Overview
Buried within the depths of the cerebrum are several large aggregates of limbic structures and nuclei which are preeminent in the control and mediation of memory, emotion, learning, dreaming, attention, and arousal, and the perception and expression of emotional, motivational, sexual, and social behavior including the formation of loving attachments. Indeed, the limbic system not only controls the capacity to experience love and sorrow, but it governs and monitors internal homeostasis and basic needs such as hunger and thirst (Bernardis & Bellinger 1987; Gloor 1992, 1997; LeDoux 1992, 1996; MacLean, 1973, 1990; Rolls, 1984, 1992; Smith et al. 1990), including even the cravings for pleasure-inducing drugs (Childress, et al., 1999).
The structures and nuclei of the limbic system are exceedingly ancient, some of which began to evolve over 450 million years ago. Over the course of evolution, these structures have expanded in size, some becoming increasingly cortical in response to increased environmental opportunities and input demands, particularly those related to olfaction. In fact, as the forebrain expanded and until as recently as 50 million years ago, the cerebrum of the ancestral line that would eventually give rise to humans, was dominated by the olfactory system, expanding over the course of evolution in response to the emotional and motivational demands of the olfactory-limbic system.
As detailed in chapter 12, the impetus for neocortical evolution was in part a direct consequence of expansions in olfactory-derived limbic structures, some of which, such as the amygdala, continue to maintain massive interconnections with almost all regions of the neocortex. In fact, it could be argued that over the course of evolution, the forebrain and much of the neocortex (and the so called "rational mind") evolved in response to and so as to better serve the limbic system and fulfill and satisfy limbic needs.
For example, much of the neocortex, is concerned with performing hierarchical analysis of stimuli that was formerly the exclusive domain and concern of the limbic forebrain. Once processed at the level of the neocortex this information may then be shunted back to the thalamus and limbic system, as well as to other neocortical structures and/or the brainstem and spinal cord for further analysis, storage in memory, or motoric expression. Like the limbic system the neocortex is largely concerned with emotion, motivation, attention, learning, memory, and visual, sexual, tactile, and gross motor activities, and is exceedingly responsive to limbic and emotional needs.
Olfaction was originally extremely important in evolutionary and phylogenetic development as it allowed the organism to be informed about the environment at a distance, in the evening, while hiding in the brush or in caves, and without the necessity of physical contact (Haberly 1990). Via olfactory cues and the detection of pheromones the organism is able to detect and track food from an incredible distance, determine the intent and social-emotional status of conspecies who may have passed by hours or even days before, as well as signal and leave chemical messages conveying it's own intent, motivation, social position, and/or sexual availability (Ackerman 1990; Blum 1985; Mayer & Mankin 1985; Michael & Keverne, 1974; Tamaki 1985; Wilson, 1980).
Moreover, through the olfactory system the creature was able to learn and remember these attributes including their location in space; and this was made possible via the evolution of the amygdala and in particular the dorsal hippocampus -a structure strongly influenced by the olfactory system (Haberly 1990), and which could use olfactory and visual cues to form a visual-olfactory map of the environment and of important landmarks.
That is, initially the dorsal hippocampus was probably exceedingly responsive to light and may well have been responsible for computing the creatures position as well as that of other creatures and objects, in visual-space -which is a function that it currently subserves in mammals; i.e. the analysis of place and position (Nadel, 1991; O'Keefe, 1976; O'Keefe & Nadel 1974; Wilson & McNaughton, 1993).
However, long before the modern hippocampus gave rise to neocortex and shifted from a dorsal to ventral position in humans, the primitive and ancient hippocamus (in conjunction with the amygdala) probably evolved the capacity to cross correlate and match olfactory/pheromonal cues with visual (and other) cues in order to create a spatial-olfactory map of the environment; as well as store and later recall that information when nessessary (chapter 12). Learning, memory and cognitive-spatial positional analysis are also attributes characteristic of the modern human amygdala and hippocampus (Enbert & Bonhoeffer, 1999; Gloor, 1997; Halgren, 1992; LeDoux, 1996; Nunn et al., 1999; Xu et al., 1998) whereas the modern limbic system still remains responsive to pheromonal chemical signals.
For example, in humans, various odors effect learning and memory and have the capability of triggering vivid recollections of some far away and past event (Hirsh 1993). Moreover, specific "positive" odors (e.g. peppermint) have been reported to improve learning, creativity, and work efficiency (Baron, 1990; Ehrlichman & Bastone 1992) and even time spent gambling (Hirsh 1993).
