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At least 289 records · Page 16Linked to original sources
PubMed2004

Excitability changes induced in the human primary visual cortex by transcranial direct current stimulation: direct electrophysiological evidence.

PURPOSE: Transcranial direct current stimulation (tDCS) has been shown to modify the perception threshold of phosphenes elicited by transcranial magnetic stimulation (TMS). The current study was undertaken to examine whether tDCS, when applied over the occipital cortex, is also able to affect visual-evoked potentials (VEPs), which characterize occipital activation in response to visual stimulation, in a polarity-specific way. METHOD: For this purpose, VEPs evoked by sinusoidal luminance grating in an on/off mode were recorded before, immediately after, and 10, 20, and 30 minutes after the end of 5, 10, or 15 minutes of anodal or cathodal tDCS of the primary visual cortex. RESULTS: Significant effects were observed only when low-contrast visual stimuli were applied. Cathodal stimulation decreased, whereas anodal stimulation increased the amplitude of the N70 component. The effect of cathodal stimulation was significant immediately after and 10 minutes after the end of stimulation, if the stimulation duration was sufficiently long (i.e., 10-15 minutes). An increase of N70 amplitude by anodal stimulation was significant only 10 minutes after the end of the 15 minutes tDCS. Cathodal stimulation tended also to affect the amplitude of the P100 component; however, the effect of stimulation was inverse. The amplitude increased immediately after the end of cathodal stimulation. In contrast, anodal stimulation did not affect the P100. The latencies of the N70 and the P100 were not affected by tDCS. CONCLUSIONS: tDCS appears to be a suitable method of inducing reversible excitability changes in a polarity-specific way, not only in the motor but also in the primary visual cortex. The duration of the induced aftereffects depends not only on stimulation duration but also on stimulation polarity. Cathodal stimulation seems to be more effective, in line with previous reports on the motor cortex.

Adult↗
PubMed1997

Hemianopic anosognosia.

In a prospective study of 32 consecutive patients with homonymous visual field defects due to ischemic infarcts we found hemianopic anosognosia (HAN), defined as the unawareness of visual loss in the homonymous hemifield (or hemiquadrant), in 20 patients (62%). HAN, although occurring predominantly in right-side lesions in 16 of 26 patients (62%) was also present in four of six patients (or 67%) with left-side lesions. This group of patients has been presented in a prior report on positive spontaneous visual phenomena. HAN was associated with somatic anosognosia in nine patients and hemineglect in 17 patients. Dissociation between somatic and hemianopic anosognosia, as well as between hemineglect and HAN, was present in several patients, indicating that these phenomena may be independent of each other. Eight patients had pure homonymous hemianopia; that is, hemianopia without cognitive, motor, or somatosensory deficits. Four of these patients (Group A) had awareness of the visual deficit, whereas three patients (Group B) had HAN. Patients in these two groups had similar anatomic lesions. Patients with phosphenes, photopsias, or visual hallucinations were usually aware of their visual field loss. We suggest that HAN is most often related to failure of discovery of the deficits, occasionally to severe visual hemineglect, sometimes to generalized cognitive impairment, or to a combination of these factors. We further conclude (1) there is no specific cortical area for conscious visual perception; (2) visual awareness is processed by a distributed network including multiple visual cortices, parietal and frontal lobes, the pulvinar, and lateral geniculate bodies (lesions localized at various nodes or centers in the network may produce similar phenomena); and (3) both hemispheres are involved in visual processing and conscious awareness.

Aged↗
PubMed2001

Suppression of perception in migraine: evidence for reduced inhibition in the visual cortex.

BACKGROUND: Results from transcranial magnetic stimulation (TMS) studies of visual cortex have confirmed visual cortical hyperexcitability in patients with migraine. It has been speculated that this may be due to deficient intracortical inhibitory tone. However, the TMS induction of phosphenes relies on the reporting of a subjective experience, and may thus be subject to bias. METHODS: Seven migraineurs with visual aura and seven sex- and age-matched controls were studied. Fifty-four different three-letter combinations were briefly displayed and followed by a magnetic pulse at 40, 70, 100, 130, 160, and 190 msec. Subjects were required to report as many letters as they thought they had recognized. RESULTS: In the migraine group, the mean proportion of correctly identified letters was significantly higher at 100 msec, as was the proportion of trials with two or three letters correctly reported. The time window in which perceptual suppression could be introduced was narrower in migraineurs compared to controls. CONCLUSION: These findings suggest that inhibitory systems are activated to a lesser extent by TMS pulses in patients. This observation is in agreement with the hypothesized deficiency of intracortical inhibition of the visual cortex, at least in migraineurs with aura.

