A Mind Impulse Unit (MIU) is a technologically complex device sacred to the Tech-priests of the Adeptus Mechanicus which is used to create a neurological interface between a human mind a piece of machinery and control it with thought alone. Mind Impulse Units are most commonly employed in the colossal Titans of the Collegia Titanica, the Knights of the Adeptus Mechanicus and the Dreadnoughts of the Space Marines, though the latter are much simpler and seem primitive in comparison. An MIU is more rarely used to operate weapons or devices mounted on the shoulders or elsewhere on the human body, freeing the hands or allowing a warrior to deliver more firepower that what he can carry in his arms. An MIU is also used by certain Tech-priests to aid them in their tasks. The MIU allows the user to instinctively and intuitively control the machine he or she is connected to, as if it were an extension of their own bodies, or to absorb digital information directly into the cerebral cortex.
It should be noted that the Space Marines do not make use of cybernetic Mind Impulse Units when linking with their Power Armour. Instead, their enhanced physiology equips them with a biological means to interface with it -- the gene-seed organ known as the Black Carapace. The Black Carapace allows the Space Marine to make maximum use of his Power Armour's capabilities. It is unknown if the Black Carapace allows a Space Marine to establish neural links with other forms of machinery.
The Astartes pilots of the Nemesis Dreadknights used by the Grey Knights Chapter make use of synaptic implants for neural interfacing. It is also said that only a few of these Astartes psykers possess the required strength of mind to use these constructs, and to master them in the heat of battle. These characteristics, and the fact that the operator is simultaneously in full control of his own body, make this interface resemble a moderately advanced Mind Impulse Unit, though the actual technology employed in these machines -- and whether it requires the use of the psychic abilities that the Grey Knights possess -- is unknown.
Dreadnought Mind Impulse Units
Internment within the cybernetic shell of a Dreadnought is a great honour for a Space Marine, and only very few of a Chapter's fallen will be chosen to receive it. A Dreadnought pilot is placed into an amniotic tank and implanted into the armoured sarcophagus. He is then wired to mind-impulse leads for sensory input and motive functions, allowing him to see and hear as if he was still his old self, to move the limbs of the shell that houses him and to utilise the weapons he is fitted with. It is unknown if these links utilise the interred Astartes' Black Carapace implant in any way. The pilot within the amniotic tank is attached to life support systems which keep his ruined body alive during the periods of his wakefulness. When the Dreadnought's aid is not required by his Chapter, it will spend its time in slumber, its shell deactivated and the pilot within placed in stasis to secure him from the ravages of time.
Titan Mind Impulse Units
A Titan is operated by an individual officer known as a Princeps, who must link with the ferocious Machine Spirit housed within the Titan's adamantium hull and subdue it in order to exert control over the mighty war machine. The Mind Impulse Unit enables this link by creating an interactive and immersive sensory space -- sometimes referred to as "the Manifold" -- in which the Princeps can comprehend his war engine and his environment when he becomes the towering war engine. The Titan is operated, in enforced cooperation with the Machine Spirit, through mental commands issued in the Manifold by the Princeps. Such neural enmeshing is extremely taxing, and most Princeps cannot bear the strain of controlling all of a Titan's systems -- and those who do, only can for short periods of time. Thus the Princeps must be assisted in operating the war machine by junior officers called Moderati, whose purpose is to operate some of the Titan's systems in their commander's stead.
When the Princeps is linked to a Titan's MIU, he feels the Titan's body as if it were his own -- his hands become weapon arms, his senses are turned into powerful Auspexes and his skin becomes the thick adamantium hull -- an experience of supremacy and escape from human frailty that addicts every Princeps who links with a Titan. The Princeps is then able to command the Titan intuitivelly -- he merely needs to think of undertaking an action and the great machine complies. During this process, the Princeps does not lose conciousness of his own body, and is able to simultaneously move and act as normal, within the limits dictated by the continued use of the Mind Impulse Unit to which he is connected.
