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This formed the basis of the “neuron doctrine,”which proposed the brain’s tissue was made up of many discrete cells,instead of one connected tissue.The neuron doctrine laid the foundation for modern neuroscience,and allowed later researchers to discover that electrical impulses are constantly converted between chemical and electrical signals as they travel from neuron to neuron.Both Golgi and Cajal received the Nobel Prize for their separate, but shared discoveries,and researchers still apply their theories and methods today.In this way, their legacies remain connected as discrete elementsin a vast network of knowledge.

While writing a book about microscopic imaging,he came across a picture of a cell treated with Golgi’s stain.Cajal was in awe of its exquisite detail—both as a scientist and an artist.He soon set out to improve Golgi’s stain even further and create more detailed references for his artwork.By staining the tissue twice in a specific time frame,Cajal found he could stain a greater number of neurons with better resolution.And what these new slides revealed would upend reticular theory—the branches reaching out from each nerve cell were not physically connected to any other tissue.

Known as the “black reaction,”Golgi’s Method finally allowed researchers to see the entire cell body of what would later be named the neuron.The stain even highlighted the fibrous branches that shot off from the cell in different directions.Images of these branches became hazy at the ends,making it difficult to determine exactly how they fit into the larger network.But Golgi concluded that these branches connected,forming a web of tissue comprising the entire nervous system.14 years later, a young scientist and aspiring artist named Santiago Ramón y Cajal began to build on Golgi’s work.

Soft nervous tissue was delicate and difficult to work with.And even when researchers were able to get it under the microscope,the tissue was so densely packed it was impossible to see much.To improve their view,scientists began experimenting with special staining techniques designed to provide clarity through contrast.The most effective came courtesy of Camillo Golgi in 1873.First, Golgi hardened the brain tissue with potassium bichromate to prevent cells from deforming during handling.Then he doused the tissue in silver nitrate,which visibly accumulated in nerve cells.

So how were these individual cells transmitting electrical signals?By studying and sketching them countless times,Cajal developed a bold, new hypothesis.Instead of electrical signals traveling uninterrupted across a network of fibers,he proposed that signals were somehow jumping from cell to cell in a linear chain of activation.The idea that electrical signals could travel this way was completely unheard of when Cajal proposed it in 1889.However his massive collection of drawings supported his hypothesis from every angle.And in the mid-1900s, electron microscopy further supported this idea by revealing a membrane around each nerve cell keeping it separate from its neighbors.

Reticular theory captivated the field with its elegant simplicity.But soon, a young artist would cut through this conjecture,and sketch a bold new vision of how our brains work.60 years before reticular theory was born,developments in microscope technology revealed cells to be the building blocks of organic tissue.This finding was revolutionary,but early microscopes struggled to provide additional details.The technology was especially challenging for researchers studying the brain.

In the late 1860s, scientists believed they were on the verge of uncovering the brain’s biggest secret.They already knew the brain controlled the body through electrical impulses.The question was, how did these signals travel through the body without changing or degrading?It seemed that perfectly transmitting these impulses would require them to travel uninterrupted along some kind of tissue.This idea, called reticular theory,imagined the nervous system as a massive web of tissue that physically connected every nerve cell in the body.

Operant conditioning is everywhere in our daily lives.There aren't many things we do that haven't been influenced at some point by operant conditioning.We even see operant conditioning in some extraordinary situations.One group of scientists showed the power of operant conditioning by teaching pigeons to be art connoisseurs.Using food as a positive reinforcer,scientists have taught pigeons to select paintings by Monet over those by Picasso.When showed works of other artists,scientists observed stimulus generalization as the pigeons chose the Impressionists over the Cubists.Maybe next they'll condition the pigeons to paint their own masterpieces.

Even though you know the mirror won't hurt,you jump out of the chair and run,screaming from the room.When you went to get a shot,the words, This won't hurt a bit,became a conditioned stimulus when they were paired with pain of the shot,the unconditioned stimulus,which was followed by your conditioned response of getting the heck out of there.Classical conditioning in action.Operant conditioning explains how consequences lead to changes in voluntary behavior.So how does operant conditioning work?

There are two main components in operant conditioning:reinforcement and punishment.Reinforcers make it more likely that you'll do something again,while punishers make it less likely.Reinforcement and punishment can be positive or negative,but this doesn't mean good and bad.Positive means the addition of a stimulus,like getting dessert after you finish your veggies,and negative means the removal of a stimulus,like getting a night of no homework because you did well on an exam.Let's look at an example of operant conditioning.After eating dinner with your family,you clear the table and wash the dishes.When you're done, your mom gives you a big hug and says, Thank you for helping me.In this situation, your mom's response is positive reinforcement if it makes you more likely to repeat the operant response,which is to clear the table and wash the dishes.

