C. W. Hess

Transcranial stimulation of the human frontal eye field by magnetic pulses

SummarySingle transcranial magnetic pulsed stimuli were applied over the cortical area of the putative right frontal eye field (FEF) in 11 healthy subjects. An especially designed figure of eight shaped twin coil was used, to focus the stimulus, the strength of which was adjusted to the individual motor threshold of the left hand muscles. Eye positions and movements were recorded by an infrared reflection technique. Three different experiments were performed: 1. Stimulation during different primary gaze position did not evoke any discernible eye movement. 2. Stimulation just prior to visually elicited horizontal saccades did not cause a significant alteration of the latency, velocity, or amplitude of the saccades. 3. Only stimulation during an antisaccade task induced a significant latency prolongation, when the stimulus was applied between 50 to 90 ms after the target flashed up. This latency prolongation was found in all subjects for the antisaccades to the right, with a statistically significant average latency difference of +66 ±55.5 ms. In contrast, the antisaccades to the left were prolonged in the female subjects only by an average of + 98 ± 41.8 ms (p=0.0064), whereas in the male subjects they did not alter with stimulation (average difference: -3 ±41.9ms, p=0.753). Significant latency prolongations were only obtained when the magnetic FEF stimuli were applied within a vulnerable period, which varied from subject to subject.

Time course of blood oxygenation level-dependent signal response after theta burst transcranial magnetic stimulation of the frontal eye field

The aim of the current study was to examine the effect of theta burst repetitive transcranial magnetic stimulation (rTMS) on the blood oxygenation level-dependent (BOLD) activation during repeated functional magnetic resonance imaging (fMRI) measurements. Theta burst rTMS was applied over the right frontal eye field in seven healthy subjects. Subsequently, repeated fMRI measurements were performed during a saccade-fixation task (block design) 5, 20, 35, and 60 min after stimulation. We found that theta burst rTMS induced a strong and long-lasting decrease of the BOLD signal response of the stimulated frontal eye field at 20 and 35 min. Furthermore, less pronounced alterations of the BOLD signal response with different dynamics were found for remote oculomotor areas such as the left frontal eye field, the pre-supplementary eye field, the supplementary eye field, and both parietal eye fields. Recovery of the BOLD signal changes in the anterior remote areas started earlier than in the posterior remote areas. These results show that a) the major inhibitory impact of theta burst rTMS occurs directly in the stimulated area itself, and that b) a lower effect on remote, oculomotor areas can be induced.

Isolated mediotegmental lesion causing narcolepsy and rapid eye movement sleep behaviour disorder: a case evidencing a common pathway in narcolepsy and rapid eye movement sleep behaviour disorder

Narcolepsy is usually an idiopathic disorder, often with a genetic predisposition. Symptomatic cases have been described repeatedly, often as a consequence of hypothalamic lesions. Conversely, REM (rapid eye movement) sleep behaviour disorder (RBD) is usually a secondary disorder, often due to degenerative brain stem disorders or narcolepsy. The case of a hitherto healthy man is presented, who simultaneously developed narcolepsy and RBD as the result of an acute focal inflammatory lesion in the dorsomedial pontine tegmentum in the presence of normal cerebrospinal fluid hypocretin-1 levels and in the absence of human lymphocyte antigen haplotypes typically associated with narcolepsy and RBD (DQB1*0602, DQB1*05). This first observation of symptomatic narcolepsy with RBD underlines the importance of the mediotegmental pontine area in the pathophysiology of both disorders, even in the absence of a detectable hypocretin deficiency and a genetic predisposition.

Visual evoked potentials in acute occipital blindness

SummaryPattern-evoked and flash-evoked responses were recorded from four patients with acute blindness due to occipital ischaemia. No responses could be obtained with pattern stimulation. Flash stimulation, however, yielded well-reproducible potentials in all four cases in spite of practically complete visual loss. In one patient vision recovered completely while the three others showed poor or minimal recovery in restricted visual field areas only. It is concluded that the flash method is not appropriate for differentiation of occipital blindness from psychogenic visual disorders. Furthermore, preserved flashevoked potentials in the acute stage of occipital blindness cannot be taken as a reliable prognostic sign for visual recovery, as has been postulated by others. A possible extrastriate origin of these flash responses is discussed in view of recent theories on a second visual system.ZusammenfassungVon vier Patienten mit akuter Erblindung infolge bilateraler occipitaler Ischämie wurden visuell evozierte Potentiale mit Schachbrettmuster-und Blitz-Stimulation abgeleitet. Während sich bei der Musterstimulation keine verwertbaren Potentiale ableiten ließen, ergab der Blitzreiz in allen vier Fällen eindeutig reproduzierte Antworten trotz praktisch vollständiger Blindheit. Bei einer Patientin bildete sich der Sehverlust vollständig zurück, die drei anderen zeigten nur geringe oder minimale Erholung in umschriebenen Gesichtsfeldbezirken. Aufgrund dieser Beobachtung scheint die Blitzmethode zur Abgrenzung kortikaler Blindheit von Psychogenen Sehstörungen ungeeignet. Zudem sprechen die Befunde gegen die von einzelnen Autoren vertretene Ansicht, daß erhaltene blitzevozierte Potentiale bei akut kortikal Erblindeten eine gute Prognose für die Erholung des Sehens erwarten lasse. Die Frage des Entstehungsortes von Blitz-VEP bei kortikaler Blindheit wird im Lichte neuerer Erkenntnisse über das visuelle System erörtert und eine mögliche extrastriäre Entstehung diskutiert.

