Clin Neurophysiol. 2010 Jul;121(7):1080-4. Epub 2010 Feb 25.
Retinal origin of phosphenes to transcranial alternating current stimulation.
Schutter DJ, Hortensius R.
Experimental Psychology, Helmholtz Research Institute, Utrecht University, Heidelberglaan 2, 3584CS Utrecht, The Netherlands. firstname.lastname@example.org
OBJECTIVE: To examine possible retinal contributions to cortically induced phosphenes by transcranial alternating current stimulation (tACS) involving the visual cortex.
METHODS: Self-reported phosphene ratings and voltage-related potentials from the canthus, supra-orbital and sub-orbital regions of the right eye were measured to 2, 10 and 20 Hz tACS at 250 and 1000 microA intensities in healthy volunteers.
RESULTS: Qualitatively similar, but more intense phosphenes were reported during frontalis-vertex tACS as compared to occiput-vertex tACS. In addition, voltage-related potentials were recorded at the canthus and orbit regions of the eye during frontalis-vertex, occiput-vertex and occiput-right shoulder tACS.
CONCLUSIONS: The experience of phosphenes during tACS involving the visual cortex is influenced by volume conductions effects of the scalp.
SIGNIFICANCE: Retinal effects should be taken into account when studying the cortical modulatory effects of tACS.
Clin Neurophysiol. 2010 Mar;121(3):376-9. Epub 2010 Jan 15.
Phosphene thresholds evoked with single and double TMS pulses.
Kammer T, Baumann LW.
Department of Psychiatry, University of Ulm, Ulm, Germany. email@example.com
OBJECTIVE: To evaluate the quantitative advantage of double pulses vs. single pulses in TMS phosphenes evoked from the occipital cortex.
METHODS: In 10 healthy subjects single pulse thresholds were compared with thresholds from double pulses of equal strength at a stimulus onset asynchrony (SOA) of 2, 5, 10, and 20ms, both with biphasic and monophasic pulse forms. In a second experiment fusion time, i.e. the double pulse SOA where the percept passes from one into two phosphenes was determined.
RESULTS: Thresholds obtained with double pulses did not depend on SOA. They were lowered to about 90% of single pulse thresholds. Biphasic pulses yielded lower thresholds (89%) than monophasic pulses. Fusion time was about 45ms but highly varied inter-individually and did not depend on stimulation intensity.
CONCLUSIONS: Although double pulses are more efficient compared to single pulses the advantage is rather small. Previous recommendations to apply double pulses in phosphene studies cannot be confirmed, at least for SOAs up to 20ms. The independence of fusion time to stimulus intensity indicates a non-linear relation between network activity and the percept of phosphene persistence.
SIGNIFICANCE: Phosphene threshold studies do not gain advantages by the application of double pulses.
Exp Brain Res. 2005 Jan;160(1):129-40.
Transcranial magnetic stimulation in the visual system. II. Characterization of induced phosphenes and scotomas.
Kammer T, Puls K, Erb M, Grodd W.
Department of Psychiatry, University of Ulm, Leimgrubenweg 12-14, 89075 Ulm, Germany. firstname.lastname@example.org
Transcranial magnetic stimulation (TMS) induces phosphenes and disrupts visual perception when applied over the occipital pole. Both the underlying mechanisms and the brain structures involved are still unclear. In the first part of this study we show that the masking effect of TMS differs to masking by light in terms of the psychometric function. Here we investigate the emergence of phosphenes in relation to perimetric measurements. The coil positions were measured with a stereotactic positioning device, and stimulation sites were characterized in four subjects on the basis of individual retinotopic maps measured by with functional magnetic resonance imaging. Phosphene thresholds were found to lie a factor of 0.59 below the stimulation intensities required to induce visual masking. They covered the segments in the visual field where visual suppression occurred with higher stimulation intensity. Both phosphenes and transient scotomas were found in the lower visual field in the quadrant contralateral to the stimulated hemisphere. They could be evoked from a large area over the occipital pole. Phosphene contours and texture remained quite stable with different coil positions over one hemisphere and did not change with the retinotopy of the different visual areas on which the coil was focused. They cannot be related exclusively to a certain functionally defined visual area. It is most likely that both the optic radiation close to its termination in the dorsal parts of V1 and back-projecting fibers from V2 and V3 back to V1 generate phosphenes and scotomas.
J Neurol Sci. 2003 Nov 15;215(1-2):75-8.
Evaluation of cortical excitability by motor and phosphene thresholds in transcranial magnetic stimulation.
Gerwig M, Kastrup O, Meyer BU, Niehaus L.
Department of Neurology, University of Essen, Essen, Germany.
Motor threshold (MT), as determined by transcranial magnetic stimulation (TMS), is used as a parameter of cortex excitability. In TMS with single or repetitive pulses, stimulus intensities in general are referred to the individual MT, although it is unclear whether MT also reflects the excitability of nonmotor cortical areas such as the visual cortex. Visual cortex excitability can be assessed by thresholds for eliciting phosphenes (phosphene threshold, PT) following TMS over the occipital cortex. The question of a different efficacy of TMS pulses in distinct cortical areas was approached by comparing motor and phosphene thresholds using single-pulse TMS applied to the primary motor and visual cortex. The aim of the study was to clarify, whether MT and PT correlate with each other and whether MT possibly serves as a reasonable measure for the excitability of the visual cortex. In 32 healthy volunteers, TMS with biphasic single pulses was applied over the motor and visual cortex with a figure of eight-shaped coil connected to a Dantec MagPro stimulator. MT and PT were individually measured (percent of maximal stimulator output). Mean PT (61.4+/-11.7%) was significantly higher than mean MT (39.4+/-5.9%) (p=0.01). MT and PT did not correlate significantly (r=0.29, p>0.1). These findings suggest that the MT does not reflect the excitability of the visual cortex. Regarding excitatory effects, the efficacy of TMS may be different over the motor and visual cortex, likely related to a different excitability of these cortical areas. This should be considered in planning and execution of TMS studies of nonmotor cortical areas.
