Well let's switch gears now and move on to a consideration of memory function. As we talk about memory very briefly here in this tutorial I think it's important to be clear really what kind of memory are we talking about. And it's possible to differentiate different categories of memory. in fact it's quite helpful to do so because as it turns out, different forms of memory are supported by different networks in the brain. So one can imagine that there is a kind of memory that is available to conscious cognition. So what I have in mind here is the kind of memory for people, for places, for the episodes of our lives, for the things that can access our language faculty and allow us to describe verbally the content of that memory. And so this kind of memory we call declarative memory. It's available to concious processing. It reflects the daily episodes of our lives. the semantic symbols that give meaning to our thoughts and we can access those symbols and apply them in the form of language. So with declarative memory we have access to our lexicon that allows us to report the content of that memory. It provides a, a personal history of our life events. So declarative memory, sometimes called episodic memory is that memory for which we can declare its content. It reflects the daily episodes of our lives. Now this is to be contrasted with another category of memory really multiple subcategories of memory that in our textbook we simply choose to call nondeclarative memory but it can be subdivided in various ways. Sometimes this is called implicit memory, Some subdivisions of this form of memory can be called motor memory so the nomenclature is actually not entirely consistent across various sources. So for those of you that are in this field and prefer a particular nomenclature that might be different from this one I hope you'll pardon my use of the nomenclature that we present in our book. But I think it's helpful to just make this broad point that it's important to differentiate from declaritive memory, these other forms of memory. So these are forms of memory that generally speaking are not available to consciousness. That is, it's much more difficult to acess our lexicon to put our semantic labels upon the content of this memory. So in this category would be motor skills for example, learning to ride a bicycle. Learning to ice skate or learning to do some other motor skill that once acquired seems to be with us, Perhaps for life, of course, depending upon how often we practice and resurrect that scale. So it's quite challenging is it not to put into words the nature of the skill that allows us to ride a bicycle? We can describe riding a bicycle but we cannot access the content of that skill through our semantic facilities. This an important clue that this kind of memory might actually be aquired and consolidated by a different kind of brain system than is operating our declarative memories. Well in addition to motor skills there are various associations that are established. either explicitely or implicitely and it's those implicit associations that are exemplars of non-declarative memory. There are various phenomena related to priming. That is a form of memory that is nondeclarative and related to motor skills there are puzzle solving skills. That are accrued through experience and practice. That like other kinds of motor skills are not readily accessible to our language faculties. Now there's another dimension of memory that can be categorized and that is the temporal dimension of memory. There's a sense of memory that's immediate and this is memory for the previous few moments that are essential for maintaining consciousness, conscious awareness. conscious deliberations over cogent thought. so this is a form of memory that most likely reflects ongoing recurrent processing throughout cortical networks. But there's another form of memory that helps us to maintain the goal of our behavior and so. This is now extending over not just the last few moments but perhaps seconds to minutes. This we call working memory. So this is the capacity to hold online the goal that is motivating behavior. So this also likely reflects recurrent networks that interact across associational regions in the front and the parietal lobes at least but also the temporal lobe. Now we think of memory probably what we have in mind really is long term memory. So this is the kind of memory that reflects the accrual of life experience and it's consolidation and the capacity to then recall this information. And have it once again enter our conscious deliberations. This kind of long-term memory is what is clearly dependent upon the associational networks of the temporal lobe and specifically the medial parts of the temporal lobe. So this gets us into parts of the brain that we've just briefly encountered in our survey of the human brain in the first unit of the course. You may recall that there is a gyral structure on the medial edge of the inferior temporal lobe called the parahippocampal gyrus. This is an important gyrus for this capacity to consolidate the moment and build long-term memory. Now deep within this gyrus on the medial edge of the cortical mantle is a structure that we saw in cross section called the hippocampus. And this as it turns out is the critical brain structure for the acquisition of declarative memory. So when we speak of long-term memory, really what we have in mind is the storage of declarative memory. And the capacity to recall those memories. So here in a phantom view of the human brain we're reminded that deep to this parahippocampal gyrus we have the structure called the hippocampus and just in front of the hippocampus is the amygdala. Now the amygdala is associated with yet other aspects of complex brain function namely emotion and we're going to talk about emotion in a separate tutorial. So for this tutorial we're going to leave the amygdala out. And focus in on the associational networks that are associated with this fascinating region of the mammalian brain called the hippocampus. So let's look more closely at the hippocampus and I'll just remind you that the hippocampus is a corticol