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PubMed2004

Phosphene induction and the generation of saccadic eye movements by striate cortex.

The purpose of this review is to critically examine phosphene induction and saccadic eye movement generation by electrical microstimulation of striate cortex (area V1) in humans and monkeys. The following issues are addressed: 1) Properties of electrical stimulation as they pertain to the activation of V1 elements; 2) the induction of phosphenes in sighted and blind human subjects elicited by electrical stimulation using various stimulation parameters and electrode types; 3) the induction of phosphenes with electrical microstimulation of V1 in monkeys; 4) the generation of saccadic eye movements with electrical microstimulation of V1 in monkeys; and 5) the tasks involved for the development of a cortical visual prosthesis for the blind. In this review it is concluded that electrical microstimulation of area V1 in trained monkeys can be used to accelerate the development of an effective prosthetic device for the blind.

Animals↗

PubMed1976

Movement phosphenes in optic neuritis: a new clinical sign.

Positive visual phenomena (phosphenes) elicited by eye movements have been described in normal individuals and in myopes with vitreous opacities. In the present paper we describe eye movement-induced phosphenes that appear to be related to optic nerve involvement in patients with optic neuritis and multiple sclerosis. This phenomenon is not associated with vitreous or retinal defects, nor is it similar to phosphenes observed in normal individuals. Instead, it shares many characteristics with the well-known Lhermitte sign and is believed to represent a similar phenomenon.

Adult↗

PubMed2006

Comparison of Proview phosphene tonometry with Goldmann applanation tonometry.

BACKGROUND: To compare intraocular pressures obtained using a handheld pressure phosphene tonometer (PPT) (Proview, Bausch & Lomb Pharmaceuticals, Inc., Tampa, Fla.) with Goldmann applanation tonometry. METHODS: Comparative case series of 30 randomly selected patients. RESULTS: The readings obtained with the pressure phosphene tonometer display a higher mean and a larger standard deviation than those obtained with the Goldmann applanation tonometer (GAT). Differences between PPT and GAT readings tended to decrease as a function of increased Goldmann levels. The relation of Proview and Goldmann readings (r = 0.32) and the scatterplot were not consistent with the hypothesis that the 2 methods are equivalent. INTERPRETATION: Our results indicate that the pressure phosphene-type handheld tonometry method, which does not appear to provide an accurate and consistent measure of intraocular pressure, is substantially less reliable than the Goldmann method.

Adult↗

PubMed2004

Phosphene threshold as a function of contrast of external visual stimuli.

Transcranial magnetic stimulation (TMS) of the occipital lobe is frequently used to induce visual percepts by direct stimulation of visual cortex. The threshold magnetic field strength necessary to elicit a visual percept is often regarded as a measure of electrical excitability of visual cortex. Using single-pulse TMS during visual motion stimulus presentation, we investigated the relationship between different degrees of visual cortical preactivation and cortical phosphene threshold (PT). The two possible, mutually exclusive, predictions on the outcome of this experiment were that a) PT increases with stronger preactivation because of a decrease in the signal-to-noise ratio, or b) that PT decreases with increased preactivation because of the increase in neuronal response towards some threshold. PTs for single-pulse stimulation of the occipital lobe were determined for eight subjects while they passively viewed a horizontally drifting luminance-modulated sinewave grating. Gratings used were of four different luminance contrasts while the spatial and temporal frequencies remained constant. PTs were shown to increase significantly as the background grating increased in contrast. These results suggest that the neural activity underlying the perception of a phosphene can be considered a type of signal that can be partially masked by another signal, in this case the visual cortical activation produced by passive viewing of drifting gratings.

Adult↗

PubMed1996

Increase of posterior cerebral artery blood flow velocity during threshold repetitive magnetic stimulation of the human visual cortex: hints for neuronal activation without cortical phosphenes.

