Hockenbury Discovering Psychology 5th txtbk

The Search for the Biological Basis of Memory263Given those two possibilities, the challenge for memory researchers has been toidentify the specific neurons involved in a given memory, a task that is virtuallyimpossible with the human brain because of its enormous complexity. What this taskrequired was a creature with a limited number of neurons that is also capable oflearning new memories.Enter Aplysia, a gentle, seaweed-munching sea snail that resides off the Californiacoast. The study of Aplysia over the past 30 years has given memory researchers importantinsights to the brain changes involved in memory. Why Aplysia? BecauseAplysia has only about 20,000 good-sized neurons. That was a key reason why memoryresearcher Eric Kandel (2001, 2006) chose this unassuming creature to studythe neuronal changes that occur when a new memory is formed for a simple classicallyconditioned response.If you give Aplysia a gentle squirt with a WaterPik, followed by a mild electricshock to its tail, the snail reflexively withdraws its gill flap. When the process isrepeated several times, Aplysia wises up and acquires a new memory of a classicallyconditioned response—it withdraws its gill when squirted with the WaterPik alone.This learned gill-withdrawal reflex seems to involve a circuit of just three neurons:one that detects the water squirt, one that detects the tail shock, and one that signalsthe gill-withdrawal reflex (see Figure 6.10).When Aplysia acquires this new memory through repeated training trials, significantchanges occur in the three-neuron circuit (Kandel, 2001). First, the functionof the neurons is altered: There is an increase in the amount of the neurotransmittersproduced by the neurons. Second, the structure of the snail’s neuronschanges: The number of interconnecting branches between the neurons increases,as does the number of synapses, or communication points, on each branch. Thesechanges allow the neurons involved in the particular memory circuit to communicatemore easily. Collectively, these changes are called long-term potentiation,which refers to a long-lasting increase in synaptic strength (Fedulov & others,2007; Malenka, 2003).The same kinds of brain changes have been observed in more sophisticatedmammals. Chicks, rats, and rabbits also show structural and functional neuronchanges associated with new learning experiences and memories. And, as you mayrecall from Enhancing Well-Being with Psychology in Chapter 2, there is evidencethat the same kinds of changes occur in the human brain (e.g., Draganski & others,2004).Aplysia, the Supersnail of MemoryResearch Eric Kandel holds Aplysia, the seasnail that is used to study how neuronschange when simple behaviors are learnedand remembered. Kandel was awardedthe Nobel Prize in 2000 for his discoverieson the neural basis of memory.SiphonGillAs the reflex is learned, theamount of neurotransmittersand the branching of neuronsincrease, allowing the circuitto communicate more easily.HeadTail neuronSiphonneuronNeurotransmittersGill-withdrawalneuronFigure 6.10 How Neurons Changeas Aplysia Forms a New Memory WhenAplysia is repeatedly squirted with water,and each squirt is followed by a mild shockto its tail, the snail learns to withdraw itsgill flap if squirted with the water alone.Conditioning leads to structural and functionalchanges in the three neuronsinvolved in the memory circuit.