structure. But it's found at the medial edge of this para hipicampal gyrus that folds in upon itself forming the floor and medial wall of the lateral ventrical as it makes it's shape into the temporal lobe. Now the hippocampus is one of those wonderful terms in neuroanatomy that I'm sure many of you know we talked about it briefly in unit one. The hippocampus is a Greek term for seahorse or perhaps sea monster depending on your interpretation of the Greek. One can imagine that this structure bears some resemblance to the body of a seahorse. Reflecting the curving back upon itself as this medial edge of the cerebral mantle took shape in the temporal lobe during the third trimester of gestation. Now interestingly if you actually dissect the hippocampus out of the human brain and just lay it out on a tray it also resembles the body of a seahorse from that view. And there are at least a few images out there that you're certainly free to Google to see that view of the hippocampus as well. perhaps for the next version of this course, I'll do that dissection myself. And be able to show you the sea monster within our temporal lobe more directly. But for this view, I would just emphasize that the hippocampus is a cortical structure but it's the simplest of all cortical regions. Philogenetically we call this archicortex, which reflects its ancient origins in the evolution of the vertebrate brain. but we retain this simple architecture even in our brains. It's a simple architecture but it's evidently sufficient to do the job of consolidating declarative memory. Now the hippocampus receives input from this parahippocampal gyrus. And I won't go into the details of exactly how all that works. You may remember though that there are inputs that enter the dentate gyrus which is this curved region where we find those dentate granule cells. That continue to harbor some neural stem cells that differentiate into neurons throughout life. So these dentate driver cells are being added to throuout our lives and so there is plasticity among that cell population. Both in terms of it's members but also of course it's synaptic weights. So the dentate gyral cells then project to other neurons in the hippocampus and there are yet additional associational connections. And it's those associational connections that we studied in unit two when we looked at the synaptic basis of long-term potentiation and long-term depression. We think that these synaptic changes are the cellular and synaptic mechanisms that are responsible for consolidating memories. A memory after all is simply a, a permanent record. So something is changed in the structure and function of circuits that enable the storage of memory. And we know that the hypocampus is an essential structure in this medial temporal lobe memory network that it is essential for the acquistion and perhaps also the recall of declarative memory. So thinking about the inputs to the hypocampus, I like to think of the hypcampus as this great funnel of the moment. Everything that defines your sense of right now. The sound of my voice. The sight of my face. The background image that you see on your device. the setting in which you find yourself in. Perhaps there are other people around you. All of these sensory stimuli define right now. And this information is being processed through the relevent sensory cortical areas. It's being fed into accosiational cortical regions in the oxipital lobe, the temporal lobe, the priatal lobe and even the frontal lobe. And from these associational cortexes inputs are being funneled into this parahypocampal gyrus. And from there they are relays from this cortex called the interinal cortex into various divisions of this hypocampal network. And from there through mechanisms that we think are inclusive of long-term potentiation and long-term depression, memories are stored. Now we know much less about the actual storage of the memory than we do about its acquisition. In terms of storage, we know that the hippocampus maintains back projections through various path ways including projections that are directed back our through the parahyppocampal gyrus. To each of these associational cortical regions that were relevant in the processing of the sensory stimuli that comprise the here and the now the moment. So these extensive interconnections betweens associational cortex in each of the lobes and this parahippocampal gyral cortex seem to be critical for the storage and the recall of information. So when we recall a memory and I would invite you all right now to recall some memory perhaps a memory from childhood. Or perhaps a memory of what you ate for breakfast today. Whatever memory you may be operating with right now reflects the back activation from this medial temporal lobe memory system. To the relevant associational cortical regions that first process that information. So there must be plasticity in the connections back from the parahippocampal gyrus cortex into these various associational regions that are associated with vision and audition. And olfaction and even gestation. perhaps even the vestibular sense. Although we know much less about that sense in terms of cortical processing and memory. So, one can imagine the kinds of problems that patients might have with damage to this critical part of the Medial temporal lobe. This produces a clinical phenomenon that we call amnesia. So amnesia refers to pathological forgetting, a failure of our Medial temporal lobe memory system to consolidate new memory. And perhaps even to recall memories that have previously been laid down in these vast associational networks. Amnesia can be in the entrograde direction that is from a moment in time of injury or disease or disorder going forward or amnesia can be retrograde. It can project back into time prior to the moment of injury or disease and disorder. Now for damage to the hippocampus we will find antrograde amnesia but also some degree of retrograde amnesia. Evidently it takes some time for memories to be consolidated and to be laid down for long term storage. Now perhaps some of you like me have suffered a concussion at some point in your life. perhaps some have