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Grasping the Pleasure Principle

The brain needs to acquire knowledge, say researchers at USC and NYU. Its reward for ‘getting’ a visual concept is a shot of natural opiates.

By Carl Marziali, USC News
23 June 2006

The brain’s craving for a fix motivates humans to maximize  the rate at which they absorb knowledge, said USC College neuroscientist Irving  Biederman.

Neuroscientists have  proposed a simple explanation for the pleasure of grasping a new concept: The  brain is getting its fix.

The ‘click’ of comprehension triggers a  biochemical cascade that rewards the brain with a shot of natural opium-like  substances, said Irving Biederman, professor of neuroscience in USC College, who  presents his theory in an invited article in the latest issue of American  Scientist [SCR: see figure below; also see related comments in New Scientist].

“While you are trying to understand a difficult theorem, it’s  not fun”, Biederman said. “But once you get it, you just feel fabulous.”

The brain’s craving for a fix motivates humans to maximize the rate at  which they absorb knowledge, he said.

“I think we’re exquisitely tuned  to this as if we’re junkies, second by second.”

Biederman hypothesized  that knowledge addiction has strong evolutionary value because mate selection  correlates closely with perceived intelligence.

Only more pressing  material needs, such as hunger, can suspend the quest for knowledge, he added.

The same mechanism is involved in the aesthetic experience, Biederman  said, providing a neurological explanation for the pleasure we derive from art.

“This account may provide a plausible and very simple mechanism for  aesthetic and perceptual and cognitive curiosity.”

Biederman’s theory  was inspired by a widely ignored 25-year-old finding that mu-opioid receptors - binding sites for natural opiates - increase in density along the ventral visual  pathway, a part of the brain involved in image recognition and processing.

The receptors are tightly packed in the areas of the pathway linked to  comprehension and interpretation of images, but sparse in areas where visual  stimuli first hit the cortex.

Biederman’s theory holds that the greater  the neural activity in the areas rich in opioid receptors, the greater the  pleasure.

In a series of functional magnetic resonance imaging trials  with human volunteers exposed to a wide variety of images, Biederman’s research  group found that strongly preferred images prompted the greatest fMRI activity  in more complex areas of the ventral visual pathway. (The data from the studies  are being submitted for publication.)

Biederman also found that repeated  viewing of an attractive image lessened both the rating of pleasure and the  activity in the opioid-rich areas. In his article, he explains this familiar  experience with a neural-network model termed ‘competitive learning’.

In  competitive learning (also known as ‘Neural Darwinism’), the first presentation  of an image activates many neurons, some strongly and a greater number only  weakly.

With repetition of the image, the connections to the strongly  activated neurons grow in strength. But the strongly activated neurons inhibit  their weakly activated neighbors, causing a net reduction in activity. This  reduction in activity, Biederman’s research shows, parallels the decline in the  pleasure felt during repeated viewing.

“One advantage of competitive  learning is that the inhibited neurons are now free to code for other stimulus  patterns,” Biederman writes.

This preference for novel concepts also has  evolutionary value, he added.

“The system is essentially designed to  maximize the rate at which you acquire new but interpretable [understandable]  information. Once you have acquired the information, you best spend your time  learning something else.”

“There’s this incredible selectivity that we  show in real time. Without thinking about it, we pick out experiences that are  richly interpretable but novel.”

The theory, while currently tested only  in the visual system, likely applies to other senses, Biederman said.

Edward Vessel, who was Biederman’s graduate student at USC, is now a  postdoctoral fellow at the Center for Neural Science at New York University.  Vessel collaborated on the studies and co-authored the American Scientist  article.

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Within the brain, the ventral visual pathway (top; red arrows) is involved in the recognition of an object or a scene. Early stages of visual processing (in areas V1 to V4) analyze an image's contours, colors and textures. Intermediate stages (the lateral occipital area and ventral occipito-temporal cortex, or VOT) integrate local information to detect surfaces, objects, faces and places. Within the VOT, a region within the collateral sulcus (CoS) responds strongly to images of places, such as buildings, houses and vistas. Later stages of recognition, in areas such as the parahippocampal cortex and rhinal cortex, are activated when the brain interprets the stimulus in the context of stored memories. Surprisingly, these visual areas also contain mu-opioid receptors (bottom; black dots), which are involved in the modulation of pain and pleasure in other parts of the brain. They are sparse in the earlier stages and grow increasingly dense in the later stages. According to the authors' hypothesis, visual stimuli that contain a great deal of interpretable information should activate many opioid receptors in the later stages and so provide the greatest pleasure. (At top, edges of ventral areas are projected onto the lateral surface and denoted by dashes. Exact locations of all areas may differ from one individual to another.)