The caudate nucleus: A final common path for integrating emotion/cognition networks into action?

July 7, 2021 at 8:42 AM
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The caudate nucleus: A final common path for integrating emotion/cognition networks into action?

Brandon Ascencio, Saleh Afoogh, Brandy Eggan, and Jim Stellar

In college, experiential learning leads to the development of skills, maturity, and arguably the most important end trait – employability. These opportunities come in the form of internships, volunteering, job shadowing, etc. and make an excellent complement to classical academic learning that occurs in the classroom. But, how do we teach these non-classroom components to make a well-rounded college graduate with their knowledge and experience integrated? We think that by diving deeper into what may be behavioral neuroscience factors in how that learning works – often from a cognitive-emotional integration perspective – could help us to connect these dots. In this blog we approach this phenomena with the addition of a philosophical perspective and focus our attention on one key and well-known brain area – the caudate nucleus.

Integration of one’s skills and academic knowledge is what employers and even students are looking for. Ideally, when they are integrated in the right balance, wise career choices, large and small, will result. But how does the brain combine this information and turn it into action? Sometimes the act and the motivation to do it are simple, e.g. in homeostasis where the decision to eat to maintain an energy balance is necessary for life. Organisms with a lesser evolved higher brain neocortex or no neocortex at all make such decisions. But higher organisms likely do it differently, integrating abstract knowledge with motivation, and here is where it gets complex. For example, consider the phenomenon of paradoxical kinesia in a classic wheel-chair-bound Parkinson patient where the caudate nucleus has lost almost all of its dopamine and the patient appears to have lost all capacity to make voluntary movements, to act. Yet the patient moves when a motivational state appears (e.g. a fire in the nursing home).

Also, higher organisms, including us as humans, can develop elaborate learned preferences, biases, likes, and attitudes that influence behavioral action. Much has been written about heuristics or implicit short-cuts in thinking that underlie economic and other decisions that may arise from emotional or motivational sources. Do we choose the chocolate cake, vanilla cake or no cake at all for dessert?  Sometimes the motivation is social. We actually have neurons, particularly in the premotor planing area of the neocortex, that fire whenever we see someone else do a task we might do, like them picking up a piece of food and putting it in their mouth. These neurons are called mirror neurons abt what that means is that you might not even be hungry for that piece of cake, but seeing someone else eating it, you might do the behavior and eat it too. Now, say you’re alone and walk into the breakroom after you just had a filling but mediocre salad for lunch and you see leftover cake from your coworker’s retirement party the day before, you sneak a piece despite being full because. Presumably, no mirror neurons are operating here. You just like cake. This behavioral action is thought to come from neurons within the mesolimbic reward system that react to learned rewards and that also plays into your motivation to act.

Our point here, and the point of this blog, is that these actions have to be organized – one behavior at a time, do it or not. You can’t half execute a behavior, half decide, change your mind every second or not decide at all. If that were the case you’d never make it through that initial interview in the hiring process or even be able to order lunch. In older neurology, this concept of integration into a single behavioral action is called the final common path, and in that path all conflicts are resolved into one motor behavior. Could the caudate nucleus be a higher level final common path for the decisions to act, particularly where motivations and cognitions are integrated – where motor behavior meets purpose and where all the cortical and subcortical input from brain networks gets resolved into a physical execution? Is this system a model for how the implicit experience of a college student working in a chosen profession combines with the more cognitive knowledge from academic learning to lead to that well-known maturity of purpose and of decision-making that leads to the graduate being hired?

A neuroscience approach to cognitive-affective integration – a final common path

The notion of a final common path had its origins in a series of papers by Sir Charles Sherington in 1906 on the “The Integrative Action of the Nervous System.” Sherington actually went on to win a Nobel Prize in Physiology in 1932 with this work that focused on the connection of the brain to spinal cord nerve cells that control muscles. More specifically, his work referred to the control over the very last neurons in the spinal cord that directly contract a muscle, like telling your leg to jerk when the doctor strikes the patellar tendon in  your knee with a reflex hammer. The neurons that control every muscle are getting messages from multiple other neurons, some sending excitatory and some sending inhibitory inputs. It is key that by the time one of these final common neuron fires, all inputs on whether to contract the muscle or not must be resolved. Now, while the final common path as discussed refers to control of a reflex, we would like to elevate the concept to mean a point of control of motor output that leads to a single complex action.  But this too is not new.

