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. email@example.com
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. firstname.lastname@example.org
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
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. email@example.com
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. firstname.lastname@example.org
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. email@example.com
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
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
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.