Adolescent↗
PubMed1983

Principal component analysis of Motokawa's data on wavelength dependence of retinal processes.

Electric excitability of the human eye as determined by measuring the threshold for a sensation of phosphene in response to electric stimulation of the eye was found by Koiti Motokawa to increase temporarily after a brief illumination (J. Neurophysiol., 1949, 112, 475-488). While changing the wavelength of illuminating light widely, he found that the time course of the variation in the eye's electric excitability after illumination differed characteristically according to the wavelength. His data on this point (Tohoku J. exp. Med., 1949, 51, 197-205) were subjected to the principal component analysis. Three components were found necessary and sufficient for their linear combinations to reproduce time courses of the excitability enhancement after illumination with lights of varying wavelengths; one of the three components makes a great contribution to the excitability enhancement by green lights, the other to the one by red lights and the remainder to the one by blue lights. This is in support of Motokawa's view that his data are interpretable as summation effects of the three retinal processes which are excited preferentially by red, green and blue lights, respectively.

Electric Stimulation↗
PubMedDate not supplied

[Transcranial magnetic stimulation. Clinical trials in psychiatry: therapeutical use].

Transcranial magnetic stimulation is the noninvasive application of localized pulsed magnetic field to the surface of the skull, to cause a depolarization of neurons in the underlying cerebral cortex (Daryl E., Bohning PH.D.). Based on Reciprocal Induction (Faraday, 1831), and the Ampere Maxwell Law, according to which electric energy is associated with magnetic energy and vice versa, transcranial magnetic stimulation has been used during the last fifteen years in the diagnosis of Central Nervous System dysfunctions, its safeness and good tolerance having been proven. Since 1876, when Dàrsonval discovered that the use of a similar apparatus caused vertigo, phosphenes and fainting, thousands of transcranial magnetic stimulation studies have been carried out in the fields of Neurology and Psychiatry. The present is a review of clinical studies carried out in Psychiatry, specifically related to Mood Disorders, Obsessive-Compulsive Disorder and Post traumatic-Stress Syndrome.

Clinical Trials as Topic↗
PubMed2002

A neurophysiological approach to effects induced by accelerated particles on the central nervous system in mice.

This experiment was designed to study mechanisms of "phosphene vision". A beam of accelerated carbon particles (energy 100 and 300 MeV/n) was delivered into retina and sensorimotor cortex of anaesthetised mice bearing chronic electrodes. Cycles of light flashes were also administered. Following accelerated particles stimulation cycles to the retina, 500 to 5000 per stimulus, 30 stimuli per cycle, stimulus time-related brain waveforms in mice cortex were recorded. Their amplitude and latency appeared to be related to the energy delivered. The short waveforms latency (approximately 30 ms), and the lack of retinic potentials and oscillatory potentials following particles stimuli suggest that the impact of accelerated particles does not produce activation of retinic chemical structures.

Animals↗
PubMed2004

[Factors related to the diagnosis of HELLP syndrome in patients with severe preeclampsia].

BACKGROUND: Each year, around 50,000 women die from preeclampsia-eclampsia worldwide. Thus, hypertensive disorders during pregnancy are public health problems in both developed and developing countries. OBJECTIVE: To identify prognosis factors associated with HELLP syndrome in patients with severe preeclampsia. MATERIAL AND METHODS: A retrospective, observational, cross-sectional, and analytical study was carried out. It included patients that suffered from severe preeclampsia, with and without HELLP syndrome. They were hospitalized at the Division of Obstetrics Dr. Manuel Gea González General Hospital, from January 1st, 1995 to January 1st, 2000 (study group). Only clinical files of patients with severe preeclampsia, without convulsions, HELLP syndrome, or who had not died during the days spent at the hospital were included in the control group; within at least 72 subsequent hours to the pregnancy termination. The connection of HELLP syndrome with the following variables was assessed in the control group: gestational age, maternal age, infant formula, prenatal control, hypertensive disorder history, headache, tinnitus, phosphen, nausea, vomiting, epigastric pain, edema, hyperreflexia, blood pressure values, hepatic biometry, platelet count, blood chemistry with hepatic function. RESULTS: Right upper quadrant or epigastric pain was the most important independent prognosis factor. There were significant differences in the admission laboratory values between those with HELLP syndrome and those without acute complications of preeclampsia. CONCLUSIONS: Although the contribution of right upper quadrant or epigastric pain to the risk status of a pregnant patient is difficult to quantify, it can be used to assess whether the patient is at high risk for development of HELLP syndrome.

