Human Theta Oscillations
Show Task Dependence
during Virtual Maze Navigation
Theta (4-8 Hz) oscillations have long been observed in
animals during spatial navigation and are thought to be
intimately involved in such tasks. However, strong evidence
of task- dependent theta in humans has-not been found.
In an attempt to evoke task-dependent theta in humans
and to characterize its functional role, we devised a
spatial navigation task, for use with humans, that resembles
tasks that elicit strong theta in rodents. We recorded
from arrays of intracranial electroencephalographic (iEEG)
electrodes during a virtual spatial navigation task. iIEEG
offers direct access to ventral brain regions as well
as improved spatial resolution and less signal distortion
and artifact than scalp EEG.
We tested three patients suffering from medically intractable
epilepsy. Arrays of intracranial electrodes were implanted
subdurally to localize the epileptogenic focus and identify
functional regions to be avoided in surgery. The placement
of the electrodes was determined by the clinical team.
We sampled a total of 171 electrodes across the patients
bilaterally in orbitofrontal and temporal cortex plus
scattered locations in occipital and parietal cortex.
Subjects learned to navigate through visually rich, computer-rendered
mazes. Each maze comprised a series of corridors, each
leading to a T-junction. The subject's aim was to use
arrow keys to navigate as quickly and accurately as possible
from a starting point to a goal position. Subjects traversed
each maze in study mode, in which arrows placed on the
walls showed the correct path, and in test mode, in which
the arrows were removed.
A dramatic feature of the raw iEEG trace, apparent upon
visual inspection, is the appearance of episodes of rhythmic
slow-wave activity interrupting the complex waveform.
These oscillations are in the theta range (4-8 Hz) and
appear during study and test trials in all three patients,
and in many brain regions.
A prominent characteristic of these oscillations is that
rather than being modulated continuously, they arise in
distinct, well-defined episodes. We compared how many
theta episodes occurred during longer, more difficult
mazes and shorter, easier mazes. For each of our subjects,
at certain cortical loci, percentage time in theta episodes
was significantly greater during long mazes than short
mazes, but no electrodes showed the reverse relationship.
Additionally, many brain regions showed significantly
more percentage time in theta episodes during test than
study trials.
Recording from subdural electrodes, we observed significant
increases in theta activity during virtual maze navigation.
The theta activity appears in distinct episodes that become
more frequent as the task becomes more difficult, suggesting
that theta oscillations play a functional role in tasks
similar to the spatial navigation tasks that evoke theta
oscillations in rodents.
