Search

The literature on the interaction of two successive identical subliminal flashes in determining the light detection threshold is reviewed. Four empirical equations, collectively termed the TEpee effect, give a good description of the data of several studies. The TEpee effect describes detection data obtained with several background luminances, with a variety of flash durations, with different colors, with decrements as well as with increments (flashes), and with both monocular and binocular viewing. The TEpee effect also describes detection data obtained with multiple identical flashes, with two electrical pulses (phosphene threshold), with response latency measures, with critical flicker frequency, and, possibly, with the interaction of two supraliminal flashes. The TEpee effect is limited to identical flashes presented foveally. It does not describe the interaction of nonidentical flashes, nor does it describe the interaction of flashes in the peripheral retina.

Electrical stimulation of the occipital cortex resulted in discrete photic sensations or "phosphenes" in two volunteers who had been totally blind for 7 and 28 years, respectively. Stimulation of multiple electrodes allowed one patient to recognize simple patterns, including letters. Both patients made an uneventful recovery, and the success of these experiments reinforces the hope that a functional visual prosthesis can be developed, although many problems remain to be solved.

In patients undergoing thalamotomy for intractable pain, multi-contact platinum electrodes were stereotaxically implanted so that the striate cortex and geniculocalcarine tract could be stimulated to evaluate the feasibility of a visual prosthesis. The parameters of stimulation were 50–300 Hz, 0.1–1 msec pulse duration and 0.1–5 mA/phase. Phosphenes were evoked in 7 out of the 17 cases studied. The projection in space of the phosphene depended upon the site of stimulation. The configuration, color and brightness of the phosphene varied from a simple white or colored light to occasionally a multicolored complex pattern. Some phosphenes appeared to blink or flutter, some were stable in position, others appeared to move away radially from the fixation point. Histological studies on 6 of the brains suggested that the phosphenes were more often evoked from the striate cortex than from the geniculocalcarine fibers.

Taking, as a criterion, the electrical phosphene evoked by electrostimulation of the human eye with use of the exponentially increasing currents of varying time-constant (RC), a study was made on the relation between the electric threshold (Vf) of the eye and the radiance (N) of monochromatic rays of differ ent wavelengths used as the adapting light.(1) In any cases of adapting light of different wavelengths excepting the red light, there were two different types of Vf-log N curve depending upon whether the time-constant was shorter than 25 msec. or longer than 75 msec. This result was the same as that obtained with the white adapting light.(2) In the case of time-constant longer than 75 msec., the effect of the red adapting light upon Vf-log N curve was quite different from that of the other adapting light; the curve showed a form of single nature, whereas in the case of time-constant shorter than 25 msec. the same red light had not such specific effect as in the former case.(3) Taking, as a criterion, the reciprocal of the radiance corresponding to the crest on I>Vf-log N N curve for 0 msec in RC (or the left edge of the horizontal part on the curve for 100 msec. in RC), a spectral sensitivity curve was obtained. This curve was proved to be in conformity with scotopic luminosity curve.(4) It was concluded that the electric threshold, as determined by the exponentially increasing current of time-constant longer than 75 msec. was equivalent to the differential threshold of brightness discrimination.(5) The experiments supported Mita's view that there might be in the retina, at least near the threshold excitation, two sorts of the neural elements which were stimulated separately depending upon the time-constant of the stimulating current. The one for longer time-constant may be the photoreceptor cells, while the other for time-constant the bipolar cells.

A systematic study on the electric stimulation of the eye by the sinusoidal alternating current was made by means of the electronic rheotome within a wide range of frequencies extending from 2 to 1, 000 c.p.s. under the various levels of illumination.1) The threshold strength-frequency curve for the dark adapted eye has at least two main minima in the vicinity of 7 and 30 c.p.s., while the curve for the light-adapted eye has a distinct minimum at about 20 c.p.s. There exists always an abrupt inflection of the strength-frequency curve at around 100 c.p.s.2) The shape of the threshold strength-illumination curve depends complicatedly upon the frequency of a.c. According to the curves' shape, all curves obtained were classified into four types, each corresponding to four ranges of frequency of 2-9, 9-25, 25-50 and 50-1, 000 c.p.s. respectively in approximateestimation.3) It was especially noted that the electric threshold for the frequency around 3 c.p.s. varies in a qualitatively parallel relation to the threshold for an adaequate stimulus with increase in illumination, and that the subjective sensation evoked by such low frequencies is also most alike to the sensation evoked by an adaequate stimulus, and that if the higher frequency is used for stimulation, only the onset and cessation of stimulation causes the electrical phosphenes to be perceived.4) It was discussed that the electrical phosphenes evoked by very low frequencies of sine waves seemed to have a close relation to the “on” elements in the retina, whereas those evoked by the higher frequencies, to the “on-off” and “off” elements.