Indeed, the old limbic brain has not been replaced and is not only predominant in regard to all aspects of motivational and emotional functioning, but is capable of completely overwhelming "the rational mind" due in part to the massive axonal projections of the amygdala upon the neocortex. Although over the course of evolution a new brain (neocortex) has developed, Homo sapiens sapiens ("the wise may who knows he is wise") remains a creature of emotion. Humans have not completely emerged from the phylogenetic swamps of their original psychic existence. Hence, due to these limbic roots, humans not uncommonly behave "irrationally" or in the "heat of passion," and thus act at the behest of their immediate desires; sometimes falling "madly in love" and at other times, acting in a blind rage such that even those who are 'loved" may be slaughtered and murdered.
Indeed, emotion is a potentially powerful overwhelming force that warrants and yet resists control-- as something irrational that can happen to a someone ("you make me so angry") and which can temporarily hijack, overwhelm, and snuff out the "rational mind." The schism between the rational and the emotional is real, and is due to the raw energy of emotion having it's source in the nuclei of the ancient limbic lobe -- a series of structures which first make their phylogenetic appearance over a hundred million years before humans walked upon this earth and which continue to control and direct human behavior.
FUNCTIONAL OVERVIEW
In general, the primary structures of the limbic system include the hypothalamus, amygdala, hippocampus, septal nuclei, and anterior cingulate gyrus; structures which are directly interconnected by massive axonal pathways (Gloor, 1997; MacLean, 1990; Risvold & Swanson, 1996). With the exception of the cingulate which is referred to as "transitional" cortex (mesocortex) and consists of five layers, the hypothalamus, amygdala, hippocampus, septal nuclei are considered allocortex, consisting of at most, 3 layers.
The hypothalamus could be considered the most "primitive" aspect of the limbic system, though in fact the functioning of this sexually dimorphic structure is exceedingly complex. The hypothalamus regulates internal homeostasis including the experience of hunger and thirst, can trigger rudimentary sexual behaviors or generate feelings of extreme rage or pleasure. In conjunction with the pituitary the hypothalamus is a major manufacturer/secretor of hormones and other bodily humors, including those involved in the stress response and feelings of depression.
The amygdala has been implicated in the generation of the most rudimentary and the most profound of human emotions, including fear, sexual desire, rage, religious ecstasy, or at a more basic level, determining if something might be good to eat. The amygdala is implicated in the seeking of loving attachments and the formation of long term emotional memories. It contains neurons which become activated in response to the human face, and which become activated in response to the direction of someone else's gaze. The amygdala also acts directly on the hypothalamus via the stria terminalis, medial forebrain bundle, and amygdalafugal pathways, and in this manner can control hypothalamic impulses. The amygdala is also directly connected to the hippocampus, with which it interacts in regard to memory.
The hippocampus is unique in that unlike the amygdala and other structures, almost all of its input from the neocortex is relayed via the overlying entorhinal cortex--a five layered mesocortex. As is well known, the hippocampus is exceedingly important in memory, acting to place various short-term memories into long-term storage. Presumably the hippocampus encodes new information during the storage and consolidation (long-term storage) phase, and assists in the gating of afferent streams of information destined for the neocortex by filtering or suppressing irrelevant sense data which may interfere with memory consolidation. Moreover, it is believed that via the development of long-term potentiation the hippocampus is able to track information as it is stored in the neocortex, and to form conjunctions between synapses and different brain regions which process and store associated memories.
The septal nuclei is in part an evolutionary and developmental outgrowth of the hippocampus (Ariens Kappers, et al., 1936; Gloor, 1997), as well as the hypothalamus, and in fact acts to link the hippocampus with this structure as well as with the brainstem (Andy & Stephan, 1968; Risvold & Swanson, 1996; Swanson & Cowan, 1979;). It consists of both lateral and medial segments; i.e. the lateral and medial septal nuclei (Ariens Kappers, et al., 1936). Presumably, via these interconnections, the septal nuclei exerts modulatory influences on the hippocampus in regard to memory functioning and arousal (Gloor, 1997).
The septal nuclei is also interconnected with and shares a counterbalancing relationship with the amygdala particularly in regard to hypothalamic activity and emotional and sexual arousal (Andy & Stephan, 1968; Swanson & Cowan, 1979). For example, whereas the amygdala promotes indiscriminate contact seeking, and perhaps promiscuous sexual activity, the septal nuclei inhibits these tendencies thus assisting in the formation of selective and more enduring emotional attachments (Joseph, 1992a, 1999b).
