An exocortex is a theoretical artificial external information processing system that would augment a brain's biological high-level cognitive processes.
An individual's exocortex would be composed of external memory modules, processors, IO devices and software systems that would interact with, and augment, a person's biological brain. Typically this interaction is described as being conducted through a direct brain-computer interface, making these extensions functionally part of the individual's mind.
Living Digital provided one description of the concept:
While [the traditional concept of] a cyborg has included artificial mechanical limbs, embedded chips and devices, another interesting concept is the exocortex, which is a brain-computer interface. In theory, the exocortex would be a computer-like processing system that would co-exist with and enhance the power of the human brain. Neuromancer is a book that has talked about such a scenario.
The noun exocortex is composed of the Greek-derived prefix exo-, meaning external or outside, and the Latin noun cortex, which originally meant bark but is used in neuroscience for the outer bark-like layer of the brain that is the site of most sophisticated cognitive information processing. It was coined in allusion to the neocortex (literally 'new bark'), the newest part of the mammalian brain (in evolutionary history), believed to be responsible for the highest human cognitive abilities including conscious thought, spatial reasoning, and sensory perception. Thus the terminology suggests a progression from reptilian thought (the older parts of the brain) through human (neocortex) to high-level human or even supra-human cognitive processing capabilities (exocortex).
Specific applications 
In 1981 Steve Mann designed and built the first general purpose wearable computer. Later on he became one of the early pioneers in using wearable computers for augmented and computer-mediated reality. Although he does not refer to it as such, his personal wearable computer could be considered an exocortex. Running applications like the remembrance agent on his wearable computer enhances his natural mental capabilities.
Intellectual background 
Computer science roots 
Within computer science, the seeds were planted by the DARPA associated researcher J.C.R. Licklider. Within his speculative 1960 paper Man-Computer Symbiosis, Licklider outlined his vision that humans and the new technology of computers, if tightly-coupled together, would prove to complement each others strengths to such a degree that many of the pure artificial intelligence systems envisioned at the time by optimistic researchers would prove unnecessary:
Man-computer symbiosis is a subclass of man-machine systems. There are many man-machine systems. At present, however, there are no man-computer symbioses. The purposes of this paper are to present the concept and, hopefully, to foster the development of man-computer symbiosis by analyzing some problems of interaction between men and computing machines, calling attention to applicable principles of man-machine engineering, and pointing out a few questions to which research answers are needed. The hope is that, in not too many years, human brains and computing machines will be coupled together very tightly, and that the resulting partnership will think as no human brain has ever thought and process data in a way not approached by the information-handling machines we know today.—Man-Computer Symbiosis, J.C.R. Licklider, March 1960.
A DARPA contemporary of Licklider, Douglas Engelbart, was thinking along similar lines in the field of computer science. In 1962, Engelbart authored Augmenting Human Intellect in which he details how to augment human intellectual effectiveness by exploiting the technology of the then emerging computer:
This is an initial summary report of a project taking a new and systematic approach to improving the intellectual effectiveness of the individual human being. A detailed conceptual framework explores the nature of the system composed of the individual and the tools, concepts, and methods that match his basic capabilities to his problems. One of the tools that shows the greatest immediate promise is the computer, when it can be harnessed for direct on-line assistance, integrated with new concepts and methods.—Augmenting Human Intellect: A Conceptual Framework, Douglas Engelbart, October 1962.
From this basis, the concept of an exocortex, the direct coupling of the human mind with computers to leverage their respective complementary strengths, can be viewed as a result of the ever increasing symbiotic coupling between human and computers.
Psychology roots 
The exocortex concept also has roots in evolutionary psychology as a result of Merlin Donald of Queen's University. Donald, in the 1990 book Origins of the Modern Mind as well as later papers, proposed an evolutionary model of the mind, from a functionary perspective, from its origins in prehistoric apes to the modern human being. Donald focuses significant attention on the use that modern humans make of external symbolic storage and manipulation systems—the range of technologies from cuneiforms, hieroglyphics, and ideograms to alphabetic languages, mathematics and now computers. From Donald's perspective, these external symbolic systems have allowed for the functional reorganization of the human mind in how it deals with the world.
The externalization of memory [via the use of external symbolic storage systems] has altered the actual memory architecture within which humans think, which is changing the role of biological memory, the way in which the human brain deploys its resources, and the form of modern culture.—Precis of Origins of the modern mind, Merlin Donald, 1996.
Thus to Donald, the human mind has long been a hybrid structure built from the vestiges of earlier biological stages and combined with our new external symbolic systems. The development of an exocortex, which could result in significant functional reallocation, again fits well within this long established trend.