Adult↗
PubMed2006

[New drugs; ivabradine].

Ivabradine lowers the heart rate by inhibiting the cardiac pacemaker current that is responsible for diastolic depolarisation. The most important side effect is the appearance of light flashes (phosphenes). The agent can be used in patients with angina pectoris when beta-blockers are insufficiently effective, provided that the heart rate is at least 60 beats per minute at the beginning of the treatment.

Benzazepines↗
PubMed1990

[Predicting visual acuity in media opacities and uncorrectable refractive errors. Assessing so-called "retinal visual acuity"].

Three different components contribute to the modulation transfer function of the visual system: (1) formation of the optical image (refractive media, pupil); (2) scattering of light in the prereceptoral layers of the retina; (3) neuronal processing in the retina und superior visual centers. In the presence of media opacities or non-correctable refractive errors, the clinical question often arises as to which macular function can be expected under the assumption of normal optical image formation (e.g. prior to cataract extraction, corneal transplantation, or vitrectomy). Simple tests such as light projection, color discrimination, and two-point discrimination cannot provide adequate information about macular function. The same holds true for the global luminance ERG. The X-ray phosphene is obsolete. The Maddox rod (with limitations), transilluminated Amsler grid, and various entoptic phenomena (Purkinje vascular phenomenon, foveal chagrin, Haidinger's brushes, blue field phenomenon) are available as qualitative subjective tests. Maxwellian view systems with pinhole aperture (potential acuity meter PAM) and the interferometers (retinometer, visometer, SITE-IRAS interferometer) provide quantitative subjective methods. The flash VECP is primarily a qualitative objective test that allows semiquantitative acuity prediction under special conditions (unilateral opacities). Psychophysical criteria that are less affected by the quality of the retinal image show promising developments in future subjective tests, e.g. optotypes in positive contrast, optotypes or targets superimposed on a background of optical noise, or hyperacuity. Future objective test developments are pattern VECP or even pattern ERG elicited by interferometric stimulation, speckle VECP and focal ERG.

Cataract↗
PubMed1987

Some contributions of I.E. Purkynĕ to the visual physiology.

In the research of binocular vision J. E. Purkynĕ pointed out the role of retinal corresponding areas. He observed binocular interaction of pressure phosphenes elicited in them and the fusion of images projected on the corresponding foveal areas even in eyes with artificial divergency. For the first time he proposed the examination of visual fields (perimetry) in several planes and he also introduced the colour perimetry. Purkynĕ analyzed profoundly the different functions of direct (foveal) and indirect (peripheral) vision. In indirect vision he anticipated the modern concept of the useful field of view. Of fundamental importance was Purkynĕ's research of the strategy and different types of ocular movements. In analysis of motion perception, he evolved the hypothesis of "general sense of space" which is very close to modern feed-back conception of "corollary discharge".

Czechoslovakia↗
PubMed1978

Receptive field density of retinal ganglion cells and cortical magnification factor in man.

A mathematical relationship is derived between the density or retinal ganglion cell receptive fields and cortical magnification factor. The derivation is based on an assumed constant density of visual fibres entering the striate cortex and on a continuous retinotopic mapping of the visual field thereto. After considering the effect of cortical ocular-dominance columns the human cortical magnification factor along the principal meridians of the visual field is calculated from the empirical estimates of receptive field density of ganglion cells in human retina. The computed estimates of cortical magnification factor agree with experimental data obtained from studies of cortically induced phosphenes, visual migraine hallucinations, and contrast sensitivity functions.

Computers↗
PubMed1982

[Gustave Flaubert's illness].

