Pleasure of reading about the pleasure of reading
In 2006, Irving Biederman and Edward A. Vessel, published a paper that gave me unforgettable pleasure to read. The article itself explained the pleasure of reading to me. In a paper titled "Perceptual pleasure and the brain", Biederman hypothesized that a gradient of opioid receptors in brain structures responsible for visual perception might contribute to the pleasure of viewing nice scenes such as beautiful landscapes. Biederman's idea seemed to explain to me what I have known for ages: learning is pleasurable. I always liked to learn, however, I never truly understood what underlies my liking in terms of brain science. Biederman's explanation was a perfect fit and it was powerfully pleasurable. It explained something that bothered my mind for a longer while. At the moment of reading, I was very self-analytical. While reading about the pleasure of reading I was trying to "feel" how the enlightenment of reading provides the pleasure. The pleasure of reading about the pleasure of reading became unforgettable.
What Biederman and Vessel proposed is monumental. Let me therefore name their thinking for simplicity: the Biederman model (name choice by seniority). In visual perception, successive layers of neurons are responsible for more abstract representations of the visual scene. Metaphorically speaking, it starts from pixels and colors, then it moves on to edges, textures and surfaces, then to objects, then to faces, places, and collections, and then to meaningful episodic scenes that, at the end of the chain, my activate a representation of "beautiful mountain" and be remembered as such with only a few details perpetuated beyond the first impression in working memory. Millions of pixels of a photograph will turn into a meaningful scene that can be verbalized in just a few sentences and remembered as such for years, at a very little neural cost.
Biederman model capitalizes on an earlier discovery (Michael E. Lewis et al., 1981) that there is a gradient of mu-opioid receptors along the visual perception pathway. The more meaning the neuron carries, the more opioid receptors it is likely to have. We know that opiates are rewarding and addictive. Biederman model is based on the hypothesis that this gradient of opioid receptors is the source of perceptive pleasure.
There is a similar hierarchical system for processing speech and music. A temporal cortex involves processing sounds from pitch to melody. However, processing rhythm involves other areas of the brain too. Chances are, all those perceptive networks work along similar principles. This is the subject of study of neuroesthetics.
Opioid vs. dopamine pleasure
There is a slight problem with the Biederman model though. The pleasure of learning can be analyzed consciously. The pleasure of reading about Biederman model, in my own case, could be decomposed and tracked down to individual components of the model. This fact implies the pleasure is integrated with conscious experience. Consciousness is a notoriously hard nut to crack for neuroscience. Most of what we know about consciousness is either speculative or based on hard and expensive experiments in which electrodes implanted in the brain can be used to elicits effects that can later, or concurrently, be reported by the affected individual. The evidence seems to be converging on the integrative model of consciousness in which an activation of several structures in the brain gets integrated and perceived as conscious self. In that line of thinking, activating a Halle Berry neuron somewhere in the cortex is not enough to bring Halle to one's consciousness. Millions of concept neurons can get activated at the same time and a thinking mind can only operate on a few pieces of the model of the perceived reality. To bring Halle to one's mind, the activation must get integrated with other components of conscious perception, including the reward of the perception.
For those reasons, opioid receptors in cortical neurons will not do much for the ultimate reward of learning. An opiod antagonist, naloxone, can take away some of the pleasure of music in some people. However, the opioid pleasure of learning should rather produce a mild bliss of first-time micro-dose heroin or morphine use. In that sense, release of endomorphins and activation of opioid receptors can make a contribution to the pleasure of learning. Nevertheless, this pleasure isn't specific enough to give one a jolt of "wow!", "aha!" or "eureka!" (Biederman calls it "click of comprehension"). For that ultimate learning reward, there must be an integrative reward experience coming from the pleasure centers in the brain.