Being linked to the MIU can also be dangerous and even lethal, as the potent Machine Spirit of a Titan can wreck an unprepared individual's conciousness and drive him or her to madness. Even when bound to the will of a Princeps, the Machine Spirit never surrenders completely, and its feral need to walk and rain destruction will prevent the Titan's pilot from terminating the link without suffering the consequences of restraining it. This, along with the addiction to the link that Princeps fall victim to, contributes to the painful withdrawal suffered in between periods of being in the mindlink. Shivers, aching bones, thudding headaches, muscle cramps, night terrors and a searing feeling of dislocation -- all the while longing for the joy of reconnection -- are the hallmarks of separation from the MIU, and those symptoms grow more severe with time. When the time of reconnection to the MIU comes, however, it is never an easy task. The Machine Spirit seem resentful at being woken, and each and every time a Princeps links with his Titan he must reestablish cooperation and regain the machine's trust -- which has been described as being similar to having to break in the same rebellious steed every time it is mounted. All this mental strain, over many years of repeated reconnections, and sometimes even through the Titan's own patient endeavour, will cause the Princeps to eventually lose his sanity. He will be broken in turn by the Titan, or completely overwhelmed by the domineering Machine Spirit, lost in the machine's mental landscape -- at which point disconnecting the Princeps from the Mind Impulse Unit will result in certain death. The Titan's Machine Spirit is at its strongest in the heat of battle, and some Princeps allow themselves to be engulfed by it, enmeshed even further, in order to lift their performance to an instinctive level. In the heat of battle it is not unknown for a Princeps to suffer a stroke, a tonic-clonic seizure or be completely lost in a screaming psychic maelstrom frothing within the Titan's mindscape.
Additionally, damage to the Titan's system circuitry can cause the MIU to go haywire, and in such circumstances, the luckiest Princeps are killed instantly by the psychic shock, while the rest of the crew are reduced to gibbering lunatics. Most of the crews' MIU's are equipped with manual emergency cut-offs, however, sometimes the Moderati and Princeps are not able to utilise them quickly enough to avoid the damage. As mentioned above, the mindlink is a very immersive and intimate connection, through which the Princeps feels the Titan as if it was his own flesh. Such a mental bond also causes the Princeps to perceive all damage inflicted upon the machine's hull as injuries to his own body, and although no physical harm is done to him, he will perceive pain and suffer from the emergence of psychosomatic wounds which can, in extreme cases, cause death.
MIU links are attached, most traditionally, via implanted cybernetic socket connectors in the cranium and neck and aided by cyber-optic implants, or in the spine -- in which case the body armour a Titan's crew wear must accomodate links to these sockets. Additionally, some Titan Legions utilise auxilliary haptic connectors, though others discard such measures as too delicate and too easily compromised to be placed in a war engine.
Internment in an Amniotic Tank
Some Titan Princeps are interred within an amniotic tank, a device filled with a jelly-like polycarbon fluid in which the individual is suspended, and equipped with augmitters and Vox-casters which allow the person within to communicate with his surroundings. The device acts as a complete life support unit, handling all of the vital functions of the Princeps within. The fluid's function is to suspend the individual within the casket, greatly lessening the strain on his motive system, and to create a stable neurochemical environment. Most usually, it is also a carrier of nutrients and oxygen, and in such a case the human within is modified to breathe it through his or her lungs. Sometimes, however, the fluid does not fulfill this function, and in this case the life support unit will provide external blood oxygenation and removal of excess carbon dioxide, which is then once again supplied into the body and into the right atrium. To facilitate such change, the Princeps' heart will possess an artificially created foramen ovale -- a hole between the upper chambers of the heart, allowing the blood flowing into the right atrium to bypass the lungs -- which perform no function while in the tank -- and enter the left atrium, from where it flows into the left ventricle and is further distributed across the body. Such an artificial circulatory system, as well as the body's suspension in amniotic fluids, closely resembles that of a gestating foetus within its mother's womb.
