Section 36–6 Neurology of vision

(Nerve impulse / Contrast enhancement / Feature detectors)

 

In this section, Feynman discusses nerve impulse and experiments on the nerve fibers of horseshoe crab (related to the principle of contrast enhancement) and frog (feature detectors).

 

1. Nerve impulse:

“First of all, we must appreciate what kind of information can come along nerves. A nerve carries a kind of disturbance which has an electrical effect that is easy to detect, a kind of wavelike disturbance which runs down the nerve and produces an effect at the other end: a long piece of the nerve cell, called the axon, carries the information along, and a certain kind of impulse, called a “spike,” goes along if it is excited at one end (Feynman et al., 1963, p. 36–9).”

 

The “spike” (disturbance) is also known as “nerve impulse” or “action potential” that moves along a nerve fiber. A nerve impulse is an electrical phenomenon that occurs due to differences in sodium ion-concentrations across the membrane of a neuron (Hodgkin, 1964). In other words, the nerve impulse is a sudden reversal of the electrical charge across the membrane of the neuron. The Nobel Prize in Physiology or Medicine 1963 was awarded to Alan Hodgkin and Andrew Huxley for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane. The electrochemical nature of the nerve impulse was investigated by Hodgkin and Huxley in their studies of squid giant axons.

“When one spike goes down the nerve, another cannot immediately follow. All the spikes are of the same size, so it is not that we get higher spikes when the thing is more strongly excited, but that we get more spikes per second. The size of the spike is determined by the fiber (Feynman et al., 1963, p. 36–9).”

 

Perhaps Feynman could have included the “all or none principle,” which means neurons will either transmit a nerve impulse over the synapse to the next neuron completely or not at all. In short, the nerve impulse is not dependent on the strength of the stimuli, but it is generated when the initial threshold is met (see below). Specifically, the membrane potential of a neuron is about -70 mV in the resting state (Patton & Thibodeau, 2018). If a stimulus is above the threshold potential, the membrane depolarises, i.e., sodium-gated ion channels open and allow sodium ions to move into the axon, making the membrane potential more positive. This principle underscores the binary nature of action potential propagation: a neuron either fires a full-sized action potential or it does not fire at all.

 

Source: Patton & Thibodeau, 2018

2. Contrast enhancement:

“We can see, perhaps, if we think about it awhile, that this is a device to enhance contrast at the edges of objects, because if a part of the scene is light and a part is black, then the ommatidia in the lighted area give impulses that are inhibited by all the other light in the neighborhood, so it is relatively weak (Feynman et al., 1963, p. 36–10).”

 

Historically, Haldan Keffer Hartline (1949) identified the lateral inhibition* phenomenon in his experiment on the compound eye of the horseshoe crab. Furthermore, he showed that its photoreceptor cells are interconnected and communicated via the mechanism of lateral inhibition: the ability of excited neurons to reduce the activity of neighboring neurons. In essence, lateral inhibition of neurons can help to enhance contrast at the edge of the objects perceived by the eye. Interestingly, Hartline discovered the phenomenon accidentally when he turned on the room lights and found that the neural response of a horseshoe crab eye decreased. Hartline was awarded the 1967 Nobel Prize in Medicine or Physiology for his research in the neurophysiological mechanisms of vision.

 

*The lateral inhibition phenomenon, which occurs in the retina was first recognized by Ernst Mach, in the year 1866.

“Figure 36–11(a) shows the compound eye of the horseshoe crab; it is not very much of an eye, it has only about a thousand ommatidia. Figure 36–11(b) is a cross section through the system; one can see the ommatidia, with the nerve fibers that run out of them and go into the brain. But note that even in a horseshoe crab there are little interconnections. They are much less elaborate than in the human eye, and it gives us a chance to study a simpler example (Feynman et al., 1963, p. 36–10).”

 

It is worth mentioning that an ommatidium is not an independent eye but a component of a compound eye; each ommatidium functions as a part of the whole, providing information about a small segment of the visual field. In addition, horseshoe crab (or Limulus), has three different types of eyes: a pair of image-forming lateral compound eyes, a pair of median ocelli, and three pair of larval eyes—lateral, median, and ventral (Tanacredi et al., 2009). However, horseshoe crabs are not really crabs (crustaceans), but are more closely related to spiders and scorpions. More important, horseshoe crabs are “living fossils,” i.e., they have existed nearly unchanged for at least 445 million years, even before dinosaurs existed. Hartline used the horseshoe crab as his first experimental model because of its compound eye, easy-to-isolate optic nerve, and individual photoreceptor cells help to understand the principle of contrast enhancement.