The anterior cingulate is considered a transitional cortex, or rather, mesocortex (also referred to as "paleocortex") as it consists of five layers (MacLean, 1990). The anterior cingulate is intimately interconnected with the hypothalamus, amygdala, septal nuclei, and hippocampus, and participates in memory and emotion including the experience of pain, misery, and anxiety, and is directly implicated in the evolution and expression of maternal behavior. It is also the most vocal aspect of the brain, becomes active during language tasks, and generates emotional-melodic aspects of speech which is expressed via interconnections with the right and left frontal speech areas, and the vocalization center in the midbrain periaqueductal gray. Thus the anterior cingulate is implicated in the more cognitive aspects of social-emotional behavior including language and the establishment of long term attachments beginning with the mother-infant bond.
Also implicated in the functioning of the limbic system are the olfactory bulb and olfactory system, the limbic striatum (nucleus accumbens, olfactory tubercle, substantia innominata, ventral caudate and putamen), the orbital frontal and inferior temporal lobes and the midbrain monoamine system. These systems and structures are also directly connected or separated by only a single synapse, and which tend to become aroused not only as a function of emotional arousal, but in reaction to olfactory input which continues to exert profound effects on the human limbic system, and upon human behavior.
HIPPOCAMPAL & AMYGDALOID INTERACTIONS: MEMORY
It has been argued that significant impairments involving short-term memory and motor learning, cannot be produced by lesions supposedly restricted to the hippocampus (Horel, 1978; see also commentary in Eichenbaum et al. 1994); though in fact it is impossible to create such "restricted" lesions. Nevetheless, ignoring for the moment that inconvenient fact, in some instances with supposed restricted lesions, good recall of new information is possible for at least several minutes (Horel, 1978; Penfield & Milner, 1958; Squire 1992).
Moreover, there is considerable evidence which strongly suggests that the hippocampus plays an interdependent role with the amygdala in regard to memory (Gloor 1992, 1997; Halgren 1992; Kesner & Andrus, 1982; Mishkin, 1978; Murray 1992; Sarter & Markowitsch, 1985); particularly in that they are richly interconnected, merge at the uncus, and exert mutual excitatory influences on one another. For example, it appears that the amygdala is responsible for storing the emotional aspects and personal reactions to events in memory, whereas the hippocampus acts to store the cognitive, visual, and contextual variables (chapter 14) whereas that the amygdala activates the hippocampus by providing excitatory input (Gloor, 1955, 1997).
Specifically, the amygdala plays a particularly important role in memory and learning when activities are related to reward and emotional arousal (Gaffan 1992; Gloor 1992, 1997; Halgren 1992; LeDoux 1992, 1996; Kesner 1992; Rolls 1992; Sarter & Markowitsch, 1985). Thus, if some event is associated with positive or negative emotional states it is more likely to be learned and remembered.
The amygdala becomes particularly active when recalling personal and emotional memories (Halgren, 1992; Heath, 1964; Penfield & Perot, 1963), and in response to cognitive and context determined stimuli regardless of their specific emotional qualities (Halgren, 1992). However, once these emotional memories are formed, it sometimes requires the specific emotional or associated visual context to trigger their recall (Rolls, 1992; Halgren, 1992). If those cues are not provided or ceased to be available, the original memory may not be triggered and may appear to be forgotten or repressed. However, even emotional context can trigger memory (see also Halgren, 1992) in the absence of specific cognitive cues.
Similarly, it is also possible for emotional and non-emotional memories to be activated in the absence of active search and retrieval, and thus without hippocampal or frontal lobe participation. Recognition memory which may be triggered by contextual or emotional cues. Indeed, there are a small group of neurons in the amygdala, as well as a larger group in the inferior temporal lobe which are involved in recognition memory (Murray, 1992; Rolls, 1992). Because of amygdaloid sensitivity to visual and emotional cues, even long forgotten memories may be evoked via recognition, even when search and retrieval repeatedly fail to activate the relevant memory store.
According to Gloor (1992), "a perceptual experience similar to a previous one can through activation of the isocortical population involved in the original experience recreate the entire matrix which corresponds to it and call forth the memory of the original event and an appropriate affective response through the activation of amygdaloid neurons." This can occur "at a relatively non-cognitive (affective) level, and thus lead to full or partial recall of the original perceptual message associated with the appropriate affect."
