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At least 19 recordsLinked to original sources
PubMed1989

Purkynĕ's description of pressure phosphenes and modern neurophysiological studies on the generation of phosphenes by eyeball deformation.

(a) When a subject indents one of his eyeballs in total darkness, he immediately perceives light extending slowly across the whole visual field of the indented eye. The appearance and the time course of these pressure or deformation phosphenes are described. (b) With simultaneous binocular indentation of the eyeballs a flickering patterned phosphene is observed. (c) A short history of the research on pressure phosphenes and its consequences for the theories of vision is presented. (d) Purkynĕ's observations of monocular deformation phosphenes are described. He repeatedly noted patterned light structures, which most observers only perceive with simultaneous binocular eyeball deformation. It is suggested that Purkynĕ's deviating observations were caused by amblyopia of one eye. (e) The neurophysiological basis of the monocular pressure phosphenes was investigated by means of microelectrode recordings from single optic tract fibers. The activity of single retinal ganglion cells (on-center, off-center neurons, latency class I [Y-neurons] or latency class II [X-neurons]), was recorded in anaesthetized cats. Eyeball deformation in total darkness led to an activation of the on-center ganglion cells, while the off-center ganglion cells were inhibited. The latency and strength of this activation or inhibition varied considerably between different neurons, but were fairly constant in the same neuron when the eyeball indentation was repeated after a pause of 1-3 min. The latency and strength of neuronal activation or inhibition seemed to be dependent mainly upon the neuron location relative to the point of eyeball indentation. Some on-center neurons also exhibited a short activation at "deformation off". (f) The antagonistic response type of on-center and off-center ganglion cells was also observed when the eyeball was deformed as a hydrostatic open system and the intraocular pressure was kept at 25 mm Hg basic pressure. (g) Dark adaptation up to 45 min affected the deformation responses of retinal neurons only to a small degree, if at all. This corresponds to the observation that deformation phosphenes in a human observer changed little during the course of dark adaptation. (h) We assume that the activation of on-center and inhibition of off-center ganglion cells by eyeball deformation are caused by retinal stretching, which also leads to horizontal cell stretch. Stretching the horizontal cell membrane probably generates an increase in membrane sodium conductivity and a depolarization of the membrane potential. This depolarization of the horizontal cell membrane potential is transmitted either directly or indirectly (via receptor synapses) from the horizontal to the bipolar cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1974

Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind.

1. To explore the feasibility of a visual prosthesis for the blind, human visual cortex has been stimulated during a series of surgical procedures on conscious volunteers undergoing other occipital lobe surgery.2. Area no. 17 seems the most effective locus for such stimulation, at least in sighted or recently hemianopic patients.3. Changes in electrode size and configuration, or in stimulus parameters, have little effect on subjective sensation.4. Thresholds do vary depending on parameters, but not electrode size, and these effects have been studied.5. Painful effects are associated with stimulation of the dura, but not of the calcarine artery and associated vessels.6. Stimulation of a single electrode usually produces one phosphene, whose size ranges from tiny punctate sensations like ;a star in the sky' up to a large coin at arm's length. Very large elongated phosphenes, like those seen by Brindley's second patient, have not been reported despite the number of patients, electrodes, and combinations of stimulus parameters tested. These large phosphenes may be an effect of prolonged blindness.7. Stimulation substantially above threshold may produce a second conjugate phosphene, inverted about the horizontal meridian.8. Stimulation of a single electrode may also produce multiple phosphenes with no differential threshold.9. Chromatic effects and/or phosphene flicker may, or may not occur. This can vary from point to point on the same patient.10. Phosphenes fade after 10-15 sec of continuous stimulation.11. All phosphenes move proportionately with voluntary eye movements, within the accuracy of our mapping techniques.12. Brightness modulation can easily be achieved by changing pulse amplitude.13. The position of phosphenes in the visual field corresponds only roughly with expectations based on classical maps showing the projection of the visual field onto the cortex.14. Patients can usually discriminate phosphenes produced by 1 mm(2) electrodes on 3 mm centres, although this seems to be close to the limit of resolution.15. Patterns of up to four phosphenes produced by four electrodes have been recognized. However, a variety of complex interactions have been reported.16. Multiple phosphenes are co-planar, although patients are unable to estimate their distance.17. Phosphenes appear immediately when stimulation is begun, and disappear immediately upon cessation of stimulation.18. Future work must concentrate on blind volunteers to explore possible differences in subjective sensation produced after prolonged blindness, and to explore more complex pattern presentation which requires substantial periods of time with any given patient.