had even more severe forms of traumatic head injury than what we might label concussion. But one common feature to traumatic injury of the brain is amnesia for the moment of impact in the case of trauma. So in my case I was in a motor vehicle accident last fall and I recall the approach of the intersection at which. I got into my motor vehicle accident but I really don't recall the moment of impact nor do I recall the approach of the vehicle that hit me from the side. I have a sense of preceding as I had right-of-way into that intersection. But the next thing I remember was coming around and realizing that I had been in an accident and now crawling out of my vehicle. So I had a bit of retrograde amnesia for the few moments leading up to the impact that resulted in my concussion. Evidently those moments leading up to that impact were peculating in the networks of the medial temporal lobe memory system and in my own head. And the trauma of the concussion was sufficient to alter the firing that was on going in that network so those moments are, are simply lost. the activity evidently cannot be reconstructed. And as a result I simply don't have access to those few moments of my life. Now the concept of amnesia is one that has gained quite a lot of interest especially among people that make movies. some of you may be familiar with several movies that came out of Hollywood in recent years on this theme of exploring the life of someone who has suffered amnesia. some of them are rather comical and distasteful and do a poor job representing the lives of such people. But there have been some that, I think have done a realistic job portraying life with limited capacity to lay down new memories. Well, I just want to be clear. What I've been talking about thus far in terms of the medial temporal lobe memory system associated with the parahippocampal gyrus. And the hippocampus itself pertains to declarative memory. The acquisition, the consolidation and then the processing that's necessary for long term storage of declarative episodic memories, the episodes of our lives. Well, what's been fascinating to discover about people that have had damage to the Hippocampal system is that they retain memories that we might categorize as non-declarative or procedural. And that's sufficient evidence to think that there are different brain systems outside of the medial temporal lobe that mediate procedural memories. So let's turn our attention there now. So the important point that I want you to be able to understand is that when it comes to nondeclarative memory to procedural memory. these forms of memory are dependent upon entirely different brain systems than are those that are responsible for the acquisition, storage and recall of declarative memory. So we've been talking about declarative memory of the hipocampus in the short term storage of those memories. Their acquisition. Their consolidation. And I mentioned that the long-term storage of declarative memory engages widespread cortical regions. Many of which were involved in the initial processing of the here and the now. So when it comes to our memory systems for nondeclaritive memory we're just now really beginning to do the kinds of experimental work. And humans and non human primates that would shed more light on to the kinds of systems that are essential for acquiring these kinds of memories. For consolidating them, and then for long term storage. Now our figure from the book indicates that well we don't know really what's involved with the short term acquisition of declarative memory. I would suggest that perhaps we do know a little bit more than what this figure would imply. And I certainly want to now restore the integrity of my cerebellar hemispheres so that I can make the point that the cerebellum appears to play an important role in the acquisition and consolidation of procedural memory. One can think of procedural memory as a form of motor learning that requires the generation of error correction signals in order to acquire a memory. And that memory then takes the form of an accrued skill. At least with respect to motor function and motor behavior. And these are functions that we associate with the cerebellum. And the means by which the cerebellum can modify the operation of our motor cortical regions here in the posterior part of the frontal lobe. So I would suggest that for the short term acquisition and storage of nondeclarative procedural memories. The Cerebellum is among the brain structures that will have an important role there. Now, we know this because in functional magnetic resonance imaging studies, for example, when human subjects are confronted with a challenging motor skill. Activity in the cerebellum is significantly modulated as these kinds of skills aer being aquired. But once a certain measure of proficiency has been establish, the activity of the cerebellum is, is much less and that would be consistent with the idea that as we learn a new skill, the generation of error signals is quite robust. But as the scale has been consolidated and now that scale has executed with a certain measure of proficiency than the need for infusing additional error signals is, is simply less. So the cerebellum is playing an important role in the early stages of the acquisition and the consolidation of at least procedural memory. The long term storage probably reflects again widespread plasticity throughout the networks that are responsible for the execution of the skill. Be it a procedural skill, a motor skill or some other aspect of implicit human behavior. So again if we were to look into the brain anatomy that might be responsible we'd be looking at components of the motor system. perhaps the cerebellum continues to play a very important role in the storage of skill. that might be especially true for the dentate nuclei in the lateral parts of the cerebellar hemispheres. But it would also likely engage circuitry that run through the basal ganglia and certainly the premotor cortex, among other sites. So this remain a active area of ongoing study. The acquisition of procedural memory or the acquisition of utter skill and how that changes the brain.
