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At least 253 records · Page 14Linked to original sources
PubMed2006

Microstimulation of V1 delays visually guided saccades: a parametric evaluation of delay fields.

Electrical microstimulation of macaque striate cortex (area V1) delays the execution of saccadic eye movements made to a visual target placed in the receptive field of the stimulated neurons. The region of visual space within which saccades are delayed is called a delay field. We examined the effects of changing the parameters of stimulation and target size on the size of a delay field. Rhesus monkeys were required to generate a saccadic eye movement to a punctate and white visual target presented within or outside the receptive field of the neurons under study. On 50% of trials, a train of stimulation consisting of 0.2-ms anode-first pulses was delivered to the neurons before the onset of the visual target. Stimulations were performed in the operculum at 0.9-2.0 mm below the cortical surface. It was found that increases in current (50-100 microA), pulse frequency (100-300 Hz), or train duration (75-300 ms) increased the size of a delay field and increases in target size (0.1 degrees -0.2 degrees of visual angle) decreased the size of a delay field. Delay fields varied in size between 0.1 and 0.6 degrees of visual angle. These results are related to the properties of phosphenes induced by electrical stimulation of V1 in humans and compared to the interference effects observed following transcranial magnetic stimulation of human V1.

Analysis of Variance↗
PubMed2000

Stormy onset of benign childhood epilepsy with occipital paroxysmal discharges.

We studied six children with ages ranging from 4 to 10 years who were affected by childhood epilepsy with occipital paroxysms and presented after a stormy onset with prolonged loss of consciousness for 6-14 h. In all these patients, seizures were preceded by visual symptoms in the form of colored circular disks. A CT scan was performed immediately after the onset of symptoms and was normal in all patients. Routine laboratory and cerebrospinal fluid examinations were normal in all patients. The interictal EEG was characterized by continuous or subcontinuous occipital spike wave discharges, which disappeared after the patients' eyes opened. We carried out a 7-year follow-up of all these patients. Only two patients were treated with antiepileptic drugs. The therapy (phenobarbital, clobazam) in the two patients did not induce changes in the EEG pattern. The first did not suffer any further seizures. The second patient had two more seizures (at 8 and 18 months from the onset) with phosphenes, confusional state, and involuntary movements followed by loss of consciousness. Among the other four patients, who did not receive any treatment, only one had any other seizures. The stormy onset of the syndrome described in our six patients emphasizes the extreme variability in the presentation of this type of childhood epilepsy. Our follow-up confirms the good prognosis of this epilepsy even when it has a stormy onset.

Child↗
PubMed1998

Confirmation of oleander poisoning by HPLC/MS.

A non-fatal case of Nerium oleander (common oleander) self-poisoning in a 45-year-old female is presented. Initial symptoms were nausea and vomiting, abdominal pain, phosphenes, cardiovascular shock and sinus brady-cardia. Blood and urine were assayed for oleandrin, the major cardiac glycoside of N. oleander, using a highly specific HPLC/MS procedure. The blood concentration of oleandrin at admission was 1.1 ng/ml. This is the first report of an oleander intoxication ascertained by the mass spectrometric identification of oleandrin in blood. HPLC/MS appears to be the method of choice for the forensic-toxicological investigation of poisonings by cardiac glycosides.

Cardenolides↗
PubMed2003

Towards the cortical representation of form and motion stimuli generated by a retina implant.

