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Magnetic coil stimulation suppresses peripheral visual perception at an extra-calcarite site
Society proceedings
12P
occipital lobes to study peripheral visual field effects in 6 subjects. The visual test pattern was a grid of asterisks around a central fixation point, and the target was the disappearance of one asterisk for a single frame of the video monitor. Using single magnetic pulses at stimulator outputs of 55-95%. robust interference effects were found at latencies of 37-87 ms, peaking around 50-75 ms. Suppression of visual perception occurred with both transverse and sagittal alignments of the stimulator. When the stimulator was moved laterally over either occipital lobe, perception of target disappearance was consistently suppressed in the contralateral visual field. Movement of the stimulator rostrally produced consistent suppression in the lower and middle field, but preferential suppression of the upper field could not be obtained. This altitudinal asymmetry is correlated with the anatomy of the occipital lobe in relation to the scalp surface. B104:
Magnetic coil stimulation suppresses peripheral visual perception at an extra-calcarine site - Charles M. Epstein and Reinaldo Verson (Department of Neurology, Emory U. School of Medicine, Atlanta, GA 30322)
ciency of stimulus parameters in evoking conscious awareness during direct electrical stimulation of the human brain. Libet’s results have never been independently verified. We examined direct cortical stimulation in 2 epilepsy patients with grids over the postcentral gyrus. Stimuli were applied to a single pair of contacts which elicited sensation in the contralateral hand. The current needed to produce sensory awareness at varying train durations (TDs) was determined using 500 ms pulses at 60 Hz. When TD increased above a critical value, there was no further decline in threshold intensity. This TD coincides with the 0.5 set utilization TD reported by Libet. We also stimulated the hand 20% above, at, and 20% below sensory perceptual threshold and recorded evoked potentials from the grid. Discrete, well localized responses were seen at and above threshold in both patients, but were not clear below threshold in one patient. These preliminary results support Libet’s findings of TD effects although conscious perception was clearly evident at higher voltage intensities with brief TDs (100 ms or less) without after-discharges or motor responses. The results partially support the presence of evoked responses to subthreshold stimuli. B201:
Magnetic coil suppression of visual perception has been repeatedly demonstrated in the central field; this interference has generally been assumed to occur at the level of the calcarine cortex. With disappearance targets, peripheral vision can also be suppressed by magnetic stimulation over the occipital lobes. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold 2.5 cm lateral to the midline, and then a level of stimulator output that produced 50% error rates for each coil. Intersection of the electric field profiles in air and in a saline model head was similar, and identified superficial occipital cortex rather than the primary visual area as the site of visual suppression. A less direct analysis involving the spatial extent of visual suppression produced the same conclusion. These results suggest that magnetic stimulation can help elucidate human visual physiology. B105:
Physiology of perception: phosphenes - K.J. Meador, (Department of Neurology, Augusta, Georgia 30912)
parameters of TMS-induced P.G. Ray and D.W. Loring Medical College of Georgia,
Transcranial magnetic stimulation (TMS) has been used to investigate the physiology of perception by examining TMS blockade of peripheral stimuli. However, the physiological parameters of TMSinduced positive perceptual phenomena have not been systematically examined in the sensory cortex. The present study investigated the parameters of TMS necessary to produce the conscious perception of visual phosphenes in 3 healthy adults (3845 years old) using a Caldwell High Speed Magnetic Stimulator. One second trains of TMS stimuli over Oz. 01 or 02 produced flickering phosphenes in all 3 subjects at rates of 10 Hz (85%-95% stimulator power output), 20 Hz (65-85%), and 40 Hz (60-90%), but, in only one subject at 5 Hz (85%). The phosphenes were described as flickering bars of light in the lower visual field, which differed individually in orientation and color. The stimulus trains were progressively decreased in duration, and phosphenes were still present with pairs of TMS stimuli (80-100% output), but not with single stimuli even at maximum output. The interstimulus interval for the pulse pairs had to be between 20-150 ms in order to produce phosphene. The results suggest that this technique may be useful in investigating the mechanisms underlying conscious perception. B106:
Conscious perception and the somatosensory cortex - P.G. Ray”, K.J. Meador” and J.R. Smithb (“Department of Neurology and bDepartment of Surgery (Neurosurgery), Medical College of Georgia, Augusta, GA 30912)
Beginning
in the 1960’s. Libet and co-workers
examined
the suffi-