To analyze the effects of low intensity repetitive transcranial magnetic stimulation over the occipital cortex on regional cerebral perfusion, bilateral simultaneous monitoring of posterior cerebral artery blood flow velocity was performed using transcranial Doppler ultrasonography in 14 healthy subjects. During 20 s of unilateral magnetic stimulation with 3 Hz and 6 Hz a significant increase of ipsilateral flow velocity was observed during both stimulus conditions (3 Hz, 10.2 +/- 3.7%; 6 Hz, 12.8 +/- 4.7%). A significantly smaller flow velocity increase occurred also in the contralateral posterior cerebral artery (3 Hz, 8.6 +/- 4.0%; 6 Hz, 10.6 +/- 4.1%). Flow velocity increases were similar to values reported in the literature for physiological activation paradigms so that excessive excitation of the visual cortex does not seem to occur during repetitive magnetic stimulation. The largest increase of flow velocity was observed in the ipsilateral posterior cerebral artery of 5 subjects who experienced phosphenes in the contralateral visual half-field during stimulation (14.3 +/- 4.1%: mean of 3 and 6 Hz stimulation). In the other 9 subjects significant velocity increases indicated cortex activation in the absence of cortical phosphenes. The occurrence of maximum velocity responses within 2-3 heart beats following the first cortex stimulus points to a fast adjustment of cerebral perfusion in response to transcranial brain stimulation.

Blood Flow Velocity↗

PubMed2003

Position, size and luminosity of phosphenes generated by direct optic nerve stimulation.

Pulses of low intensity current, delivered through a cuff electrode chronically implanted around the optic nerve of a blind retinitis pigmentosa patient generate visual sensations. These phosphenes are obtained at lower thresholds for a train of stimuli than for single pulses, which suggests the existence of a spatial and temporal integrating mechanism. The perceptions are much smaller than those predicted from model simulations. A set of equations are derived which show the effect of pulse current, duration, number and frequency on the position, size and, to some extent, luminosity of the resulting phosphenes.

Adult↗

PubMed2005

Evaluation of the pressure phosphene tonometer as a self-tonometer.

PURPOSE: Different from conventional tonometers, the pressure phosphene tonometer (FPT) measures intraocular pressure (IOP) through the upper eyelid without corneal applanation. We evaluated the usefulness of the FPT as a self-tonometer by comparing FPT IOP readings with those obtained with the Goldmann applanation tonometer (GAT). We also evaluated the influence of central corneal thickness (CCT) on IOP measurements obtained with the two different devices. METHODS: We confirmed the repeatability of FPT measurements in a preliminary study. The main investigation formed part of a prospective clinical trial, in which IOP was measured in 101 eyes of 101 participants (55 normal and 46 glaucomatous eyes) using GAT and FPT. FPT measurements were self-acquired by each participant. CCT was measured with an ultrasonic pachymeter. The agreement between FPT and GAT measurements was evaluated by the method of Bland and Altman. Using individual IOP and CCT values, we determined the correlation coefficients and performed regression analysis. RESULTS: FPT met the British Standard criteria for reproducibility. Among 101 participants, seven patients with glaucoma were unable to detect the pressure phosphene and to measure IOP using the FPT. By the statistical method of Bland and Altman there was a significant difference between FPT readings self-measured by the remaining 94 participants and GAT readings obtained by an ophthalmologist. There was no correlation between FPT and CCT readings in 65 participants with no prior history of anti-glaucoma medications or glaucoma surgery. CONCLUSIONS: Self-tonometer FPT readings differed from GAT readings. However, we suggest that the FPT may be a clinically acceptable device because it enables patients to self-measure their IOP easily and safely. As FPT readings are not affected by CCT, this tonometer may be of clinical and practical value for the at-will measurement of IOP in patients with corneal changes.

Adult↗

PubMed1982

Auditory evoked phosphenes in optic nerve disease.

Five patients with optic neuropathy, four vascular and one demyelinating, are described who each complained of an unusual symptom. Bright flashes of light (phosphenes) occurred in the affected eyes and were evoked by sudden unexpected sounds. Movement of the eye alone did not reproduce the symptom. In all patients the phenomenon was sufficiently prominent to interfere with sleep and was the main complaint of one patient. An anticonvulsant (phenytoin) greatly reduced the frequency and intensity of the phosphene in one patient.

Acoustic Stimulation↗

PubMed1979

Phosphenes induced by sound.

Three adults with acquired unilateral visual impairment noticed phosphenes when they heard noises. They witnessed them only when resting in a dark or dimly illuminated room. The hallucinations persisted for days in a postkeratoplasty patient, for weeks in a patient with optic neuritis, and for months in a patient with compression of the optic nerve. The sound-induced phosphenes in these cases seemed to be a pathologic variety of hypnagogic hallucination. We theorize that under conditions of altered excitability and visual deafferentation of the brain, cells capable of responding to both visual and auditory stimuli become hyperresponsive to sounds.