BACKGROUND: A retina implant for restoring basic visual perception in patients who are blind due to photoreceptor loss should not only evoke focal phosphenes at high resolution, but should also generate cortical representations of form and motion. We are currently exploring these potential capabilities in anaesthetised cats. METHODS: Fibre electrodes were inserted through a small scleral incision onto the retinal surface for stimulation. For the recording of cortical population activities we placed up to 16 fibre electrodes in areas 17 and/or 18. Retinal and cortical electrodes were adjusted to corresponding sites, i.e., overlapping receptive fields. Electrical stimuli were charge-balanced impulses (200 micros, 10-100 microA). Basic form stimuli were generated by the selective and synchronous activation of some of the seven retinal stimulation electrodes. Movement stimuli were generated by spatio-temporal shifting of form stimuli. From multiple microelectrode recordings we computed stimulus-related spatio-temporal cortical activation profiles. We used these profiles to estimate the relations between stimulation distance and spatial resolution (form) and between stimulus velocity and spatio-temporal resolution (movement). Influences by the retino-cortical pathway were assessed by comparing cortical activations evoked by true form or motion stimuli with synthetic responses composed by superpositioning of responses to appropriate subsets of form and motion stimuli. In addition, we compared cortical responses to form and motion stimuli by a receptive-field-based backprojection of cortical activities. RESULTS: We confirmed our previous finding that electrical retina stimulation may yield a spatial resolution of 1-5 degrees visual angle and a temporal resolution of about 20 ms. We found that the spatio-temporal cortical activation profiles are commonly related to retinal form and motion stimuli. Cortical activity analyses showed that for two-point form stimuli the neuronal interaction depends on the stimulation electrodes' distance and that local cortical group activities can exhibit some tuning to the directions or the velocities of moving electrical bars'. Projections of cortical activations to visual space were consistent with electrical form and motion stimulation of the retina. CONCLUSIONS: Our data indicate that retinal stimulation with electrical form and motion stimuli can lead to spatio-temporally related cortical activations. However, the selective activation of single cortical neurones with specific visual tuning properties by electrical retina stimulation and the potential adaptation of the visual system to long-term stimulation with retina implants should be addressed in future work.

Animals↗
PubMed2006

Masking visual stimuli by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) applied over the occipital pole can suppress visual perception. Since its first description in 1989 by Amassian et al., this technique has widely been used to investigate visual processing at the cortical level. This article presents a review of experiments masking visual stimuli by TMS. The psychophysical characterization of TMS masking, the dependence on stimulus onset asynchrony between visual stimulus and TMS pulse, and the topography of masking within the visual field are considered. The relation between visual masking and the generation of phosphenes is discussed as well as the underlying physiological mechanisms.

Evoked Potentials, Visual↗
PubMedDate not supplied

Retinal vasculitis caused by adult T-cell leukemia/lymphoma.

BACKGROUND: To report a case of lymphomatous infiltration and bilateral retinal vasculitis observed among 83 cases of adult T-cell leukemia (ATL) treated in the University Hospital Center in Fort-de-France (Martinique, French West Indies) between 1984 and 2003. CASE: A complete clinical ophthalmologic examination was performed in this patient along with fluorescein angiography. OBSERVATIONS: After being checked for diffuse adenopathies, myodesopsias, and phosphenes, the 35-year-old patient was diagnosed with ATL. The ocular impairment, present since the onset of ATL as peripheral subretinal infiltrates, spread progressively and afferently to the rest of the retina in the form of an essentially venous vasculitis. Impairment of the vitreous was noted only in the end stages of disease progression. As ocular lesions progressed, the general state of the patient degraded at the same time despite chemotherapeutic measures. CONCLUSION: Among the more than 300 seropositive for human T-cell lymphotropic virus type 1 (HTLV-1) or patients with HTLV-1-associated myelopathy/tropical spastic paraparesis treated at our hospital in the last 20 years, and among the 83 cases of ATL, only this single case of retinal vasculitis associated with HTLV-1 was observed (1/83, 1.2%) in Martinique, confirming the geographic variability of the clinical phenotype of HTLV-1 infection. The incidence of retinal vasculitis in ATL patients may signify an even worse prognosis than initially indicated.

Adult↗
PubMed1980

Recording the visual electrically evoked potential (VEEP).

A system for recording cortical responses to electrical stimulation of the visual system (electrical phosphene) is described. The system included a rotatable electrode pair for stimulation, with which artifacts caused by the relatively large stimulating currents could be reduced to suitable levels. Optical isolation on both the stimulating and recording sides of the equipment provided safety for the subject and made precise application of the stimulating current possible.

Electric Stimulation↗
PubMedDate not supplied

Retino-cortical information transmission achievable with a retina implant.

