Box

NONGRADED ACQUISITION OF MEMORY: FOOD AVERSION AND IMPRINTING

Food Aversion

While most forms of long-term memory exhibit graded acquisition, some types of learning are so critical to an animal's survival that extremely robust learning mechanisms have evolved to subserve them. One striking example of this is conditioned food avoidance. Generally, if an animal consumes a novel foodstuff that subsequently causes sickness, even after a single such experience the animal will exhibit a lifelong aversion to that particular food. For animals in the wild, the survival value of this type of learning is obvious, but the phenomenon can have unintended consequences. For example, I once got food poisoning after eating a bowl of New England clam chowder; to this day, even the sight of a can of New England clam chowder on the grocery store shelf is enough to send me scurrying to the next aisle. This is a textbook case of conditioned food avoidance—being from Alabama, I had never had clam anything until that day. I certainly will fastidiously avoid future clam encounters of any kind.

NONGRADED ACQUISITION OF MEMORY: FOOD AVERSION AND IMPRINTING

Although I have not personally experienced it, hatchling chicks exhibit a robust form of learning termed imprinting. A newborn bird will develop a strong, long-lasting affinity for whatever it sees in the first hour after hatching. In one famous example, a group of young geese imprinted on the experimental ethologist Konrad Lorenz. In experimental situations, chicks will even imprint on inanimate objects such as red boxes or dolls. Of course, in the wild this type of learning serves a useful purpose, as hatchlings will almost always first see their mother and imprint upon her. The chicks will then stick close by the mother as she guides and protects them through the perilous fledgling period. Photo of chick courtesy of Reuben Clements.

For example, a single repetition (or training trial) may elicit a memory that lasts only a few minutes, whereas repeated stimulations will likely result in memory lasting hours to days. Repeated presentations of multiple training trials can elicit memory lasting for even more prolonged periods, up to the lifetime of the animal. Thus, the acquisition of memory is a graded phenomenon (see Figure 4).

One exciting area of contemporary learning research is to try to understand the basis for this attribute. It is intriguing to wonder how repeated presentations of the identical environmental stimulus can uniquely elicit a long-lasting behavioral alteration, especially when one considers that the behavioral output (e.g., enhanced responsiveness) is identical in the short-and long-lasting forms. This phenomenon is still fairly mysterious at present for the various mammalian systems that we will be discussing; however, at several points in subsequent chapters, I will describe current thinking in this area.

Long-term memory also has the general attribute that it undergoes a period of consolidation. Decades ago, researchers discovered that, for a period of time after the training period, generally on the order of hours, memories that were normally destined to become long-term memories were susceptible to disruption. Disruption of nascent long-term memories can be brought about by trauma, for example, or in a more refined manipulation application of inhibitors of protein synthesis can block memory consolidation (Figure 5). Thus it is clear that some set of molecular processes is occurring for some period of time after the training trial, which are necessary for memory to be established as truly long-lasting. After the critical time window has passed, the same disruptive manipulations have no effect on memory storage. Aside from the insight that protein synthesis inhibitors can block memory consolidation, not much is known concerning the specific molecular underpinnings of this fascinating process. Studies of the cellular and molecular mechanisms contributing to the consolidation of hippocampus-dependent long-term memory will be an area of emphasis in Chapters 6-9 and Chapter 12 of this book.

There has been a resurgence of interest in the consolidation phenomenon lately because several groups have reported that previously stored memories are subject to disruption in certain circumstances.

Aside Box Formation

FIGURE 4 Graded acquisition of memory. Multiple training trials typically result in more robust and long-lasting memory formation. In this case, sensitization of the gill-withdrawal reflex was measured by quanti-tating the duration of gill withdrawal in response to a slight touch (duration of withdrawal, Y-axis). Delivery of a tail shock to the animal elicits sensitization, an increase in the magnitude of the protective gill withdrawal reflex (see Box 3 and text). Increasing the number of training trials (tail shocks) increases both the duration of the memory (days of duration) and the magnitude of the learned response. Adapted with permission from Kandel (14). Copyright 2001 American Association for the Advancement of Science.