Acoustic Stimulation↗

PubMed1973

Contour-like phosphenes from electrical stimulation of the human eye: some new observations.

1. The contour-like phosphenes that may be seen with electrical stimulation of the human eye are described.2. It is found that a moving edge is particularly effective in generating these patterns.3. The relation between the velocity of the edge, the frequency of the current and the resultant spatial frequency of the lines is as if one line were generated at the edge for every cycle of the current.4. The frequency-dependence of the phase-shift between phosphene and current is such as would be expected if the light was delayed by some 30 msec relative to the current at the site of interaction.5. Evidence is brought forward suggesting that this phenomenon is the result not of interference-like processes, but of coupling interactions of the retinal neurones, dividing the population into domains responding in opposite phase to the electrical stimulation.

Accommodation, Ocular↗

PubMed2003

Manipulation of phosphene thresholds by transcranial direct current stimulation in man.

Transcranial direct current stimulation (tDCS) can modulate the excitability of the human motor cortex, as revealed by the amplitude of the motor-evoked potentials (MEP). The aim of our study has been to produce localized changes of cerebral excitability of the visual cortex in the intact human by weak anodal and cathodal stimulation. For quantification of current-induced excitability changes, we measured phosphene threshold (PT) using short trains of 5-Hz transcranial magnetic stimulation (TMS) pulses in nine healthy subjects before, immediately after, 10 min, and 20 min after the end of tDCS. PTs are suggested as representative values of visual cortex excitability changes. Reduced PT was detected immediately and 10 min after the end of anodal stimulation, while cathodal stimulation resulted in an opposite effect. Our results show that tDCS elicits a transient, reversible excitability alteration of the visual cortex, thus representing a promising tool for neuroplasticity research.

Adult↗

PubMed2004

The Proview phosphene tonometer fails to measure ocular pressure accurately in clinical practice.

PURPOSE: To evaluate the Proview Eye Pressure Monitor as a medical instrument and as a technique for enabling a patient to obtain an accurate measure of his or her intraocular pressure (IOP). DESIGN: An experimental laboratory evaluation and an independent prospective clinical study to test the reproducibility and accuracy of the Proview technique relative to Goldmann applanation tonometry. PARTICIPANTS: For the laboratory study, we analyzed 3 tonometers, each packaged as a Proview Eye Pressure Monitor by Bausch & Lomb. In the independent prospective experimental study, 137 subjects participated, consisting of healthy volunteers and glaucoma patients. METHODS: For laboratory testing, we held each tonometer with a micrometer to assure controlled positioning and pressed its sensing tip against a force meter that produced a calibrated, digital force reading. For clinical testing, we taught subjects (n = 137) to use the Proview technique in accordance with the manufacturer's instructions. Each subject obtained 5 measurements with each of the 5 different Proview devices. A clinician measured the IOP using Goldmann applanation tonometry. MAIN OUTCOME MEASURES: We measured the absolute value, linearity, and repeatability of the force meter readings on the tonometers during the instrument laboratory evaluation. The accuracy was evaluated by comparing the Proview measurements to the Goldmann applanation measurements. Reproducibility of clinical Proview measurements was also measured. All measurements were in mmHg during the clinical evaluation. RESULTS: Laboratory: There was a linear relationship between the pressures read by the Proview tonometers and known forces. The Proview tonometers read the maximum pressure applied. Clinical: The Proview technique is simple to use because it was comfortable and reproducible, with an average variance of the measurements by the same patient of 3.4 mmHg(2). Other variables besides IOP seem to affect the Proview pressure measurements, as seen in the large scatter in our data, measured by our correlation coefficient of r = 0.41. The sensitivity of the Proview technique to detect patients with high IOP (which we defined as a Goldmann pressure of >/=22 mmHg) is low; the Proview pressure identified only 18% (4/22) of these patients. CONCLUSIONS: The Proview instrument and technique were reproducible. However, the Proview tonometer seems not to be reliable as an indicator of IOP. The sensitivity for detecting high IOP was low in this cohort, and the agreement with Goldmann applanation was poor for some individuals. This brings into question the underlying assumption that a force proportional to the IOP generates phosphenes.

Adolescent↗

PubMed2004

Comparison of the Proview pressure phosphene tonometer performed by the patient and examiner with the Goldmann applanation tonometer.

