Joan B. Cracco

Suppression of visual perception by magnetic coil stimulation of human occipital cortex

Magnetic coil (MC) stimulation percutaneously of human occipital cortex was tested on perception of 3 briefly presented, randomly generated alphabetical characters. When the visual stimulus-MC pulse interval was less than 40-60 msec, or more than 120-140 msec, letters were correctly reported; at test intervals of 80-100 msec, a blur or nothing was seen. Shifting the MC location in the transverse and rostro-caudal axes had effects consistent with the topographical representation in visual cortex, but incompatible with an effect on attention or suppression from an eyeblink. The MC pulse probably acts by eliciting IPSPs in visual cortex. The neural activity subserving letter recognition is probably transmitted from visual cortex within 140 msec of the visual stimulus.

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation

Human transcallosal responses (TCRs) were elicited by focal magnetic coil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8-12.2 msec, a duration of 7-15 msec, and an amplitude which reached up to 20 microV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.

TRANSCRANIAL MAGNETIC STIMULATION IN STUDY OF THE VISUAL PATHWAY

The authors critically reviewed experiments in which transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS) of the higher visual pathway were used. Topics include basic mechanisms of neural excitation by TMS and their relevance to the visual pathway (excitatory and inhibitory effects), TMS and rTMS of calcarine cortex (suppression, unmasking, and phosphenes), TMS of V5 (suppression), TMS and rTMS of higher level temporoparietooccipital areas (perceptual errors, unmasking, and inattention), the role of frontal lobe output in visual perception, and vocalization of perceived visual stimuli (role of consciousness of linguistic symbols).

Measurement of information processing delays in human visual cortex with repetitive magnetic coil stimulation.

Previous work disclosed that single magnetic coil (MC) pulses applied over human calcarine cortex could suppress perception of letters briefly presented, e.g. 80-100 ms earlier. 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, nor or random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.

The spinal evoked response in infants and children

Summated responses to peroneal nerve stimulation were recorded from surface electrodes placed over the spine of 60 infants and children. These potentials generally were greater in amplitude in infants than in older children. Over the cauda equina and rostral cord, initially positive triphasic potentials were recorded. Over the caudal cord, complex potentials were recorded in children less than three years of age. The conduction velocity of the response from midlumbar to lower cervical recording sites was less in infants than in older children and progressively increased with age, reaching adult values after the fourth year.

Intracranial stimulation of facial nerve in humans with the magnetic coil

Using ourselves as subjects, maximal compound motor action potentials (CMAPs) were evoked in ipsilateral nasal and orbicularis oculi muscles (onset latency 4.9-5.4 msec) by a magnetic coil (MC) tangentially oriented over parieto-occipital scalp. The facial nerve was also electrically stimulated sequentially at the posterior tragus near the stylomastoid foramen, anterior tragus and 3 cm more distally. Onset latency of the CMAP elicited at posterior tragus ranged from 1.0 to 1.3 msec less than that elicited by the MC over scalp. Because the measured distal facial nerve motor conduction velocity was 50-60 m/sec, the locus of impulse generation induced by magnetic coil stimulation was estimated to be approximately 6.5 cm proximal to the site of electrical stimulation at the posterior tragus, i.e., closer to the exit of the facial nerve from the brain-stem than to its entrance into the internal auditory meatus. This non-invasive technique should be useful in evaluating patients with peripheral facial nerve disorders including Bells palsy.

Cerebral function revealed by transcranial magnetic stimulation

Although transcranial magnetic stimulation (TMS) has been introduced only recently, it is safe and provides a painless, inexpensive noninvasive method for the evaluation of brain function. Determining central motor conduction time (CMCT) permits assessment of the corticospinal pathways. Mapping the central representation of muscles provides a method for investigating the cortical reorganization that follows training, amputation and injury to the central nervous system. Such studies of human plasticity may have important implications for neurorehabilitation. TMS also provides a method whereby cortical excitability can be noninvasively evaluated, which is likely to have important implications in the study of epilepsy, movement disorders and related conditions. TMS is useful in tracking the flow of information from one brain region to another and in investigations of cognition and functional localization, thereby complementing information obtained using functional imaging techniques, which have superior spatial but inferior temporal resolution. Finally, TMS is currently being investigated as a method for establishing cerebral dominance and as a therapeutic tool in the treatment of depression. Investigations for treatment of other neurologic and psychiatric conditions are likely to be undertaken.

Comparison of scalp distribution of short latency somatosensory evoked potentials (SSEPs) to stimulation of different nerves in the lower extremity

SSEPs to stimulation of the CPN at the knee and PTN, PN and SN at the ankle were recorded from 15 cephalic sites and compared in 8 normal subjects. The configuration, amplitude, peak latency and distribution of P27, N35 (CPN) and P37, N45 (PTN, PN and SN) were analyzed. The configuration and distribution of SSEPs to stimulation of the 3 nerves at the ankle were similar across subjects. Both P37 and N45 were greatest in amplitude at the vertex and at recording sites ipsilateral to the side of stimulation. At contralateral sites either negative (N37) or negative, positive, negative potentials were recorded. The peak latency of N37 was the same or slightly less than that of P37. CPN-SSEPs were lower in amplitude and their configuration and scalp distribution showed much greater intersubject variability. This suggests that complex mechanisms which variably interact with one another are reflected in scalp SSEPs to CPN stimulation at the knee. The larger amplitude plus the minimal intersubject variability in morphology and topography of PTN-SSEPs indicate that this nerve is the most suitable for routine clinical use.

Stimulation of the human nervous system using the magnetic coil.

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) pertubation 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.

Intrauterine subdural hemorrhage

Subdural hemorrhage (SDH) detected postnatally in the newborn infant is usually related to trauma at the time of vaginal delivery. With improvement in obstetric methods, the incidence of this problem has declined. However, with advances in obstetric ultrasonography, it has been recognized that SDH may occur in utero before the onset of delivery. Intrauterine SDH remains a rare event. A review of the literature revealed only 32 cases to date (Tables I and II). Neonatal SDH is venous in origin and associated with trauma in most cases. Etiopathogenesis of neonatal SDH is explained by shearing of bridging veins or other venous structures caused by trauma. Instrumental delivery (forceps or vacuum extraction) causes distortion of the infants cranium with elongation of the falx and angulation of the tentorium, leading to tearing of posterior fossa venous structures and bleeding into the subdural space. Rupture or tearing of large venous channels, such as the great vein of Galen, straight or transverse sinus or falx laceration with rupture of the sagittal sinus, result in accumulation of blood in both the supratentorial and infratentorial compartments. However, etiopathogenesis of intrauterine SDH is less certain. The fetus is usually well protected from direct abdominal trauma by maternal structures and amniotic fluid. Fetal intracranial vessels may be susceptible to shearing or acceleration/deceleration forces because of the following features: first, the head is large and neck muscles weak; this allows more rotational movement with angular acceleration. Second, the subarachnoid space is larger, allowing the brain to move within the cranium easily. Third, the fetal brain has a higher water content that increases its mass and allows it to develop more momentum when acceleration is applied. These unique conditions could make the fetus more susceptible to developing subdural hematomas and allow the development of subdural hematomas without obvious trauma.