Brain waves, consciousness, and the perceptual moment.

Imagine this experiment. A brain scientist flashes a dim light in front of you and asks whether or not you see the light flash. The intensity of the light is adjusted so that you see it only fifty percent of the time. But why is there this variability? If all conditions are identical from trial to trial, shouldn't you see the light during all the trials or during none at all?

Here's where brain waves may come in. Ever since 1929 when Hans Berger reported the first human EEG recordings (which were made from his son's scalp), we know that voltage on the surface of our brain rises and falls in a rhythmic fashion. The frequency of these voltage oscillations varies in different regions of the brain and with different brain states. The oscillations probably reflect synchronous changes in membrane currents and voltages produced by many neurons located near the surface of the cerebral cortex. When each neuron's membrane voltage is most positive or most depolarized, it has the greatest chance of firing a nerve impulse while when its membrane voltage is most negative or most hyperpolarized, the neuron is least likely to fire. Thus, the firing potential of our cortical neurons may vary in a rhythmic way.

Seeing or not seeing the light may depend upon when during your voltage oscillations the flash occurs. Indeed, in some studies, scientists recorded EEGs from their subjects and could predict whether or not their subjects would see a dim light flash based on when the stimulus appeared during the voltage cycle. In another study, a magnetic pulse was used to directly excite the visual cortex. Following this stimulus, subjects reported seeing a phosphene, an illusory flash of light. The strength of the magnetic pulse was adjusted so that the subjects reported seeing the phosphene in only about half the trials. Again, there was a particular phase during voltage oscillations in the cortex when the phosphene was most likely to be seen. The chances of seeing the phosphene varied by about fifteen percent depending upon when during the voltage wave the magnetic pulse occurred.

In all these studies, neurons in the visual cortex were excited during each trial, yet the subjects saw the light only some of the time. We are not always aware of the sensory input coming into our brain. Indeed, our perceptual abilities may rise and fall along with our brain waves many times per second. These experiments provide insights into but also deepen the mystery of what makes us conscious.