In an earlier report (3), the authors have provided experimental evidence that, under certain circumstances, the configuration of the induction takes its form in accordance with what a subject perceives in the visual field. If the pre-illuminating patterns I, II, III and a point in IV of Fig. 1 are presented in succession and fixation is shifted from the left mark to the right one between the presentation of III and that of IV, the indirect induction of Y (yellow) in I is observed at the points 1 and 4 etc. in IV and the neutralization, i.e., the arrival of the induction initiated by B (blue) in II is demonstrated at the point 2 (the pattern P in Fig. 2). Let us consider the moment when IV is presented. The region of the retina that is pre-illuminated by I, II and III, if projected to the plane where the pre-illuminating patterns are presented, is given by the dotted figure in Fig. 2, whereas the full lined figure indicates the place where I, II and III are presented, i.e., where these patterns are perceived in the visual field. Hence, the only possible interpretation may be as follows. In so far as I, II, III and IV are presented, the configuration of the induction does not exist around the pre-illuminated region of the retina, but it does take its form around the region in the visual field where the pre-illuminating patterns are perceived. This is what we named the P-result and the discovery was so unexpected that the need of its confirmation from every possible angle was pressing.In a series of experiments described in this article, the pre-illuminating patterns, the shift of fixation, the delivery of the electrical pulse to the eye, the definition of the induction (contrast effect, CE) etc. were exactly the same as those employed in the previous experiments (3). However, the psychophysical method to obtain the electrical threshold for evoking the phosphene was changed. Namely, in place of the method of limits in the descending series, the one in the ascending series was employed in Experiment 3 (Fig. 3), and the constant method in Experiment 4 (Fig. 6). And it became clear, irrespective of the method, the P-result was always obtained under the conditions mentioned above. In passing, the goodness of fit of the φ(γ) function to the data in the case of the constant method was satisfactory as shown in Fig. 4 and in Fig. 5.So far, the interval of time between the cessation of IV and the delivery of the electrical pulse was fixed at 1.5sec. since only existence or non-existence of the induction of Y was at issue. In Experiment 5, however, the interval was changed from 1 to 3sec. for the purpose of obtaining the so-called ζ-time curve. The P-result was also ascertained because two distinctively different types of curves appeared as shown in Fig. 7. The one (at the points 1 and 4) is of typical type for existence of the induction and the other (at the point 2) for occurrence of the neutralization.

Chronic thalamic electrodes were implanted via the occipital lobe in three patients for the relief of intractable pain. Electrical stimulation of the electrode tract passing through the visual system led to the production of central phosphenes. These phosphenes were described as round, square, triangular, or rectangular. Some were white, some were patterned in black and white, and others were colored. Good visual angle constancy was demonstrated for one phosphene. Phosphenes were reliably evoked over the period of implantation. The temporal history of stimulation affected the location, brightness and threshold of the phosphene. The type of phosphene evoked by stimulation in the optic radiation area depends on the site of stimulation and the pulse frequency employed. Some phosphenes appear to move away from the fixation point, while others are stable for the duration of stimulation.

The color-processes around the spot of Mariotte were investigated by Motokawa's method of electrostimulation with electrical phosphenes as index. 1. Near the blind spot colorless sensations were evoked by colored lights, and curves of electrical excitability obtained in this region were found to be independent of wave-lengths. 2. At a certain distance from the blind spot a curve characteristic of the wave-length was obtained. The zone around the spot within which no such curve characteristic of the wave-length was obtained, was called the “color-blind zone.” The extent of the zone fairly coincided with the one subjectively determined. 3. The color-blind zones for red and green lights were wider than those for yellow and blue lights. 4. Within the color-blind zone the maximum of the excitability curve shifted as the distance from the blind spot increased. But in case of blue light, no such shift occurred. 5. The distributions of cone- and rod-processes around the blind spot were investigated, and it was found that within the color-blind zone the rod-process was predominant over the cone-process.

