436
for efficient learning and memory. This has largely
resulted from evidence of interference from chol-
inergic antagonists; the effects of cholinergic agon-
ists have been less than convincing and deterred a
formulation for a ‘cholinergic role’. However a
series of ERP studies using visual discriminations
of varying difficulty by Callaway, Halliday et al.
showed that antagonists like scopolamine slow N1,
P3 and often reaction time, interfering particularly
if the task was relatively easy and more so with
the target than the irrelevant stimulus (Brandeis et
al. 1992). This led them to suggest that choliner-
gic activity promotes parallel processing and the
more automatic processes by which salient features
capture attention.
The widespread involvement of inhibitory (e.g.
GABA, Gly) and excitatory amino-acid transmit-
ters (e.g. Glu, Asp) in local circuitry in association
areas as well as in some long feedback loops (e.g.
basal ganglia) implies their inevitable participation
in the generation of ERP components and compa-
rator processes essential to attentional mechanisms.
It is therefore surprising when anticholinergics and
neuroleptics can affect several ERP components,
respectively monotonically or differentially, if an
agent such as diazepam slows P3 processing se-
lectively (Ray et al. 1992). Less surprising is the
finding that schizophrenics with varied cognitive
(and other) problems, possibly with cerebral atro-
phy, display impaired synthesis, release and up-
take of glutamate and GABA in various parts of
the CNS (Sherman et al. 1991; Simpson et al.
1992).
Conclusions
ERP subtraction waves (e.g. MMN, PN, Nd) are
helping to inform on the nature and on the locus in
time and space of the mechanisms that go to make
up the series of operations we call attention. The
judicious study of psychopathology and psycho-
pharmacology is starting to show which anatomi-
cal systems (pathways and transmitters) may be
involved in which processes. But perhaps still at
the youngest stage of development is the knowl-
edge that operational measures in one test reflect
similar functions in another. Attentional process-
ing is widely distributed but the mechanisms are
localised; transmitter systems modulate and me-
diate, where the degree to which any one is in-
volved is dependent on the situational demands.
The further study of schizophrenic patients should
help to improve this understanding (e.g. on im-
paired fronto-temporal cross-talk); it is to be hoped
that they too will eventually benefit from this im-
provement.
Acknowledgments
I am grateful to Professor C. Eggers for his sup-
port and I thank Dieter Zerbin and RobertWindel-
schmitt for technical assistance.
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