Paul Reber, professor of psychology at Northwestern University, responded to this question submitted to Scientific American:
Link | Image: US Department of Health and Human Services
The human brain consists of about one billion neurons. Each neuron forms about 1,000 connections to other neurons, amounting to more than a trillion connections. If each neuron could only help store a single memory, running out of space would be a problem. You might have only a few gigabytes of storage space, similar to the space in an iPod or a USB flash drive. Yet neurons combine so that each one helps with many memories at a time, exponentially increasing the brain’s memory storage capacity to something closer to around 2.5 petabytes (or a million gigabytes). For comparison, if your brain worked like a digital video recorder in a television, 2.5 petabytes would be enough to hold three million hours of TV shows. You would have to leave the TV running continuously for more than 300 years to use up all that storage.
Link | Image: US Department of Health and Human Services
It emphasizes how pitifully little of our brains we utilize on a regular basis."
I love this myth that we don't use the considerable amount of our brain capacity. Abstract thought of even the most basic sort is the result of the carefully controlled cooperation of numerous disparate regions of the brain. Our brain don't just have huge dead zones.
As for this romantic idea of the autistic savant, it's also silly. The normal human brain receives and processes a massive amount of data every second. It discards the input it deems irrelevant and focuses on the most important. If it didn't, you would be overloaded constantly with trivial, distracting information that impaired your higher functions. This is essentially what disables the autistic. While they may like Temple Grandin be geniuses about very specific topics, they are unable to function on more abstract levels such as comprehending social interaction.
Actually, there are over 100 Billion neurons in the brain and since the connections for each neuron multiply exponentially (eg. 2 neurons connect twice, 4 neurons connect 64 ways, 8 neurons connect 32,000 ways, etc.) After all is said and done, there are more possible synaptic connections in the human brain than there are particles in the known universe. Far, FAR more than a trillion which really is quite meager compared to the numbers we are talking about here.