A Sherrington contemporary, John Hughlings Jackson, wrote of influences of “higher centers” on “lower ones” in the generation of behavior. For example, when a positive symptom appears such as excessive speech as a result of a brain lesion, one presumes that the now damaged brain center held another brain center in check. Today’s neuroscientists tend to stay away from the word “center” and prefer the word “system” out of respect for the incredible connective complexity of the nervous system. This concept of a network is something about which we have written previously focusing initially on the well-known default mode network, on skill learning network, and then network integration properties.

As mentioned, here we focus on the idea that an important final common path control over lower systems may be the caudate nucleus. The caudate receives input from the entire neocortex giving it the potential to act as an integration station. The caudate is well known as being necessary for the basic initiation of movement as mentioned with Parkinson’s Disease above. Caudate-related emotional structures of the larger basal ganglia (e.g. the accumbens) could be a place where emotion inputs are integrated with cognitive inputs to create a pressure to act. If so, the caudate could be a final common pathway for cognitive/emotional integration in action about which we keep writing in this general blog. However, we also acknowledge that networks themselves could have different ways of implementing the same behavioral decision-making without this notion of a fixed structural final common path where conflict  between brain circuits is resolved, and we may return to that in future blogs.

A philosophical approach to cognitive-affective integration

To make some decisions, higher order thinking is necessary. The most recent action theorists in the field (i.e. Kanthians) have claimed that higher order decision making is preceded by cognitive attitudes (ex. perception of the cake; cake is eaten for dessert) while others (i.e. Humeans) believe that the motivation is preceded by the affective attitudes (ex. desires or passions; I like cake for dessert). There is also a new contextualism approach which says, the motivation comes out from a mixture of cognitive and affective attitudes which need to weigh into one ultimate decision making location in the human brain.

That leads us to the question, “what is the origin of motivation toward actions?” or what makes someone motivated to act? With this question being multifactorial, it makes sense that multiple brain regions would be involved in this. Moreover, it is broadly accepted that the potential origins, either cognitive or affective attitudes, play a central role in the deliberation process, which leads to some decisions toward a specific action and the physical execution of the action which lives in yet another brain region. Moreover with these brain regions having important individual roles it must also be noted that a decision purely based on “I did this because I like it” (e.g. I did cocaine. It feels good.) is driven by motivation in the reward pathway, but that pathway itself extends inches through the brain and stops at multiple nuclei along the way including the ventral tegmental area, nucleus accumbens, and medial prefrontal cortex. What that means is that even one individual component of motivation to act comes from multiple brain regions.

As contextualists, we believe that the decision to execute a behavior will be affected by both cognitive and affective attitudes, yet to different extent and degrees, in different contexts. For example, in a mathematics class we will usually be affected more by our cognitive attitudes to participate in the class and learn new mathematical ideas. However, there might be a kind of passion or desire for learning mathematics. On the contrary, we are more affected by our passions and desires when we decide to eat ice cream in a hot summer. The information that we received, and the way we deliberate to make a decision is pretty context-relevant.

Moreover, in addition to the conscious cognitive, and conscious affective attitudes, there are still some unconscious causes in human agents, which might affect the deliberation and decisions making process. For example, sometimes, I decide to use a red pen to solve a mathematical problem, or decide to buy a red ice-cream, without any conscious motivative reason in my mind; but, a psychologist bystander who knows me well, may confirm that the causes for my choosing red pen or red ice-cream is the unconscious cause of red-color bias in me; that is an unconscious physiological bias (in addition, the affective and cognitive attitudes) will play a role in my decision making as well.

Adding on the unconscious brain and its influence over decision making also adds yet again more regions to this long list of regions driving the motivation of even our most basic decisions. Bringing it back to the theme of wanting students to get jobs after they graduate, they will get those jobs if they make good decisions. But, again look at all of the components that weigh into one decision. It can even be as simple as a decision about what to wear to the interview – desire to look professional and not stick out (cognitive), but having a bias towards your favorite good luck Hawaiian shirt, maybe under your blazer (affective). These are the kinds of little decisions that students must make as they move from the student world to the world of work.

