Redwood Neuroscience
Title " Human
Prefrontal Cortex and Attention”
Robert T. Knight, M.D.
Helen Wills Neuroscience
Institute and Department of Psychology
Abstract:
Humans fluidly move attention
across the temporal domain rapidly shifting from the present state to past or
future mental representations. This
capacity requires rapid engagement of parallel inhibitory and excitatory neural
processes. Electrophysiological, fMRI and electrocorticographic evidence accrued from controls and
neurological patients reveals that these processes aredependent
on lateral prefrontal cortex (PFCx). PFCx patients fail to inhibit sensory input to primary
sensory cortices in the first 25-50 msec of sensory
processing resulting in a distractible PFCx patient functioning
in a noisy internal milieu. Deficits in inhibitory control
extends to the cognitive domain in PFCx
patients and is most readily observed as intrusions of previous information
into current cognitive operations. In
addition to the critical role in inhibitory control of sensory flow and
cognitive processing, PFCx exerts excitatory input in
all sensory modalities which provides the critical substrate for voluntary
attention. In the visual modality, PFCx controls
three parallel top-down excitatory mechanisms to extrastriate
processing during the initial 500 milliseconds of stimulus processing. PFCx provides a
tonic attention independent input to extrastriate
cortex onsetting at 100 msec,
a sustained attention dependent input from 150-400 msec
and a third input engaged from 200-800 msec during
detection of a target stimulus. Deficits in inhibitory and excitatory
modulation of distributed neural networks after PFCx
damage involving all sensory modalities leads to a breakdown in voluntary
attention and working memory. Automatic attention allocation as indexed by
detection of novel environmental events is also dependent on a rapidly
habituating prefrontal-hippocampal network. PFCx initiates this
novelty detection network within 150 msec of
presentation of a novel event. Direct
cortical recording in neurosurgical patients reveals that ultra-high gamma
bursting (60-160 Hz) is generated to automatically detected
deviant sounds as well as other salient auditory events. The capacity to detect novelty and produce
novel behaviors is crucial for learning, creativity and mental flexibility
which are all impaired after lateral PFCx damage.
Thus, the devastating human prefrontal syndrome can be viewed as a failure in
rapid PFCx control of multiple, parallel neural
networks subserving both voluntary and automatic
attention.