in neonates with electricallyPlacental abnormalities confirmed seizures - Mark S. Scher, Guilana S. Trucco, Marquita E. Beggarly, Doris A. Steppe and Trevor A. Macpherson (Developmental Neurophysiology Laboratory, Department of Pediatrics, and Department of Pathology, Magee-Womens Hospital; Department of Pediatrics, Division of Neurology, Children’s Hospital of Pittsburgh, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213)
Placental specimens were reviewed from 76 singleton pregnancies of women whose offspring received neonatal EEG studies. 43 neonates with electrically confirmed seizures (23-43 weeks) were compared to 33 healthy preterm and fullterm infants of comparable maturity without electrographic seizures. Placental specimens and EEGs were reviewed without knowledge of the presence or absence of seizures or placental findings, respectively. Acute lesions (i.e., c72 hours in duration) consisted of chorioamnionitis, edema, meconium staining, and retroplacental hematoma. Chronic lesions (i.e., >72 hours) consisted of dysmaturity, infarction, or villitus. Logistic regression analyses calculated the odds ratio of having acute or chronic placental lesions in each group as a function of increasing postconceptional age. For the seizure group, the odds of having chronic placental lesions increased by a factor of 1.2 for each postconceptional week. Over a 15 week interval, the odds of having chronic lesions for the seizure group increased by a factor of 12.1 (P value c.003). Ratios were not significant for acute lesions in the seizure group, or for either chronic or acute lesions in the group without seizures. As reflected by placental pathological findings, chronic rather than acute pathophysiological events in utero are significantly associated with electrically-confirmed seizures in near term and term infant. Supported by NS 1110, NS26793, NS26946, RR00084. B202: EEG features of pediatric syndromes that present with serial spasms - Jane F. Donat (Department of Pediatrics (Neurology), Children’s Hospital, University of Washington School of Medicine, Seattle, WA 98105) Serial spasms (SS), once thought to occur exclusively as ‘infantile’ spasms in West syndrome, are now known to sometimes occur as ‘juvenile’ spasms in Lennox-Gastaut syndrome and sporadically in other children. This report outlines the EEG features of a large series of SS patients (I month - I8 years). SS were documented by video-EEG in 192 patients (mean age 9 months; 25% 22 years old). Hypsarrhythmia occurred in 61%. and was found only <5 years old. Generalized slow spike-wave (SW) occurred in 16% and was not found <7 months old. Multifocal SW occurred in 20%. at all ages. Other interictal EEGs occurred in 6%: 3 patients 1 year old had generalized atypical SW; 1 patient < I8 months old had no interictal SW. Hypsarrhythmia coexisted with slow SW in 4
12P
occipital lobes to study peripheral visual field effects in 6 subjects. The visual test pattern was a grid of asterisks around a central fixation point, and the target was the disappearance of one asterisk for a single frame of the video monitor. Using single magnetic pulses at stimulator outputs of 55-95%. robust interference effects were found at latencies of 37-87 ms, peaking around 50-75 ms. Suppression of visual perception occurred with both transverse and sagittal alignments of the stimulator. When the stimulator was moved laterally over either occipital lobe, perception of target disappearance was consistently suppressed in the contralateral visual field. Movement of the stimulator rostrally produced consistent suppression in the lower and middle field, but preferential suppression of the upper field could not be obtained. This altitudinal asymmetry is correlated with the anatomy of the occipital lobe in relation to the scalp surface. B104:
Magnetic coil stimulation suppresses peripheral visual perception at an extra-calcarine site - Charles M. Epstein and Reinaldo Verson (Department of Neurology, Emory U. School of Medicine, Atlanta, GA 30322)
ciency of stimulus parameters in evoking conscious awareness during direct electrical stimulation of the human brain. Libet’s results have never been independently verified. We examined direct cortical stimulation in 2 epilepsy patients with grids over the postcentral gyrus. Stimuli were applied to a single pair of contacts which elicited sensation in the contralateral hand. The current needed to produce sensory awareness at varying train durations (TDs) was determined using 500 ms pulses at 60 Hz. When TD increased above a critical value, there was no further decline in threshold intensity. This TD coincides with the 0.5 set utilization TD reported by Libet. We also stimulated the hand 20% above, at, and 20% below sensory perceptual threshold and recorded evoked potentials from the grid. Discrete, well localized responses were seen at and above threshold in both patients, but were not clear below threshold in one patient. These preliminary results support Libet’s findings of TD effects although conscious perception was clearly evident at higher voltage intensities with brief TDs (100 ms or less) without after-discharges or motor responses. The results partially support the presence of evoked responses to subthreshold stimuli. B201:
