One of the big questions around "free will" and "choice" is that it certainly seems to most folk like WE make choices. Continuing insights from cognitive neuroscience, particularly neurotransmitter chemistry, make the meaning of "choice", particularly as an informed, reliable and "objective" process, a highly questionable concept.
The figure on the left shows the areas that increased in activity as the dopamine levels were increased in "high effort, low probability" trials. Fig. A shows left caudate and ventromedial prefrontal cortex (vmPFC) left to right respectively. Fig. B shows left lateral prefrontal cortex and temporal cortex, left to right, respectively.
These figures on the right show the increase in the proportion of "high effort/low probability" choices (horizontal axis) as dopamine levels (shown as % delta BP on vertical axis) increased for the vmPFC (C) and left caudate (D). These charts refer to Fig. A above.
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| Michael Treadway |
A new paper by Michael Treadway, and associates at Vanderbilt University, entitled "Dopaminergic Mechanisms of Individual Differences in Human Effort-Based Decision-Making" published in the May 2, 2012, Journal of Neuroscience sheds some light on what role dopamine plays in our decision making processes.
we all know that there are great variations in how different folk make value judgments on the "cost" of some activity versus its "benefits". The "costs" in this context are how much to work, how much to risk, and/or how long to wait for the "benefits", the rewards, which have some probability of being achieved. Just how does that work?
The cost/benefit psychology has been linked to personality by Richards, J.B. et al., in "Delay or probability discounting in a model of impulsive behavior: effect of alcohol". J Exp Anal Behav 71:121–143.1999 and psychopathology by Bickel and Marsch in "Toward a behavioral economic understanding of drug dependence: delay discounting processes" Addiction 96:73–86. 2001, and by Forbes, et. al. in "Alterations in reward-related decision making in boys with recent and future depression." Biol Psychiatry 61:633– 639, 2007. However, the neurochemical mechanisms underlying these processes remained poorly understood, until now.
Animal studies have point strongly to the role of the neurotransmitter dopamine as playing a/the key role in cost/benefit decision making. For example, when rats are deciding between whether to choose "low effort" food, which is of low quality but easy to reach (our twinkies), or "high effort" food of high quality (our Thai stir fry w/brown rice), healthy rats choose the high effort, high quality food.
If, however, pertinent dopamine availability is restricted, the rats will choose "low effort/low quality" food. If dopamine availability is increased, their willingness to work increases significantly. Similarly, if dopamine is restricted, rats are less willing to go after larger, riskier rewards. This all points to increased dopamine being the factor that determines whether rats will choose to overcome the "costs"/resistance of higher effort, lower probability of success and having to wait, so that they can achieve a greater reward.
Recent human studies have shown that smokers, with restricted diets, who consequently have decreased levels of dopamine precursors, have a decreased willingness to expend effort to get more cigarettes. In another study, increased dopamine increased participants’ willingness to expend more effort at lower probabilities of success to obtain rewards.
The protocol that was used by the Vanderbilt team was the "effort expenditure for rewards task" (EEfRT or “effort” - grooaaan!...c'mon Vandy...you're the "Harvard of the South"). The EEfRT is a multi-trial game in which participants choose between two different task difficulties with varying monetary rewards and probabilities of success.
Each trial presents a choice between a “hard task ” (high-effort option) and an “easy task ” (low-effort option). Easy-task choices gave rewards of $1.00 if the task was successfully completed. Hard-task choices gave higher rewards that varied from $1.24 to $4.30 (remember that a Harvard study in a recent blog gave rewards of 1 - 4 cents. - booo, Harvard.) Subjects were provided with three different, accurate probabilities of success, 12%, 50% and 88%.
The study focused on how dopamine levels affected the cost/benefit decision-making preferences and the brain centers that were involved. The neuroimaging work was done by Positron Emission Tomography/PET scanning. PET uses radioactive tracers selected for the specific purposes of the study; Fluorine 18 was used in this work.