OBJECTIVE: To compare the results of Proview pressure phosphene tonometry (PPPT) performed by the patient and an examiner with Goldmann applanation tonometry (GAT). METHODS: A comparative case series of 96 (192 eyes) consecutive patients from a glaucoma clinic was conducted. Intraocular pressure (IOP) was measured with GAT by one examiner, PPPT by another examiner, and PPPT by the patient. All examiners were masked to the results from any preceding IOP measurement. RESULTS: The coefficient of repeatability for repeated measurements for the GAT was +/-0.48 mmHg. The limits of agreement for self-measurement of IOP with the PPPT and examiner measured IOP with PPPT were 6.3 mmHg and 4.8 mmHg, respectively. The limits of agreement between GAT and self assessed PPPT were +/-11.8 mmHg (mean difference of 0.63 mmHg). When the same comparison was made between GAT and examiner assessed PPPT, the results were limits of agreement of +/-10 mmHg (mean difference of 2.86 mmHg). No significant difference was identified in the agreement of the GAT and the PPPT when subanalysed for age of patient or diagnosis (P > 0.05). The limits of agreement between self-assessed IOP with the PPPT and the GAT were +/-8.2 for those with IOP <20 mmHg and +/-14.9 mmHg for those with IOP>20 mmHg. CONCLUSIONS: Poor agreement exists between IOP measured by GAT and PPPT measured by an examiner or by the patient.

Adolescent↗

PubMed2005

Level of disagreement between Proview phosphene tonometer and Goldmann applanation tonometer intraocular pressure readings.

PURPOSE: To evaluate the agreement of intraocular pressure (IOP) readings obtained with the Proview phosphene tonometer and those obtained by Goldmann applanation tonometry as well as the effect of regular use of the Proview on patients' anxiety about their glaucoma. METHODS: One hundred thirty-five consecutive patients with glaucoma, 35 designated as controls, were enrolled in a 10-month randomized prospective clinical trial. The study patients but not the controls used the Proview outside the office. At office visits IOP was measured by an ophthalmologist with the Goldmann applanation tonometer (GAT) as well as by examiners and patients using the Proview. Patients in the treatment group (n = 100) were asked to measure and record their IOP at home as well. Using ANOVA, we examined the relationship between demographic variables and the level of agreement between the Proview readings and those measured by GAT at the final office visit. All participants also were asked to complete a questionnaire regarding their anxiety about their IOP at the baseline and final office visits. The primary outcome measure was the level of agreement of the Proview measurements with those obtained by GAT. Secondary outcome measures included patients' anxiety about their glaucoma. RESULTS: The absolute mean difference between GAT and Proview readings at the final visit was 3.5 +/- 2.9 mm Hg (median, 2.8 mm Hg). The treatment group reported significantly less anxiety about their glaucoma after 4 to 6 weeks of using the Proview (P = .024). CONCLUSIONS: There was considerable discrepancy between Proview and GAT readings. However, regular use of the Proview tonometer significantly reduced patients' anxiety about their glaucoma.

Adolescent↗

PubMed2006

Pressure phosphene self-tonometry in a patient with Posner-Schlossman syndrome.

Studies have shown that pressure phosphene tonometry (PPT) may be applied to glaucoma patients. It has the potential for patients to self-monitor the intraocular pressure (IOP) at home. We describe the successful use of this approach in a patient with Posner-Schlossman syndrome. In the first part of the study, we investigated whether our patient could use PPT accurately and reliably. In the second part of the study, our patient performed daily home self-tonometry, and was educated to seek ophthalmic assessment if his self-measured IOP was >or=21 mmHg. The outcome measurements included IOP and the number of unscheduled attendances for ophthalmic assessment. The use of PPT reduced his number of unscheduled hospital visits from 0.82 to 0.47 visits per month. PPT can be applied with accuracy, reliability and safety in patients with Posner-Schlossman syndrome, with potential benefits in reducing unnecessary ophthalmic attendances by nearly 50%.

Adult↗

PubMed2006

Higher variability of phosphene thresholds in migraineurs than in controls: a consecutive transcranial magnetic stimulation study.

