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Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ “Learning” in Planarians Learning is described as a process by which we acquire knowledge about the world. In experimental science, the way we can measure this phenomena is by a behavioral bioassay. Learning can be defined as a change in behavior due to experience. This definition excludes behavioral changes due to developmental maturation or by drugs. The behavioral change has to come from exposure to the environmental medium that has acted on sensory systems to produce stimuli. While some researchers exclude very simple experience-induced behavioral modifications from the definition of “true learning”, most neuroscientists identify the category of non-associative learning to include sensitization, habituation, and dishabituation. There are two types of sensitization. (1) a stimulus that originally elicited a weak or no response, evokes an increased response with repeated presentations. or (2) following one stimulus, a different stimulus evokes an increased response than would have been otherwise observed. For example, after a very loud crash sound, smaller noises can startle a person, which otherwise would go almost unnoticed. In the case of habituation, repeated presentation of the same stimulus produces decreasing responses to it. In the example of the loud crash, if it keeps sounding repeatedly every twenty seconds, the startle to it will decrease with additional presentations. Behaviorally, dishabituation is very much like the 2nd form of sensitization in that one stimulus evokes an increased response to another stimulus, however dishabituation is different from sensitization because it restores the strength of a response that was previously decreased by habituation. Because the same stimulus can evoke an increased response in either habituated or nonhabituated responses, dishabituation was thought to represent a special instance of sensitization in which the increase in response is simply superimposed on a habituated response level. However, by considering the ontogeny (see your 4 questions of animal behavior) researchers were able to determine that dishabituation and sensitization reflect separate mechanisms (see your 4 questions of animal behavior). Rankin and Carew (1988) studied the withdraw reflex in the sea slug Aplysia throughout development and found that dishabituation was present in all developmental stages, but sensitization was not seen until several weeks later. The identification of molecular mechanisms that distinguish between sensitization and dishabituation continues to be an area of current research (e.g. Mongeluzi and Frost (a Reedie), 2000; Hawkins et al., 2006) and will be presented in lecture. For the purpose of this lab, we will distinguish between sensitization and dishabituation according to a behavioral definition. We will refer to any stimulus that restores (or exceeds) normal response to stimulus as a “dishabituating” stimulus only when that animal has previously been habituated (i.e. trained with repeated stimulus that resulted in 1 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ a decremented or reduced response). Much of the behavioral analysis, as well as the study of the molecular mechanisms underlying non-associative learning, was conducted using the marine mollusk (slug) Aplysia (visit Steve Arch’s lab if you would like to see one). The nervous system of the slug is easily accessible and they have very large neurons for electrophysiology, however these animals are difficult to maintain in lab so we will use a different model organism to study simple forms of learning. simple learning 18 A B C D 16 response strength 14 12 10 another stimulus 8 6 4 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 trail number trial number Figure 1 presents the average response strength determined for three animals that were each tested with a repeated stimulus for 21 trials (error bars indicate standard error). In your own words, in your lab notebook, describe which simple form of learning is represented during phase A, B, C and D. Planarians belong to the order Tricladida (named for their three branched gut structure) in the phylum Platyhelminthes (Flatworms). Although this phylum has retained a sac like gut with only one opening, flatworms are triploblastic (have tissues derived from three embryonic germ layers) but have no coelom. Planarians can be found in most bodies of fresh water, they are the free-living relatives of parasitic flatworms such as tapeworms and flukes. Most planarians are hermaphrodites and can therefore reproduce sexually as either males or females (simultaneously). They also reproduce asexually by fission. In nature they do this by holding tight to the substrate at the tail end while the head end crawls away, but in the lab they show remarkable regenerative capabilities and have recently become a powerful model for cell differentiation (for a review see Salo, 2006). More significant to their utility in research on behavior, these animals have a central nervous system. In the head of a planarian, there is a concentration of nervous tissue that 2 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ can be called a “brain” which allows more complex behaviors. By