The caudate as a center for integration

The caudate nucleus is a major part of a larger brain area called the striatum whose components interact. The striatum has long been studied in regards to its function in regulating motor behaviors in response to pleasant or aversive stimuli, but inputs from higher cortical structures also influence the degree to which these behaviors are executed. Many tracing studies from decades ago have shown connectivity all around the brain and this could occur in a way that could integrate information from multiple brain circuits that are home to conscious cognitive and affective aspects that motivate the decision making process. And, there is neuroscientific evidence to suggest that this final decision of cortical input may reside in the caudate nucleus.

For example, research has shown that the striatum participates in parallel corticobasal ganglia thalamocortical loops that interconnect the different territories with the area of the frontal lobe. In simple terms, that means connectivity between our regions exists, but what is more interesting is that smaller regions of the striatum including the caudate alone are not necessarily linked to a single cortical region. There are specific patterns of coativation in different portions of the higher cortex that suggest again that it is networks that feed into caudate as a central hub. As in the link above, this has been seen in fMRI studies and specifically with specific physiological tasks involving cognitive and emotional processing, lending more evidence to the idea that humans with lesions in the caudate typically have cognitive and behavioral abnormalities. Additionally, as stated in the first line, PET scan studies show that activity (metabolism) of frontal lobe structures including the dorsolateral and medial prefrontal cortex as well as the anterior cingulate gyrus is significantly decreased with lesions to the caudate. These are interconnected networks that regulate higher order thinking, motivated decision making, e.g. the decision that “I probably shouldn’t eat the cake because I’m on a diet”.

Another interesting piece of research shows that the caudate is activated during the anticipation of a potential reward. If you’re a fan of recent social media, you’ve probably seen the old study and current challenge where parents leave their children in a room, put a tasty snack in front of them and tell them not to eat it until they come back to the room and then they will get even more. This is a delay of gratification test and has been long used in the field of neuroscience and has been claimed to be an indicator of future behavior in adolescents. It wasn’t until recently that fMRI studies (see above) show that among the age 10-14 age participants, increased activity in the caudate led to improved scores, meaning that they were more willing to wait for the delayed gratification and larger reward later. These data give evidence to the caudate as a sort of weight between “I like it” and “should I do it or wait for a larger reward later” networks in the brain and its activity in these decisions suggests again that it could be acting as a final path for this information.

Trying to dissect the cognitive and affective components of behavior out of the caudate is difficult. One additional piece of research supporting this central hub for this incoming network information is a study that implicates the caudate as an integrator for uncertain visual evidence (cognitive) and reward context (affective) as a guide to adaptive decision-making. In this experiment, monkeys were trained to make eye saccade decisions based on noisy visual motion evidence and asymmetrical reward contexts. Activity in the caudate was measured related to both quickness to saccade and reward context / size and showed that the activity of many neurons in the caudate were sensitive to changes in these variables. Additionally, microstimulation of these neurons changed the responsiveness to reward suggesting a coordinated adjustment that the monkeys naturally make in response to a change in reward value. Thus, the caudate is clearly playing a role in processing the networked information from the cognitive sensory system (including visual, evidence also exists for the auditory) and the affective or reward system.

Furthermore, it was even said in 2018 that gene expression could link functional networks across the cortex and the striatum. The network-associated expression is very consistent across independent human datasets and evolutionarily conserved in non-human primates. Patterns of gene expression could even show similarities across the brain structures functionally connected by spatially and anatomically distinct (like the cortex and the caudate). The “shared enrichment” of genes among cortical and subcortical components of a network could even reveal the network preferential presence of a particular cell type, which would yield deep insight into underlying biological mechanisms.

Back to a philosophical defining of the value behind a decision

Value, in a broad and commonsensical sense, considered as the worth, utility, or goodness of something or someone. In a technical sense, it might include a normative sense of worth, utility, or goodness as such. It has also been discussed that there are two senses of value, intrinsic and extrinsic. The intrinsic refers  to the value that a thing has “in itself,” or “for its own sake,” or “as such,” or “in its own right.” (e.g. the value of justice) while the extrinsic value refers to an instrumental kind of values which are worth something not “for its own sake, but for the sake of something else to which it is related in some way,” e.g. the value of helping a person in defense of her rights.

