Related Topic(s):

The Treatment of Attention Deficiency

A chapter from the Textbook of Neurofeedback, EEG Biofeedback and Brain Self Regulation
edited by Rob Kall and Joe Kamiya

Clinical Applications of Subliminal Auditory Stimulation:
The Treatment of Attention Deficiency

Paul G. Swingle

formerly of University of Ottawa, McLean Hospital, and Harvard Medical School
, now

Suite 630, 1190 Melville Street, Vancouver, British Columbia, V6E 3W1 Canada
Tel: (604) 608-0444 Fax: (604) 684-7659

Some deficiencies in attention are associated with anomalous slow-wave activity in the brainwave frequency spectrum. A common feature of many treatment protocols is the suppression of EEG activity in the 4-8Hz frequency band. An auditory subliminal treatment has been developed which appears to s uppress Theta activity. This treatment is self- administered in situ without interfering with ongoing activities. Several preliminary cases are discussed in the context of patient self-administered treatment adjunctive to more established protocols. The use of auditory subliminal stimulation to reduce autonomic arousal with patients with stress exacerbated attention deficits is also discussed.

Broadly speaking, there appear to be three approaches to the modification of the brainwave spectrum. The first is EEG biofeedback in which specific frequencies or frequency bands are displayed which patients are instructed to enhance or inhibit. For conditions characterized by cortical underarousal, patients are trained to enhance faster frequencies and/or inhibit low frequency activity. For conditions of hyperarousal or insufficient low frequency amplitudes patients are taught to enhance low frequency amplitudes. What is striking about EEG biofeedback is the generality of the technique and the range of disorders effectively treated. In the treatment of attention deficits, hyperactivity, anxiety, pain, learning disabilities, epilepsy, sequela of closed head injury, sleep disorders, and asthma, the unifying factor underlying the conditions appears to be some abnormal EEG pattern. Treatment protocols reported as effective for these disorders vary considerably with regard to electrode location, size, type, and electrode montage.

Feedback frequencies likewise differ considerably with some clinicians favouring single frequencies while others favour feedback frequency bands. Further, the frequency bands used in training differ among various clinical settings, with apparently equivalent results. This suggests that the different protocols may train the same mechanism but perhaps through different processes. If the anomalous EEG activity is excessive low frequency activity then training to inhibit 4-8 Hz, or to enhance 12-15 Hz, 16- 20 Hz, or 14 Hz, activity may all be effective. The fact that the effects of training can be accomplished with different electrode placements suggest that change is being effected subcortically.

The second approach involves administering auditory, visual or microelectrical stimulation that entrains brainwaves. The rationale for these procedures is that if psychophysiological states are associated with identifiable brainwave densities, then these states can be enhanced by entraining the appropriate EEG frequency with auditory, visual or somatosensory stimulation. Research has indicated that alpha brainwave activity is associated with subjective euphoria (Lucas, 1991), relaxation (Benson, 1983), increased creativity (Hardt & Gale, 1993), decreased pain (Melzack & Perry, 1975), decreased anxiety and increased hypnotic susceptibility (Delmonte, 1984). Depression, on the other hand, has been found to be related to reduced Beta and enhanced Theta in the frontal cortex (John et al., 1989; Schatzberg et al., 1986).

Auditory and visual stimulation of set brainwave frequencies have been found to enhance the amplitude of brainwaves at the same frequency (Barlow et al., 1960; Ohatrian et al., 1960). The rationale for the light and sound machines which present visual and auditory rhythmic stimulation in the range of about 4 to 30 Hz, is that enhancement of the amplitude of particular frequencies will be accompanied by subjective psychophysiological changes associated with the increases in brainwave densities. Thus, stimulation in the Alpha range should have a calming and relaxing effect. Theta stimulation might enhance susceptibility to suggestions while stimulation in the Beta ranges might have a stimulating influence.