Theta burst transcranial magnetic stimulation is associated with increased EEG synchronization in the stimulated relative to unstimulated cerebral hemisphere

Theta burst transcranial magnetic stimulation (TBS) may induce behavioural changes that outlast the stimulation period. The neurophysiological basis of these behavioural changes are currently under investigation. Given the evidence that cortical information processing relies on transient synchronization and desynchronization of neuronal assemblies, we set out to test whether TBS is associated with changes of neuronal synchronization as assessed by surface EEG. In four healthy subjects one TBS train of 600 pulses (200 bursts, each burst consisting of 3 pulses at 30 Hz, repeated at intervals of 100 ms) was applied over the right frontal eye field and EEG synchronization was assessed in a time-resolved manner over 60 min by using a non-overlapping moving window. For each time step the linear cross-correlation matrix for six EEG channels of the right and for the six homotopic EEG channels of the left hemisphere were computed and their largest eigenvalues used to assess changes of synchronization. Synchronization was computed for broadband EEG and for the delta, theta, alpha, beta and gamma frequency bands. In all subjects EEG synchronization of the stimulated hemisphere was significantly and persistently increased relative to EEG synchronization of the unstimulated hemisphere. This effect occurred immediately after TBS for the theta, alpha, beta and gamma frequency bands and 10-20 min after TBS for broadband and delta frequency band EEG. Our results demonstrate that TBS is associated with increased neuronal synchronization of the cerebral hemisphere ipsilateral to the stimulation site relative to the unstimulated hemisphere. We speculate that enhanced synchronization interferes with cortical information processing and thus may be a neurophysiological correlate of the impaired behavioural performance detected previously.

Horizontal and vertical dimensions of visual extinction: a theta burst stimulation study

After a lesion of the posterior parietal cortex (PPC), the perception of a contra-lesional stimulus in presence of a simultaneous, ipsilesional stimulus may be impaired, a phenomenon referred to as visual extinction. In the present study, visual extinction was transiently induced in healthy subjects by interfering with the function of the right PPC by means of continuous theta burst stimulation (TBS). We investigated to which extent the horizontal and vertical position of visual stimuli influenced the extinction rate. A single TBS train over the right PPC induced a significant increase of left visual extinctions of at least 30 min. Left visual extinction rate was higher when the left sided visual stimulus was presented at a more eccentric position on the horizontal axis (irrespective of right sided visual stimulus position) and in the lower part of the visual field. The results are discussed within the framework of current explanatory models and of putative inter- and intrahemispheric mechanisms directing visuospatial attention.

Mapping of direction and muscle representation in the human primary motor cortex controlling thumb movements.

Larger body parts are somatotopically represented in the primary motor cortex (M1), while smaller body parts, such as the fingers, have partially overlapping representations. The principles that govern the overlapping organization of M1 remain unclear. We used transcranial magnetic stimulation (TMS) to examine the cortical encoding of thumb movements in M1 of healthy humans. We performed M1 mapping of the probability of inducing a thumb movement in a particular direction and used low intensity TMS to disturb a voluntary thumb movement in the same direction during a reaction time task. With both techniques we found spatially segregated representations of the direction of TMS‐induced thumb movements, thumb flexion and extension being best separated. Furthermore, the cortical regions corresponding to activation of a thumb muscle differ, depending on whether the muscle functions as agonist or as antagonist for flexion or extension. In addition, we found in the reaction time experiment that the direction of a movement is processed in M1 before the muscles participating in it are activated. It thus appears that one of the organizing principles for the human corticospinal motor system is based on a spatially segregated representation of movement directions and that the representation of individual somatic structures, such as the hand muscles, overlap.

Loss of exploratory vertical saccades after unilateral frontal eye field damage

Despite their relevance for locomotion and social interaction in everyday situations, little is known about the cortical control of vertical saccades in humans. Results from microstimulation studies indicate that both frontal eye fields (FEFs) contribute to these eye movements. Here, we present a patient with a damaged right FEF, who hardly made vertical saccades during visual exploration. This finding suggests that, for the cortical control of exploratory vertical saccades, integrity of both FEFs is indeed important.

Reversible callosal disconnection syndrome in internal hydrocephalus

A 74 year old woman was referred to the neurological department for evaluation of progressive gait disturbance. On admission she complained of alienation and loss of control of her left arm for six months. There were no spontaneous movements without the patients intention, but she had always to rely on visual cues. For example, when she was cooking, eating, or doing exercises with her home trainer she had to watch her left arm to be sure of its movement. On examination she was alert, fully oriented, and cooperative. Snout and palmomental reflexes were positive. There was no visual, somatosensory, or auditory extinction. Motor examination revealed a mild left sided facial droop and a left sided pronator drift but strength was full and symmetric and there was no lack of spontaneous movement in the left upper limb. The plantar response was extensor on the left. Gait was slow, unsteady, and wide based. The steps were short with reduced step height. Neuropsychological assessment showed fluent speech without dysarthria. Comprehension and reading were intact. Performance in verbal and non-verbal fluency tasks was diminished, and colour-word interference was slightly increased. Long term memory was slightly deficient for verbal and non-verbal material. Visuo-constructive abilities were normal and there was no …