In this regard, it appears that the amygdala is responsible for emotional memory formation whereas the hippocampus is concerned with storing verbal-visual-spatial and contextual details in memory. Thus, in rats and primates damage to the hippocampus can impair retention of context, and contextual fear conditioning, but it has no effect on the retention of the fear itself or the fear reaction to the original cue (Kim & Fanselow 1992; Phillips & LeDoux 1992, 1996; Rudy & Morledge 1994). In these instances, fear-memory is retained due to preservation of the amygdala. However, when both the amygdala and hippocampus are damaged, striking and profound disturbances in memory functioning result (Kesner & Andrus, 1982; Mishkin, 1978).
Therefore, the role of the amygdala in memory and learning seems to involve activities related to reward, orientation, and attention, as well as emotional arousal and social-emotional recognition (Gloor, 1992, 1997; Rolls, 1992; Sarter & Markowitsch, 1985). If some event is associated with positive or negative emotional states it is more likely to be learned and remembered. That is, reward increases the probability of attention being paid to a particular stimulus or consequence as a function of its association with reinforcement (Gaffan 1992; Douglas, 1967; Kesner & Andrus, 1982).
Moreover, the amygdala appears to reinforce and maintain hippocampal activity via the identification of motivationally significant information and the generation of pleasurable rewards (through action on the lateral hypothalamus). However, the amygdala and hippocampus act differentially in regard to the effects of positive vs. negative reinforcement on learning and memory, particularly when highly stressed or repetitively aroused in a negative fashion. For example, whereas the hippocampus produces theta in response to noxious stimuli the amygdala increases its activity following the reception of a reward (Norton, 1970).
LATERALITY.
It is now very well known that lesions involving the mesial-inferior temporal lobes (i.e. destruction or damage to the amygdala/hippocampus) of the left cerebral hemisphere typically produce significant disturbances involving verbal memory--particularly as constrasted with individuals with right sided destruction. Left sided damage disrupts the ability to recall simple sentences, complex verbal narrative passages, or to learn verbal paired-associates or a series of digits (Frisk & Milner 1990; Milner, 1966, 1970, 1971; Squire 1992).
In contract, right temporal destruction typically produces deficits involving visual memory, such as the learning and recall of geometic patterns, visual or tactile mazes, locations, objects, emotional sounds, or human faces (Corkin, 1965; Milner, 1965; Nunn et al., 1999; Kimura, 1963). Right sided damage also disrupts the ability to recognize (via recall) olfactory stimuli (Rausch et al. 1977), or recall emotional passages or personal memories (Cimino et al., 1991; Wechsler, 1973).
It appears, therefore, that the left amygdala and hippocampus are highly involved in processing and/or attending to verbal information, whereas the right amygdala/hippocampus is more involved in the learning, memory and recollection of non-verbal, visual-spatial, environmental, emotional, motivational, tactile, olfactory, and facial information. These issues and the differing roles of these nuclei in memory formation, as well as amnesia and repression will be discussed in greater detail in chapters 29, 30.
THE PRIMARY PROCESS AMYGDALA & PLEASURE
The amygdala maintains a functionally interdependent relationship with the hypothalamus in regard to emotional, sexual, autonomic, consumatory and motivational concerns. It is able to modulate and even control rudimentary emotional forces governed by the hypothalamic nucleus. However, the amygdala also acts at the behest of hypothalamically induced drives. For example, if certain nutritional requirements need to be meet, the hypothalamus signals the amygdala which then surveys the external environment for something good to eat (Joseph, 1982, 1992a). On the otherhard, if the amygdala via environmental surveilance were to discover a potentially threatening stimulus, it acts to excite and drive the hypothalamus as well as the basal ganglia so that the organism is mobilized to take appropriate action.
When the hypothalamus is activated by the amygdala, instead of responding in an on/off manner, cellular activity continues for an appreciably longer time period (Dreifuss et. al., 1968). The amygdala can tap into the reservoir of emotional energy mediated by the hypothalamus so that certain ends may be attained (Joseph, 1982, 1992a)
AMYGDALA & HIPPOCAMPAL INTERACTIONS DURING INFANCY HALLUCINATIONS
The amygdal-hippocampal complex, particularly that of the right hemisphere, is very important in the production and recollection of non-linguistic and verbal-emotional images associated with past experience. In fact direct electrical stimulation of the temporal lobes, hippocampus and particularly the amygdala (Gloor, 1990, 1997) not only results in the recollection of images, but in the creation of fully formed visual and auditory hallucinations (Gloor 1992, 1997; Halgren 1992; Halgren et al., 1978; Horowitz et al., 1968; Malh et al., 1964; Penfield & Perot, 1963), as well as feelings of familiarity (e.g. deja vus).