There are few, if any, disagreements on the existence or of intrinsic values in human life; there is a variety of worthy and good things in our lives such as moral goodness, monetary value, beauty, etc. we respect them and take them as criteria in our decisions and actions. However, there is a significant metaphysical question on the origins of the normativity of values, which says, where does a normative value come from? Some philosophers, like Kant and Aristotle centuries ago, believed that there exist some cognitive entities, e.g., universal considerations, or moral virtues that work as a source of values. On the contrary, some philosophers like Hume and Hobbes believed that a valuable thing is a desirable one, which is a pretty subjective and affective attitude – that is, what you desire to have/do.

Regardless, of the long historical debate between the philosophers on the normativity of values, it seems that we, human beings, follow a mixture of ways in our life and use both cognitive and affective attitudes as a source of our values. For example, given that there is a cognitive value for marriage (which guarantee the continuation of human life on the earth), I love to marry Sara. But I marry not only because there is that cognitive value for marriage, but also, and here, more importantly, I marry her for the sake of three personal affective values in Sara, i.e., kindness, being smart, and her blue eyes.

There is also a more basic value in the emotional or affective input to the caudate. Setting affective value is what the midline Ventral Tegmental dopamine neurons in their projections to the accumbens shell.  That projection then finds its way to the accumbens core, and eventually back to the substantia nigra and the all-important dopamine input that keeps the cortical-caudate loop working to allow for action. We have written about this dopamine story in a previous set of blogs focusing on the complex decision making process that drives the Pavlovian classic “I can train a dog to salivate to a bell” behavior because the dog likes the food.

Finally, there is another value that is computed abstractly in the lateral prefrontal cortex that underlies estimates of risk and reward. This cortical calculation can find its way to the caudate and spur action.  However, the value at a higher level represents a yet-not-understood integration of the emotional value with that cognitive value. It is in the potentially apocryphal moment of eureka of Archimedes who understood the value of his discovery. According to legend, he knew he had an answer to a difficult scientific problem. The story persists because of the joy of discovery in science and those words “joy of discovery” represent the kind of convergence that leads to action with conviction.

The caudate as a potential hub for wisdom?

To take our discussion again back to that college student completing an internship, in that internship, they are learning the basic skills they need, the hands on, unconscious technical feel required to get by. Examples range from the carpenter trying to hit a nail on the head without crushing their fingers to the doctor pushing with the right pressure to make an incision through the flesh but not deep enough to hit the bone. It’s easy for someone to tell them “aim here, push this hard” or for that intern to read about it, view pictures, and learn that the hammer should be gripped three inches below the head for the ultimate force transfer or the incision should be 2.5 mm under the surface and 4.5 mm long. But doing it – executing it – learning to make it flawless requires the complicated network of interacting brain regions that we have discussed above and that potentially converge into the caudate as the final hub for action. With that being said, the action will likely be incorrect numerous times and with that comes learning and modifications to these brain regions in the network. Moreover, it also puts stress on the caudate to adapt, perhaps with every time the young carpenter hit their thumb with the hammer the brain began to weigh that “ouch” input as more important than the desire to get that board nailed to the wall. We think this kind of learning builds professional wisdom and if that action is correct, then we might even say that was a moment of insight or wisdom.

Is the caudate with its cortical and subcortical convergence and its proximity to action the final common path for this integration? Does it represent the cognitive-emotional convergence, the “mixture of ways” referred to above? To properly answer from a behavioral neuroscience perspective will take much thinking and many more studies combining rigorous psychological tests with activity brain scanners, involving studies of normal people and those with specific brain damage. Much of this research has already begun but is beyond the scope of this discussion. Yet despite its richness, it still has a long way to go before we get to a good neuroscience understanding of the brain circuits involved.

In the meantime, we will have to make due with exploring the convergence of implicit and explicit thinking and with the linking of emotional logic with cognitive processing. Also, we will have to focus practically on continuing to develop a college education that allows the simultaneous acquisition of direct experience in a career path converged (implicit learning) with a major area of academic study (explicit learning). Such a combination may even aid the development of long-held core educational values, such as critical thinking and writing, as students get inspired and pay more attention to their academic studies. Our hope here is that, perhaps, the idea of a caudate nucleus as a final common path for convergence between implicit/emotional and explicit/cognitive processing in student will be a useful model for our thinking.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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