Blind subjects with photoreceptor degeneration perceive phosphenes when their intact retinal ganglion cells are stimulated electrically. Is this approach suitable for transmitting enough information to the visual cortex for partially restoring vision? We stimulated the retina of anesthetized cats electrically and visually while recording the responses in the visual cortex. Transmission of retino-cortical information T was quantified by information theory. T was 20-160 bit/s (per stimulation and recording site) with random electrical or visual impulse stimulation at rates between 20 and 40 s-1. While increasing spatial density of independent electrical stimulation channels T did not saturate with 7 electrodes/mm2 retina. With seven electrodes up to 500 bit/s was transmitted to 15 cortical recording sites. Electrical stimulation basically employs temporal stimulus patterns. They are intimately linked with intensity/contrast information coded by the spike density of retinal ganglion cells. From the cortical information spread we estimated the spatial resolution as 0.5mm cortex corresponding to 0.5-1.0 degrees visual angle. If the human cortex can receive and decode the information transmitted by a retina implant, our quantitative results measured in cats suggest that visuo-motor coordination and object recognition in many in- and out-door situations will be possible.

Action Potentials↗
PubMed2004

Electric field properties of two commercial figure-8 coils in TMS: calculation of focality and efficiency.

OBJECTIVE: To compare two commonly used TMS coils, namely the Medtronic MC-B70 double coil and the Magstim 70 mm double coil, with respect to their electric field distributions induced on the cortex. METHODS: Electric field properties are calculated on a hemisphere representing the cortex using a spherical head model. The coil designs are characterised using several parameters, such as focality, efficiency and stimulation depth. RESULTS: Medtronic and Magstim coils exhibit similar focality values and stimulation depths, despite very different coil designs. However, the Medtronic coil is about 1.2 times more efficient compared to the Magstim coil. This difference corresponds to different motor and visual phosphene thresholds obtained in previous physiological studies, thereby validating the chosen coil modelling approach. Focality of the Medtronic coil changed less with varying coil-cortex distance compared to the Magstim coil, whereas both coils exhibited similar dependencies on changes in cortex radius. CONCLUSIONS: The similar values for focality and stimulation depth indicate that both coil types should evoke similar physiological effects when adjusting for the different efficiencies. The different physiological thresholds of the two coils can be traced back to differences in coil design. Ideally, focality should depend neither on coil-cortex distance nor on cortex radius in order to allow for an inter-subject comparability. In particular, in motor mapping experiments the size of the resulting maps is affected by these two parameters. Consequently, they are at least partially the cause of the variability across subjects seen in these experiments.

Cerebral Cortex↗
PubMed2006

Safety of rTMS to non-motor cortical areas in healthy participants and patients.

OBJECTIVE: rTMS is increasingly being used for stimulation to non-motor areas, but available safety guidelines are derived from experience with motor cortex rTMS. We reviewed the literature and our own data to assess the safety of rTMS to non-motor areas. METHODS: We reviewed for adverse effects all articles published from January 1998 to December 2003 that applied rTMS to non-motor areas, and analyzed data from our own studies from January 1997 to December 2003. RESULTS: Adverse effects were infrequent and generally mild. Headache was the most common, occurring in 23% of the subjects and more frequent with frontal rTMS. More serious adverse effects were rare and consisted of two seizures and four instances of psychotic symptoms induced by rTMS to the dorsolateral prefrontal cortex in patients with depression. CONCLUSIONS: Overall, as currently applied rTMS to non-motor areas appears to be safe with few, generally mild adverse effects. In future studies, we recommend systematic reporting of adverse effects and careful documentation of machine type, coils used, and actual intensity as a function of maximum stimulator output. Phosphene threshold might be used to index stimulation intensity when rTMS is applied to the visual cortex, and research should be directed to identifying other indexes of intensity for TMS to other non-motor areas. SIGNIFICANCE: rTMS under the present guidelines is safe, with minimal adverse effects, when applied to non-motor areas.

Adult↗
PubMed2004

Brain stimulation and conscious experience.