virtue of abundant sensory cells, specialized sense organs and a centralized, cephalized nervous system, planarians show varied behavior with rapid and precise responses. Planarians are negatively phototactic (avoid light), positively rheotactic (move toward water current) and respond to different chemical cues. In lab we will test the hypothesis that planarians exhibit simple forms of learning. When fully extended, a planarian can readily be characterized as having a “head” and “body”. When fully contracted, a planarian appears to be an undifferentiated, roughly circular blob. A planarian crawling in a Petri dish initially exhibits a reflexive contraction response when water is dropped onto or near its anterior region (hereafter referred to as its head). This response declines in magnitude over repeated trials but shows partial or full recovery following moderate physical agitation (dishabituation). Table 1. Scoring System for Planarian for Contraction Response Score Behavior Description 0 No response The planarian remains in extended swimming position 1 Simple flinch The planarian responds but without visible shortening of the body 2 Head Retraction Visible widening of the body and decrease in head distinction 3 Full Contraction Near or complete loss of head-body distinction MATERIALS: Each team of 4 students will have: plastic disposable pipettes 100 – 1000 micropipettor + blue tips Petri dishes (60mm) 15 flat worms (12 experimental, 3 practice) Planarian water in a 15 ml tube 2 white plastic squares stopwatch instructions for setting a stopwatch to beep every 10 seconds: upper left button = Reset \Select upper right button = Start\Stop\Set Lower right button = Chrono/Timer Chrono/Timer switch from Chrono to Timer Select 3 times Set 1 time Select 3 times press-Start Stopwatch will beep every 10 seconds and count the number of 10-second intervals. press-Stop Reset 3 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ PROCEDURE Practice drawing the planarian into and expelling it from the disposable pipette gently. Draw a substantial amount of water into the plastic pipette along with the planarian to prevent it from clinging to the pipette’s inside surface. This is how you will move one animal at a time in and out of the experimental arena. By rapid repetition, this is also the stimulus that will be used for sensitization and dishabituation. Working in teams of 4 students, establish interobserver reliability. Get three practice worms Fill the Petri dish bottom with 3 ml. of Planarian water (use a 100-1000 micropipettor to measure). This is sufficient to submerge the planarian but not allow it to avoid the impact of a water stimulus. (Before adding the worm shake the water to break surface tension and cover the bottom of the plate) Using a practice planarian before beginning the experiment proper, transfer one planarian to the center of the Petri dish on top of the white plastic square. Set the stopwatch to beep every 10 seconds. (instructions above) For each trial, expel a drop of water from the disposable pipette onto the planarian’s head from approximately 1 cm above the water’s surface. Draw the stimulus water from the Petri dish so as not to add more volume. Increasing volume would introduce a confounding factor. Repeat this for 5 trials at 10 second intervals. In your lab notebook, record your response using the scoring system in table 1. As a team, compile the scores in the following table to calculate reliability. Trial / 1 2 3 4 5 Obs. 1 2 3 4 ____ ____ ____ ____ ____ How many observers agreed on each trial? Interobserver-reliability is calculated as (total # in agreement)/ (total # of observations) * 100 In order to achieve 90% inter-observer reliability, your team must score 18 agreements for the set of 20 observations. Discuss your results. Repeat the test until the team achieves 90% interobserver reliability. Experimental Methods A habituation trial will be conducted with 10 second intervals for 15 trials. The 10 second interval should allow time for the planarian to resume its gliding motion. Be careful to not disturb the Petri Dish, if the dish is disturbed, skip this data point, because the individual has received additional stimulus that is not part of the experimental design. Dishabituation/Sensitization stimulus: The planarian is stimulated by drawing it into and expelling it from the disposable pipette 5 times in rapid succession. This stimulus typically produces the effect without risk of injury to the subject. If you are working with a very large planarian you may need to cut the tip off of the disposable pipette to provide a large enough opening to not damage the animal. 