MEASURE OPTIC NERVE IMPULSES ACCURATELY

The most accurate measurements ever made of the manner in which the optic nerve transmits nerve impulses from the eyes to the brain have been made recently at the laboratories of the Medical School and have thrown new light on the laws governing the mechanism of vision.

The experiments, conducted under the direction of Assistant Professors E. L. Chaffee and W. T. Bovic of the Physics Department, were made with extremely delicate electrical apparatus which amplified about 500 times the nerve impulses passing from the eye to the brain.

It has long been known that light passing through the lens of the eye makes an image on the retina at the back of the eyeball, as on a photographic plate, and that the impulse which then carries the message from the retina along the optic nerves to the brain is in the nature of an electrical disturbance. Such men as Walter and Jolly in England and Einthoven in Holland have been able to measure coughly the intensity of this electrical impulse. They have represented its intensity by a curve, and have found that when a light is flashed upon the eye, the curve rises sharply, then dips, and then rises again gradually. This second rise corresponds with the phenomenon known as after-vision", which may be noticed by anyone who looks at a bright light and then closed his eyes. He continues to "see" the light, because the optic nerve is still transmitting impulses to the brain.

Register Impulses Electrically

At the University laboratories these investigations have now been pushed much farther and made more exact. The recent experiments were performed upon the eyes of frogs. The frog was killed and his eye was then immediately removed. The eyeball was cut in two and the optic nerve, which ordinarily connects it with the brain, was attached instead to two electrodes connected with a vacuum-tube amplifier such as is used in wireless telegraphy.

This was connected with a string galvanometer so adjusted that it registered the slightest electrical disturbance. In other words, whatever the frog's eye "saw" was reported electrically, not to the frog's brain, but to this galvanometer, and amplified 500 times.

This apparatus, attached to the frog's eye, proved far more sensitive to light than any hitherto constructed, showing that the eye is a more sensitive instrument than any which it has been possible for man to construct. It measured the impulse from light a great deal too faint for ordinary human vision. The galvanometer, for instance, registered a stimulus from the light, as faint as would be that of a candle set 1200 feet away from the frog's eye, and did this, furthermore, with the front of the frog's eyeball cut off so that there was no lens to collect the diffused light.

Discover Perception of Color

Studying the mechanism of vision with this extraordinarily delicate machine, the experimenters found that the curves which had been roughly measured by previous investigators could be plotted with much more accuracy. They discovered that the smoothness and regularly of these curves are disturbed by a large number of small oscillations, and that these oscillations have to do with the perception of color.

How color-impulses were transmitted from the tiny "cones" on the retina to the brain has not previously been known. Professor Chaffee and Professor Bovie discovered that all impulses passing along the optic nerve are rhythmical, that they go in a series of extremely rapid pulsation's or separate impulses, and that the nature of this rhythm varies with the color of the light seen.

White light, for instance, when flashed from the frog's eye, causes impulses to pass along the optic nerve to the galvanometer in groups, while blue light causes regularly recurring impulses. As the investigators alter the color of the light cast upon the frog's eye, the rhythm of the impulses varies accordingly. Incidentally, it has been noted that the frog is practically red blind; the frog's eye transmits only very faint impulses from red light.

The observations made at the University are said to confirm the accuracy of many of the laws of vision worked out through totally different methods by psychologists. The recent investigators have measured carefully the way in which the response of the eye varies according to the intensity and duration of the light which shines upon it. By thus finding out more about the natural laws which govern the transmission of messages from the eyes to the brain, they hope in some degree to make it easier for the medical profession in the future to understand and treat defects of vision.