Radiat Prot Dosimetry. 2003;106(4):349-56.
Dosimetry considerations in the head and retina for extremely low frequency electric fields.
Taki M, Suzuki Y, Wake K.
Department of Electrical Engineering, Tokyo Metropolitan University 1-1, Minami-osawa, Hachioji, Tokyo 192-0397, Japan. email@example.com
Magnetophosphenes are investigated from the viewpoint of electromagnetic dosimetry. Induced current density and internal electric fields at the threshold of perception are estimated by analytical and numerical calculations, assuming different models. Dosimetry for electrophosphenes is also discussed and compared with that for magnetophosphenes. The distribution of current density and internal electric fields is consistent with the experimental observation that flashing sensations reach their greatest intensity at the periphery of the visual field, for both electro and magnetophosphenes. The estimated thresholds in internal electric fields are consistent for magnetophosphenes and for electrophosphenes, respectively. The magnitudes of the thresholds, however, differ by about 10-fold. The thresholds in induced current density are critically dependent on the conductivity of the eye assumed for the calculations. The effect of thin membrane structure is also discussed with regard to the difference between electric field and magnetic field exposures.
Clin Neurophysiol. 2001 Nov;112(11):2015-21.
The influence of current direction on phosphene thresholds evoked by transcranial magnetic stimulation.
Kammer T, Beck S, Erb M, Grodd W.
Department of Neurobiology, Max-Planck-Institute for Biological Cybernetics, Spemannstrasse 38, D-72076, Tübingen, Germany. firstname.lastname@example.org
OBJECTIVES: To quantify phosphene thresholds evoked by transcranial magnetic stimulation (TMS) in the occipital cortex as a function of induced current direction.
METHODS: Phosphene thresholds were determined in 6 subjects. We compared two stimulator types (Medtronic-Dantec and Magstim) with monophasic pulses using the standard figure-of-eight coils and systematically varied hemisphere (left and right) and induced current direction (latero-medial and medio-lateral). Each measurement was made 3 times, with a new stimulation site chosen for each repetition. Only those stimulation sites were investigated where phosphenes were restricted to one visual hemifield. Coil positions were stereotactically registered. Functional magnetic resonance imaging (fMRI) of retinotopic areas was performed in 5 subjects to individually characterize the borders of visual areas; TMS stimulation sites were coregistered with respect to visual areas.
RESULTS: Despite large interindividual variance we found a consistent pattern of phosphene thresholds. They were significantly lower if the direction of the induced current was oriented from lateral to medial in the occipital lobe rather than vice versa. No difference with respect to the hemisphere was found. Threshold values normalized to the square root of the stored energy in the stimulators were lower with the Medtronic-Dantec device than with the Magstim device. fMRI revealed that stimulation sites generating unilateral phosphenes were situated at V2 and V3. Variability of phosphene thresholds was low within a cortical patch of 2x2cm(2). Stimulation over V1 yields phosphenes in both visual fields.
CONCLUSIONS: The excitability of visual cortical areas depends on the direction of the induced current with a preference for latero-medial currents. Although the coil positions used in this study were centered over visual areas V2 and V3, we cannot rule out the possibility that subcortical structures or V1 could actually be the main generator for phosphenes.
Biomed Tech (Berl). 1992 Mar;37(3):42-5.
Psychological aspects of perception of magnetophosphenes and electrophosphenes.
[Article in German]
Reissenweber J, David E, Pfotenhauer M.
Institut für Normale und Pathologische Physiologie, Universität Witten, Herdecke.
Scientific research at the Helmholtz Institute for Biomedical Engineering at Aachen University indicates that ELF (extremely-low frequency) electric and magnetic fields may generate visual perceptions in the human retina which are similar to the pressure phosphenes. During our own experiments we found that 90% of subjects undergoing a blind experiment reported various visual sensations which were mostly colored and moving. Our findings indicate that the psychological component of the perception of electric and magnetic phosphenes must not be underestimated. It is possible that there is a connection between retinal noise in the dark (due to metabolic processes [5, 8]) and magnetic or electric phosphenes.
Optom Vis Sci. 1991 Jun;68(6):427-40.
Magnetostimulation of vision: direct noninvasive stimulation of the retina and the visual brain.
School of Optometry, University of California, Berkeley.
The history of magnetophosphenes and their closely related predecessor, electrophosphenes, is described from the mid-18th century to the present time. The current era of magnetic stimulation started in 1985 with the development of a practical capacitor-discharge electromagnetic stimulator by Barker and his colleagues at the University of Sheffield, and their application of it to the brain with Merton and Morton at the National Hospital, London. The safety of magnetostimulation of the brain is discussed as well as the advantages of magnetostimulation over electrostimulation. Principles of magnetostimulation of nerves and magnetic measurement are considered. Effects on motor and sensory systems of the brain are described including magnetic perceptual suppression in the visual cortex and other pioneering work of Amassian, Cracco and Maccabee at SUNY Health, Brooklyn. Magnetophosphenes from retinal and cortical magnetostimulation are distinguished. Now that visual cortical stimulation is possible with the strong magnetic pulses generated by capacitor-discharge instruments, the functional viability of the visual cortex may be tested directly and noninvasively.