814 LIGHT AND LIFE 



you keep it light-adapted, with strong illumination being presented at inter- 

 vals, in some units if we just turn off all the light after awhile you just 

 stop getting any activity. There are other units which instead of responding 

 with a fairly steady train of impulses during the on and off situation, like 

 I showed you in the slide, give bursts, an on-burst and an off-burst and 

 nothing in between. There are still other units which are spontaneously 

 active in the dark which will be excited to more activity during the flashes 

 o£ light at one wavelength. This activity will be inhibited at other wave- 

 lengths. We use this on inhibition instead of the off response as a criterion 

 for threshold, and the kind of curves you get is the same. 



Dr. Buck: Actually, the question I had in mind did not require such a 

 detailed answer. I am intrigued at the idea you get impulses in the dark 

 after red flashes. Does this mean that they are seeing red in the dark? 



Dr. MacNichol: Well, you can't keep them in a rosy glow indefinitely 

 because as they dark-adapt the situation changes. This is a condition in 

 which the flashes are presented sufficiently frequently to keep the animal at 

 a fairly constant level of light adaptation. 



Dr. Buck: Is there any other instance known where you have this sort of 

 negative vision? 



Dr. MacNichol: Well, this isn't negative vision exactly. We have a 

 whole population of ganglion cells, and some of these ganglion cells are 

 excited when the light goes on at one end of the spectrum and inhibited 

 when the light goes on at the other end of the spectrum. You have also 

 other ganglion cells in which just the reverse takes place. I suppose as one 

 studies the properties of this population one would find out just how this 

 could fit into a system of color perception. We are now in the process of 

 gathering the evidence. We would like to get a lot more evidence in before 

 we do much speculating on this point. 



Dr. Hartline: I can answer Dr. Buck's question partially. All the verte- 

 l)rate retinas tested have 'on' response ganglion cells and 'off' response 

 ganglion cells. The eye of a scallop, Pecten, has a double retina composed of 

 two layers of primary receptors. One layer is an 'on' retina, and the other is 

 an 'off' retina. So 'on' and 'off' responses may be fairly universal in visual 

 systems. 



Dr. Rushton: Donner and I found that ganglion cells could receive con- 

 irilnitions from receptors which were shown by the Stiles-Crawford effect 

 to be cones (yellow-sensitive) and rods (blue-sensitive) . The spectral sensi- 

 tivity of these ganglion cells was iiuermediate between that of the rods and 

 the cones alone, and it was unaffected by adaptation to colored lights. The 

 results followed quantitatively from the assumption that each receptor 

 output is a linear function of the logarithm of the light absorbed by that 

 receptor, and that the ganglion cell is excited by the weighted mean of these 

 two outputs. 



For the effect of adaptation upon one excitability will be to shift the 



