Figure 4: Five technological axes of BCIs

Each axis is a trade-off: both ends have strengths, and moving right, towards the greatest potential for augmentation, comes at a price in safety, portability, computing power or equipment. The axes are linked but do not all move together: more invasive devices tend to reach deeper and record finer detail, but cover less of the brain, while combining modalities usually needs hardware too bulky to implant or wear. No current technology sits at the right-hand end of every axis.

1. Invasiveness

   How close the device sits to neural tissue

   Non-implantableSafer and easier to deploy,
   but weaker signal

   ImplantablePrecise recording,
   but requires surgery

2. Number of modalities

   Physical principles employed: electrical, magnetic, optical, mechanical

   SingleCompact,
   but less information

   MultipleMore information,
   but bulkier hardware

3. Spatial and temporal resolution

   How precisely activity is read or stimulated in space and time

   LowLight computing,
   but coarse detail

   HighFine detail,
   but heavy computing

4. Brain coverage

   How much of the brain is reached at once

   NarrowLess equipment,
   but small area mapped

   BroadWhole-brain dynamics,
   but more equipment

5. Depth of brain access

   Which brain functions can be reached

   SurfaceSensory and motor areas

   DeepMemory and mood circuits