hallucinations

[Stephen Grossberg]

The following article is available in HTML, PDF, and  Gzipped Postscript at
http://www.cns.bu.edu/Profiles/Grossberg

Grossberg, S. (1999).  How hallucinations may arise from brain mechanisms
of learning, attention, and volition. Journal of the International
Neuropsychological Society, in press.

ABSTRACT: This article suggests how brain mechanisms of learning,
attention, and volition may give rise to hallucinations during
schizophrenia and other mental disorders. The article suggests that normal
learning and memory are stabilized through the use of learned top-down
expectations. These expectations learn prototypes that are capable of
focusing attention upon the combinations of features that comprise
conscious perceptual experiences. When top-down expectations are active in
a priming situation, they can modulate or sensitize their target cells to
respond more effectively to matched bottom-up information. They cannot,
however, fully activate these target cells. These matching properties are
shown to be essential towards stabilizing the memory of learned
representations. The modulatory property of top-down expectations is
achieved through a balance between top-down excitation and inhibition. The
learned prototype is the excitatory on-center in this top-down network.
Phasic volitional signals can shift the balance between excitation and
inhibition to favor net excitatory activation. Such a volitionally-mediated
shift enables top-down expectations, in the absence of  supportive
bottom-up inputs, to cause conscious experiences of imagery and inner
speech, and thereby to enable fantasy and planning activities to occur. If
these volitional signals become tonically hyperactive during a mental
disorder, the top-down expectations can give rise to conscious experiences
in the absence of bottom-up inputs and volition. These events are compared
with data about  hallucinations. The article predicts where these top-down
expectations and volitional signals may
act in the laminar circuits of visual cortex, and by extension in other
sensory and cognitive neocortical areas, and how the level of abstractness
of learned prototypes may covary with the abstractness of
hallucinatory content. A similar breakdown of volition may lead to
declusions of control in the motor system.

Key Words: hallucinations, learned expectations, attention, learning,
adaptive resonance theory

Preliminary version appears as Boston University
Technical Report, CAS/CNS-TR-99-020.