4 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ Protocol – Read through the Protocol and make a plan of attack. Use a work flowchart or outline in your lab notebook, and decide who will drop the water, who will do the recording, and be sure you know how to set the timer. Experiment 1: Habituation/Dishabituation Protocol Get 6 flatworms. Each team of 4 will habituate 6 individual planarians in separate dishes with repeated water drops. The team can split into pairs. The pairs should work in parallel, each habituating 1 of the group’s planarians at a time. One student administers the water drop while the other records the response. The recorder should start the timer when both the two droppers in your team are ready, and the recorders should enter data directly into the lab notebook. It is important to run these protocols in parallel so that the “no treatment” in the control is the same amount of time as the dishabituation stimulus. HDH HCH Three of the habituated planarians will be “dishabituated” with the dishabituation stimulus immediately following the habituation trials. As soon as this animal has resumed its gliding motion, immediately repeat the full 15 trials for habituation. Trial 1 … Trial 15 dishabituation Trial 16 … Trial 30 The other three of the habituated planarians will be the “control group” and will not be treated with the dishabituation stimulus. This animal will sit undisturbed for a time equal to the dishabituation stimulus. This animal will then be retreated with the full 15 trials for habituation in parallel with the experimental animal. Trial 1 … Trial 15 no treatment Trial 16 … Trial 30 After the experiment, return the used Planarians to the jar labeled “used planarians”. Experiment 2: Sensitization/Habituation/Dishabituation/Habituation Protocol Get 6 new flatworms BEFORE you do anything, do one water drop test to get a baseline measure for each animal (experimental and control). Record this baseline as trial 0. Each team will “sensitize” 3 new individual planarians with the disposable pipette and immediately habituate these animals with the repeated water drop stimulus. Each team will also run 3 control planarians that do not receive the sensitization stimulus. SHDH CHCH Three of sensitized/habituated planarians will be “dishabituated” with the disposable pipette and then retested for persistence of habituation with repeated water drops. Trial 0 sensitization Trial 1 … Trial 15 dishabituation Trial 16 … Trial 30 The other three control planarians will be the “control group” and will not be treated with the sensitization or dishabituation stimulus. This animal will sit undisturbed while the other pair in your team is sensitizing and later dishabituating their test animal. This animal will then be retreated with the habituation trial in parallel with the experimental animal. Trial 0 no treatment Trial 1 … Trial 15 no treatment Trial 16 … Trial 30 5 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ DATA ENTRY StatView Instructions Log in to the computer and the Courses Server as YOURSELF. Enter your data before starting the statistical analysis section. Drag the Expt 1+2 worm data template data template file from the Courses Server to your desktop. Open the Expt 1+2 worm data template that is on your desktop. Save As Joe_Th_Carey or Sue_F_Ned etc. Enter your results, one row for each worm, in the “Joe_Th_Carey” file on your desktop. Save this file on your desktop and also put this file in the drop box. You will need this file for part A of the assignment. The class will also be waiting for your results for the class data file. STATISTICAL ANALYSIS What are we controlling by including a control group that does not receive the stimulus? Was there an experimenter artifact? There should be no difference between the experimental and control animals prior to the dishabituating stimulus. But, there could be a strong effect of water dropping technique that might cause a different degree of habituation. In some experimental designs it is important to randomize all sources of variation, but sometimes it is not possible, or it may introduce too much confusion to the experimental design. When it is not possible to control these effects or randomize the design, it is important to examine the data to ensure that significant results are due to biology that you are studying and not due to your experimental design. Below are listed several interesting questions you could answer by interrogating the data from this very simple experiment. The questions are listed on the left and the statistical tests are listed on the right. In your group decide which tests you would need to do to answer each question. If your group has additional questions, write them in your lab notebook and decide what parts of the data set you will need to use to answer them. Experiment 1-Which comparison and statistical test would you use with data from experiment 1 in order to answer the following questions? Match A-E with 1-5. 1) Did the animals habituate to repeated stimulus? A) paired t-test trial 1 compared to trial 2, 3, or 4. 2) Did the dishabituation stimulus cause a significant B) paired t-test for the first mean habituation increase in response? score compared to the second mean habituation score for experimental animals. 3) Is the response to stimulus fully restored by the C) paired t-test trial 16 and trial 1 dishabituation stimulus? (not significantly different) experimental animals only. 4) Is habituation after dishabituation the same as the D) t-test (trial 16 minus trial 15) original habituation? (use mean score for 15 trials as experimental compared to control. the characteristic of mean habituation) 5) Did the animals show sensitization? E) paired t-test for trial 1 and trial 15 all (According to the earlier description of sensitization, animals. how is this different than the sensitization stimulus that we use in experiment 2?) 