Tapetum lucidum and visual acuity

[Apollyon]

Where I work here in Hawai'i, there are some feral cats, and I go out and look for them each night when I am working (ER doc). As such, I see the tapetum lucidum, of course, and I had a question for which I have searched, but cannot find the answer.

I know the mechanism of how the tapetum lucidum increases night vision - by sending the light stimulus through the neuron a second time. As such, I would think that a brighter light source might overwhelm the optic neurons, such as when I shine my flashlight into their eyes. Now, in humans, with a bright light source, we will look away if it is too bright, too quickly. However, when shining a light into the eyes of an animal with a tapetum lucidum, they don't look away.

Thus is my question: does a bright light at night into the eyes of an animal with a tapetum lucidum decrease their visual acuity, either just for the moment that the light is in the eye, or by bleaching the visual purple? Is it irritating or bothersome for the animal?

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[Armymutt25A]

I'm not sure how one would even measure this. It's not like you can ask the animal if it can still read line 7 while shining a bright light in its eyes. I think our eye testing is pretty much limited to pupillary light reflex and menace response. Neither measure acuity just function of various nerves and associated structures. I would imagine the intensity of the light might play a role in what you're looking for. Good luck getting close enough to a feral cat to ensure that your light's main point of maximum intensity is hitting the retina. From what I've seen, most animals react to a beam of light by freezing and staring directly at it - i.e. deer in the headlights. Evolution hasn't quite caught up to the 20th century.

 

[Apollyon]

Hawai'ian feral cats live on average 15 years, vs. 2 on the mainland. Although I'm not going to do it, if you put down food for feral cats, they'll come right up to it and eat it, and may even rub up against you while doing so, without any aggressive posture (I've seen it - it's a program of the Hawai'ian Humane Society to spay/neuter feral cats and have the local neighborhood people maintain the community).

As how to test it, I would guess (being a health professional, but not vet) to have an animal with a tapetum lucidum, and shine a bright light into its eyes, then, after removal of the stimulus, approach it with variable other stimuli that are known to cause a reaction, and see how the animal responds.

The deer in the headlights, sure - but can they see afterwards? That is the question.

 

[Minnerbelle]

Not the most scientific explanation, but when I come home at night and turn the lights on in a completely dark room, my cats squint for a little bit. From that, I'm inclined to say that yes, bright light does bother them that way it does us. This is totally speculation, but if anything, I'd think that the tepetum would exacerbate the prob because more of whatever light you have will hit the rods due to reflection. Not sure if that would make much of a diff, esp in the context of other differences in eye anatomy/physiology between people and cats.

As for why cats don't look away when a car with bright lights is coming towards them? Totally pulling this out from my *****, but if a really bright something was coming towards me really fast, I'd probably try to keep looking at it no matter how bright it was too.

 

[jmo1012]

As for why cats don't look away when a car with bright lights is coming towards them? Totally pulling this out from my *****, but if a really bright something was coming towards me really fast, I'd probably try to keep looking at it no matter how bright it was too.

or from a behavioral point, it would be idiotic and suicidal to look away from a predator?

 

[Armymutt25A]

So what you are talking about is the degradation of rhodopsin in the presence of light and the corresponding time period of darkness required for its synthesis. Yes, that does occur. Whether this is enhanced by the tapetum I can't say. I don't think it would. It seems like it's more of a low light enhancement than an amplifier. The closest analogy I can come up with is night vision devices. They can amplify star light to allow me to see in the dark, but if you turn on the light, I don't get blinded.

 

[Minnerbelle]

or from a behavioral point, it would be idiotic and suicidal to look away from a predator?

same diff. to-may-ter, to-mah-toe

 

[Apollyon]

So what you are talking about is the degradation of rhodopsin in the presence of light and the corresponding time period of darkness required for its synthesis. Yes, that does occur. Whether this is enhanced by the tapetum I can't say. I don't think it would. It seems like it's more of a low light enhancement than an amplifier. The closest analogy I can come up with is night vision devices. They can amplify star light to allow me to see in the dark, but if you turn on the light, I don't get blinded.

However, by anatomical definition, it is an amplifier, as the light source goes through the same neuron twice.