Libet discovered that a substantial duration (> 0.5-1.0 s) of direct electrical stimulation of the surface of the somatosensory cortex at threshold currents is required before human subjects can report that a conscious somatosensory experience had occurred. Using a reaction time method we confirm that a similarly long stimulation duration at threshold currents is required for activation of elementary visual experiences (phosphenes) in human subjects following stimulation of the surface of the striate cortex. However, the reaction times for the subject to respond to the cessation of the visual experience after the end of electrical stimulation could be as brief as 225-242 ms. We also carried out extensive studies in cats under a variety of anesthetic conditions using the same electrodes and parameters of stimulation employed in the human studies to study the patterns of neuronal activity beneath the stimulating surface electrode. Whereas sufficiently strong currents can activate neurons within milliseconds, stimulating currents close to threshold activate sustained neural activity only after at least 350-500 ms. When currents are close to threshold, some neurons are inhibited for several hundreds of millisecond before the balance between inhibition and excitation shifts towards excitation. These results suggest that the prolonged latencies, i.e., latencies beyond 200-250 ms, for the emergence of conscious experience following direct cortical stimulation result from a delay in the sustained activation of underlying cortical neurons at threshold currents rather than being due to any unusually long duration in central processing time. Intracellular records from cortical neurological cells during repetitive electrical stimulation of the surface of the feline striate cortex demonstrate that such stimulation induces a profound depolarizing shift in membrane potential that may persist after each stimulus train. Such a depolarization is evidence that extracellular K+ concentrations have increased during electrical stimulation. Such an increase in extracellular K+ progressively increases cortical excitability until the threshold for sustained activation of cortical neurons is reached and then exceeded. Consequently, the long latency for threshold activation of cortical neurons depends upon a dynamically increasing cortical facilatory process that begins hundreds of milliseconds before there is sustained activation of such neurons. In some cases, this facilatory process must overcome an initial stimulus-induced inhibition before neuronal firing commences.

Adult↗
PubMedDate not supplied

Behavioral and neuroplastic changes in the blind: evidence for functionally relevant cross-modal interactions.

The study of blind individuals provides insight into the brain re-organization and behavioral compensations that occur following sensory deprivation. While behavioral studies have yielded conflicting results in terms of performance levels within the remaining senses, deafferentation of visual cortical areas through peripheral blindness results in clear neuroplastic changes. Most striking is the activation of occipital cortex in response to auditory and tactile stimulation. Indeed, parts of the "unimodal" visual cortex are recruited by other sensory modalities to process sensory information in a functionally relevant manner. In addition, a larger area of the sensorimotor cortex is devoted to the representation of the reading finger in blind Braille readers. The "visual" function of the deafferented occipital cortex is also altered, where transcranial magnetic stimulation-induced phosphenes can be elicited in only 20% of blind subjects. The neural mechanisms underlying these changes remain elusive but recent data showing rapid cross-modal plasticity in blindfolded, sighted subjects argue against the establishment of new connections to explain cross-modal interactions in the blind. Rather, latent pathways that participate in multisensory percepts in sighted subjects might be unmasked and may be potentiated in the event of complete loss of visual input. These issues have important implications for the development of visual prosthesis aimed at restoring some degree of vision in the blind.

Adaptation, Psychological↗
PubMed2006

Activation of color-selective areas of the visual cortex in a blind synesthete.

Many areas of the visual cortex are activated when blind people are stimulated naturally through other sensory modalities (e.g., haptically; Sadato et al., 1996). While this extraneous activation of visual areas via other senses in normal blind people might have functional value (Kauffman et al., 2002; Lessard et al., 1998), it does not lead to conscious visual experiences. On the other hand, electrical stimulation of the primary visual cortex in the blind does produce illusory visual phosphenes (Brindley and Lewin, 1968). Here we provide the first evidence that high-level visual areas not only retain their specificity for particular visual characteristics in people who have been blind for long periods, but that activation of these areas can lead to visual sensations. We used fMRI to demonstrate activity in visual cortical areas specifically related to illusory colored and spatially located visual percepts in a synesthetic man who has been completely blind for 10 years. No such differential activations were seen in late-blind or sighted non-synesthetic controls; neither were these areas activated during color-imagery in the late-blind synesthete, implying that this subject's synesthesia is truly a perceptual experience.