6 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ Experiment 2- Which comparison and statistical test would you use with data from experiment 2 in order to answer the following questions about sensitization? Match A-E with 1-5. 1) Did the sensitization stimulus cause an increase A) paired t-test (trial 1 minus trial 0) compared to in response? (trial 16 minus trial 15) experimental animals. ** 2) Does the dishabituation stimulus cause a B) compare the animal’s mean score for the entire significant increase in response to stimulus? (same habituation session before the dishabituation as in experiment 1) stimulus to the mean score for the entire habituation session afterwards using a paired t-test and experimental animals only 3) Does the dishabituation stimulus cause a greater C) paired t-test compare trial 0 to trial 1 difference in response than the sensitization experimental animals only. stimulus? 4) Is the habituation after dishabituation different D) t-test (trial 16 minus trial 15) experimental than habituation after sensitization? (use mean compared to control. score for 15 trials as the characteristic of mean habituation) 5) Did the sensitization stimulus cause greater E) t-test (trial 1 minus trial 0) experimental sensitization than simple repeated stimulation with compared to control. the same stimulus? Combining Data, a “Meta Analysis” Which comparison and statistical test would you use with data from experiment 1 and experiment 2 in order to answer the following questions about sensitization? Match A-B with 1-2. 1) Is habituation following dishabituation A) compare the mean for trials 16 through 30 stimulus affected by an earlier sensitizing stimulus? experiment 1 experimental animals compared to the mean for trials 16 through 30 experiment 2 experimental animals. 2) As a necessary control for a meta analysis you B) t-test trial 1 experiment 1 compared to trial 0 must show that the animals used (and baseline experiment 2. ** conditions) in the two experiments are not different. --Why do we not use a “paired t-test” and only a “t-test” in some of these analyses? --Record your reasoning in your lab notebook and include any other interesting comparisons you could do to answer additional questions about the simple learning in planarians. ** tests marked with a double asterisk are tricky, ask for help if you need it. 7 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ HAND IN (Due at the end of lab) A: Hand in two graphs of your team’s data, one for experiment 1 and another for experiment 2. On each graph, the average score for the 3 experimental animals and also the average score for the 3 control animals should be plotted on the Y-axis and the trial number for that experiment should be plotted on the X-axis. After you have entered your data and put your file in the drop box begin this part of the assignment. StatView Instructions for data graph Drag the score by trial data template from the Courses Server to your Desktop. Open the score by trial data template file that is now on your Desktop. Copy the scores for trials 0-30 for all 6 animals in experiment 1 from your saved version of Expt 1+2 worm data template (Joe_Th_Carey) on your desktop. In the score by trial data template, select the entire area where the data will go and use Paste Transposed from the Edit menu to move trial scores from horizontal to vertical form. The text entries (habituation, sensitization, dishabituation) in your data set will automatically be assigned numbers as placeholders, but you DO NOT want to graph these, so you must first delete them. Find the trial number 15.5 and delete the placeholder numbers in the data field. Repeat these steps for all 6 animals in experiment 2. The placeholder numbers that need to be deleted now correspond to trials 0.5 and 15.5. You will calculate the mean for the 3 control and the three experimental animals for each experiment separately in the appropriately named 4 columns to the far right. Use the Source:Dynamic Formula to enter the formula to calculate the average score for each set of 3 worms. Choose Analyze / New View to make the graphs. Using Bivariate Plots / Line Chart plot the average scores for the experimental and the average scores for the control animals on the Y axis (Y Variable) and plot the trial# for the appropriate experiment on the X axis (X Variable) B: Choose 3 hypotheses from above (or ones generated by your team). 1) Apply the appropriate statistical test to the class data set. 2) Present your results in a graph. 3) Include a figure legend to briefly state the method (not the result). 3) Summarize the results of your statistical analyses and explain whether your data clearly answer the question you asked. Graph your team’s data and discuss the hypotheses that you wish to test before going on to the statistical analysis. You will have to wait for the complete lab dataset in order to have a sample size appropriate for the t-tests. 