Studies by

Studies by Byrne and colleagues, also in the snail Aplysia, have suggested still another mechanism of synaptic memory. Their studies have shown that stimuli from separate sources acting on a single neuron, under appropriate conditions, can cause longterm changes in membrane properties of the postsynaptic neuron instead of in the presynaptic neuronal membrane, but leading to essentially the same memory effects. LongTerm Memory There is no obvious demarcation between the more prolonged types of intermediate longterm memory and true longterm memory. The distinction is one of degree. However, longterm memory is generally believed to result from actual structural changes, instead of only chemical changes, at the synapses, and these enhance or suppress signal conduction. Again, let us recall experiments in primitive animals where the nervous systems are much easier to study that have aided immensely in understanding possible mechanisms of longterm memory. Structural Changes Occur in Synapses During the Development of LongTerm Memory Electron microscopic pictures taken from invertebrate animals have demonstrated multiple physical structural changes in many synapses during development of longterm memory traces. The structural changes will not occur if a drug is given that blocks DNA stimulation of protein replication in the presynaptic neuron; nor will the permanent memory trace develop. Therefore, it appears that development of true longterm memory depends on physically restructuring the synapses themselves in a way that changes their sensitivity for transmitting nervous signals.

Then the

Then the monkey is taught to recognize different objects with its right eye while its left eye is covered. Next, the right eye is covered and the monkey is tested to determine whether its left eye can recognize the same objects. The answer to this is that the left eye cannot recognize the objects. However, on repeating the same experiment in another monkey with the optic chiasm split but the corpus callosum intact, it is found invariably that recognition in one hemisphere of the brain creates recognition in the opposite hemisphere. Thus, one of the functions of the corpus callosum and the anterior commissure is to make information stored in the cortex of one hemisphere available to corresponding cortical areas of the opposite hemisphere. Important examples of such cooperation between the two hemispheres are the following. Cutting the corpus callosum blocks transfer of information from Wernickes area of the dominant hemisphere to the motor cortex on the opposite side of the brain. Therefore, the intellectual functions of Wernickes area, located in the left hemisphere , lose control over the right motor cortex that initiates voluntary motor functions of the left hand and arm, even though the usual subconscious movements of the left hand and arm are normal. Cutting the corpus callosum prevents transfer of somatic and visual information from the right hemisphere into Wernickes area in the left dominant hemisphere. Therefore, somatic and visual information from the left side of the body frequently fails to reach this general interpretative area of the brain and therefore cannot be used for decision making.

An animal

An animal builds up strong memory traces for sensations that are either rewarding or punishing but, conversely, develops complete habituation to indifferent sensory stimuli. It is evident that the reward and punishment centers of the limbic system have much to do with selecting the information that we learn, usually throwing away more than per cent of it and selecting less than per cent for retention. Specific Functions of Other Parts of the Limbic System Functions of the Hippocampus The hippocampus is the elongated portion of the cerebral cortex that folds inward to form the ventral surface of much of the inside of the lateral ventricle. One end of the hippocampus abuts the amygdaloid nuclei, and along its lateral border it fuses with the parahippocampal gyrus, which is the cerebral cortex on the ventromedial outside surface of the temporal lobe. The hippocampus and its adjacent temporal and parietal lobe structures, all together called the hippocampal formation has numerous but mainly indirect connections with many portions of the cerebral cortex as well as with the basal structures of the limbic systemthe amygdala, the hypothalamus, the septum, and the mamillary bodies. Almost any type of sensory experience causes activation of at least some part of the hippocampus, and the hippocampus in turn distributes many outgoing signals to the anterior thalamus, hypothalamus, and other parts of the limbic system, especially through the fornix, a major communicating pathway.

This was

This was done by inserting a blunt, thinbladed knife through a small opening in the lateral frontal skull on each side of the head and slicing the brain at the back edge of the prefrontal lobes from top to bottom. Subsequent studies in these patients showed the following mental changes: The patients lost their ability to solve complex problems. They became unable to string together sequential tasks to reach complex goals . They became unable to learn to do several parallel tasks at the same time. Their level of aggressiveness was decreased, sometimes markedly, and, in general, they lost ambition . Their social responses were often inappropriate for the occasion, often including loss of morals and little reticence in relation to sex and excretion. The patients could still talk and comprehend language, but they were unable to carry through any long trains of thought, and their moods changed rapidly from sweetness to rage to exhilaration to madness. The patients could also still perform most of the usual patterns of motor function that they had performed throughout life, but often without purpose. From this information, let us try to piece together a coherent understanding of the function of the prefrontal association areas. Decreased Aggressiveness and Inappropriate Social Responses. These two characteristics probably result from loss of the ventral parts of the frontal lobes on the underside of the brain. As explained earlier and shown in Figures and , this area is part of the limbic association cortex, rather than of the prefrontal association cortex.