Adult↗
PubMed1990

On the history of deformation phosphenes and the idea of internal light generated in the eye for the purpose of vision.

Deformation phosphenes are light sensations evoked by deformation of the eyeball in total darkness. They were first reported in Western literature by Alcmaeon of Croton in the fifth century B.C. The phenomenon of deformation phosphenes was instrumental in prompting some pre-Socratic philosophers and Plato to conceive the idea that efferent light is emitted from the eye for the purpose of vision and a 'cone of vision' is formed by interaction with the external light. In the theories of vision this cone of vision played an important role as a signal-transmitting structure and was also used by the Greek opticians as a geometrical construction to explain optical properties of vision. The impact of the deformation phosphene experiment on the ideas of visual sensation can be followed from Greek antiquity through the period of Roman dominance and Galen's medical teaching on to medieval times and up to the late Renaissance when, based on the anatomy of the eye as illustrated by Felix Platter, the image formation on the retina was correctly described for the first time by Johannes Kepler. In the generations following, deformation phosphenes were still employed as an important argument in defence of the theories of vision. However, the idea of physical light generated by eyeball deformation was rejected with increasing frequency during the 17th and 18th centuries. The literature on this topic is discussed, comprising the contributions of the Arabic philosophers and physicians of the 9th and 10th centuries A.D., the Franciscan and Dominican philosophers of the 13th century, Nicolaus Cusanus of the 15th century, several anatomists of the 16th and 17th centuries, Kepler, Plempius, Descartes, Boyle, Newton and others. After Kepler, the mechanical interpretation of the deformation phosphene being caused by direct action of the eyeball deformation onto the retina slowly became dominant, and the idea that physical light is generated in the eye disappeared. The experimentum crucis in this matter was performed by Giovanni Battista Morgagni (1682-1771) and repeated and extended by Georg August Langguth (1711-1782). On the basis of their results, the case for physical light being generated in the eye by deformation was refuted definitively and slowly vanished thereafter from scientific literature. Deformation phosphenes were used in the 19th and 20th centuries as an instructive example of the percepts evoked by inadequate stimulation of a sense organ. J.E. Pŭrkyne in particular contributed to the study of deformation phosphenes, and finally in 1978, F. Tyler devoted a careful study to the differences between monocular and binocular deformation phosphenes.(ABSTRACT TRUNCATED AT 400 WORDS)

History, 15th Century↗
PubMed2001

The influence of current direction on phosphene thresholds evoked by transcranial magnetic stimulation.

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.

Adult↗
PubMed2005

The pressure phosphene tonometer--a clinical evaluation.

AIMS: Pressure phosphene tonometry is said to assess intraocular pressure by inducing a pressure phosphene. This study compared the results of this relatively new technique with Goldmann applanation tonometry. METHODS: A total of 100 patients (196 readings) in a general ophthalmology clinic at Dunedin Hospital who consented to take part in this study were randomised to receive by different examiners either pressure phosphene tonometry by a Proview eye pressure monitor (Bausch & Lomb Inc., Tampa, FL, USA) or Goldmann tonometry first. There was no communication between the examiners regarding results. RESULTS: Of the 196 attempted readings, pressure phosphene tonometer readings were only able to be obtained for 136 eyes (69%) compared to all 196 (100%) eyes with the Goldmann tonometer. The mean IOPs were 18.5 mmHg using the pressure phosphene tonometer and 16.0 mmHg using the Goldmann tonometer. The mean difference was +2.43 mmHg (95% confidence interval: 10.37 mmHg below to 15.22 mmHg above Goldmann readings). CONCLUSION: This study found that 31% of patients could not perceive a pressure phosphene using the Proview eye pressure monitor. Data obtained from those who could perceive the phosphene indicated that large discrepancies between pressure phosphene tonometry and Goldmann tonometry were common.