Indeed, it has long been know that tumors invading specific regions of the brain can trigger the formation of hallucinations which range from the simple (flashing lights) to the complex. The most complex forms of hallucination, however, are associated with tumors within the most anterior portion of the temporal lobe (Critchley, 1939; Gibbs, 1951; Gloor 1992, 1997; Halgren 1992; Horowitz et al. 1968; Tarachow, 1941); i.e. the region containing the amygdala and anterior hippocampus.
Similarly, electrical stimulation of the anterior lateral temporal cortical surface results in visual hallucinations of people, objects, faces, and various sounds (Gloor 1992, 1997; Halgren 1992; Horowitz et al., 1968)--particularly the right temporal lobe (Halgren et al. 1978). Depth electrode stimulation and thus direct activation of the amygdala and/or hippocampus is especially effective.
For example, stimulation of the right amygdala produces complex visual hallucinations, body sensations, deja vu, illusions, as well as gustatory and alimentary experiences (Weingarten et al. 1977), whereas Freeman and Williams (1963) have reported that the surgical removal of the right amygdala in one patient abolished hallucinations. Stimulation of the right hippocampus has also been associated with the production of memory- and dream-like hallucinations (Halgren et al. 1978; Horowitz et al. 1968).
The amygdala also becomes activated in response to bizarre stimuli (Halgren, 1992). Conversely, if activated to an abnormal degree, it may in turn produce bizarre memories and abnormal perceptual experiences. In fact, the amygdala contributes in large part to the production of very sexual as well as bizarre, unusual and fearful memories and mental phenomenon including dissociative states, feelings of depersonalization, and hallucinogenic and dream-like recollections (Bear, 1979; Gloor, 1986, 1992, 1997; Horowitz et al. 1968; Mesulam, 1981; Penfield & Perot, 1963; Weingarten et al. 1977; Williams, 1956). In addition, sexual feelings and related activity and behavior are often evoked by amygdala stimulation and temporal lobe seizures (Halgren, 1992; Jacome, et al. 1980; Gloor, 1986, 1997; Remillard, et al. 1983; Robinson & Mishkin, 1968; Shealy & Peele, 1957), including memories of sexual intercourse (Gloor 1990) or severe emotional trauma and abuse (Gloor, 1997).
Moreover, intense activation of the temporal lobe and amygdala has been reported to give rise to a host of sexual, religious and spiritual experiences; and chronic hyperstimulation (i.e. seizure activity) can induce some individuals to become hyper-religious or visualize and experience ghosts, demons, angels, and even God, as well as claim demonic and angelic possession or the sensation of having left their body (Bear 1979; Gloor 1986, 1992; Horowitz, Adams & Rutkin 1968; MacLean 1990; Mesulam 1981; Penfield & Perot 1963; Schenk, & Bear 1981; Weingarten, et al. 1977; Williams 1956).
LSD.
As is well known, LSD can elicit profound hallucinations involving all spheres of experience. Following the administration of LSD high amplitude slow waves (theta) and bursts of paroxysmal spike discharges occurs in the hippocampus and amygdala (Chapman & Walter, 1965; Chapman et al. 1963), but with little cortical abnormal activity. In both humans and chimps, when the temporal lobes, amygdala and hippocampus are removed, LSD ceased to produce hallucinatory phenomena (Baldwin et al. 1959; Serafintides, 1965). Moreover, LSD induced hallucinations are significantly reduced when the right vs. left temporal lobe has been surgically ablated (Serafintides, 1965)
Overall, it appears that the amygdala, hippocampus, and the neocortex of the temporal lobe are highly interactionally involved in the production of hallucinatory experiences. Presumably, it is the neocortex of the temporal lobe which acts to interpret this material (Penfield & Perot, 1963) as perceptual phenomena. Indeed, it is the interrelated activity of the temporal lobes, hippocampus and amygdala which not only produce memories and hallucinations, but dreams. In fact, the amygdalas involvement in all aspects of emotion and sexual functioning, including associated memories, the production of overwhelming fear as well as bizarre and dream-like mental phenomenon, may well account for why this type of unusual stimuli, including personal and innocuous memories also appears in dreams.
