This form of computer processing technology was essentially a positronic brain but included a holographic matrix. Initial problems faced with its development was the matrix collapsing when the cycle had to be refreshed. It was later determined that the holographic matrix would not only be less susceptible to cascade anomalies which gave the android it was equipped with a greater chance of survival during the early stages of its development but also made it possible for the brain to accommodate more complex neural net pathways. Thus, theoretically, such androids were capable of evolving very quickly.
The substructures of holotronic processing unit were similar to the earliest experiments that were conducted by Doctor Noonien Soong. Initial concepts for the system included particular emotion emulation and information absorption subroutines though he was never able to successfully develop such a unit. The work conducted on these units used studies that were produced by Professor Emil Vaslovik. The unique requirements meant that certain bodies were incapable of supporting a holotronic system.
In the 24th century, Starfleet conducted experiments in the development of a holotronic android which included top specialists in the field. This included Bruce Maddox along with the aid of Lewis Zimmerman but also Reginald Barclay all of which made numerous successes during the program. Reginald Barclay was known to had contributed to the central processing center or as it was called by Bruce Maddox, the holotronic brain. By 2374, Professor Vaslovik had joined the developmental project on the holotronic android but planned to abduct the unit for his own goals. He was responsible for not only the androids disappearance but also the purging of the project records. It was later discovered that Starfleet officer Rhea McAdams was actually a holotronic android.
In the same year, it was discovered that a number of androids had survived on the planet Exo III who had been released by Emil Vaslovik and Ira Graves. These machines studied the crashed space ship of the late Doctor Roger Korby and used it to develop their own vessel after which they secretly gathered intelligence on the Federation and Starfleet for years. At some point, they became aware of the holotronic android project and found an answer to their own dilemma which they intended to use as a template to create a superior new generation of Exo III whereupon they would transfer their minds to their new bodies. To accomplish this, they abducted McAdams and intended to use her to accomplish their goals. (TNG novel: Immortal Coil)