(iv) The Gender of the Subjects Must Be Reported
Because gender affects many electrophysiological measurements,
the investigator must report how many of the subjects were male
and female, and must ensure that any group effects are not con-
founded by differences in the female0male ratios across groups.
When studying normal subjects, the investigator should generally
use either a similar number of female and male subjects or subjects
of one gender only. It is often worthwhile to include gender as an
experimental variable. If the experiment compares normal subjects
with subjects with a clinical disorder that is more common in one
gender, the male0female ratios should be approximately equivalent
across the two groups.
(v) Sensory and Motor Abilities Should Be Described
for the Stimuli Being Presented and the
Responses Being Recorded
This recommendation is to ensure that subjects can perceive the
stimuli normally. For most studies of normal young subjects, it is
sufficient to document that all subjects reported normal hearing or
vision ~with correction!. Such self-report is usually correct about
normal sensory ability. However, the accuracy of self-report will
depend on the type of questions asked. The answer to “Do you
have normal hearing?” is much less informative than answers to a
set of questions about hearing under different situations ~Coren &
Hakstian, 1992!.
In experiments designed specifically to evaluate perceptual
function, particularly in studies of disordered perception, more
intensive evaluations should be used to clarify what is normal or
to categorize levels of abnormality. For auditory stimuli, sub-
jects should be screened for normal hearing at 20 dB HL at the
frequencies tested. For visual stimuli, acuity should be measured
~with refractive correction! at a distance appropriate for the stim-
uli used. Because most visual stimuli are presented at close dis-
tances, acuity normally would be checked using Jaeger rather
than Snellen charts. If stimulus color is manipulated during the
experiment, color vision should be checked ~e.g., using one or
several Ishihara plates!. Unfortunately, there are no widely ac-
cepted quantitative screening tests for normal somatic, taste, or
smell sensations.
When subjects are making motor responses during the experi-
mental paradigms, the investigator should provide some basic de-
scription of the subjects’ ability to perform the task. It is usually
sufficient to ensure that the subjects report no history of weakness.
In all studies using motor responses, the handedness of the subject
should be reported and preferably measured using a validated
questionnaire.
(vi) The Subjects’ Cognitive Abilities Relevant
to the Tasks Being Studied Should Be Described
The experimenter should provide some basic assessments of the
subjects’ ability to perform the tasks being evaluated. In normal
subjects, the educational level is a reliable indicator of general
cognitive abilities, and descriptions of the subjects such as “un-
dergraduate students” is sufficient. However, this approach is in-
adequate in the context of clinical patients, children, and the elderly,
for whom more specific evaluations should be provided. For ex-
ample, mental status tests should be used when evaluating the
ERPs of demented patients, standardized reading assessments when
ERP paradigms that require reading are used in children, and neuro-
psychological tests of memory when ERPs are used to study mem-
ory disorders in the elderly.
(vii) Clinical Subjects Should Be Selected According to Clear
Diagnostic Criteria and the Clinical Samples Should Be Made
as Homogeneous as Possible
The selection criteria for clinical subjects should be explicitly stated.
The Diagnostic and Statistical Manual of the American Psychiat-
ric Association ~American Psychiatric Association, 1994! provides
criteria for most psychiatric disorders. Diagnostic criteria for neuro-
logical disorders can be found in the relevant literature. When the
clinical disorder is heterogeneous ~e.g., schizophrenia, attention
deficit disorders!, the experimenter should attempt to limit the
subjects to one of the various subtypes of the disorder or to stratify
the patient sample according to the subtypes. The sample should
also be made as homogeneous as possible in terms of both the
duration and the severity of the disease process. It is never possible
to devise pure patient groupings. Nevertheless, some attempt should
be made to limit heterogeneity and any residual sources of hetero-
geneity should be described. In addition, the sample should be
characterized carefully with respect to demographic and psycho-
metric variables. For example, in a study of patients with dementia
of Alzheimer type, the investigator should include information
about the age and gender of each subject, along with data on
current mental status ~e.g., Mini-Mental State Examination!, pre-
morbid intelligence ~e.g., National Adult Reading Test!, and mem-
ory function ~e.g., selected subtests of the Wechsler Memory Scale!.
For patients with focal brain lesions, such data should also include
detailed information about the location and nature of the lesions.
(viii) Medications Used by Subjects Should Be Documented
In ERP studies of normal subjects, the investigator should make
sure that the subjects are not taking prescription medications that
may affect cognitive processes. It is probably also worthwhile not
to use subjects who have taken alcohol or other recreational drugs
within the preceding 24 hours. Because clinical patients are com-
monly treated with medications, it is often difficult to disentangle
the effects of the clinical disorder from the effects of the treatment.
Wherever possible, some control for medication should be at-
tempted. In some cases unmedicated patients can be studied. If the
patients have various dosages of medication, the level of medica-
tion should be considered in the statistical analysis, or preferably
in the experimental design ~e.g., selecting different subgroups of
patients with different medication levels!. Unfortunately, it is not
possible to use an analysis of covariance to remove the effects of
different medication levels ~or other variables! from other group
differences ~Chapman & Chapman, 1973, pp. 82–83; Miller, Chap-
man, & Isaacks, submitted!. An analysis of covariance can be used
to reduce the variability of measurements in groups that vary ran-
domly on the variable used as a covariate. However, if groups
differ on each of two variables, covarying out the effects of one
variable will distort any measurement of the effect of the other.
(ix) In Clinical Studies, Control Subjects Should Be Chosen
so That They Differ From the Experimental Subjects
Only on the Parameters Being Investigated
The selection criteria for the control subjects should be stated
clearly, as should the variables on which the control subjects and
patients have been matched. In general, the groups should be
matched for age, gender, socioeconomic status, and intelligence.
The premorbid intelligence of the patient group may be compared
with the actual intelligence of the control group using educational
level or some more formal psychological assessment such as the
National Adult Reading Test. Both control and experimental sub-
jects should be evaluated on standardized behavioral, psycholog-
ERP guidelines 131