Adult↗
PubMed1991

[Phosphenes elicited by subcortical stimulation in man].

Electrical stimulation of a point in the visual pathway can evoke a visual sensation which is called a phosphene. The phosphenes elicited by intracerebral stimulation were investigated in twenty-three subjects. One hundred and seven phosphenes were reported and all of them appeared in the visual field contralateral to the side of stimulation. The exception was a single case where a diffuse flashing sensation appeared in the whole visual field. Thirteen patients reported white phosphenes and nine patients reported coloured phosphenes. In the medial area (10-15 mm from the midline) of the occipital lobe, stimuli above the calcarine fissure resulted in phosphenes in the lower quadrant of the visual field. In the lateral area (16-32 mm from the midline), however, stimuli above the level of the calcarine fissure tended to produce phosphenes in the upper quadrant. These findings appear to conflict with traditional concept of the physiological anatomy of the visual pathway in man. The possible mechanism of this phenomenon produced by intracerebral stimulation is discussed in relation to the phosphenes produced by cortical stimulation.

Adult↗
PubMed2005

Transcranial magnetic stimulation in the visual system. II. Characterization of induced phosphenes and scotomas.

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.

Adult↗
PubMed1990

The radiation phosphene.

A low flux of X-rays below the Cerenkov energy threshold generates a phosphene by direct action on the retina without a fluorescence in the ocular media. X-rays above the Cerenkov threshold can generate only a faint luminescence in the lens and cornea. From experimental work on humans in 1905 with unencapsulated radium, it is known that approximately 80% of the intensity of the radium phosphene is from the beta-ray component and approximately 20% from the gamma-ray. From calculations of the photon yield due to Cerenkov radiation in the eye from radium, one finds intensities of approximately 90% and approximately 10% for beta and gamma-rays, respectively, if only Cerenkov radiation is considered. Thus, one may conclude that the dominant mechanism of the radium phosphene is Cerenkov radiation, primarily from electrons and not fluorescence as previously speculated. The term "radium phosphene" is a misnomer and should be subsumed along with the X-ray phosphene and particle induced visual sensations under the name "radiation phosphene".

Dose-Response Relationship, Radiation↗
PubMed1998

A new tonometer--the pressure phosphene tonometer: clinical comparison with Goldman tonometry.

OBJECTIVE: This study aimed to compare the results of pressure phosphene tonometry, a new tonometric technique, with Goldmann applanation tonometry. DESIGN: Comparative case series. PARTICIPANTS: A total of 100 consecutive patients (192 eyes) without diagnostic specificity, plus a separate subset of 14 eyes with intraocular pressure (IOP) above 19 mmHg, participated. INTERVENTION: Intraocular pressure was measured with Goldmann tonometry by one examiner and pressure phosphene tonometry by a different examiner. There was no communication between the examiners regarding test results. MAIN OUTCOME MEASURES: Intraocular pressure. RESULTS: In the group without diagnostic specificity, the mean difference between the two techniques was 0.3 mmHg. Fifty-one percent of the measurements were within +/-1 mmHg, 74.9% of the measurements were within +/-2 mmHg of each other, and the average deviation for 100% of the data was 1.8 mmHg. For pressure phosphene tonometry, the mean IOP was 15.2 mmHg with a standard deviation of 2.9 and a range of 18 mmHg (minimum, 10; maximum, 28). With Goldmann, the mean was 15.5 mmHg, the standard deviation was 3.1, and the range was 20 mmHg (minimum, 8; maximum, 28). The t test for paired data showed a Gaussian approximation with a P value of 0.05. A comparison between the results of the two techniques gave a correlation coefficient of 0.71. Results for the separate 14 patients with higher IOPs yielded a statistically significant mean difference of 0.1 mmHg between the two techniques (P < 0.05, t test for paired data). For pressure phosphene, the mean was 21 mmHg with a standard deviation of 5.5 and a range of 24 mmHg (minimum, 14; maximum, 38). For Goldmann, the mean was 21.1 mmHg with a standard deviation of 4.7 and a range of 19 mmHg (minimum, 15; maximum, 34). The correlation coefficient was 0.73. CONCLUSIONS: The close agreement between the two techniques suggests that pressure phosphene tonometry offers an alternative method for measuring IOP. It has the advantages that it is simple, noninvasive, and inexpensive. Potential uses of the pressure phosphene tonometer may include self-administered home testing, in outreach clinics by nonophthalmic technicians, and in patients with corneal conditions that preclude the use of Goldmann tonometry.