DREAMING
When hallucinations follow depth electrode or cortical stimulation, much of the material experienced is very dream-like (Gloor 1990, 1992; Halgren et al., 1978; Malh et al., 1964; Penfield & Perot 1963) and consists of recent perceptions, ideas, feelings, and other emotions which are similarly illusionary and dream-like. Indeed, the right amygdala, hippocampus, and the right hemisphere in general (Broughton, 1982; Goldstein et al., 1972; Hodoba, 1986; Humphrey & Zangwill, 1961; Kerr & Foulkes, 1978; Meyer et al. 1987) also appear to be involved in the production of deam imagery as well as REM sleep (chapter 10). For example stimulation of the amygdala triggers and increases ponto-geniculo-occipital paradoxical activity during sleep (Calvo, et al. 1987), which in turn is associated with REM and dreaming. In addition, during REM, the hippocampus begins to produce slow wave, theta activity (Jouvet, 1967; Olmstead, Best, & Mays, 1973; Robinson et al. 1977), which is associated with long-term potentiation which is associated with learning and memory (see chapter 14). Presumably, during REM, the hippocampus and amygdala act as a reservoir from which various images, emotions, words, and ideas are drawn and incorporated into the matrix of dream-like activity being woven by the right hemisphere. It is probably just as likely that the right hippocampus and amygdala serve as a source from which material is drawn during the course of a daydream.
The Right Hemisphere & Dreams.
There have been reports of patients with right cerebral damage, hypoplasia and abnormalities in the corpus callosum who have ceased dreaming altogether, suffer a loss of hypnogic imagery or tend to dream only in words (Botez et al. 1985; Humphrey & Zangwill, 1951; Kerr & Foulkes, 1981; Murri et al. 1984). However, there have also been some report that when the left hemisphere has been damaged, particularly the posterior portions (i.e. aphasic patients), the ability to verbally report and recall dreams also is greatly attenuated (e.g., Murri et al. 1984). Of course, aphasics have difficulty describing much of anything, let alone their dreams.
Electrophysiologically the right hemisphere also becomes highly active during REM, whereas, conversely, the left brain becomes more active during N-REM (Goldstein et al. 1972; Hodoba, 1986). Similarly, measurements of cerebral blood flow have shown an increase in the right temporal regions during REM sleep and in subjects who upon wakening report visual, hypnagogic, hallucinatory and auditory dreaming (Meyer et al. 1987). Interestingly, abnormal and enhanced activity in the right temporal and temporal-occipital area acts to increase dreaming and REM sleep for an atypically long time period (Hodoba, 1986). Hence, it appears that there is a specific complementary relationship between REM sleep and right temporal electrophysiological activity.
Interestingly, daydreams appear to follow the same 90-120 minute cycle that characterize the fluctuation between REM and NREM periods, as well as fluctuations in mental capabilities associated with the right and left hemisphere (Broughton, 1982; Kripke & Sonneschein 1973). That is, the cerebral hemisphere tend to oscillate in activity every 90-120 minutes -- a cycle which appears to correspond to the REM-NREM cycle and the appearance of day and night dreams.
Forgotten Dreams.
Most individuals, however, have difficulty recalling their dreams. This may seem paradoxical considering that hippocampal theta is being produced. However, this is theta punctuated by high levels of desychronized activity, which is not conducive to learning. In this regard, theta activity may represent the reverberating activity of neural circuits formed during the day, such that the residue of day time memories come to be inserted into the dream. Conversely, due to the high level of desychronization occuring in the hippocampus (as it is so highly aroused), although it contributes images and the days memories, it does not participate in storing these dream-like experiences into memory.
Consider the results from temporal lobe, amygdala, and hippocampal electrical stimulation on memory recall and the production of hallucinations. Although personal memories are often activated at low intensities of stimulation (memories which are verified not only by the patient but family), if stimulation is sufficiently intense, the memory instead will become dreamlike and populated by hallucinated and cartoon like characters (Halgren, et al. 1978). That is, at low levels of stimulation memories are triggered but these memories become increasingly dream-like with high levels of activity. Moreover, once these high levels of stimulation are terminated, patients soon become verbally amnesic and fail to verbally recall having had these experiences (Gloor, 1992; Horowitz, et al. 1968). However, these memories can be later recalled if subjects are provided with specific contextual cues (Horowitz, et al. 1968). The same can occur during the course of the day when a fragment of a conversation, or some other experience, suddenly triggers the recall of a dream from the previous night which had otherwise been completely forgotten. Presumably it had seemingly been forgotten because the hippocampus did not participate in their storage and thus could not assist in their retrieval (see chapters 29, 20).