And isn't it a later evolution of night vision to close an iris when bright light is encountered? Don't cheaper and/or older devices indeed blind you if a bright light is shown into them?

 

[jmo1012]

same diff. to-may-ter, to-mah-toe

er, you know i was agreeing with you, right? 🙂

 

[RadRadTerp]

However, by anatomical definition, it is an amplifier, as the light source goes through the same neuron twice.

And isn't it a later evolution of night vision to close an iris when bright light is encountered? Don't cheaper and/or older devices indeed blind you if a bright light is shown into them?

Completely pulling this out of my poor memory from Physio 2's eye section and too lazy to look it up on the interwebz, but...

You can't really think of our nervous system as a piece of analog equipment. A neuron fires a signal or it doesn't. I was always under the impression that the tapetum lucidum only "amplified" in so much as to increase the probability that a signal would be generated at all. Without low-light "amplification", most images would appear dark as fewer signals reach the brain, but if an animal can send light back around for a second trip, then there's a bigger chance that one of them will get stimulated to generate some type of image. Something appears brighter only because a larger number of neurons are stimulated in the same region of the retina, not because they each got hit with a stronger/(or multiple) signal(s). But don't take my word for it...

 

[Armymutt25A]

My GEN I Russian NODs don't blind me in bright light. If the TL had this effect, the animal wouldn't be able to see in daylight because they would get overloaded by the bouncing photons.

 

[Apollyon]

My GEN I Russian NODs don't blind me in bright light. If the TL had this effect, the animal wouldn't be able to see in daylight because they would get overloaded by the bouncing photons.

Since I don't know the specifics (as in whether the Gen I Russian devices have an automatic gated power supply), I don't know how to evaluate this. I thought that the Gen I American devices (like the Starlight scope) WOULD blind one if a bright light was shown into it.

The second statement, though, makes sense.

Still, even though an academic discussion with vet students is good, an actual vet could help (I would hope).

 

[eventualeventer]

In a research setting, one could design a study to determine visual acuity by training the animal that it will get a reward by picking the correct stimulus (consistent from trial to trial) and introducing distractors that were closer and closer in shape to the target stimulus. As an example, this is how neurobiologists study color perception in animals.

I found this in a lit search:

The eyes of animals to which an acute perception of light is a necessity can be differentiated from animals which need excellent visual acuity.
...
While increasing retinal sensitivity in dim illumination, the tapetum lucidum may harm visual acuity in bright illumination.4
...
Animals without a tapetum lucidum (e.g. primates, squirrels, birds, red kangaroo and pig) generally have diurnal habits, and a red or orange to pale gray fundus reflection (Fig. 2, lane 1).7,8
...
Birds are apparently the only large group of animals in which the tapetum is consistently absent.16

Ollivier, F. J., Samuelson, D. A., Brooks, D. E., Lewis, P. A., Kallberg, M. E. and Komáromy, A. M. (2004), Comparative morphology of the tapetum lucidum (among selected species). Veterinary Ophthalmology, 7: 11–22. doi: 10.1111/j.1463-5224.2004.00318.x

Unfortunately, I don't seem to have online access to reference 4:
Braekevelt CR. Fine structure of the tapetum cellulosum of the grey seal (Halichoerus grypus). Acta Anatomica (Basel) 1986; 127: 81–87.

I think there are also ways to quantify retinal pigmentation and bleaching in vivo, but I haven't found any studies addressing your specific question. Maybe when I stop shedding zillions of URI virions I will ask one of our veterinary ophthalmologists.

Regarding whether bright light is aversive, most horses don't seem to appreciate it when you put them in a dark stall and shine a penlight in their eyes for an ophthalmic exam, but I don't know that this has anything to do with having a tapetum as opposed to us not being able to explain what's happening; no one likes being encountered with bright light suddenly after being in the dark.

 

[breenie]

We were told point blank in our phys class that the tapetum does lower bright light acuity.

 

[RadRadTerp]

We were told point blank in our phys class that the tapetum does lower bright light acuity.

Not for this guy:

 

[Minnerbelle]

Or Armymutt's cat:

 

[Armymutt25A]

Maybe a dog for me - it is well known that I'm not a fan of cats.

 

[breenie]

Those cats are way cooler than I'll ever be...