Humans↗
PubMed2007

Cellular mechanisms underlying the pharmacological induction of phosphenes.

Visual sensations evoked by stimuli other than luminance changes are called phosphenes. Phosphenes may be an early symptom in a variety of diseases of the retina or of the visual pathways, but healthy individuals may perceive them as well. Phosphene-like phenomena are perhaps the most common side effect reported in clinical pharmacology. Ivabradine, a novel anti-anginal drug that reduces heart-rate by inhibiting the hyperpolarization activated current expressed in cardiac sinoatrial node cells (I(f)) induces phosphenes in some patients. One hypothesis is that ivabradine interacts with the visual system by inhibiting hyperpolarization-activated current in retinal cells (Ih). An Ih current with properties similar to cardiac I(f) has been reported in retinal neurones. Under normal circumstances most of the random fluctuations generated within the retinal circuits do not reach the level of conscious perception because they are filtered out. Presumably, filtering occurs mostly within the retina and one serious candidate for this action is the ability of Ih to act as a negative-feedback mechanism. Ih activation in the membrane of visual cells causes dampening of responses to slow noisy inputs thus tuning the visual system to perceptually more relevant signals of higher frequency. Ih inhibition, by altering at the retinal synapses the filtering of signals generated by thermal breakdown of rhodopsin or other fluctuations, is expected to increase the probability of phosphene occurrence. It is the purpose of the present paper to outline and discuss the features of the visual system and the pharmacological conditions relevant to phosphene perception.

Animals↗
PubMed1998

Magnetic stimulation of visual cortex: factors influencing the perception of phosphenes.

Using transcranial magnetic stimulation of occipital cortex, the authors studied the stimulus parameters that generate phosphenes in healthy volunteers. Single pulses or trains of stimuli readily elicited phosphenes in all subjects. The threshold current needed to elicit perception of phosphenes was essentially the same for stimulus trains from 250 msec to 2000 msec in length, but increased dramatically for trains of shorter duration. The effect of stimulus frequency was variable, with each subject having a distinctive "frequency tuning curve," but overall, the threshold current necessary to produce phosphenes decreased as frequency of stimulation increased. Using paired pulses, the perceptual threshold was flat for interstimulus intervals between 2 msec and 100 msec, but increased rapidly as the interstimulus interval was increased above 100 msec. Stimulation of sites lateral to the midline elicited phosphenes in the contralateral visual field. Phosphenes were dominant in the lower and peripheral aspects of the visual fields. The findings are discussed in relation to similar studies of electrical stimulation of somatosensory cortex.

Adult↗
PubMed1994

Phosphenes induced by magnetic stimulation over the occipital brain: description and probable site of stimulation.