Baumgardt E. IV. Le phosphène électrique comme moyen d'analyse des mécanismes de contraste et des illusions d'optique. In: L'année psychologique. 1952 vol. 52, n°1. pp. 137-145.

Using different warning signals and threshold stimuli, the thresholds, as determined by a method of limits, were found to rise monotonically as the interval between warning signal and threshold stimulus increased from I to 9 sec. It was found that the variability of the threshold did not increase as the threshold increased. Similar results were obtained for phosphene and auditory thresholds and with visual and auditory warnings; therefore the effect was considered to be central. Motokawa's finding of a minimum in the phosphene threshold 2 sec. after a flash of white light was not repeated. The rise in threshold was not obtained when the warning intervals were randomized and so seemed to depend on the use of fixed warning intervals. A model was developed relating threshold level to accuracy of anticipation of the end of the warning interval.

In the previous articles (1, 2), highly unexpected results were reported which the authors encountered in a series of experiments employing a particular set of pre-illuminating stimuli (Fig. 1 in 2) and dealing with the indirect induction. Namely, the authors made discoveries strongly suggesting the interpretation that, under certain circumstances, the configuration of the indirect induction can not be understood in terms of the pattern of the retinal stimulation. In short, if, I, II, III and IV of Fig. 1 in (2) are presented in succession and fixation is shifted from the left mark to the right one at an appropriate moment prior to the presentation of IV, existence or non-existence of the induction at each point follows the pattern designated P in Fig. 2 in (2). The P-result seems to be accounted for only by the assumption that the configuration of the induction takes its form in accordance with what a subject perceives under the given conditions. On the contrary, if I, II and IV are presented in succession and fixation is shifted in the same way, the results turn to what designated R in Fig. 2 in (2). The R-result is easy to understand in terms of the retinal stimulation pattern.In one of the present experiments, the configuration of the direct induction was examined with the same pre-illuminating stimuli (Fig. 1 in 2). As shown in Fig. 1, the results were exactly parallel to those obtained in the case of the indirect induction. That means, when III was included, the result doubtlessly supported the P-hypothesis, whereas, when III was omitted, the R-hypothesis became tenable.In order to obtain further informations concerning these findings, a series of experiments were designed with the pre-illuminating stimuli as shown in Fig. 2 which involve so-called refraction of a beam of the induction initiated from B in II. When III was included, the arrival of the induction from B to the point 2 was demonstrated by the neutralization of induction of Y at this point (Fig. 5). This is what we called the P-result accompanying the refraction in addition. It is astonishing that the refraction takes place in the region of the retina which isnot stimulated at all by the prism-shaped yellow figure in I. Besides, the refraction index was discovered to coincide with that obtained by Motokawa in the retinal region actually pre-illuminated by the prism shaped figure. On the other hand, when III was omitted, the neutralization was observed at the point 6 and the R-result was obtained (Fig. 7). Hence, it may be said that the results are highly consistent throughout all the experiments reported so far.

Entoptic imagery, which refers to reproducible visible phenomena arising within the human eye, is not a neglected subject. With Purkinje9,10it was a major field of investigation; his fiery rings are related to the present study, but he did not describe the present phenomenon proper. Helmholtz5mentions the phosphene of quick eye motion and pictures it, but he also fails to discuss it adequately. Duke-Elder3has a chapter on entoptic phenomena, but it does not include the eye-movement phosphene. Adler1devotes nine pages to entoptic imagery and does not mention the phenomenon. The closest recent approach to the present description is given by Friedman,4who states that in the dark the effects of optic nerve traction are recognized as the fiery rings of Purkinje. The circles include within their arc the blind spot. Modern abstract journals list from 5 to 30 papers per year dealing