8 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ General StatView Instructions for t-tests The exact steps that you use will depend upon the questions you choose to answer. Both types of t-tests can be done using the worm_data_day_room file (example: worm_data_T_Ned) found on the Courses Server. When you need to separate experiment 1 from experiment 2, you will use Manage-> Create Criteria When you need to separate experimental from control values, you will select the expt/control column and use the Split By button in the Variables window. When you need to calculate the difference between two columns, you will need to do this in a new column. Use Manage-> Formula to calculate the difference between two columns and place the results in a new column. The mean of trials 1-15 and the mean of trials 16-30 are done for you in columns at the end of the file. t-test: used to compare the means of two distributions given the between group and within group variances Factorial ANOVA is an extension of the t-test for >2 groups. Analyze-> New View Unpaired Comparisons Unpaired t-test Hypothesized difference: 0 95% Confidence Interval Tail: Both OK Assign a continuous variable and a nominal variable with the Add button. Paired t-test: used when multiple response variables are measured for each individual Repeated measures ANOVA is an extension of the paired t-test for > 2 groups. Analyze-> New View Paired Comparisons Paired t-test Hypothesized difference: 0 95% Confidence Interval Tail: Both OK Assign two continuous variables with the Add button. To make your graphs, you will use Cell Plot->Point Chart to plot the two means with 95% Confidence Intervals. While the t-test tables are still selected, Point Chart will use the same two variables already selected. 9 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ WORM RUNNERS In 1955 two psychologists, Thompson and McConnell, published a paper claiming that they had successfully trained individuals of the planarian species Dugesia dorotocephala (black planarian) with a classical conditioning task. Planarians had hardly ever been used before in learning experiments, but the Thompson and McConnell experiment was a relatively straightforward extension of behavioral psychology techniques to a new organism. The subject of planarian learning did not become the subject of controversy until after the publication of further, dramatic experiments. As mentioned above, planarians can regenerate into viable flatworms if they are cut into pieces. McConnell used this regenerative property to ask exciting questions about the retention of memory. He trained worms, cut them in half, allowed them to regenerate and then tested them to see how much of their previous training they had retained. Surprisingly, worms that regenerated from the brainless tail sections remembered as much as if not more than head section regenerates (McConnell et al., 1959). A further characteristic of the particular species used by McConnell is that it is cannibalistic. In an even more daring and controversial paper, McConnell fed pieces of trained worm to hungry untrained worms (McConnell, 1962). Their results indicated that a behavioral tendency or a “memory” had been transferred from the trained to the untrained worms. “Knowledge, it seemed, was edible!” Others went on to identify the mechanism of memory transfer and focused their efforts on RNA by injecting a recipient with intact RNA isolated from previously conditioned worms. The training could be transferred if the RNA was first degraded (Jacobson et al., 1966). For a time, experimenters in this field published a special-interest newsletter titled “Worm Runner’s Digest”. The results of those studies are still controversial. However, recent advances in our understanding of gene regulation including additional functions of RNA molecules have renewed scientific interest in this protocol. PROCEDURE In the back of the room there is a container of planarians to be used for a long-term learning experiment (labeled “for training”). Each day these flatworms will be trained to go right (Carey’s class) or to go left (Ned’s class). 1. Hook the 9 volt battery to the Train-a-Tray. Attach a wire between the positive battery terminal to the + sign on the Train-a-Tray. Attach the other wire to the Train-a-Tray only. You will manually touch the loose end to the negative terminal of the battery in order to deliver a “punishment.” 2. Fill the Y-trough with 3 ml of pond water. Fill the pre-training holding pen with pond water 3. Gently place your 3 worms in the pre-training holding pen using the disposable pipette as before. 4. Get 5-6 worms from your habituation experiments and allow them to crawl around the Train-a-Tray trough to get it good and slimy. (~5minutes) 5. Remove the slime contributors. 6. Place your first flatworm for training at the base of the Y-Maze with its head pointing toward the Y and watch it crawl. 10 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ 7. When it reaches the Y and is between the center contact point and the contact point in one Y arm: a. If it goes down the wrong arm, it is punished with a quick shock. When it stretches out to crawl again, use the disposable pipette to return it to the “already trained” container for training tomorrow. 8. Each worm should receive 3 training trials 9. Each group of 4 students should train 3 flatworms (or more if there are < 6 student groups in class). 10. When you are finished, return all worms to the “already trained” container for training tomorrow. 