There is also some evidence to suggest that different regions of the hippocampus show different levels of arousal during paradoxical sleep. For example, it appears that the posterior hippocampus becomes activated during paradoxical sleep and shows theta activity, whereas the more anterior portions become inhibited (Olmstead et al. 1973). As the anterior portions are more involved in new learning (at least in humans), whereas the posterior hippocampus is more concerned with old and well established memories, this would suggest that the posterior hippocampus is contributing older or already established memories to the content of the dream--which explains why theta, which is associated with learning and memory, is also produced during the dream--that is, it is replaying various fragmentary memories. Conversely, the inhibition of the anterior region would prevent this dream material from becoming rememorized.
DREAMS & INFANCY
In the newborn, and up until approximately 6-9 months, there are two distinct stages of sleep which correspond to REM and N-REM periods demonstrated by adults (Berg & Berg, 1978; Dreyfus-Brisac & Monod, 1975; Parmelee et al. 1967). Among infants, however, REM occur during wakefullness as well as during sleep. In fact, REM can be observed when the eyes are open, when the infant is crying, fussing, eating, or sucking (Emde & Metcalf, 1970). Moreover, REM is also observed to occur within a few moments after an infant begins to engage in nutritional sucking and appears identical to that which occurs during sleep (Emde & Metcalf, 1970).
The production of REM during waking in some respects seems paradoxical. Nevertheless, it might be safe to assume that like an adult, when the infant is in REM, he or she is dreaming, or at least, in a dream-like state. Possibly, this state corresponds to what Freud has described as the Primary Process. That is, when produced when the infant is crying or fussing, it is dreaming of whatever relief it seeks. Correspondingly, REM which occurs while eating or sucking may have to do with the limbic structures which are involved not only in the production of dream-like activity, but the identification, learning and retention of motivationally significant information (i.e. the amygdala and hippocampus).
Presumably this relationship is a consequence of REM as well as eating and sucking being mediated, in part, by the amygdala as well as other limbic nuclei, which are also concerned with forming motivationally significant memories. Hence, when hungry, the hypothalamus becomes aroused which activates the amygdala which is responsible for the performing environmental surveillance so as to attend, orient to, identify and approach motivationally significant stimuli and eat. However, because the infants brain is so immature and as its resources for meeting its limbic needs are quite rudimentary, under certain conditions prolonged hypothalamus induced amygdala activation results in the formation and recall of relevant memories which may be experienced as hallucinations of the desired object. That is, previously formed neural networks become activated and the infant begins to dream and hallucinate food and will then suck and smack its lips as if eating or sucking when it is awake, in REM, and there is no food present.
THE PRIMARY PROCESS
The hypothalamus, our exceedingly ancient and primitive Id, has an eye that only sees inward. It can tell if the body needs nourishment but cannot determine what might be good to eat. It can feel thirst, but has no way of slacking this desire. The hypothalamus can only say: "I want", "I need", and can only signal pleasure and displeasure. However, being the seat of pleasure, the hypothalamus can be exceedingly gracious in rewarding the organism when its needs are met. Conversely, when its needs go unmet it can respond not only with displeasure and feelings of aversion, but with undirected fury and rage. It can cause the organism to cry out.
Nevertheless, the cry does not produce the immediately desire relief or reduction in tension. There is thus a pressure on the limbic system and the organism to engage in environmental surveillance so as to meet the needs monitored by the hypothalamus.
Over the course of the first months of life, as the amygdala and then hippocampus develop, the organism begins to develop an eye that not only sees outward, but which can register and recall events, objects, people, etc., associated with tension reduction, pleasure and the satiaty of the infants internal needs (e.g. the taste, smell, feeling of mother's breast and milk, the experience of sucking and relief, etc.). This is called learning.
With the maturation of these two limbic nuclei the infant is increasingly able to differentiate what occurs in the external environment based on hypothalamically monitored needs and the emotional/motivational significance of that which is experienced. The infant can now orient, selectively attend, determine what brings satisfaction, and store this information in memory.
PRIMARY IMAGERY
Although admittedly we have no direct knowledge as to the psychic interactions in the neonate, it does seem reasonable to assume that as the neocortex and underlying structures and fiber pathways mature, neural "prgorams" are formed which correspond to the repeated registration of experiences which are deemed significant (e.g. pleasurable). That is, neural pathways which are repetitively fired, deactivated or activated in response to specific sensory and affective activities and experiences, become associated with that activity, such that an associated neural circuit is formed (chapter 14); i.e. a memory is created. Eventually, if this circuit is reactivated, the "learned" pattern is reexperienced; i.e. the organism remembers.