The effect of ionizing radiation was investigated upon the electrical threshold of the eye as determined with electrical phosphenes as an index. The present experiment dates back to a chance observation that an enor-mous increase in electrical threshold of the eye and a conspicuous rise in standard deviation of threshold values occurred after a fluorography of the chest and continued for several days. 1. In 10 normal subjects, who were beginners in such measurements, definitely positive responses were obtained to X-ray irradiation with a dose of 50 mr. 2. It was found that the standard deviation in percentage of the mean of thresholds was a better indicator of the effect of radiation than the threshold value itself. This measure was designated the “phosphene index” and denoted by Pi. 3. The phosphene index was found decidedly higher in a group of 21 X-ray-technicians and physicians than in the control group of as many university students. 4. The time-course of the effect was investigated after momentary exposure to 50 mr, and two phases, acute and chronic, were distinguished. The acute effect lasted for a few days, declining rapidly. A residual effect of several per cent of the maximum remained for over 10 days (chronic phase). 5. There was an approximately linear relationship between the maximum acute effect and the logarithm of the applied dose within a range from 1 mr to 50 mr.

Physiological induction transferred from one eye to the other or binocular induction was studied by the method of electrostimulation with electrical phosphenes as an index. 1. The properties of curves of induction were the same for monocular and binocular inductions and correspond well to the properties of monocular and binocular contrasts. 2. There is, however, a striking difference between monocular and binocular induction in the rate of development. The latency of binocular induction was found to be about 300 msec., while that of monocular induction was about 40 msec. 3. It took about 4, 000 msec. for full development of binocular induction, but such a short period of about 100 msec. was sufficient for monocular induction to develop in full magnitude. 4. The correlation between physiological induction and psychological contrast was discussed.

Spontane oder durch elektrische Impulsreizung bei geschlossenen Augen auftretende subjektive Lichterscheinungen sind als Phosphene bekannt; die bei elektrischer Anregung vorkommenden Formen lassen sich in etwa 15 verschiedene Gruppen einteilen, die samtlich einfache geometrische Muster darstellen. Es wird gezeigt, das mittels einer in Kopfnahe angebrachten Reizspule auch durch magnetische Induktion Phosphene angeregt werden konnen, wobei die notige Induktion im encephalographischen Frequenzbereich etwa 200–1000 Gaus betragt. Die mit 30 Vpn (Studenten) gefundenen 39 Muster sind samtlich mit geometrischen Phosphenformen identisch, die schon von elektrischen Reizversuchen her bekannt sind [14]; jedoch ist die prozentuale Haufigkeit der magnetischen Muster von der der elektrischen Muster verschieden. Eine zweite Gruppe von Versuchspersonen (20 Pilotenanwarter) zeigt ahnliche Resultate. Die gemessenen Reizparameter definieren einen Anregungs- oder Existenzbereich magnetischer Phosphene, der dem Existenzbereich elektrischer Phosphene [22, 23] analog ist. Eine Betrachtung der Leitfahigkeitsverteilung im Schadelmodell fuhrt zu der Vermutung, das als Ursache fur die magnetische Phosphenanregung eine Dichte-erhohung der induzierten Reizstrome in der Nahe gut leitender Medien (z.B. der Bulbi und damit der Retina) in Frage kommt.

In the first half of the article it was reported that we succeeded to confirm the discoveries made by Motokawa concerning with the electrical excitability of the eye, e.g., the indirect induction, its propagation and the neutralization. The experiments consisted in determining under various conditions the threshold for the phosphene which was evoked by a single constant pulse of 0.1sec. given through the silver electrodes attached to the brow and cheek. A kind of the method of limits in the descending series was employed and special caution was taken against the possibility that expectation of the experimenter or of the subject influences the result.The excitability of the eye, the reciprocal of the threshold, is enhanced if a flash of light is presented before the pulse is delivered and this enhancement is expressed by an index ζ. Then, the induction is an increase observed in ζ when the white light is preceeded by a colored light. The induction vanishes, however, if a flash of the complementary color intervenes between the two flashes, the colored and the white, and this is what is called the neutralization. The propagation of the induction is demonstrated by the fact that the neutralization is observed even when the complementary one is given to a part of the retina apart from the part where the colored one is presented.A series of the experiments were designed to make a thorough inquiry into the nature of the coordinate system according to which the propagation of the induction takes its path and a part of the results was described in the second part. Two alternative hypotheses were proposed as shown in Fig. 5, where R denotes the one that the propagation takes place in accordance with the pattern of the retinal stimulation and P the other that configuration of the induction comes to form in accordance with what the subject perceives under the given conditions. Then it was demonstrated that there existed the case where the P-hypothesis holds. For instance, under the condition that fixation shifts from the left mark to the right one between the presentation of the figures III and IV of Fig. 4, the induction initiated by Y was observed with the point 1 and the induction initiated by B arrived to the point 2 to neutralize the induction evoked by Y. These are exactly what are expected from the P-hypothesis because the point 1 is seen above the place where Y is perceived in I and the point 2 below the place where B is perceived in II. If III is omitted and the fixation shifts between II and IV, however, the induction propagates in terms of the R-hypothesis. The results were obtained with all of the three subjects.