Due to their near-complete uselessness, the hands and feet of a Princeps interred in an amniotic tank are sometimes amputated and replaced with augmetic haptic sheaths that serve as a part of the broader Mind Impulse Unit. In addition, the eyes are occasionally removed, and the sense of sight, along with others, is replaced by the sensors and Auspexes of the Titan and those built into the amniotic casket. Eye sockets then serve as another medium of connection for the individual to the Mind Impulse Unit, possibly making use of the conveniently placed ocular nerves. The cybernetic link created via an amniotic interface is much deeper than that attained using hard-plugs, which makes disconnection from the MIU much harder. It is a complex ritual that can only be performed by a senior Tech-priest, and if not carried out appropriately the Princeps in question can suffer severe mental and neural trauma.
Internment in an amniotic tank allows the Princeps within to enmesh with the Titan's Machine Spirit more completely than he would ever be able to utilising only hard-plug connection. This greater and even more immersive connection grants an advantage in comprehension of the war engine and speed of reaction over a Princeps using the more traditional connection methods. A Princeps utilising amniotic interfacing is also at all times in remote communion with his war engine, even when the Mind Impulse Unit is deactivated and the Titan itself is shut down. Because of this permanent neural connection, these Princeps suffer none of the symptoms of withdrawal or the pain of separation which are the domain of pilots using standard hard-plug connection. However, due to this constant mindlink, and because of the atrophied state of their bodies, they are more exposed to the Mind Impulse Unit's effects on the psyche. This is only natural, for as the body atrophies and grows weaker from lack of use, the mind further withdraws into the depths of the Manifold and the personality is degraded. The human within slowly but surely is detached from his humanity, and eventually becomes nothing more than a dysfunctional part of the machine, his consciousness lost in the artificial mental landscape created by the Mind Impulse Unit.
For this reason, many Princeps hesitate to embrace amniotic immersion despite its advantages and fear the consequences it would bring, while some other members of a Titan's crew view such individuals with a measure of unease and uncertainty. Still, amniotic interfacing offers considerable advantages, and for this reason its use is standard practice in some Titan Legions, and some patterns of Titans require it by design. Princeps will also be modified for internment in an amniotic tank in order to save their lives, for example after they sustain great physical or mental trauma, as often occurs after surviving the destruction of a Titan, or when their addiction to the link grows severe enough that they may not be able to bear disconnection from the MIU. Amniotic immersion is then used to preserve their ruined bodies and to keep their sanity intact through the permanent connection to the Mind Impulse Unit it allows.
The Tau make use of neural interface technology which allows the pilots of their Battlesuits to directly receive information from sensors and provide advanced targetting mechanisms. This allows the pilot to have greater control over the Battlesuit, and also allows for the pilot to use larger Battlesuits such as the XV8 Crisis Battlesuit and variants without actually moving inside them. It is believed that the pilot of a Battlesuit sits in a specialised compartment built into the torso section and his arms, legs and head are kept close to his body in a foetal position. What enemies often mistake for the "head" of the Battlesuit is really the primary sensor cluster which feeds information from built-in sensory systems to the MIU of the Fire Warrior pilot inside.
- Codex: Grey Knights (5th edition), pg. 34
- Codex: Tau (3rd Edition), pg. 16
- Codex: Titanicus, pp. 7, 15, 18, 22
- Imperial Armour Volume Two - Space Marines and Forces of the Inquisition, pg. 132
- Imperial Armour Volume Seven - The Siege of Vraks - Part Three, pg. 49
- Inquisitor Rulebook, pg. 87
- Titan Legions, pg. 17
- Titanicus (Novel), by Dan Abnett
- Mechanicum (Novel), by Graham McNeill
- Helsreach (Novel), by Aaron Dembski-Bowden