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Mapping the Receptive Field Laboratory: Week 1
Description of the Laboratory
One of the most important techniques for studying the brain in the past century has been the development of single cell recording. In this method, an extremely think electrode, called a microelectrode, is lowered into the brain until it actually penetrates a neuron, usually an axon is what is recorded from because of the action potentials. The researcher knows that a cell has been penetrated usually by listening to the clicks generated by action potentials over a loudspeaker.
Once a cell has been penetrated, the researcher tries to understand what the cell responds to. In this laboratory, the cells are part of the visual cortex. More particularly, the striate cortex or V1. So the task is to determine what visual stimuli causes the cell to respond the most. In essence, this lab is a simplified version of the research of Hubel and Wiesel that has been discussed in class. In their research, they held on to a cell as long as they could and then presented as many different stimuli to find those features of the world that the cell responded to best.
Goals for the Laboratory
- To develop further your skills in systematic observation, the hallmark of science
- To develop further your skills in making conclusions from data, a fundamental skill in science
- To develop skills in communicating your findings and conclusions
- To understand more fully one method of understanding the brain, single cell recording.
Directions for the Laboratory
Before beginning the laboratory, download and copy the spreadsheet that has been developed for data collection. It will help guide you in your attempts to determine the optimal stimulus for the cells that you are recording from. Bring a copy on a thumb drive to class or use your vault account.
Then open the activity from the link at the bottom of the page. You will be presented with a full screen to run the simulation. The screen is structured as seen below:
On the far left is the menu that will allow you to pick a stimulus to use to stimulate your cell. The choice are a black field (default), a white field, a white dot, a white bar, a black bar, and a moving bar (white). The center of the screen is the area of the retina that you can stimulate. To simplify matters, it is also the receptive field. For this lab, you do not need to hut for the receptive field.
Around and below the receptive field are controls for the stimulus. The sliders to the right and immediately below the receptive field control the position of the stimulus. You can also move the stimulus by clicking or dragging your mouse over the receptive field. You can adjust the width of the bar and the diameter of the dot using the Stimulus Size slider. For the bar stimuli, you can control the tilt or orientation using the Stimulus Orientation slider. Finally, for ease, you can center your stimulus in the receptive field by simply clicking the Center Stimulus button.
Below the stimulus controls is the button you need to press to get a new cell to examine. In front of the New Cell button are buttons you can use to make a map of yoru receptive field. The first two check boxes indicate whether you are mapping excitator areas, those that increase the firing rate, or inhibitory areas, those that decrease the firing rate. You make a mark of the indicated type by right clicking on the receptive field area you wish to mark. If you made a mistake and want to remove a mark, you can use the Remove Last button to remove marks starting with the most recent one place on the screen.
The right side of the screen has a graph of the cells firing rate. The height of the bar indicates the number of action potentials a second. The current value is plotted as part of the x-axis legend. Also, to smooth out the values, the average of the last 10 readings is printed out as well.
This screen is your experimental world for this experiment. Your goal is to try and determine the optimal stimulus for 10 different cells. Your lab partner will do the same giving you 20 cells together to have to characterize.
The spreadsheet that you downloaded has been developed to help guide you through this process. However, do not let it limit you. Use it as a starting point. A sample spreadsheet is shown below (Given space, some of the labels might be a bit different in the actual excel sheet).
| Cell | Black Field | White Field | Dot Center | Dot Surround | White | Black | Orient | Width | FR Optimal |
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| 2 | |||||||||
| 3 | |||||||||
| 4 | |||||||||
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| 10 |
The cell simply indicates which cell the data belongs to. The number is to indicate the order you recorded from the cell. The next four columns are to indicate firing rate to some basic stimuli: a complete black screen, a completely filled white screen, a small dot (choose a small size) in the center of the receptive field, and a small dot outside the center. The next two columns are where you indicate whether the cell responds to a white or dark bar in the center better. The next column, indicate the preferred orientation of the stimulus. Next, indicate the width of the preferred stimulus. The final column you indicate the firing rate of the preferred stimulus. Let me reiterate. This is a minimal summary chart. There is much else to learn here. In no way let this chart dictate what you discover and what you report.
Issues to consider:
- What can you learn from the way that the cell responds to both full-field black and white stimuli?
- What can you learn from the response to the dot?
- How sensitive is the cell to the stimulus orientation?
- How important is the width of the bar?
- How does the cell respond to a moving stimulus? (You need to make this observation and describe its results for at least two cells: one that favors a white bar and one that favors a black bar).
Laboratory Write-up
The point of a laboratory report is rhetorical. What I mean by this is that you are writing a document to convince those that read it that you have accurately describe something about reality. You have both made good observations and you have made a meaningful and sound summary of those observations.
So, you are not merely reporting findings. You are trying to convince. Hubel and Wiesel were able to convince their readers that they have made and fundamental set of discoveries about the organization of the visual cortical areas V1, V2 and V3, particularly V1. They used almost no statistics and were very descriptive. They used few tables and mostly relied on examples to illustrate their results to summarize their findings. They were very meticulous in how they described their findings because other researchers were able to make the same findings and also build upon those findings.
So in this report, you need to take the 20 cells that you and your partner have and describe what you think you have found. For some context, Hubel and Wiesel's papers usually reported studies that had 100's of cells that they had recorded from. In this report, you will write what is in essence a results section and a discussion. You will report your findings and discuss their importance.
Results
First, before you can write your results you need to spend time with your data and see what conclusions you can make. This may seem odd, but your reporting of your results need to be thorough, but also organized to help your readers understand your conclusions. So, first decide what you have found. In addition, to the questions above, some questions you might want to consider are:
- Are there more, over all, cells responsive to white or black bars?
- Are there cells sensitive to all orientations or are there just a few orientations where cells are sensitive at their peak?
- Are all sizes represented by receptive fields or just a few?
In other words, don't just think about the cells you recorded from but what do your findings tell you about all of the cells possible to record from?
So your task is to describe your findings using tables, figures, and text to explain to a reader who was not there when you collected your data what you have found.
Discussion
In this section, you bring together and interpret all that you have found. Here you present your answers to the questions that you have considered. No, not by a simple format of: here is a question the prof asked and here is my answer. Make it interesting to read.
One group will present these results to the class so we can all learn more about making a good description of the results. These data will be combined with the information from the next lab for what will be handed in.
We will discuss this more in class.
Links to lab simulation and spreadsheet
Click here to open the laboratory simulation.
Click here to get the sample spreadsheet for data collection.
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