Current applicability 
Cognitive science origin 
In November 1998 the specific term exocortex was coined by researcher Ben Houston. Houston coined the term to refer concisely to tightly-coupled cognition-level brain-computer interface technologies in the spirit of Licklider's and Engelbart's original visions.
exocortex (eks'o kor'teks) n. Latin -- an organ that resides outside of the brain that aids in high level thinking. .... This will not be a prominent term until prefrontal cortex neural implants become widespread. (emphasis in original)—early exocortex definition, Ben Houston, May 2000 .
Use in science fiction 
Speculative devices which fit the definition of exocortices were described in hard science fiction long before the term was coined; examples appear in Neuromancer by William Gibson and in The Peace War by Vernor Vinge, both published in 1984. More recently Vinge, in A Fire Upon the Deep and several short stories, described the functional effects of what are essentially several kinds of exocortices - both those composed of computational elements, and those enabled by high-bandwidth communication between groups of beings. Peter F. Hamilton's Night's Dawn trilogy also describes in detail similar technological beings.
Charles Stross, the Hugo Award-nominated hard science fiction writer, has led the adoption of the term exocortex within science fiction circles. Beginning in 2004, Stross made use of the term in Elector, a short story published in the September issue of Asimov's Science Fiction. Stross made more extensive uses of the term exocortex and its derivatives in Accelerando, his 2005 novel.
While Stross himself does not provide an explicit definition of the term, a few passages indicate his meaning:
About ten billion humans are alive in the solar system, each mind surrounded by an exocortex of distributed agents, threads of personality spun right out of their heads to run on the clouds of utility fog – infinitely flexible computing resources as thin as aerogel – in which they live. (emphasis added)—Accelerando, Charles Stross, 2005.
Sometimes he isn't certain he's still human; too many threads of his consciousness seem to live outside his head, reporting back whenever they find something interesting. .... And it's too early for anyone out there to be trying to hack exocortices... isn't it? Right now, the external threads of his consciousness are telling him that they like Annette.... (emphasis added)—Accelerando, Charles Stross, 2005.
The Wikibooks Accelerando Technical Companion provides this explanation:
An EXOcortex can best be described as the portion of a trans- or posthuman entity's brain (or cortex) which exists outside of that entity's primary computing structure, usually the brain inhabiting a person's 'meatbody.' For example, a person's exocortex could very well be composed of all the external memory modules, processor, and devices that the person's biological brain interacts with on a realtime basis, thereby in effect making those external devices a functional part of the individual's 'mind.' (emphasis in original)—Accelerando Technical Companion, Wikibooks.
Popular usage 
While initial recognition of the exocortex concept was nonexistent, this has changed as a result of Charles Stross's recent publications and the growing awareness of brain-computer interfacing. The term and concept of an exocortex has both been applied (i.e. "Suffered a Stroke in my Exocortex") and noted as a novel interesting word (i.e. "Found Words: Exocortex") by various bloggers. Here are some additional examples of proper contextual usage: , and . The concept has been described in the March 2006 issue of Living Digital. James Hughen wrote in an essay entitled "What comes after humans?" that appeared in the Nov 16, 2006 issue of the New Scientist:
To remain the web’s weavers and not its ensnared victims, we must merge with our electronic exocortex, wiring greater memory, thought processing and communication abilities directly into our brains.
Future prospects 
If one widens the definition of an exocortex, one can see that computational elements are already used as supporting elements of biological brains, and growing dependence on parts of the Internet that serve cognitive functions has brought what could be considered a proto-exocortex into existence. Wikipedia itself is an example, as technological interfaces enable inter-brain co-operation on high-level cognitive tasks. The fulfillment of the initial vision of Licklider and Engelbart suggests that continued development along this path is likely.
Currently, true exocortices remain speculative. The main issue is that the required underlying technology is yet to be produced by the scientific research fields of (1) cognitive neuroscience, (2) computational neuroscience and (3) neural engineering.
See also 
- Brain-computer interface
- Cerebral cortex
- List of emerging technologies
- Project Xanadu
- Man-Computer Symbiosis paper, JCR Licklider, March 1960.
- Augmenting Human Intellect paper, Douglas Engelbart, October 1962.
- Origins of the Modern Mind book, Merlin Donald, 1990, ISBN 0-674-64484-0.
- Exocortex definition, Ben Houston, May 2000.
- Elector short story, Charles Stross, 2004.
- Accelerando novel, Charles Stross, 2005, ISBN 0-441-01284-1.
- Accelerando Technical Companion, WikiBooks, 2005.
- Found Words: Exocortex False Positives blog posting, Ian Irving, July 2005.