The relation between the threshold intensity, as determined with the light-sensation evoked by electrical stimulation of the eyes, and the frequency of the stimulating currents was studied in varying adaptation states of the eyes. The strength-frequency curve of the light-adapted eyes has 5 minima at frequencies of 7, 20, 35, 54 and 74 c. p. s. and that of the darkadapted eyes has 3 minima at frequencies of 7, 35 and 65 c. p. s. The two kinds of strength-frequency curves taken under light- and dark-adapta-tion differ from each other most greatly at 20 c. p. s., moderately about 55 c. p. s., and only slightly at 7 and 35 c. p. s. Even though a small circumscribed area of the retina is illuminated, the strength-frequency curve varies its form in various manners according to the brightness and wavelengths of light used. In general, red light of extremely long wave-lengths has only a slight effect on the form of the curve, but blue-violet light is far more effective. It was discussed that 20 c. p. s. may be the optimum frequency for electrical stimulation of those portions of the optic organs connected to rods.

Electrical stimulation of the palpebral conjunctiva of normal subjects was achieved using gold ribbon electrodes imbedded in scleral lenses. Various electronic parameters and electrode configurations were altered to find the most efficient combination necessary to produce full lid closure. The impedance found for transconjunctival stimulation was one tenth that of percutaneous stimulation. Frequency of stimulation below 25 pulses per second (pps) was painful. Threshold current necessary for full lid closure via transconjunctival stimulation was similar to percutaneous stimulation. Transconjunctival stimulation avoids phosphene induction but does produce elevation of the globe. Finally, transconjunctival-stimulating currents of greater than 100 pps produced lid anesthesia.

1. In this experiments a special apparatus to compress the eyeball was devised, and effects of pressure on the electrical excitability of the dark-adapted eye were investigated. 2. The intensity of the electrical test stimulus, a rectangular pulse of 0.1 second was increased step by step from a subthreshold level, and threshold voltages necessary to evoke the minimal electrical phosphene were determined. The electrical excitability or reciprocal of threshold was expressed in percentage of the resting level. 3. When pressure of 100-400g. was applied to the eyeball, the electrical excitability decreased rapidly within a few seconds, and then remained constant for about 4-5 seconds, to be followed by a second rapid decrease. The steady level mentioned above remained always above 50% of the resting level. There was a linear relation between the intensity of pressure and the decrease of excitability caused by the pressure. 4. The effect of pressure of 1-10 seconds in duration and 100-200g. in intensity remained unchanged about 3 seconds after removal of the pressure, and it took 5-10 seconds for complete recovery. 5. With the same pressure apparatus the critical value of pressure required to stop retinal circulation was determined by means of an ophthalmoscope in three subjects, and values 460-480 g. were obtained. 6. In order to investigate the effect of pressure on the photosensitivity, dark-adaptation curves were determined under pressure by Nagel's adaptometer, and it was found that light thresholds were increased by pressure in both cone- and rod-branches of dark-adaptation curves. All these findings have provided evidence that the electrical excitability of the eye is determined by the state of the retina. The writer is greatly indebted to Prof. K. Motokawa, under whose guidance this investigation was performed, for his generous advice and valuable suggestions throughout the course of the experiment and the preparation of the manuscript.

(1959). Phosphene Threshold as Related to Age and Sex. The Journal of Psychology: Vol. 47, No. 2, pp. 189-198.