Pleasure of association
That ultimate pleasure jolt of discovery will come from a meaningful association. It can be explained using the pleasure of understanding the Biederman model itself. When thinking about the model, we activate two important concepts in our minds: (1) a gradient of meaning (derived from understanding neural structures involved in visual perception), and (2) a gradient of pleasure (derived from the observation on the content of opioid receptors in visual pathways). Once these two concept come up in mind, there is a glue of analogy: the concept of "gradient". That glue helps bring up the association that gives a jolt of pleasant enlightenment: MEANING = PLEASURE! That's exactly what I experienced when reading Biederman's paper. For that jolt to happen, it is not enough that there are more opiate receptors associated with the concept of the gradient of pleasure than with gradient's mathematical underpinnings or its association with the word "gradient". It is not enough that there is more opiate associated with the novel concept of "gradient of meaning" than with the often used term "meaning". The jolt happens when those two highly priced concepts collide: meaning + pleasure.
Biederman noticed that the gradient of receptors proceeds far into the associative areas, incl. the parahippocampal cortex. We may remember that further downstream, in the hippocampus we have found the Halle Berry neuron. To illustrate the difference between the opioid pleasure and the associative pleasure, let us imagine meeting Halle on a beautiful beach. While walking on a beach, we may experience a delicate heroin-like breeze of bliss, which comes from the realization that our environment is perceptively beautiful: "the beach I walk on feels great". Once Halle shows up on a horizon, visual analysis may provide another breeze of opioid pleasure coming from the signal "beautiful lady approaching". Then the visual processing unit may identify the lady as Halle, which might activate cortical representation of Halle, which could be opioid-rich. However, only the ultimate association of Halle and "my beach" would trigger a major discovery, perhaps an atavistic reproductive dream: "Halle walks the same sand like me!". This is where the reward from the ventral striatum and the nucleus accumbens might come to play in "liking" the situation, and a jolt of dopamine might trigger a behavioral program of "wanting". The details of that behavioral "wanting" program have been cut out from this text by censorship. Nevertheless, execution of that program would inevitably be halted in highly-developed individuals by executive signals from the prefrontal cortex. In short, an injection of dopamine in the pleasure centers of the brain may give the brain some indecent ideas, while the release of opioid peptides might just result in an associative bliss.
The pleasure of learning does not need to involve attractive representatives of the opposite sex. Halle showed up in my example only because of the discovery of the Halle Berry neuron. For the pleasure of learning, all that is needed is a powerful and highly-valued association of ideas that activates the pleasure centers in the brain. The pleasure happens each time we learn something new, and the jolt is most powerful when we learn something of high value. The pleasure of discovering the Biederman model came from high valuations of the pleasure of learning itself in my knowledge valuation network.
Impact of memory on the pleasure of learning
I would also add to Biederman's hypotheses on desensitization, i.e. the decline in pleasure with repeated exposure. Biederman suggests that children love repetitive videogames because of the gambling factor. However, gambling is no less potent in adults. I posit that children enjoy repetitive learning more because of childhood amnesia. Some of the repeat pleasure may come from limited comprehension, but some will simply be explained by accelerated forgetting. Poor comprehension and forgetting are the primary differentiators between the adult and the child brains.
We should also notice that a great deal of decline in pleasure of review will come not from competitive learning but from long term-memory consolidation that might result in signals flowing efficiently in the system. Competitive learning may be important in pattern recognition but in associative learning, it will be high retrievability that will undermine the pleasure of repeated exposure.
Stages of learn drive evolution
When I hypothesized on the emergence of powerful learn drive in humans, I had in mind the direct channel from knowledge to reward centers. It would ultimately be a higher level of learn drive than the one implied by the Biederman model. Each time receptors are involved, evolution has a simple and grateful material to work with. Receptor gradient has originally been discovered in a rhesus cortex. Similar mechanisms might be involved in simpler brains or even more primitive nervous systems deprived of central control. I have no idea what an ant thinks or how it feels, but finding a great food source must definitely be a source of some kind of ant pleasure. From this we can conclude that the pleasure of learning might not be much phylogenetically younger than the nervous system itself. However, in the course of evolution, the drive has built up new layers of functionality and efficiency. Playful creativity seems to emerge only with some birds and with mammals. That evolutionary process might have ultimately peaked as human learn drive. This will naturally, at some point, be implemented in thinking machines. Understanding the power of the learn drive will be vital for survival of humanity: both in its need for artificial intelligence and the threat of having AI turn against mankind.