It is an exciting time to be a neuroscientist. The last fifty years have revealed more about the brain than the preceding two million years of human history. The ability to map the electrical activity of brain cells has exponentially increased the understanding of their function. The October issue of the Scientific American contained an interesting article by two neuroscientists, Terry Sejnowski and Tobi Delbruck. The authors believe that neuroscientists are close to a breakthrough in understanding the biological underpinnings of thought. This breakthrough is on the heels of experiments that are revealing how electrical and chemical signals in the brain convey information.
Most people know that the brain contains billions of specialized cells (nearly a trillion), called neurons, that are highly interconnected. Once an electrical impulse travels down a wire (axon) to the main body of the neuron, it may or may not send a subsequent signal to an adjacent neuron. Neuroscientists have already discovered how individual neurons function, but not how the neurons interact to yield useful information. They lacked an understanding of why groups of neurons suddenly discharged electricity, as well as how this excitation ultimately led to thoughts. By examining the firing of many neurons at the same time, a recent achievement, neuroscientists believe it is the timing of electrical spikes that encodes information and solves complex problems. The variable rate of when the spikes occur may convey discrete components of information about the physical world. For example, nerve cells in the retina of the eye appear to coordinate firing with a change in light intensity, which may also occur with a change in spatial orientation or color. The human brain appears to yield processing time to several neurons switching on at the same time, since a random or accidental grouping of nerves firing is very unusual.
To be more specific, the observation of just one neuron in isolation reveals a pattern of random electrical activity. Observing the electrical activity in tracts of nerves that connect the eye with middle and hind portions of the brain, researchers note that groups of neurons will fire when the spatial orientation of an object changes. Neurons in the mid-brain will not fire with input from one, two or even three neurons located in the eye. Four neurons discharging electricity from the eye to the mid-brain will cause it to relay information to the visual cortex at the back of the brain. Similarly, exciting single neurons in the retina of the eye produces a random firing pattern. Once the adjacent neurons in the eye are excited as well, the firing of this single neuron decreases as it becomes synchronized to the firing of the entire group. As mentioned, it is believed that these synchronized firing patterns respond to a discrete visual components; eventually amassing the data to form an image in our consciousness. Spaced only a few milliseconds apart, the rhythmic firing of widely dispersed cortical neurons is necessary to yield a visual perception.
It has been observed for some time that when mammals attend to some stimulus, the synchronized firing of multiple neurons increases, and the rate at which these neurons discharge electricity increases as well. People who suffer with either schizophrenia or autism appear to have a decreased level of this neuronal firing pattern when attending to a stimulus. The deficit has been traced to specialized cells in the cortex that control the timing of adjacent neural circuits. Too much excitation or inhibition may cause this attention circuit to be less efficient or imprecise. Disruption to the pattern of neuronal firing within the most frontal regions of the brain has been a hypothesized etiology of these disorders for decades. To be sure, many other theories have been advanced as well.
In terms of encoding new memories, the synchronization of neuronal firing is also very important. If one neuron in the brain causes another to consistently fire within just ten milliseconds, the first neuron in the chain will tend to discharge electricity with greater frequency. Conversely, if the second neuron in the chain consistently fires less than ten milliseconds before the first neuron, the synchronization between the two declines. The more groups of neurons become biologically accustomed to firing together, the greater their interconnections, the greater their rate of firing, and greater is the probability that this pattern of synchronous electrical discharges will be encoded into proteins for long-term storage. As was said in graduate school, neurons that fire together…wire together.