This study will examine whether direct current (DC) polarization (electrical stimulation) of
the visual cortex can cause a temporary improvement of vision in an amblyopic eye of an
adult. Amblyopia (also called lazy eye) is reduced vision in an eye, caused by abnormal
brain processing of visual information. In amblyopia, the visual cortex (the part of the
brain that processes visual information) favors the other eye and suppresses the image from
the amblyopic eye.
Amblyopia in children is treated by patching or blurring the good eye, which forces the
child to use the amblyopic eye and overcome suppression by the brain. This treatment only
works in children 8 years old and younger, however. Electrical stimulation of the brain can
temporarily change the function of the visual cortex in adults with good vision, but its
influence on the visual function of people with amblyopia is unknown. If DC polarization can
improve vision in amblyopic eyes in adults, it would show that the visual cortex is still
plastic, and it might help researchers develop a treatment for adults with amblyopia in the
Patients 18 years of age and older with amblyopia caused by crossing in or turning out of
the eyes in childhood or by a difference in near- or farsightedness between the eyes may be
eligible for this study. Candidates are screened with a medical history and complete eye
examination, including a glaucoma screening and checks of vision, in- or out-turning of the
eyes, depth perception, need for glasses, and the interior structures of the eyes.
Participants undergo two study sessions, scheduled at least 24 hours apart, involving the
- Examination: Before each session, the patients' distance vision, contrast sensitivity
(ability to see fading letters), and ability to read small print are checked in both
- DC polarization: Patients receive either 20 minutes of electrical stimulation or 20
minutes of sham stimulation (each patient will receive both electrical and sham
stimulation on different days).
- Repeat examination: Immediately after the stimulation and again 20 minutes later,
patients undergo repeat visual function testing. Those who show any differences in
visual function 20 minutes after the stimulation are examined again 1 hour after the
stimulation. Patients in whom the effect continues after 1 hour are examined again
after 1 week.
Background: Amblyopia is a disorder of reduced visual function in an eye, without ocular
disease, which arises as a result of an abnormal visual experience in early life. It is
attributed to a cortical suppression of the image from the amblyopic eye caused by blur or
diplopia. Amblyopia is the leading cause of preventable monocular vision loss in the US
affecting 2% to 5% of the population. In addition to significant visual deficits, amblyopes
are more likely to loose sight in the other, healthy eye as a result of accidents or ocular
diseases. Accepted treatment of amblyopia, which includes patching or pharmacological, or
optical penalization of the fellow eye, is limited to young children. This treatment is
considered ineffective beyond 8 years of age, and no treatment is offered to older
individuals, who comprise 72% of the population of amblyopes.
Recent evidence suggests that some cortical plasticity might be present in older individuals
beyond the accepted age limit for the amblyopia treatment. Spontaneous improvement of visual
acuity in the amblyopic eye was reported in some adults, who lost sight in the good eye
following trauma or disease. However, the chance of spontaneous improvement of visual acuity
to a usable level is relatively low. Therefore, it is important to search for modalities to
improve the vision in amblyopic individuals to prevent incapacitating loss of function
following loss of sight in a sound eye.
Weak direct current (DC) stimulation is a noninvasive method able to induce cortical
excitability changes. It has been previously safely applied in many animal and several human
studies, and was able to modulate activity of primary motor and prefrontal cortices. These
studies revealed that cathodal stimulation reduces spontaneous firing rates of cortical
cells, most likely by hyperpolarizing cortical neurons, whereas anodal stimulation results
in a reverse effect. The number of studies applying DC stimulation over the human visual
cortex is still limited, however these have shown that DC stimulation can induce modulation
in the excitability of the visual cortex, similar to what was observed in motor and
prefrontal cortex. This modulation is reflected by changes in both psychophysical and
electrophysiological parameters of normal volunteers. Because DC stimulation can induce
acute, as well as prolonged modulation in the cortical excitability and activity, it could
be employed as a tool for studying neuroplasticity, and might be beneficial in conditions
accompanied by pathological changes in cortical excitability, such as amblyopia.
In the proposed study we will assess a short-term effect of DC stimulation of the occipital
cortex on the visual function of 10 adult amblyopic subjects. If the study will demonstrate
a favorable effect, then DC stimulation could be used in the future to induce lasting
benefits in amblyopes, because the duration of the after-effects depend on the duration,
intensity, and polarity of stimulation.
Aims: To gather preliminary data on whether DC stimulation of the occipital cortex can
modulate visual function in adult amblyopic subjects. We will check the short-term effect of
the DC stimulation on the plasticity of the visual cortex and its ability to temporarily
alter the impaired visual functions in amblyopia.
To confirm on amblyopic adults that DC stimulation of the visual cortex is safe and well
tolerated, similar to what has been previously established on normal volunteers.
Methods: In the current study we plan to enroll 10 amblyopic subjects ages 18 and older.
Based on the previous reports, anodal stimulation has an excitatory effect, and cathodal
stimulation has an inhibitory effect on the cortex. Therefore, only anodal stimulation of
the visual cortex will be performed on the amblyopic subjects. The stimulation will be
applied over the occipital cortex, duration 20 min at 2mA. As a placebo, a sham stimulation
of the same duration will be applied on the same area of the scalp of all subjects at least
24 hours before or after the anodal stimulation, in a random order. The stimulation will be
done in the masked fashion, so that neither the study subjects, nor the investigator
assessing their visual function will be aware of the type of stimulation.
Assessment of visual function will be done before, and immediately and 20 min after the DC
stimulation, and will include measuring visual acuity, contrast sensitivity, reading speed
and reading acuity. In each session both amblyopic and fellow eye will be assessed starting
with the amblyopic eye. If any differences from the pre-stimulation visual function will be
detected 20 min after the stimulation, the patients will also be examined 1 hour after the
stimulation and, if the effect persists, 1 week following that.
- INCLUSION CRITERIA:
Amblyopia as defined by at least 2 logMAR units difference in the visual acuity between
the two eyes as measured by ETDRS chart
VA in the sound eye 20/40 or better
VA in amblyopic eye 20/40-20/400
Amblyopia due to anisometropia, strabismus or both
No ocular cause of reduced VA and no prior ocular surgery
Structural ocular disease or prior ocular trauma/surgery
Current serious medical or psychiatric condition of any kind
History of any significant trauma or medical condition affecting the brain or skull
History of epileptic seizure
History of significant psychiatric illness, i.e., requiring medication or hospitalization
Current use of neuroactive medication or recreational drugs
Presence of metal in the head other than dental hardware
Broken skin in the area of the stimulating electrodes
Presence of implantable pacemaker or defibrillator