Using, as an index of excitation, the electrical phosphene elicited by the electric current running through the eyeball, we made a systematic study of accommodation of the retina to the exponentially increasing current, whose time-constant extended up to several hundred msec. The relation between the electric threshold (final voltage Vf) and the time-constant (RC) , namely, the so-called accommodation curve, was determined at six different levels of adaptation. The results obtained are as follows:1) The accommodation curve for the dark-adapted eye is of duplex nature, namely, it is divided into two sections by a clear break at a time-constant of about 70-90 msec.2) It is noticed that two sorts of subjective sensation of the electrical phos-phene are perceived in accordance with two sections of the accommodation curve.3) The accommodation curve changes its form depending upon the intensity of illumination, where both sections of the accommodation curve shift separately upwards and sidewards respectively.4) It is found that there are two types of the electric threshold strength-log-illumination curve. The one appears in cases of RC being shorter than 30 msec., while the other appears in cases of RC being longer than about 75 msec.5) All these results stated from (1) to (4) may be explained by assuming two different seats of electrical excitation.6) Hill's λ was calculated from each of both sections of the accommodation curve; the value of λ ranged between 366 and 18 msec. according to the experi-mental condition of illumination.7) It is discussed that, if the time-constant of the exponentially increasing current is equal to a half period of the sinusoidal alternating current, both kinds of these stimuli have almost identical effects with regard to electrical excitation of the eye.

Using light sensation (electrical phosphenes) produced by the sinusoidal alternating current flowing through the eyeball as an indicator for determination of the electric threshold of the eye, we studied dependence of the electric threshold upon intensity of illumination as well as the time course of the electric threshold during dark adaptation. Frequencies of electric current ranging from 2 to 20 cps were used for stimulation. 1. The form of the electric threshold strength-log-illumination curve varies according to the frequency of the alternating current. Among the curves obtained, the curves for 2, 3 and 4 cps are of duplex nature and their form is quite similar to Ferry-Porter's law of critical fusion frequency (CFF), namely, the electric threshold for frequencies around 3 cps has the closest correlation with CFF. But the same correlation is not found in case of the other higher frequencies. 2. The electric threshold for frequencies at 2, 3 and 4 cps decreases progressively in the course of dark adaptation. This relation is, though qualitatively, in accordance with the progressive decrease of the light threshold during dark adaptation. There appears an abrupt inflection on the electric threshold strength-time curve for 2-4 cps, when the preillumination is sufficiently intense and long. This inflection is not coincident in time for appearance with the so-called “Kohlrausch's Knick” lying on the normal dark adaptation curve obtained by the adequate stimulus. 3. It is suggested that the structures stimulated by the sinusoidal alternating current at 3 cps may be the cone and rod cells, while those stimulated by the current at 15 or 20 cps may be supposedly the bipolar cells.

1. An array of radio receivers, connected to electrodes in contact with the occipital pole of the right cerebral hemisphere, has been implanted into a 52-year-old blind patient. By giving appropriate radio signals, the patient can be caused to experience sensations of light (;phosphenes') in the left half of the visual field.2. The sensation caused by stimulation through a single electrode is commonly a single very small spot of white light at a constant position in the visual field; but for some electrodes it is two or several such spots, or a small cloud.3. For weak stimuli the map of the visual field on the cortex agrees roughly with the classical maps of Holmes and others derived from war wounds. With stronger stimuli, additional phosphenes appear; these follow a map that is roughly the classical map inverted about the horizontal meridian.4. The phosphenes produced by stimulation through electrodes 2.4 mm apart can be easily distinguished. By stimulation through several electrodes simultaneously, the patient can be caused to see predictable simple patterns.5. The effects of the duration and frequency of stimulating pulses on the threshold have been explored.6. For cortical phosphenes there is no sharp flicker fusion frequency, and probably no flicker fusion frequency at all.7. During voluntary eye movements, the phosphenes move with the eyes. During vestibular reflex eye movements they remain fixed in space.8. Phosphenes ordinarily cease immediately when stimulation ceases, but after strong stimulation they sometimes persist for up to 2 min.9. Our findings strongly suggest that it will be possible, by improving our prototype, to make a useful prosthesis.