The figure on the left shows the areas that increased in activity as the dopamine levels were increased in "high effort, low probability" trials. Fig. A shows left caudate and ventromedial prefrontal cortex (vmPFC) left to right respectively. Fig. B shows left lateral prefrontal cortex and temporal cortex, left to right, respectively.
These positive correlations between dopamine level and the willingness to expend effort for larger rewards was strongest during these high-effort, low-probability trials, when subjects had to overcome both effort and low likelihood of success. Two regions showing this association—the striatum and vmPFC—are known to be critically involved in many types of cost/benefit decision making as demonstrated in 5 different studies in the last four years.
This figure shows the decrease in the proportion of "high effort" choices (horizontal axis) as the dopamine level in the right insula (vertical axis) increased. (Both left and right insula showed similar behavior.) This strong inverse correlation between dopamine levels in the insula and the willingness to make "high effort" choices demonstrates how critical the insula is.
Other neuroimaging studies have shown the insula to be important in mediating motivation, cost/benefit decision making, processing response costs, and anticipation of losses. A recent fMRI study exploring effort-based decision-making in humans found that increased bilateral insula activation was a strong predictor of choosing a low-effort option. Although these studies did not directly assess dopamine levels or function, as Treadway's work does, the speculation is that these activation patterns may reflect dopamine signaling because the insula has a rich dopamine "innervation".
So this leaves us with the understanding that as our dopamine levels in our key decision-making centers change, whether we will choose to make a lot of effort, or a little, to get a reward, will change. Also, our willingness to choose to make the effort to get a given reward in the face of a low probability outcome will change. Some key decision-making centers will respond positively to increased dopamine, while others will respond negatively, to impact this process.
None of this is w/in our control, or direct personal knowledge. How then do we believe we have informed, reliable and meaningful choices? How high is your dopamine now? How high will it be when you make the next risky and difficult decision? Will "you" choose the low effort, high effort, high probability, low probability, high reward or low reward option? It depends on what your dopamine level is.
I believe that I have clear choices because that has been the basis of my spiritual guidance for twenty years. The existence of neural correlates to any given behaviour or cognitive process is merely that: a correlate. It is easy enough to relax yourself taking a few breaths, or by bringing the mind to attention on something before you. That there are nueural correlates to what is happening, and a vast array of physiological processes which you not directly control, does not negate the fact that there has ben a choice made to become present. There is much that we do not have contriol over, but this is not evidence of a pervasive absence of choice.
ReplyDeleteBTW, even Benjamin Libet did not discount free will. he beleived there was a brief moment in time between a thought and an action where free will existed.
One of the big problems with all this science, in my opinion, is that even though it is impressive in an immediate sense, the bigger picture is that our cognitive science is still primitive. The dominant models of the nature of consciousness, and the nature of space, time and cosmos are still infantile on a cosmic scale. Our empirical science is barely 300 years old, give or take a couple of hundred years depending on where you want to posit the beginnings of modern science. Our species is about 2 million years old, earthly life about a billion years, the earth about 4 billion years and the universe is about 13.7 billion years old. That puts it in a bit of perspective.
Once recent paper I briefly perused in biology reviewed the most cited literature in the field during a certain time frame a couple of decades ago - over a ten year period or so. About 40% of the findings in those most cited papers subsequently turned out to be highly flawed or dead wrong. I wonder how flawed will these neuro-imaging papers be in a couple of decades, a couple of centuries, a few hundred thousand years later? I am not dismissing scientific knowledge. I am simply putting forward the posssibility that, even despite impressive improvements, modern science remains highly limited in its current expression, particularly in the understanding of mind and intelligence.
Marcus:
ReplyDeleteThanks for your interest.
This comment raised so many interesting issues that it formed the basis for a reply in the form of a separate blog post, "Free will vs neuroscience; belief vs science" @ http://happinessbeyondthought.blogspot.com/2012/05/free-will-vs-neuroscience-belief-vs.html.
stillness
gary