Blueprint MCAT Full-Length 1: Psych/Soc Passage 6 – Illusion

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Session 236

I struggled a bit with last week’s discrete set, but hopefully you learned something from it. Now, we head on back to passages with one on optical illusions.

We’re joined by Dorothy from Blueprint MCAT. Sign up for a free account and get access to their amazing new flashcards on top of a free half-length diagnostic,  free full-length one, and free amazing study planner tool. If you would like to follow along on YouTube, go to premed.tv.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[02:28] Passage 6 (Questions 31 – 34)

Optical illusions illustrate several principles of human perception. People rely on past experience when perceiving images, even when they are incomplete. This reliance on past information allows individuals to quickly interpret what is being seen. This phenomenon is illustrated as follows in the Kanizsa triangle.

Figure 1 The Kanizsa triangle

Notes:

Looking at the image, it looks like there’s some sort of 3D element even though it’s all on one 2D screen. Something to highlight here are “rely on past experience when perceiving images even when they are incomplete” and “quickly interpret what is being seen.”

[03:43] Paragraph 2

Another optical illusion that invokes previous information are those that involve depth perception. Two same-sized figures are placed so that it seems one figure is closer to the viewer than the other figure. The figure which is judged to be farther back appears bigger than the one that seems closer.

Notes:

This paragraph is talking about depth perception. And we’re given an example here where there are two same-sized figures, but one looks closer. And so, the figure that’s judged as further back appears bigger than the one that seems closer.

[06:05] Paragraph 3

People typically perceive depth through stereopsis, in which binocular neurons in the striate cortex respond most strongly when visual information is perceived in different locations in each eye. This perceived difference in location is called a retinal disparity, and contributes strongly to depth perception. These processes are also influenced by other cues, such as loss of clarity in objects farther away or objects appearing to move faster when a person turns their head.

Notes:

So we’re still talking about depth perception here, but a little bit more specifically to what happens in terms of the neurons as well.

[07:32] Paragraph 4

Optical illusions also involve color perception. If a person stares at a green image for a length of time, and then looks at a white background, the previous image appears as a red residual image. The cones in the retina, responsible for perception of green, are activated when staring at the image initially and after staring these cones become desensitized. When the person then looks at the white background, white combined with the absence of perception of green gives a perception of a red residual image.

Notes:

Now, we’re talking about color perception. So we’ve moved beyond depth perception. We’re given this red residual image phenomenon. It means that when someone stares at a green image for a really long time, and then quickly goes to look back at the white background, they tend to see this residual image happen. It’s because the cones in the retina are activated, and then become desensitized.

[09:18] Paragraph 5

A study was conducted to determine the effect that the length of time staring at an image had on the perceived intensity of the residual image. Groups of 10 individuals each viewed a green image for 20, 40 or 60 seconds. They then rated the intensity at which they perceived the residual image at 10-second intervals for 1 minute. The averages of each group at each time point are presented in the line graph as follows:

Figure 2 Intensity of the residual image over 60 seconds based on exposure time to the original image

Notes:

When looking at figures, you want to look at the figure description. Get oriented with what information is given in the figure. Look at our variables in our axes. The x-axis shows the length of exposure to the image while the y-axis shows the intensity. So that is our dependent variable there on a scale of 1 to 10. Then look at the legend to make sure you understand which lines are what. Then you could translate all of that into a story.

Based on the figure, we’ve reached some sort of bax desensitization with the 40 seconds and the 60 seconds doesn’t really add much to it. And because the 40 and 60-second lines are basically on top of each other the whole time, this is the threshold for desensitization of those cones.

[12:48]  Question 31

It can be inferred from the results of the study that 20 seconds is sufficient time for what to have occurred in green cones in the retina?

1. Desensitization
2. Residual image
3. Binocular vision
4. Excitation

Thought Process:

C – Binocular vision has nothing to do with this.

A – It’s right in the passage talking about the desensitization of the green cones that’s making it happen. So this is the correct answer right off the bat.

Dorothy says a lot of students sometimes get tempted by B and D. And those are both wrong for their own reasons. Residual image is what happens because of the desensitization of the cones. So they just switched up the causality there.

Then D. Excitation happens when you look at a green image. But they’re asking specifically about that 20-second timeframe in terms of that threshold for desensitization. The fact that you see a residual image after 20 seconds shows that they are way past the point of excitation. They have been desensitized at that point.

Correct Answer: A

[14:39] Question 32

A possible explanation for results found in the study is:

A.there was a difference between groups in the number of green rods desensitized after 20 seconds as opposed to 40 or 60 seconds.

B.there was a difference between groups in the number of green cones desensitized after 20 seconds as opposed to 40 or 60 seconds.

C.there was a difference between groups in the number of green cones desensitized after 40 seconds as opposed to 60 seconds.

D.the group who viewed the green image for 40 seconds must have had, on average, more sensitive green cones than the group who viewed the green image for 60 seconds.

Thought Process:

A –  this is talking about rods but nowhere in the passage are they talking about rods, so this is out. Rods are also for sensitivity in lower light conditions and peripheral vision, whereas cones are more for color perception.

C- This goes against what the graph tells us.

D – This also goes against what the graph is telling us.

Correct Answer: B

[17:01] Question 33

It can be inferred that a person who has lost sight in one eye, will come to predominantly use which of the following type of information to perceive depth?

1. Binocular vision
2. The clarity of objects

III. Retinal disparities

1. I only
2. II only
3. III only
4. I and II only

Thought Process:

I and III both rely on having two eyes. So answer choice II is the only one that you can do with one eye. So the correct answer is B. II only.

Correct Answer: B

[17:49] Question 34

When a subject views the word “red” written in green ink it takes longer for the subject to recognize the word than when viewing the word “red” written in red ink. This phenomenon is known as:

1. the James-Lange theory.
2. cognitive dissonance.
3. attribution theory.
4. the Stroop effect.

Thought Process:

A – It’s one of the emotion theories so it’s not relevant to the situation here.

B – This is the contradiction between the two beliefs.

C – Attribution theory is when you’re giving yourself more grace in terms of looking at your situational factors because it’s your own situation versus assigning more dispositional traits to other people when they do something wrong.

D – The Stroop effect is when it is harder for someone to reconcile two different pieces of information that are both color-related. And so, if you’re looking at those questions, then it essentially says you have the red written in green. It’s then a little harder for our brains to process that because we’ve got two color words that are associated there.

Correct Answer: D

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