (2) the theoretical relationship between electric field distribution and excitation of distal peripheral nerve, nerve root, cranial nerve, and motor cortex; (3) the effect of focal MC stimulation of motor and visual systems; (4) perturbation of sequential digit movements by MC stimulation of human premotor cortex; (5) activation of frontal motor areas related to speech; (6) elicitation of a sense of movement in an ischemic paralyzed limb by focal MC cortical stimulation; and (7) the effect of stimulation of the human visual system to (a) suppress and unmask visual perception using single MC stimuli and (b) prolong visual suppression using short trains of MC stimuli. In the future, prolongation of MC action by using repetitive stimuli should be useful in further investigating functions concerned with language, speech, and cognition.
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Magnetic Coil stimulation of human visual cortex: studies of perception.
Electroencephalography and clinical neurophysiology. Supplement, 1991Co-Authors: Maccabee Pj, Alan P Rudell, Amassian Ve, Cracco Rq, Cracco Jb, Eberle Lp, ZemonAbstract:: The effects of Magnetic Coil (MC) stimulation of human visual cortex on the foveal perception of briefly presented letter trigrams include: (1) letters were nearly always reported correctly at visual stimulus-MC pulse intervals less than 60-80 msec or greater than 120-140 msec. Thus, by 120-140 msec, information related to letter recognition is relayed from calcarine cortex. (2) Presentation of equiluminant chromatic stimuli (specifically green letters against a red background) results in suppression curves which commence at longer latencies than those obtained with achromatic stimuli. (3) At a stimulus-MC pulse interval of 100 msec, shifting the MC laterally or rostrally resulted in suppression of the contralateral or caudal-most letter respectively. This implies a focal, topographical effect on visual cortex. (4) Two trigram stimuli separated in time (e.g. 100 msec) resulted in classical backward masking in which S1 (the target) was suppressed by S2 (the mask), using an S2/S1 luminance-contrast ratio of 4:1. When the MC was subsequently discharged 80-100 msec after S2, and S2 was suppressed, the response to S1 was easily retrieved (unmasked). Presumably, by 160 msec, S1 has been transmitted to the next processing, extrastriate level. (5) The unmasking phenomenon has been used to track information flow from visual cortex to higher cortical centers (e.g. Wernicke's, Broca's, and related areas). (6) Using a prototype repetitive stimulator, a consecutive train of single MC pulses given 70, 143 and 216 msec following a brief alphabetic trigram stimulus elicited a significant reduction in letter perception. This notably contrasts with the absence of suppression when a single MC pulse was given 70 or 143 msec following presentation of the alphabetic trigram. The results with 3 pulses suggest that the first MC pulse (at 70 msec) delays but requires repetition to prevent processing and/or transmission of information from visual cortex.
