#19 - Light is registered in the retina when photons hit

[Dave Killoran]

Complete Question Explanation

Must be true. The correct answer choice is (B)

This is a very challenging question. Part of the difficulty with this problem is the scientific subject
matter. Many people are intimidated by the mention of rhodopsin, with which they are unfamiliar. As
with the flavonoids in problem #1 of the chapter text, you do not need to know what rhodopsin is to
complete the problem.

The stimulus is a fact set, and can be broken into several easily digestible parts:

Premise: Light is registered in the retina when photons hit rhodopsin molecules and the
molecules change shape.

Premise: Due to normal molecular motion, rhodopsin molecules sometimes change
shape without having been hit by light. This change causes errors in the visual
system.

Premise: The amount of molecular motion is directly proportional to the temperature of
the retina.

Answer choice (A): The stimulus does not indicate that the temperature of the retina depends on the
amount of light. It could easily be affected by other factors, such as body temperature. About 15% of
test takers fell prey to this answer.

Answer choice (B): This is the correct answer and just over 40% of test takers correctly chose
this answer. To prove this answer you must link together several pieces of information. First, the
last sentence of the stimulus shows that the amount of rhodopsin molecular motion is directly
proportional to the temperature of the retina (meaning that when temperatures are lower, molecular
motion is lower; when temperatures are higher, molecular motion is higher), and the second sentence
of the stimulus shows that this motion causes visual errors, so the higher the retinal temperature, the
greater the motion, the more errors in the visual system. The answer choice ties body temperature
(remember, the retina is a body part, and animals with visual systems would have retinas) to the
temperature of the surroundings and then rightly notes that hot surroundings would cause more
visual errors than cold surroundings if body temperature matched those surroundings.

Answer choice (C): This was the most popular incorrect answer, and just under a quarter of test
takers fell for this answer. The answer is wrong because we do not know that temperature causes the
rhodopsin to react more slowly. Higher retinal temperature causes the rhodopsin molecules to change
shape, but no mention is made of reaction time. This answer falls under the “New information”
category.

Answer choice (D): Another New Information answer choice. Similar to answer choice (C), this
answer is not proven because no information is given about the surface area of the retina. Answer
choices (C) and (D) are great examples of how an answer can contain information unmentioned by
the stimulus. These answers are somewhat attractive because there is nothing actively wrong about
them and thus they could be true. To avoid them, always keep in mind that your goal is to find the
answer that must occur based on the information in the stimulus.

Answer choice (E): While the stimulus focuses on rhodopsin, no indication is given that rhodopsin is
the only naturally occurring pigment molecule—there could be others.

[Garrett K]

Hello I'm having trouble understanding the answer to question 19. For some reason I don't understand how the answer can be E. Can you please walk me through the correct thinking process for this question?

Thanks!

[Ron Gore]

Hi Garrett!

Thanks for your question. The correct answer choice for this question is (B), not (E). Assuming, though, that you identified answer choice (E) in your post accidentally, I'll walk you through it.

This is a Must Be True stimulus that contains causality. Retinas register light when the rhodopsin molecules change shape due to being hit by photons. So, the photons cause the molecules to change shape and register light.

We are told that the molecules can also change shape due to normal molecular motion, which is directly proportional to the temperature of the retina. So, increased retinal temperature is an alternate cause the molecules to change shape.

Because of this, when the molecules change shape it can either be because of the presence of photons or the presence of heat, which can cause a false positive for light even when photons are not present. That is why answer choice (B) is correct. If an animal's body temperature, and therefore retina, increases as the temperature around the animal increases, then that animal will tend to have more false positives for light in hot surroundings, because alternate cause of heat will be present and affect the shape of the molecules. In cold temperatures, that alternate cause will not be present and there will not be as many, if any, false positives.

Please let me know if I can be of further assistance.

Thanks,

Ron

[anthonycarral]

I do not understand the assumption in the rationale of the correct answer to this (most strongly supported) question.

[edited: copyrighted question content removed]

Excerpt from rationale: "Firstly, the last sentence of the stimulus shows that the amount of rhodopsin molecular motion is directly proportional to the temperature of the retina (meaning that when temperatures are lower, molecular motion is lower; when temps are higher, molecular motion is higher).

My confusion: How does one induce the proportional relationship between temperature and molecular motion to be more active in warmer climates than in colder ones? Is it not possible based upon the information given in the stimulus that this relationship could be reverse, meaning more active in cold climates and less active in hot?

Please help. I believe this to be a failure on part of the Makers of the test to define how the temperature "relationship" works. Leaves it up to the test takers to make the assumption that the correlation is as above mentioned in the rationale, but it could plausibly exist in the opposite direction.