Phosphenes were elicited by brief, intense magnetic pulses directed to the occipital area of the brain with two different magnetic stimulators and various coils. The observed phosphenes were described or sketched by the subject. Phosphenes were usually wedge-shaped flashes of light in the midperiphery, although occasionally structured phosphenes were reported (stripes or grids). The depth of effective stimulation was measured by determining the phosphene threshold for two different size coils. Additional depth measurements were made at the cortical motor strip for threshold finger twitches. The visual stimulation site was clearly deeper (approximately 4 cm) than the site for motor stimulation (approximately 2 cm), and lay near the midline. Both foveal and peripheral phosphenes had identical stimulation depths, implying a subcortical stimulation site, possibly in the optic radiation fibers adjacent to the posterior tip of the lateral ventricles. Fibers closest to the ventricle, representing the horizontal meridian of the visual field, would be preferentially stimulated, in agreement with experimental results.

Adult↗
PubMed1991

Magnetic stimuli applied over motor and visual cortex: influence of coil position and field polarity on motor responses, phosphenes, and eye movements.

Transcranial magnetic stimulation was performed over the motor and visual cortex with the Novametrix 12-cm diameter stimulation coil. The influence of coil position on the size of electromyographic responses and on the intensity and position of phosphenes in the binocular visual field was studied. Furthermore, it was investigated whether stimulation over the visual cortex or over the frontal eye field evoked or disturbed eye movements. Coil position was recorded by constructing grids on the skull surface using extracranial bony landmarks, and was then related individually to underlying cerebral sulci by analysis of magnetic resonance images of the brain. Motor responses. Excitatory effects were maximal when the coil windings in the middle of the coil ring lay over the particular motor representation field of area 4. The response size depended on the direction of the coil currents passing over the motor cortex. For example, coil currents passing over the lateral part of the precentral gyrus from the front and transversely to the central sulcus elicited maximal responses in hand muscles. With the currents passing over the same area in the opposite direction, response amplitudes were much smaller, suggesting activation of different inputs to cortical motoneurons. Phosphenes. Fields of fixed, white and unstructured phosphens occurred in the lower half of the visual field when the coil center was placed about 7 cm anterior to the inion on the inion-nasion line. Counterclockwise or clockwise coil currents elicited phosphenes within the left or right lower quadrant of the binocular visual field, respectively, which could be attributed to an activation of the right or left primary visual cortex (area 17). The 'cortical' phosphenes moved with voluntary eye movements, but not during caloric and optokinetic nystagmus. Phosphenes resulting from an excitation of the optic nerve rather than the retina could be evoked by stimulation over frontal parts of the skull. Eye movements. The application of single magnetic field pulses over the frontal eye field or over the visual cortex did not elicit eye movements except for small vertical eye movements as part of a magnetically elicited blink.

Adolescent↗
PubMed1978

A method for plotting the optimum positions of an array of cortical electrical phosphenes.

The cortical visual prosthesis provides one approach to the substitution of vision in blind people. Usable visual information is provided in the form of phosphenes, and in order to make use of the prosthesis the positions of the phosphenes in visual space must first be determined. Such phosphene maps have to be constructed from observations of the angle and distance between various pairs of phosphenes. Because of the variation in repeated observations of the same phosphene pair, some method is needed to provide the 'best' fitting map to the observations. By formulating the problem as one involving the minimization of a function of many variables, an algorithm is constructed which determines a two-dimensional co-ordinate for each phosphene by minimizing one of two criteria indicating the fit of the map to the observations. The minimization algorithm employed is, essentially, a 'steepest descent' procedure, and initial co-ordinate values are provided by a triangulation method.

Blindness↗
PubMed2002

Phosphene thresholds evoked by transcranial magnetic stimulation are insensitive to short-lasting variations in ambient light.

Transcranial magnetic stimulation applied over the occipital pole is able to induce the perception of light flashes called phosphenes. For phosphene detection and threshold measurement subjects are usually blindfolded or investigated in the dark. The question that we posed here is whether phosphene thresholds change with variations in ambient light. In six subjects we measured thresholds under four different conditions: closed eyes (5 min adaptation) and 0.5, 100, and 3,200 cd/m(2) background illumination. No systematic change in phosphene thresholds was observed with different lighting conditions. In three subjects we repeated the measurements after one week and again found similar values with no systematic modulation. Our data show that cortical excitability does not change with different light adaptation levels. This confirms that the main adaptation to light takes place at subcortical levels, namely at the retina. The practical conclusion is that it is unnecessary to blindfold subjects when determining phosphene thresholds.