Phosphenes have been produced by direct stimulation of the dorsal mesencephalon in the region of the superior colliculus of an alert man. Electrodes were implanted in the midbrain for the purpose of localizing the ascending sensory pathways in a patient who was suffering from intractable phantom limb pain prior to a stereotactic lesion to relieve the pain. One electrode was situated just beneath the left colliculus. Electrical bipolar stimulation was done over a period of several weeks and resulted in the patient experiencing phosphenes in the contralateral visual fields which were associated with lateral conjugate eye movements toward the side of the visual phenomena. These observations point to the possible role of superior colliculus in the organization of visual phenomena in man.

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 mm2 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.

SUMMARY:A feasibility study for the development of a human visual prosthesis has led several workers to observe the effects of electrical stimulation of the human visual cortex. Experience with such stimulations of three normal-sighted patients is reported. The results confirm some of the findings of other workers, but do not show that multiple phosphenes were experienced by our patients, using strictly limited parameters of stimulation.

Using 25 msec single current pulse or repetitive square waves of varying frequencies, the electrical sensitivity of the eye (the reciprocal of the eye's current threshold) was determined under states of a complete dark-adaptation and extrafoveal illuminations of reduced intensities with the least perceptible electrical phosphene as the index. The increase in electrical sensitivity caused by illumination was quantitatively expressed by K, which was defined by the formula K=100 (E-E0)/E0. In this case, E represents electrical sensitivity of the light-adapted eye to the stimulating current with given parameters, and E0, the electrical sensitivity of the dark-adapted eye to the current with the same parameters.1. The K-value for 25 msec single current pulse exhibited a maximum at a level of 10-3 to 10-2 mL, whereas K-values for the repetitive square waves of 20 cps were nearly zero or slightly negative for all the levels of illumination. The findings indicate that the K for 25 msec single shock reflects more sensitively the rod response to suprathreshold light stimuli than K for the current at 20 cps. This justifies our proposed view that pulse width plays a more fundamental role in selective stimulation of the rod elements than frequency of repetition.2. The K-values were determined for varying durations of the stimulating current consisting of repetitive square pulses. A maximal K was obtained by a current of 25 msec in duration, irrespective of the number of pulses and the pulse width.3. The K was measured with double shocks with varying intervals. The K took the maximal value when the pulse interval was about 25 msec. In addition, a close similarity in configuration was demonstrated between the scotopic visibility curve of HECHT and WILLIAMS and the spectral distribution curve of K for the twin shocks separated by about a 25 msec interval. Thus, YONEMURA and NANGO'S finding that 25 msec single current pulse can exert a selective stimulating effect upon the rod elements has been reproduced to a certain extent by using twin shocks with about a 25 msec interval.

Electrical reaction of the retina was noted as a result of the action of Co60 γ-rays upon the isolated frog eye. This reaction and some of its regularities are very similar to the electroretinogram appearing in light stimulation of the eye. Analysis of the data here and literature data has led to a suggestion that photochemical processes are of great significance in the mechanism of radiation phosphene formation.

WE reported earlier1,2 a method which permits reproducible electrical stimulation of phosphenes—that is subjective light patterns, visible with closed eyes—without surgery. It was shown that these patterns resembled line drawings of ‘geometrical’ forms, and that a great number of them could be stimulated within a rather small bandwidth (±10 per cent of the mean excitation frequency) within the electro-encephalographic frequency range. In the meantime, the dependence of this effect on the shape of the stimulating current3, the identification and lifetime of single phosphenes4 and the influence of hallucinogenic drugs on their shape5 have also been investigated. In addition, copies of 520 phosphenes observed by 313 subjects have been collected3: it was possible to classify these into 15 form groups (‘phosphene-Linnaeus’, 1–15, first two columns of Table 1).

The perception of patterns not resulting from viewing external objects but stimulated by cranial electrodes with pulse currents within the electroencephalographic frequency range (‘phosphenes’) has been investigated in earlier work. The experiments described in this paper were undertaken to observe in one subject effects of chemical stimulation (by mescaline, psilocybin and LSD) on electrically-induced phosphenes as to their bandwidth, shape, number and intensity. An increase of two electrically-induced ‘control’ patterns in intensity and bandwidth and the production of a great number of new phosphene patterns with large bandwidth and intensity have been observed. These new drug-induced phosphenes prevailed during simultaneous electrical and maximum chemical stimulation (Fig. 22). From the experimental results a cybernetic model of a pulse-driven phosphene resonator can be derived.