[Nikki Siclunov]

Hi anthonycarral,

Thanks for your question. Note that I've removed portions of your post in which you copied and pasted portions of the original question, to comply with LSAC's copyright regulations.

Onto your question: the last sentence of the stimulus states that the amount of molecular motion of the molecule rhodopsin is directly proportional to the temperature of the retina. In other words, the warmer the retina, the more molecular motion there is, and - inversely - the colder the retina, the less molecular motion. This is the essence of direct proportionality: we have a positive correlation between motion and temperature. If the relationship was reversed, the last sentence would have said, "The amount of molecular motion is inversely proportional to the temperature of the retina."

So, if the visual system of the animal matches that of its surroundings, then the molecular motion will be higher when the surrounding temperature (and, hence, the animal's own body temperature) is higher. Given the information presented in the second sentence of the stimulus, the visual system of such an animal would be more error-prone.

This is a perfectly valid question from a logical standpoint. In fact, all questions on the LSAT are logically valid; the few that weren't, at the time they were administered, were subsequently removed from scoring. The LSAT is an exceptionally well-made test, so resist the urge to argue with it You'll never win.

Hope this helps! Let me know.

Thanks!

[anthonycarral]

I came to this realization soon after I posted the question. I guess my confusion stemmed from the meaning of "directly proportional". Thanks for the quick response!

[srcline@noctrl.edu]

Hello Ron,

The part that is throwing me off is the "directly proportional part". so does this mean that the amount of this molecular motion is dependent to the temperature of the retina? I am failing to see how temperature affects the argument here.

Thankyou
Sarah

[Nikki Siclunov]

Sarah,

Thanks for your question. If "the amount of this molecular motion is directly proportional to the temperature of the retina," then yes - the two are in perfect correlation, and the former depends on the latter. This is precisely what Ron discussed in his previous post.

Let us know if you have any other questions.

Thanks,

[kingivory76]

Onto your question: the last sentence of the stimulus states that the amount of molecular motion of the molecule rhodopsin is directly proportional to the temperature of the retina. In other words, the warmer the retina, the more molecular motion there is, and - inversely - the colder the retina, the less molecular motion. This is the essence of direct proportionality: we have a positive correlation between motion and temperature. If the relationship was reversed, the last sentence would have said, "The amount of molecular motion is inversely proportional to the temperature of the retina."

So, if the visual system of the animal matches that of its surroundings, then the molecular motion will be higher when the surrounding temperature (and, hence, the animal's own body temperature) is higher. Given the information presented in the second sentence of the stimulus, the visual system of such an animal would be more error-prone.


Thanks!

=====
I am writing because I also had an issue with the vocabulary of this question, due to the usage of terminology, and not based on the logical structure of the stimulus.
After reading Nikki's explanation, and then conducting research, I have to continue to argue that the usage of "directly proportional" is terminology largely used in the scientific and mathematical communities, and not used much elsewhere. Therefore, it cannot be expected to be commonly known that "directly proportional" can only be a positive correlation between temperature and molecular motion. Logically, someone who is not familiar with the usage of this terminology in the scientific and mathematical communities could assume that temperature and motion are definitely proportional, but not that one definitely only increases when the other increases. As anthonycarral states, a person not knowledgeable in the mathematical and scientific usage of this terminology could logically deduce that motion decreases when temperature increases, or it could be vice versa - therefore choosing one direction over the other would be perceived by the test taker as guessing an answer, or adding outside information to the stimulus.
Logically, if this additional clarifying information had been presented in the stimulus, I would have had no problem selecting the correct answer provided. However, without having this information clarified in the stimulus, choosing the correct answer out of the choices provided became a guessing game, as none of the choices could be proven/supported fully by the stimulus without incorporating outside information.

[Emily Haney-Caron]

Hi kingivory,

Thanks for the question!

I hear you; it doesn't seem like you should be expected to know the meaning of that term, which feels like more of a term of art to you. Unfortunately, the makers of the LSAT don't agree; there are several other questions that have used the term "directly proportional" in tests over the years. This is one of those things that, unfortunately, you might not know on test day unless you had a reason to be familiar with the term--and in fact, other LSAT questions also require some understanding of math/math terms, as you are asked to identify reasoning flaws related to numbers and percentages, etc. The good news is, now you know this term! You'll be ready next time it comes up, and you'll be prepared to see this on test day. This is one of the reasons a good prep course or plan is so important, because you're exposed in the course of studying to things just such as this; you may get tripped up on the practice question, but then you'll know it after that. Great job really thinking through this and digging in; the more you invest understanding questions you got wrong, the more prepared you'll be in the future, and the more you'll get out of your studying!