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ERPs are classified according to the nature
of the stimulus: visual, somato-sensory, and auditory; they can also be
classified according to the latency at which their components occur
after stimulus presentation: short latency (<100msec) and long latency
(>100msec) potentials.
The shorter latency components are generated
during the sensory stimulus processing stages (exogenous components).
The longer latency components represent the cortical processing stages,
which are less determined by the physical features of the stimulus
(endogenous components).
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Exogenous potentials |
Endogenous potentials |
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- Depend on physical
features of the sensory stimulus. |
- Do not depend on
physical features of sensory stimulus. They can be evoked, just
with stimulus expectancy, even in the absence of stimulus.
|
|
- Do not depend on the
subjects’ level of consciousness. |
- Can change depending
on the level of attention, its relevancy during the task and
resources required for stimulus processing. |
|
- Are not influenced by
cognition processes. |
- Related to cognition
processing. |
The classification into early or late
components ERPs is useful in practical terms, however it is more
theoretical than realistic since ERPs generation is a continuous
process.
In this session the stimulus used to evoke
the responses is auditory, so the responses will be auditory related
potentials.
Early auditory related potentials
include five positive waves that occur during the first 10 msec after
stimulus presentation and are labeled from I to V according to their
order of appearance. They are very stable in shape, amplitude and
latency in subjects with no hearing impairment. It is well proven that
these components are generated as a result of the activation of brain
stem nuclei of the auditory pathway during auditory stimuli information
processing. Due to their stereotyped behaviour, even during sleep and
unconsciousness states, these potentials have been very helpful as an
objective functional evaluation of auditory system in newborns and
psychological deafness.
Long latency potentials are referred
to those components that appear after 100 msec of stimulus presentation
and are thought to represent cortical information processing. They are
affected by level of attention, stimulus significance, task relevance
and stimulus processing requirements.
We are going to record P100 (first
positive [P] component appearing 100 ms after the stimulus) using
auditory stimuli, although this component can also be evoked visually;
the most important factor is that the stimulus must be unpredictable
in time.
This kind of potential has been used for
psychophysical assessment in patients with cognitive and attention
disorders such as Alzheimer’s dementia, schizophrenia, and speech
disorders. |