Nobody trains a dog to salivate over some steak.However, when we pair an unconditioned stimulus like food with something that was previously neutral,like the sound of a bell,that neutral stimulus becomes a conditioned stimulus.And so classical conditioning was discovered.We see how this works with animals,but how does it work with humans?In exactly the same way.Let's say that one day you go to the doctor to get a shot.She says,;Don't worry, this won't hurt a bit,and then gives you the most painful shot you've ever had.A few weeks later you go to the dentist for a check-up.He starts to put a mirror in your mouth to examine your teeth,and he says, Don't worry, this won't hurt a bit.

In the 1890's, a Russian physiologist named Ivan Pavlov did some really famous experiments on dogs.He showed dogs some food and rang a bell at the same time.After a while, the dogs would associate the bell with the food.They would learn that when they heard the bell,they would get fed.Eventually, just ringing the bell made the dogs salivate.They learned to expect food at the sound of a bell.You see, under normal conditions,the sight and smell of food causes a dog to salivate.We call the food an unconditioned stimulus,and we call salivation the unconditioned response.

When we think about learning,we often picture students in a classroom or lecture hall,books open on their desks,listening intently to a teacher or professor in the front of the room.But in psychology, learning means something else.To psychologists, learning is a long-term change in behavior that's based on experience.Two of the main types of learning are called classical conditioning and operant, or instrumental, conditioning.Let's talk about classical conditioning first.

Some neural fibers have as many as 100 layers of myelin.Others only have a few.And fibers with thicker layers of myelin can conduct signals 100 times faster or more than those with thinner ones.Some scientists now think that this difference in myelination could be the key to uniform conduction time in the brain, and consequently, to our mental synthesis ability.A lot of this myelination happens in childhood,so from an early age, our vibrant imaginations may have a lot to do with building up brains whose carefully myelinated connections can craft creative symphonies throughout our lives.

The problem is that some neurons are much farther away from the prefrontal cortex than others.If the signals travel down both fibers at the same rate,they'd arrive out of sync.You can't change the length of the connections,but your brain, especially as it develops in childhood,does have a way to change the conduction velocity.Neural fibers are wrapped in a fatty substance called myelin.Myelin is an insulator and speeds up the electrical signals zipping down the nerve fiber.

The mental synthesis theory proposes that like a puppeteer pulling the strings,the prefrontal cortex neurons send electrical signals down these neural fibers to multiple ensembles in the posterior cortex.This activates them in unison.If the neuronal ensembles are turned on at the same time,you experience the composite image just as if you'd actually seen it.This conscious purposeful synchronization of different neuronal ensembles by the prefrontal cortex is called mental synthesis.In order for mental sythesis to work,signals would have to arrive at both neuronal ensembles at the same time.

One hypothesis, called the Mental Synthesis Theory,says that, again, timing is key.If the neuronal ensembles for the dolphin and pineapple are activated at the same time,we can perceive the two separate objects as a single image.But something in your brain has to coordinate that firing.One plausible candidate is the prefrontal cortex,which is involved in all complex cognitive functions.Prefrontal cortex neurons are connected to the posterior cortex by long, spindly cell extensions called neural fibers.

If you try to imagine a pineapple later,the whole ensemble will light up,assembling a complete mental image.Dolphins are encoded by a different neuronal ensemble.In fact, every object that you've seen is encoded by a neuronal ensemble associated with it, the neurons wired together by that synchronized firing.But this principle doesn't explain the infinite number of objects that we can conjure up in our imaginations without ever seeing them. The neuronal ensemble for a dolphin balancing a pineapple doesn't exist.So how come you can imagine it anyway?

like a collage made from fragments of photos.The brain has to juggle a sea of thousands of electrical signals getting them all to their destination at precisely the right time.When you look at an object,thousands of neurons in your posterior cortex fire. These neurons encode various characteristics of the object: spiky, fruit, brown, green, and yellow.This synchronous firing strengthens the connections between that set of neurons, linking them together into what's known as a neuronal ensemble, in this case the one for pineapple.In neuroscience, this is called the Hebbian principle,neurons that fire together wire together.

Imagine, for a second,a duck teaching a French class,a ping-pong match in orbit around a black hole,a dolphin balancing a pineapple. You probably haven't actually seen any of these things,but you could imagine them instantly.How does your brain produce an image of something you've never seen?That may not seem hard, but that's only because we're so used to doing it.It turns out that this is actually a complex problem that requires sophisticated coordination inside your brain.That's because to create these new, weird images,your brain takes familiar pieces and assembles them in new ways,