All those interested in Gustave Flaubert's illness, during his lifetime as well as after his death, have agreed that he had epilepsy. The one important exception is Jean-Paul Sartre, who, in the 2800 pages of his "Idiot de la famille" claimed that Flaubert was a hysteric with very moderate intelligence who somatized his neurosis in the form of seizures. These, in Sartre's views, were moreover probably hysterical, but possibly epileptic resulting from the existence of a psychogenic epilepsy bred from the neurosis. The basis for this neurosis could have originated at the time of Gustave's birth, as this occurred between those of two brothers who both died young, and as his mother had wished for a daughter. Further development of the neurosis might have taken place during a temporary phase of learning difficulties, exaggerated and exploited by his father to make his youngest son the idiot of a family in which the eldest son was the dauphin. Destroyed in this way, Gustave would have sought refuge in passivity and could have developed a hatred for his father and for his elder brother, who he would have liked to kill before killing himself. But, unable to carry out his wishes and desiring both to die and to survive, Gustave, adolescent, might have chosen the pathway of "false deaths", as exemplified by the seizures. Modern epileptology data enables not only to confirm the epileptic etiology and to discount the hysterical nature of the fits, but also: 1. to establish precise details of the site and nature of the cerebral lesions responsible for the attacks: neonatal atrophy or vascular malformation of the occipitotemporal cortex of the left hemisphere, the only lesion capable of provoking: a) the phosphenes marking the onset of the seizures; b) the intellectual manifestations (forced thoughts or flight of ideas), affective features (panic terror), and psychosensory (ecmnesic hallucinations) or psychomotor (confusional automatism) symptoms accompanying some attacks; c) the loss of speech preceding the loss of consciousness and terminal generalized convulsions. 2. to establish the consequences of the lesion and of the temporal fits on the behaviour of the writer; a) associated hypoactivity (slowness of ideation and writing) and paroxysmal impulsiveness (violent unmotivated angry outbursts); b) disturbed verbal functions (difficulty in finding words); and c) diminished sexuality. As a conclusion, it must be admitted; a) that Flaubert suffered from an organic and not a psychogenic epilepsy, the existence of the latter currently being disputed; b) that this epilepsy modified the behaviour of the author without affecting his genius; c) that, as a result of this, the epilepsy of Flaubert, as that of Dostoïevski, should serve as a defense witness for the unfortunate epileptics thought to be destined for intellectual deterioration only because of the repetition of their seizures.

Conversion Disorder↗
PubMed1980

An exploration of the ability of macaques to detect microstimulation of striate cortex.

With its head steadied within a form-fitting mask, a macaque was first taught to signal when it detected the application of 0.2-ms electrical pulses at 50 Hz through electrodes chronically implanted within its striate cortex. Stimuli were then applied via a movable microelectrode and the threshold for the animal’s detection determined at intervals of 50-250 micrometers. With permanently implanted 130- 200-micrometers diameter electrodes such thresholds range between 50 and 250 microamperes (and are highly stable), whereas with the microelectrodes sites were encountered, estimated to be primarily within cortical layers V-VI, where the monkey could reliably detect as little as 2-4 microamperes. The threshold at most sites within striate cortex with the microelectrode, however, was 15-25 microamperes. Background unit activity recorded with the microelectrode varied greatly in different laminae and survived the microstimulation, but has so far provided no clear basis for predicting threshold. It is tentatively hypothesized that the relatively rare points where the threshold is as much as 10 times less than that in the surround arise because the giant, solitary cells of Meynert provide the exclusively effective output for the behavioral response. This hypothesis would also explain the singular uniformity of sensation (a "phosphene") evoked in human subjects by such stimuli, and the equivalence of all such stimuli in striate cortex found for the macaque.

Animals↗
PubMed1983

Visual phenomenology of the LSD flashback.

One hundred twenty-three persons with a history of LSD use were studied for the presence of the LSD flashback phenomenon and compared with 40 control subjects. A syndrome emerged that included ten distance visual disturbances. It had lasted for five years in half of the population, was treatable with benzodiazepines, exacerbated by phenothiazines, and precipitated by 19 different stimuli, most commonly emergence into a dark environment. Sensitivity to LSD as determined by flashbacks appears to divide the study sample into three discrete subgroups. There may be a genetic basis to LSD sensitivity.

Acute Disease↗