11. Rinse your training tray. (DO NOT USE SOAP.) ANALYSIS Determine the number of worms making correct choices and the number of worms making wrong choices observed by your team. Record these in the lab data collector for today’s lab. http://collector.reed.edu/ You will enter: 1) your name 2) your partners' names 3) class (Ned/Carey) 4) day (T,W,TH,F) 5) the total # of correct choices. 6) the total # of incorrect choices. At the end of the week, the data will be available from all classes, so that we can use lab lecture to determine if the worms have learned. We will see if the worms made significantly more correct choices on Friday than on Tuesday. No assigned write up other than your lab notebook record. References (will be available in lab if you are interested) Thompson, R. and McConnell (1955) Classical Conditioning in the Planarian Dugesia dorotocephala, J. Comp Phsyiol Psych 48:65-68. McConnell, J.V., Jacobson, A.L. and Kimble, D.P. (1959) The Effects of Regeneration upon Retention of a Conditioned Response in the Planarian. J. Comp Phys Psych 52:1-5. McConnell, J.V. (1962) Memory Transfer Through Cannibalism in Planarians J Neuropsychiatry 3:s42. Jacobson, A.L. Fried, C., Horowitz, S.D. (1966) Planarians and Memory. Nature 209:599-601. Hawkins, R.D., Tracey E. Cohen, T.E., and Kandel, E.R. (2006) Dishabituation in Aplysia can involve either reversal of habituation or superimposed sensitization Learning and Memeory 13:397-403. Rankin, C.H. and Carew, T.J. (1988) Dishabituation and sensitization emerge as separate processes during development in Aplysia. Journal of Neuroscience 8:197-211. 11 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ Mongeluzi,D.L. and Frost, W.N. (2000) Dishabituation of the Tritonia Escape Swim Learning and Memory 7:43-47 Appendix Learning can be defined as a process that expresses itself as an adaptive use information to cause a change in behavior resulting from experience. Though information can be learned that does not lead to the change in behavior, that learning is very difficult to detect in most animals. The stages of learning include acquisition, consolidation, retrieval and extinction. Memory can be defined as the encoding, storage, and retrieval (or forgetting) of information from past experiences. Memory is necessary if learning is to take place. Non-associative learning occurs after repeated presentation of a stimulus. In the first part of this lab, you explore simple forms on non-associative learning, sensitization and habituation. Habituation is fundamentally different than fatigue, the loss of efficiency in the performance of motor act due to physiological run down. It is also different than sensory adaptation that occurs at the level of the sensory receptor, when it simply stops sending impulses after repeated stimulation. In habituation, the sensory cell is still sending signals to the CNS, but they are essentially ignored. Associative learning is a broad category that refers to many of our daily activities: learning to be afraid, learning to talk, learning a foreign language, or learning to play the piano. In essence, associative learning involves the formation of associations among stimuli and /or responses. It is usually subdivided into classical conditioning and operant conditioning. Classical Conditioning is a type of associative learning, also known as Pavlovian conditioning. Ivan Pavlov described the learning of conditioned behavior as being formed by pairing stimuli to condition an animal to give a certain response. The simplest form of classical conditioning, is reminiscent of what Aristotle would have called the law of contiguity, which states that: “When two things commonly occur together, the appearance of one will bring the other to mind." This type of conditioning is induced by a procedure in which a generally neutral stimulus, termed the conditioned stimulus (CS), is paired with a stimulus that generally elicits a response, termed the unconditioned stimulus (US). At first the unconditioned stimulus will elicit the unconditioned response (UR). After repeated pairing of a conditioned stimulus with the unconditioned stimulus in order to evoke the unconditioned response, the conditioned stimulus alone will be sufficient to evoke a response that is now termed the conditioned response (CR). Operant Conditioning is the type of associative learning made famous by B.F. Skinner. In operant conditioning, learning occurs when an animal performs an action in the course of normal behavior and this particular action is reinforced (either positively or negatively). After sufficient pairing, the animal learns to associate its action (the operant) with the reinforcement. This is the type of learning we examine with the Train-a-Tray in the second part of lab. For both associative and non-associative learning, the information is encoded in the animal’s nervous system through modifications of biophysical properties of neurons, as 12 of 13 Lab#2 Animal_behavior_Renn_2007_REVISED-Fri. NAME ________________ well as the strength of synaptic connections among neurons. These changes are often referred to as a “memory trace” or “engram”. In humans there is no single universal system for learning and memory. Instead, different memory systems can use different mechanisms, and a single memory system will include multiple mechanisms. We will discuss these mechanisms in lecture. 13 of 13