The aim of this study was to characterize the temporal course of phosphene thresholds (PT) using transcranial magnetic stimulation (TMS) in control subjects and in subjects with migraine and to observe whether changes in PT over time can predict a subsequent migraine attack. PTs were measured in 16 migraineurs [nine with aura (MA) and seven without aura (MoA)] and nine controls five times over an approximately 10-week period. Mean PTs were not significantly different between migraineurs and controls; however, there was a trend in MA showing lower thresholds. The minimum threshold values were also smaller in MA subjects than in MoA or control subjects. Generally, PTs had higher variance in migraineurs than in controls, revealing a significant increase in standard deviation of PTs in MA subjects. There was no significant difference of thresholds from the first to the last stimulation in controls and in MoA subjects, but the 3rd, 4th and 5th measurements of MA subjects were significantly lower than their first measurements. Four migraineurs experienced headache within 1 day after one of the measurements. They had either very low or very high PTs compared with the PT values which were not followed by a migraine attack. Our results imply that migraineurs show a higher variability among PT measurements over time than controls, revealing unstable excitability levels in these patients. Additionally, both particularly high and low PTs might predict a subsequent headache in some individuals.

Adolescent↗

PubMed2004

Pressure phosphene self-tonometry: a comparison with goldmann tonometry in glaucoma patients.

PURPOSE: To evaluate whether the pressure phosphene tonometer (PPT) is suitable for self-tonometry in patients with glaucoma or ocular hypertension. METHODS: This was a prospective comparative study of 102 eyes of 102 patients with chronic glaucoma or ocular hypertension. Intraocular pressure (IOP) measurements by the Goldmann tonometer (GT) were compared with self-measured readings with the PPT. Patients evaluated the ease of home use of the PPT. The last 15 patients were asked to stop their glaucoma medications, and the ability of the PPT to detect an elevated IOP during self-tonometry was studied. RESULTS: The mean +/- SD difference between PPT and GT readings was -0.24 +/- 1.57 mm Hg. Comparing the PPT with the GT, 86% of the readings were within +/- 2.0 mm Hg, and 91% were within +/- 3.0 mm Hg. Spearman's correlation coefficient was 0.91. A Bland-Altman plot showed that the 95% limits of agreement between the two methods lay between 2.90 and -3.38 mm Hg. Within-subject coefficients of variation for the GT and the PPT were 4.4% and 7.3%, respectively. In detecting an elevated IOP of more than 21 mm Hg, the sensitivity and the specificity of the PPT were 92.3% and 98.6% respectively. The mean satisfaction score for home use of the PPT was 87.4 +/- 16.3 (maximum 100). CONCLUSIONS: With proper training and technique, self-tonometry with the PPT appears to be accurate up to at least 25 mm Hg and is reproducible. The PPT was sensitive and specific in detecting an elevated IOP of more than 21 mm Hg. As patients were expected to seek ophthalmic care before the self-measured IOP reaches 25 mm Hg, the PPT may have a value for self-monitoring. Patients rated the PPT as satisfactory for home use. Because the PPT is portable and relatively inexpensive and requires no topical anesthesia or direct contact with the eyeball, it may have potential as an instrument for home self-tonometry.

Adult↗

PubMed2002

Motor and phosphene thresholds to transcranial magnetic stimuli: a reproducibility study.

OBJECTIVES: As repetitive transcranial magnetic stimulation (rTMS) is often applied on different days, it is of interest to know whether motor (MT) and phosphene (PT) thresholds are reproducible across time and whether the intensity determined on the first day can be used in subsequent sessions. METHODS: We studied MT and PT over 5 separate recordings in 10 healthy volunteers using a focal coil and a Magstim(Rapid stimulator. After the initial recording (session 1), the others (2 to 5) were performed respectively after 1 day, 7 days, 1 month and 4 months. RESULTS: Mean MT at rest were 65.30 +/- 5.54%, 65.7 +/- 7.18%, 60.4 +/- 4.27%, 61.8 +/- 4.34%, and 63 +/- 9.1% at sessions 1 to 5. Mean PT were 71.43 +/- 6.68%, 66.29 +/- 10.67%, 60.71 +/- 8.64%, 60.57 +/- 8.08%, and 68.71 +/- 15.48% at sessions 1 to 5. MT and PT were reproducible (ANOVA analysis), however, as shown by coefficients of variation, variability between the first 3 sessions exceeded 10% for MT in 3 subjects and in 4 subjects for PT. CONCLUSIONS: It seems preferable to determine thresholds and adapt output intensity of the stimulator at each rTMS session.

Adult↗