 

3. Frog’s feature detectors:

“The most recent picture of the operation of the frog’s eye is the following. One can find four different kinds of optic nerve fibers, in the sense that there are four different kinds of responses. These experiments were not done by shining on-and-off impulses of light, because that is not what a frog sees. A frog just sits there and his eyes never move, unless the lily pad is flopping back and forth, and in that case his eyes wobble just right so that the image stays put. He does not turn his eyes. If anything moves in his field of vision, like a little bug (he has to be able to see something small moving in the fixed background), it turns out that there are four different kinds of fibers which discharge, whose properties are summarized in Table 36–1 (Feynman et al., 1963, p. 36–11).”

 

Feynman mentions that there are four different kinds of nerve fibers, but summarizes five types of neural responses as Sustained edge detection, Convex edge detection, Changing contrast detection, Dimming detection, and Darkness detection. He adds that the neural responses seem to be rather complicated to classify, however, Smoorenburg (1987) identified the four responses as Sustained Contrast detector, Convexity detector, Moving Edge detector, and Net-Dimming detector. Interestingly, toads, a sub-classification of frogs, have only three primary types of responses in their visual processing system: (1) Sustained edge detector, (2) Convexity detector, and (3) Moving edge detector. While frogs exhibit four types of neural responses, including dimming detectors, the toads’ system has evolved for their specific ecological needs.

 

“Now these responses seem to be rather complicated to classify, and we might wonder whether perhaps the experiments are being misinterpreted. But it is very interesting that these same classes are very clearly separated in the anatomy of the frog! By other measurements, after these responses had been classified (afterwards, that is what is important about this), it was discovered that the speed of the signals on the different fibers was not the same, so here was another, independent way to check which kind of a fiber we have found! (Feynman et al., 1963, p. 36–12).”

 

According to Lettvin and his collaborators’ (1959) study, there are four types of nerve fibers or feature detectors in the frog’s eyes. The four types of feature detectors can be classified as follows: (1) Contrast (Sustained Edge Detector): respond to the presence of edges in the visual field and continue to signal as long as the edge is visible. (2) Convexity (Convex Edge Detector): sensitive to convex shapes, which might resemble preys. (3) Changing contrast (Change-in-Contrast Detector): fire in response to moving edges or changes in contrast but do not maintain the signal if the object is stationary. (4) Darkness (Dimming Detector): increase their firing rate when the light intensity decreases. They also suggested the idea of “bug detectors” that respond to small, dark, and moving objects. However, the frog’s eye behaves like a quantum sensor because it is possible to use a quantum light source to show that the frog’s light-sensitive rod cells can detect single photon (Phan et al., 2014).

 

Review Questions:

1. How would you explain the neural impulse using the all-or-none principle?

2. How would you relate the lateral inhibition phenomenon to the principle of contrast enhancement?

3. How would you classify the frog’s feature detectors (or types of neural response in nerve fibers)?

 

The moral of the lesson: The lateral inhibition (or contrast enhancement) in horseshoe crabs and feature detections (three, four, or five) in frogs illustrate that brain-eye systems and neural responses are dependent on how environments evolve.

 

References:

1. Ewert, J. P. (1974). The neural basis of visually guided behavior. Scientific American, 230(3), 34-43.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

3. Hartline, H. K. (1949). Inhibition of activity of visual receptors by illuminating nearby retinal areas in the Limulus eye (abstract), Federation Proceedings, 8, 69.

4. Hodgkin, A. L. (1964). The ionic basis of nervous conduction. Science, 145(3637), 1148-1154.

5. Lettvin, J. Y., Maturana, H. R., McCulloch, W. S., & Pitts, W. H. (1959). What the frog's eye tells the frog's brain. Proceedings of the IRE, 47(11), 1940-1951.

6. Patton, K. T., & Thibodeau, G. A. (2018). Anthony's Textbook of Anatomy & Physiology-E-Book: Anthony's Textbook of Anatomy & Physiology-E-Book. Elsevier Health Sciences.

7. Phan, N. M., Cheng, M. F., Bessarab, D. A., & Krivitsky, L. A. (2014). Interaction of fixed number of photons with retinal rod cells. Physical Review Letters, 112(21), 213601.

8. Smoorenburg, G. F. (1987). Discussion of physiological correlates of speech perception. In The psychophysics of speech perception (pp. 393-399). Dordrecht: Springer Netherlands.

9. Tanacredi, J. T., Botton, M. L., & Smith, D. R. (Eds.). (2009). Biology and conservation of horseshoe crabs. New York: Springer.