Association↗
PubMed2001

Tickling the brain: studying visual sensation, perception and cognition by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is a means of stimulating the brain from outside the skull with little, and occasionally no discomfort for the subject. A single TMS pulse, lasting less than 1 ms, can briefly disrupt the normal activity of a targeted region of the brain for tens of milliseconds, allowing the effects of disruption on specific perceptual and cognitive tasks to be measured behaviorally. Rapid, repeated pulses can disrupt activity for correspondingly longer periods. The reversibility of the effects make it possible to create 'virtual patients' who can be tested in the same way as actual patients with real brain damage in order to explore regional functional specialization. Although several aspects of TMS continue to be evaluated, such as its safety, the extent and localization of the effective region of induced electrical current, the importance of the waveform of the pulse, the configuration and positioning of the coil, its productivity has been firmly established in little more than 10 years of systematic use. Examples of the latter are given from investigations of the nature of visual phosphenes produced by TMS applied to different regions of the visual cortex in normal subjects and subjects with occipital or ocular damage in an attempt to reveal the role of visual cortex in visual awareness.

Brain↗
PubMed1982

[Benign spike-wave occipital epilepsy in children].

Among the partial epilepsies of childhood with a mainly if not exclusively functional focus, a place must be given, next to epilepsy with rolandic spikes, to 'benign epilepsy with occipital spike-waves' featuring: (1) from the EEG standpoint: (a) normal background activity; (b) interictal high amplitude S.W. occurring only when the eyes are closed and recurring more or less rhythmically in the occipito-posterotemporal region of one or both hemispheres (in the last case synchronously or asynchronously, symmetrically or asymmetrically); (c) ictal localized self-maintaining discharges over one occipital lobe, even when the interictal S.W. are bilateral; (2) from the clinical standpoint: (a) between seizures: normal neuropsychiatric, neuroradiological, CT and ophthalmological status; (b) during seizures: visual symptoms (amaurosis, phosphenes, complex hallucinations or illusions) which are: (i) isolated when the ictal neuronal discharge remains confined to the occipital region; (ii) followed by hemiclonic or automatic or cephalalgic manifestations when the occipital discharge extends to the central or temporal regions or to the brain stem. The observed symptomatology, associated with a family history of epilepsy in 36% of cases, confirms that this syndrome is indeed true epilepsy and not basilar migraine with severe epileptiform abnormalities as supposed by Camfield et al. (1978). Benign epilepsy of childhood with occipital S.W. is essentially functional in nature, depending on epileptic predisposition acting to uncover a minimal brain lesion probably acquired at birth. The prognosis is good, although less satisfactory than that of benign epilepsy with rolandic spikes.

Adolescent↗
PubMed2004

An aura of confusion: 'seeing auras-vital energy or human physiology?' Part 1 of a three part series.

The first of three papers that considers claims made for the perception or detection of vital energy. Many systems of Complementary and Alternative Medicine (CAM) assume the existence of a vital force that mediates therapeutic efficacy, for example chi or qi in Traditional Chinese medicine. Vital energy directly perceived or imaged that surrounds living organisms is frequently termed the aura. This paper aims to show how phenomena that arise as a consequence of the normal functioning of the human visual system can be inappropriately offered as support of claims for the direct perception of vital energy or the aura. Specifically, contrast and complementary colour phenomena, entoptic phenomena and the deformation phosphene, the 'flying corpuscle effect', the blind spot and the 'reverse telescope effect' are explained and discussed.

Color↗
PubMed1974

Electrical stimulation of the human visual cortex; preliminary report.

A feasibility study for the development of a human visual prosthesis has led several workers to observe the effects of electrical stimulation of the human visual cortex. Experience with such stimulations of three normal-sighted patients is reported. The results confirm some of the findings of other workers, but do not show that multiple phosphenes were experienced by our patients, using strictly limited parameters of stimulation.

Electric Stimulation↗
PubMed2000

Transcranial magnetic stimulation and the human brain.

Transcranial magnetic stimulation (TMS) is rapidly developing as a powerful, non-invasive tool for studying the human brain. A pulsed magnetic field creates current flow in the brain and can temporarily excite or inhibit specific areas. TMS of motor cortex can produce a muscle twitch or block movement; TMS of occipital cortex can produce visual phosphenes or scotomas. TMS can also alter the functioning of the brain beyond the time of stimulation, offering potential for therapy.

Brain↗