FIGURE 4 Graded acquisition of memory. Multiple training trials typically result in more robust and long-lasting memory formation. In this case, sensitization of the gill-withdrawal reflex was measured by quanti-tating the duration of gill withdrawal in response to a slight touch (duration of withdrawal, Y-axis). Delivery of a tail shock to the animal elicits sensitization, an increase in the magnitude of the protective gill withdrawal reflex (see Box 3 and text). Increasing the number of training trials (tail shocks) increases both the duration of the memory (days of duration) and the magnitude of the learned response. Adapted with permission from Kandel (14). Copyright 2001 American Association for the Advancement of Science.

FIGURE 5 Protein synthesis inhibitors block consolidation of long-term memory. Inhibitors of protein synthesis typically block the ability of learned information to be consolidated into a long-lasting form. In this experiment rats were trained in a step-down avoidance paradigm (see Chapter 2). Animals are placed on an elevated platform in the middle of an electric grid and receive a mild foot shock when they step down from the platform. On the training day, animals that received a saline infusion (CONTROL) or the protein synthesis inhibitor anisomycin (INHIBITOR) both quickly step down from the platform (latency to step-down, Y-axis). Twenty-four hours later, the control animals exhibit a much longer latency to step-down, indicating that they have learned to avoid the electrified floor. Animals treated with protein synthesis inhibitor have not consolidated their memory for the step-down training and exhibit a short latency to step-down just as they did on the first day. Additional experiments (not shown) have demonstrated that anisomycin treatment immediately after training is also effective at blocking memory consolidation, indicating that consolidation is a post-training phenomenon (5). Adapted from reference 5. Copyright 2001 National Academy of Sciences, USA.

Specifically, for some types of memory an event already learned and stored in long-term memory is selectively subject to disruption when it is recalled. The basic experimental observation is that while protein synthesis inhibitors do not wipe out stored memory, the same protein synthesis inhibitor treatment will disrupt memory if the subject is simultaneously required to recall the information (see references 5 and 6). Thus, pairing protein synthesis inhibitors with a behavioral task requiring information recall can lead to a selective loss of a previously stored memory from long-term stores. The intriguing but still ill-defined process underlying this observation is currently referred to as reconsolidation of memory.

Finally, to round out our terminology, we need to introduce two terms related to the loss of memories: extinction and forgetting. Forgetting is woefully familiar to most of us, and its basis is essentially unexplored. Extinction is the specific erasure of a memory in response to an environmental stimulus. Extinction has largely been studied in the context of reversal of learning. For example, if your cafeteria serves hamburgers every Monday, you will over time learn that the cafeteria always serves hamburgers on Monday. If at some later point they quit serving hamburgers on

Monday, it will take a while to readjust. Over time, you will no longer assume that if it's Monday that means hamburgers; similarly, you will no longer infer that if you're having hamburgers then it is Monday. This disassociation is an example of extinguishing a previously learned response. You will have extinguished your memory that Monday means hamburgers. Similar to forgetting, the mechanisms of extinction have not been extensively studied. One intriguing speculation is that the reconsolidation mechanism may be involved in some cases, the thinking being that perhaps reconsolidation is the process that has evolved to allow specific erasure of previously learned material, by opening up a period of susceptibility upon recall (5, 6).

Was this article helpful?

0 0
Eliminating Stress and Anxiety From Your Life

Eliminating Stress and Anxiety From Your Life

It seems like you hear it all the time from nearly every one you know I'm SO stressed out!? Pressures abound in this world today. Those pressures cause stress and anxiety, and often we are ill-equipped to deal with those stressors that trigger anxiety and other feelings that can make us sick. Literally, sick.

Get My Free Ebook


Post a comment