Magnetic coil suppression of visual perception has been repeatedly demonstrated in the central field; this interference has generally been assumed to occur at the level of the calcarine cortex. With disappearance targets, peripheral vision can also be suppressed by magnetic stimulation over the occipital lobes. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold 2.5 cm lateral to the midline, and then a level of stimulator output that produced 50% error rates for each coil. Intersection of the electric field profiles in air and in a saline model head was similar, and identified superficial occipital cortex rather than the primary visual area as the site of visual suppression. A less direct analysis involving the spatial extent of visual suppression produced the same conclusion. These results suggest that magnetic stimulation can help elucidate human visual physiology. B105:
Physiology of perception: phosphenes - K.J. Meador, (Department of Neurology, Augusta, Georgia 30912)
parameters of TMS-induced P.G. Ray and D.W. Loring Medical College of Georgia,
Transcranial magnetic stimulation (TMS) has been used to investigate the physiology of perception by examining TMS blockade of peripheral stimuli. However, the physiological parameters of TMSinduced positive perceptual phenomena have not been systematically examined in the sensory cortex. The present study investigated the parameters of TMS necessary to produce the conscious perception of visual phosphenes in 3 healthy adults (3845 years old) using a Caldwell High Speed Magnetic Stimulator. One second trains of TMS stimuli over Oz. 01 or 02 produced flickering phosphenes in all 3 subjects at rates of 10 Hz (85%-95% stimulator power output), 20 Hz (65-85%), and 40 Hz (60-90%), but, in only one subject at 5 Hz (85%). The phosphenes were described as flickering bars of light in the lower visual field, which differed individually in orientation and color. The stimulus trains were progressively decreased in duration, and phosphenes were still present with pairs of TMS stimuli (80-100% output), but not with single stimuli even at maximum output. The interstimulus interval for the pulse pairs had to be between 20-150 ms in order to produce phosphene. The results suggest that this technique may be useful in investigating the mechanisms underlying conscious perception. B106:
Conscious perception and the somatosensory cortex - P.G. Ray”, K.J. Meador” and J.R. Smithb (“Department of Neurology and bDepartment of Surgery (Neurosurgery), Medical College of Georgia, Augusta, GA 30912)
Beginning
in the 1960’s. Libet and co-workers
examined
the suffi-
in neonates with electricallyPlacental abnormalities confirmed seizures - Mark S. Scher, Guilana S. Trucco, Marquita E. Beggarly, Doris A. Steppe and Trevor A. Macpherson (Developmental Neurophysiology Laboratory, Department of Pediatrics, and Department of Pathology, Magee-Womens Hospital; Department of Pediatrics, Division of Neurology, Children’s Hospital of Pittsburgh, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213)
Placental specimens were reviewed from 76 singleton pregnancies of women whose offspring received neonatal EEG studies. 43 neonates with electrically confirmed seizures (23-43 weeks) were compared to 33 healthy preterm and fullterm infants of comparable maturity without electrographic seizures. Placental specimens and EEGs were reviewed without knowledge of the presence or absence of seizures or placental findings, respectively. Acute lesions (i.e., c72 hours in duration) consisted of chorioamnionitis, edema, meconium staining, and retroplacental hematoma. Chronic lesions (i.e., >72 hours) consisted of dysmaturity, infarction, or villitus. Logistic regression analyses calculated the odds ratio of having acute or chronic placental lesions in each group as a function of increasing postconceptional age. For the seizure group, the odds of having chronic placental lesions increased by a factor of 1.2 for each postconceptional week. Over a 15 week interval, the odds of having chronic lesions for the seizure group increased by a factor of 12.1 (P value c.003). Ratios were not significant for acute lesions in the seizure group, or for either chronic or acute lesions in the group without seizures. As reflected by placental pathological findings, chronic rather than acute pathophysiological events in utero are significantly associated with electrically-confirmed seizures in near term and term infant. Supported by NS 1110, NS26793, NS26946, RR00084. B202: EEG features of pediatric syndromes that present with serial spasms - Jane F. Donat (Department of Pediatrics (Neurology), Children’s Hospital, University of Washington School of Medicine, Seattle, WA 98105) Serial spasms (SS), once thought to occur exclusively as ‘infantile’ spasms in West syndrome, are now known to sometimes occur as ‘juvenile’ spasms in Lennox-Gastaut syndrome and sporadically in other children. This report outlines the EEG features of a large series of SS patients (I month - I8 years). SS were documented by video-EEG in 192 patients (mean age 9 months; 25% 22 years old). Hypsarrhythmia occurred in 61%. and was found only <5 years old. Generalized slow spike-wave (SW) occurred in 16% and was not found <7 months old. Multifocal SW occurred in 20%. at all ages. Other interictal EEGs occurred in 6%: 3 patients 1 year old had generalized atypical SW; 1 patient < I8 months old had no interictal SW. Hypsarrhythmia coexisted with slow SW in 4