Thus, infants as young as 2 days of age can learn to suck at the mere sight of a bottle (see Piaget, 1954) and in order to receive milk as a reinforcement, infants can even modify their sucking response (Sameroff & Cameron, 1979). Hence, they are suceptible to classical conditioning (Sameroff & Cavanagh, 1979), although the possibility of operant conditioning has not been established. Nevertheless, the fact that they can recognize the bottle and suck (as well as cry and shed tears) indicates that various regions of the limbic system, especially that of the amygdala is functional and that learning and the creation of specific, context specific neural circuits have been formed very early in life.
Thus, when the amygdala/hippocampus are stimulated by a hungry hypothalamus, the events and images associated with past experiences of pleasure can not only be searched out externally, but recalled in imaginal form. For example, as an infant experiences hunger and stomach contractions as well as it own cries of displeasure, these states become associated with the sound, smell, taste, etc. of mother and her associated movement and other stimuli which accompany being fed (cf Piaget, 1952, pp. 37, 407-408). Repetitively experienced, the sequence from hunger to satiety evokes and becomes associated with the activation of certain neural pathways and the creation of a specific neural network subserving that memory (chapter 14).
Eventually, when the infant becomes hungry, if prolonged there is the possibility that the entire neural sequence associated with hunger and feeding (i.e. hunger, mother, food, satiaty), may become involuntarily triggered and activated (via association) such that an "image" of being fed is experienced. The activation of these rudimentary and infantile memory-images is probably what consititutes, at least in part, the primary process.
Behaviorally this is manifested by REM and via sucking and tongue movements as if eating, when in fact there is no food present (cf, Piaget, 1952). That is, when hungry, the infant will begin to cry, rapid eye movement (REM) might be observed, and then the infant will stop crying and smack its lips and make sucking movement (mediated by the amygdala) as if it were being fed. The infant experiences the experience of being fed in the form of a dream (Joseph, 1982) or hallucination, although it is awake.
In that the brain of the human infant is quite immature for in fact several years, which in turn restricts information reception and processing (chapters 23-28), and given the limited amount of reality contact infants are able to achieve, these rudimentary memories and images (even when occurring during waking, i.e. REM), are probably indistinguishable from actual experience simply because they are experience.
Like a dream, when replayed, the infant presumaly reexperiences to some degree the sensations, emotions, etc., originally linked to tension reduction. Thus, the young infant, as yet unable to distinguish between representation and reality, responds to the image as reality (Freud, 1900, 1911), even while awake--as manifested by REM. When hunger is prolonged the association linked to feeding are triggered and for a brief time period the infant behaves as if its hunger has been sated. Reality is replaced by an image, or rather, a "dream". This is the primary process.
Since the hypothalamus (which monitors internal homeostasis) is not conscious that the dream images experienced are not real, it initially accepts the memory/dream images transmitted from the amygdala and hippocampus and ceases to cry, i.e. it responds to the imagined sources of nourishment just as it responds to a cue-tone associated with a food reward (Nakamuar & Ono, 1986; Ono et al., 1980). However, the hypothalamus is not long fooled, for the primary process does not offer effective long lasting relief from tension. As the pain of hunger remains and increases, limbic activity is increased, and the image falls away to be replaced by a cry of hunger (Joseph, 1982). The amygdala and hippocampus are thus forced to renew their surveillance of the environment in search of sources of tension reduction. Cognitive development is thus promoted.
"Whatever was thought of (desired) was simply imagined in an hallucinatory form, as still happens today with our dream-thoughts every night. This attempt at satisfaction by means of halluciantion was abandoned only in consequence of the absence of the expected gratification, because of the disappointment experienced. Instead, the mental apparatus had to decide to form a conception of the real circumstances in the outer world and to exert itself to alter them...The increased significance of external reality heightened the significance also of the sense-organs directed towards the outer word, and of the consciousness attached to them; the later now learned to comprehend the qualities of sense in addition to the qualities of pleasure and "pain" which hitherto had alone been of interest to it. A special function was instituted which had periodically to search the outer word in order that its data might be already familiar if an urgent need should arise; this function was attention. Its activity meets the sense-impressions halfway, instead of awaiting their appearance. At the same time there was probably introduced a system of notation, whose task was to deposit the results of this periodical activity of consciousness--a part of that which we call memory" (Freud, 1911, pp. 410-411).
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