Adaptation, Ocular↗
PubMed2005

Modulation of phosphene perception during saccadic eye movements: a transcranial magnetic stimulation study of the human visual cortex.

Saccadic suppression allows for perceptual stability during rapid movements of the eyes. One of the neural mechanisms may involve saccade-related modulation of neural activity in the visual cortex. Using the perception of phosphenes induced by transcranial magnetic stimulation (TMS) applied over the visual cortex (VC) as an index of cortical excitability, we sought to determine if VC excitability was modulated at varying times relative to saccade onset. We used two measures of excitability: (1) stimulator intensity required to induce phosphenes in 50% of trials, also called the phosphene threshold (PT), and (2) the subjective intensity of the phosphene. We found that there was no change in PT for different saccade-TMS onset asynchronies while there was an increase in perceived phosphene-intensity near the time of saccade onset (F(7,42) = 4.34, P = 0.001). Contrary to what would be expected from a saccadic suppression model, our results suggest that excitability of the visual cortex is slightly enhanced at the time of saccade onset.

Adult↗
PubMed2006

Evaluation of residual retinal function by pupillary constrictions and phosphenes using transcorneal electrical stimulation in patients with retinal degeneration.

BACKGROUND: To evaluate inner-retinal function by pupillary constrictions and phosphenes evoked by transcorneal electrical stimulation (TES) in patients with hereditary retinal degeneration. METHODS: Consecutive 20 eyes of 20 patients (16 with retinitis pigmentosa (RP); and four with cone-rod dystrophy (CRD)) whose visual acuity was equal to or worse than 20/2000 at Osaka University Hospital and eight eyes of eight healthy subjects were enrolled. TES was performed on with a contact lens stimulating electrode. The electrically evoked pupillary response (EEPR) was recorded by a pupillometer, and the phosphenes by the subjective responses. Three electrical current thresholds were determined: T1, threshold current for initial phosphene; T2, threshold for eliciting a phosphene extending into the central field; and P, threshold for a relative pupillary constriction > or = 3%. The EEPR and phosphene thresholds were compared with the visual acuity or the visual field. RESULTS: All T1, T2 and P were significantly higher in patients than in normals (Mann-Whitney, P<0.001). Both T1 and T2 were not correlated with visual acuity but depended on the area and location of the residual visual field. T1 and T2 in RP eyes with a EEPR was significantly lower than that in RP eyes without an EEPR. During TES, all subjects and patients had no pain, and no complications except for a slight corneal superficial punctuate keratopathy. CONCLUSIONS: The safety and the efficacy of TES to estimate the residual inner-retinal function in patients with retinal degeneration indicate that TES can be used as one of the most important test to select candidates for retinal prostheses.

Adult↗
PubMed1995

Migraine phosphenes and the retino-cortical magnification factor.

Quantitative observations on the shape and position of migraine phosphenes within the visual field were obtained by controlled "perimetric" drawings of the phosphenes performed every 1-2 min during the aura state. The visual field eccentricity of the "fortification" or zig-zag patterns scintillating at about 10 Hz was plotted as a function of observation time. It is well described by an exponential function of time. This exponential function is the product of a first-order linear differential equation determined by the distribution of the retino-cortical magnification factor across the visual field and a constant diffusion speed of the cortical pathophysiological process leading to the migraine phosphene patterns. The observed "particle" size of the phosphene pattern and the width of the scotoma trailing the scintillating phosphenes could also be easily predicted from these assumptions. A model in which the main components are an increase in extracellular potassium concentration, a decrease in extracellular calcium concentration and the constant speed diffusion of the ions along the extracellular space of the stripe of Gennari within the primary visual cortex explains the observations.

Calcium↗