Measurement of the electrical excitability of the eye provides a useful means for analyzing visual function. Determination of electric thresholds of the eye necessitates, however, the following special precautions. (1) A single constant current pulse of 0.1 sec. in duration is adequate as an electric stimulus, because it can stimulate uniformly all kinds of retinal elements. On the contrary, a brief condenser discharge and alternating currents of definite frequency stimulate selectively a certain kind of element. (2) A weak phosphene caused by an electric stimulus near the threshold can hardly be distinguished from intrinsic light of the retina unless the effect of the stimulus in question is compared with that of a stimulus far below the threshold. (3) The threshold for disappearance of phosphenes can be determined more accurately than that for appearance. (4) Retinal elements of different kind show different thresholds, and this is the most important thing to be borne in mind in determination of the electrical excitability-of the eye following illumination.

IT has long been known that the sensation of light (phosphene) can be produced by the effect of ionizing X-radiation on the retina1–3. The threshold dose for a phosphene falls with dark adaptation but always corresponds to the simultaneous arrival of many X-ray photons. Little attention has, however, been paid to the question of whether single fast particles can produce phosphenes, with the exception of the pioneering attempts by D'Arcy and Porter4 to detect single relativistic cosmic-ray muons.

A ‘phosphene’ is an illusion of a flash of light, which you get when an electric current is passed through your brain in a certain way. I shall assume that for every particular phosphene anyone has, there is such a thing as the (one and only) physiologically describable event with which the phosphene could be identified if it could be identified with any such event. For if you could equally well identify something with this or that thing, then you could by no means identify it with either.2

1. Experiments are described in which the phosphene produced by passing alternating current of frequency 100 Hz through the eye is matched with a patch of light having the same apparent size. 2. Matches of this type have been made against different background light intensities, and at various times after a strong retinal bleach. 3. To match the phosphene under particular conditions of this type, it is found that the patch must be set some 1·5 log td brighter than its own threshold under the same conditions. Exceptions to this rule occur with very bright backgrounds, or very soon after bleaching. 4. The increment threshold for a small spot of light on the phosphene in the dark is some 0·5 log td higher than for the same spot on a patch of light matched in appearance to the phosphene under the same conditions. 5. The process linking electrical stimulation of the eye to firing of fibres in the optic nerve is deduced to be substantially unaffected by the state of adaptation of the eye, save perhaps in the exceptional conditions described in (3). 6. It is therefore argued that this phosphene is the result of stimulation both of the retinal visual pathways lying central to the variable-gain element commonly accepted to explain the facts of dark adaptation, and also of the input to this element that is supposed to alter its gain.

Mechanical pressure of variable intensities was applied to the eyeball, to study its effect upon the electrical excitability of the human eye, and upon the ERG. The electrical excitability was measured with electrical phosphenes as an index. 1. The electrical excitability was lowered by 5, 25, 45 and 55 per cent under pressure of 50, 100, 200 and 300g., respectively in 15 seconds. The decrease caused by each pressure was not steadily progressive, but took place in two steps; the first decrease happened within 4 seconds and the second at 7 seconds from the onset of pressure. 2. It is known that the electrical excitability shows a temporary rise following an illumination. This effect of light was found to be abolished by pressure of about 200g. 3. The time taken for the electrical excitability to reach a maximum is different according to the wave-length of the light used for illumination. This color effect disappeared under pressure of about 200g. 4. In the experiments mentioned above, pressure was applied to a doughnut-shaped disc which was held in contact with the eyelids, the outer and the inner diameters of the disc being 25mm. and 18mm. respectively. Therefore it was necessary to compare the data obtained in this way with those obtained by other methods. Pressure values of 30 and 50g. in Bailliart's dynamometer were found to be equivalent to 130 and 230g. in our apparatus. The retinal arterial blood pressure was found 85g. with the dynamometer, and 480g. with our appratus. 5. ERGs of our subjects suffered from marked reduction under dynamometric pressure of 30g. 6. From these findings it was concluded that the observed decrease in the electrical excitability of the eye and especially the abolition of the color effect were due to local asphyxia caused by pressure.

Recent discoveries on the ommatidium mosaic structure of the human retina for sharp vision in colour are shortly reviewed and discussed in terms of new experimental results of electric phosphene threshold measurements with sinusoidal and pulse-shaped current.