17 Lab 7: Goggles Demonstration and Horizontal Slices
Erin Mazerolle and Sherry Neville-MacLean
Learning Objectives
Type your learning objectives here.
- Consider the impact of displaced visual input on motor output
- Review brain structures involved in vision and movement
- Review structures visible in two horizontal dissection slices of the sheep brain
Displacement Goggles Demonstration
We will need some volunteers for a friendly game of bean bag tossing at targets. Why not get involved and have some laughs?
One group or a few groups of students will participate in the game. Each participant will select a pair of goggles to wear while throwing bean bags of one colour into a large box. The objective is to get the bean bags as close to the center of the box as possible. In fact, there is a second box inside the larger box; we will call this smaller box the innermost box. Participants score 3 points if a bean bag lands in the innermost box, 1 point for a bean bag that lands in the larger box (but not in the innermost box), and 0 points if the bean bag falls outside the box. The participant with the highest score wins.
Let’s play fair. Use only your eyes to guide your throwing. Please do not look at your hands while tossing the bean bags. Also, be sure participants remain at the same distance from the targets. We can use a line marked on the floor to maintain that distance.
Everyone else will watch the participants, make observations to assist with answering homework questions, and await the results of the competition.
Then, we will allow participants who want a “do-over” a second (or third) chance if time permits.
What Happens during the Demo
Your eyes provide input to your brain. Wearing the goggles results in the input being wrong. It’s displaced! Based on the throws you saw, is the input displaced to the right or the left? How much is it displaced? You can describe the amount of displacement using the concept “visual angle.”
Review: What route does the visual input take to get to the brain?
When the brain gets the wrong input, the output will be wrong, too. That’s why it was so hard to throw the bean bags accurately with the goggles on.
Review: Where does the visual cortex send signals to in order to throw the bean bag at the target?
If you wore the goggles for 20 minutes or so, you would adapt: Your throws would become accurate. What would happen if you took the goggles off after that time? Would you be able to throw accurately? Why or why not? If not, make a prediction about where your bean bag would land.
Horizontal Slices
In this final dissection lab, you will be investigating the specimens in the horizontal or transverse plane. Familiarize yourself with identifying and/or locating these structures: 1) pineal body (See image below.) and 2) thalamus. You can use these structures to identify where you should position your scalpel to make each dissection slice. For the pineal body, review your work on the mid-sagittal slice. For the thalamus, you might review your work on coronal slices 3a and 3b.
Procedure:
- Place the hemisphere you selected for horizontal slices from your whole specimen in a dissection tray. Look at the midsagittal view, and locate the pineal body. Because it would be easier to cut the specimen while the mid-sagittal view is on the wax of the dissection tray, we suggest you use two pins to identify the anterior and posterior points of the brain you would need to cut to dissect in a straight line through the pineal body. In other words, decide on some markers on the lateral view that will allow you to cut through the pineal body, and place pins in those markers. Then, lay the specimen on its mid-sagittal side.
- Cut anterioposteriorly based on the markers you identified when looking at the pineal body.
- After making your slice, you can view the structures on the inferior surface of that cut you made. This is horizontal slice 1 – the most superior dissection cut you will make during this lab.
REVIEW BOX
On the following image, and on your own slice, be sure you can identify these structures:
- Head of the caudate
- Lateral ventricle (Identifying the ventricle may be more easily accomplished with the use of a probe on your specimen than by looking at the image.)
- Cerebellum
- Corpus callosum
- A gyrus
- A sulcus
- Cortical gray matter
- White matter
- Septum pellucidum (While this structure is depicted in horizontal slice 1 of our resource books, it is not visible in the dissection slice depicted. Look to horizontal slice 2 image, instead.)
4. Next, considering what you know about the thalamus in relation to visible structures on the mid-sagittal slice or lateral view, select some markers that will allow you to cut through at about the middle of the thalamus, and use the pins as previously discussed (to help you envision a straight dissection line through the thalamus).
5. Next, cut anterioposteriorly along the imagined line connecting the markers you identified. The inferior surface is horizontal slice 2.
REVIEW BOX
On the following image, and on your own slice, be sure you can identify these structures:
- Lateral ventricle
- Cerebral aqueduct (in the image)
- Cerebellum
- Corpus callosum
- A gyrus
- A sulcus
- Cortical gray matter
- White matter
- Lateral geniculate nucleus (LGN)
- Medial geniculate nucleus (MGN)