Cranial Electrical Stimulation (CES) refers to the transcranial application of electricity in the microamperage range. Devices approved by the United States Food and Drug Administration usually deliver less than 1.0 mA at a pulsating frequency of 100 Hz. As with EEG biofeedback, CES treatment also appears robust to variations in electrode type and placement. Although the stimulation frequency is well beyond those frequencies commonly associated with EEG treatment, research has indicated that CES treatment increases the amplitude of the slow EEG frequencies in the frontotemporal (Jarzembski, 1985) and in the occipital cortex (Cox & Heath, 1975).

The third general approach is that of teaching the patient self-regulatory behaviours that have an effect on the brainwave spectrum. Hypnotic states, for example, are associated with increases in central Theta and decreases in Alpha in posterior regions (Schwartz et al., 1993). Breath control and PCO2 feedback normalizes the EEG of seizure sufferers by attenuating Theta and increasing Alpha (Fried, 1993). And, of course, meditation has long been recognized as a method for decreasing cortical arousal as reflected in increases in Alpha and Theta (Delmonte, 1984; Hardt, 1993). Experienced meditators exhibit these increases even when not meditating and very advanced meditators (no thought) appear to inhibit the electrocortical activity of both hemispheres (Meissner & Pirot, 1983). Further, many relaxation protocols are associated with increased Alpha with attendant reductions in stress and anxiety (Benson, 1983).
As the above brief review indicates, the curious aspect of the burgeoning field of neuronal regulation is that so many apparently different treatment protocols appear effective for a rather wide range of disorders. The unifying concept for many of these treatment protocols is that irregularities in the EEG spectrum are rendered more normalized as a result of treatment. The concept of neuronal regulation as a treatment modality is based on the premise that anomalies in the brainwave spectrum are associated with specific disorders, and that these anomalies can be normalized with attendant reduction in the associated symptomatology. The great variation in treatment protocols that appear effective for normalizing the EEG and/or reducing symptomalogy is indicative that a more fundamental neurological change is taking place than surface EEG recordings reveal.

The notion that a fundamental abnormality is being redressed by the various treatment protocols is also related to findings that EEG abnormalities often vary with regard to location. In substance abusers for example, EEG anomalies are sometimes observed in the frontal, temporal, parietal, and occipital lobes (Braverman et al., 1990). Further, the fact that anticonvulsant medication has been found useful for many different psychiatric disorders (Post et al., 1989) also suggests some fundamental underlying neurological abnormality. The fact that many of these disorders respond to treatments that vary in terms of site of intervention and training frequency suggests that the various treatments have the effect of regularizing the central nervous system. This regularization presumably can occur as a result of protocols that inhibit or potentiate brainwave frequencies that are anomalous. Hence, elevated Theta during cognitive tasks such as reading or arithematic indicates deficient attention and can be remediated with interventions that inhibit excessive Theta activity.

In clinical practice, one encounters many manifestations of attention deficiency. Some patients cannot concentrate because they are too anxious whereas others complain of inability to concentrate based on depressed emotional states. Many patients in treatment for psychological distress also complain of physiological conditions that interfere with attentional focus. These complaints vary. Some maintain that their vision becomes "foggy" or "blurred" which accompanies a feeling of mental "fuzziness". Others do not report physiological sensations but rather state that they often forget what they are doing or find that they are very easily distracted. Still others report simply that they learn very slowly and easily forget what they have been taught.
Attention deficiencies that are related to physiological underarousal or overarousal have been effectively treated with relaxation protocols, EMG biofeedback (Carter & Russell, 1985; Potashkin & Beckles, 1990) and sensory stimulation (Mangina & Beuzeron-Mangina, 1992). The latter procedure is particularly interesting. It involves the presentation of tones to the right and/or left ear whenever the electrodermal activity (EDA) level falls below 6.5lmhos on either hand. Tone presentation is contralateral to the EDA measurement. When the EDA of either hand exceeds 8.5 lmhos, various relaxation techniques are used to reduce the EDA level. When the bilateral EDA is within the 6.5 to 8.5 lmhos window, visual or auditory learning tasks of increasing complexity are presented.
This procedure has been found to be a very effective treatment for "learning disabled" students. Improvements in school grades are marked and are sustained at two-year post- treatment follow-up. However, like brainwave treatment, many sessions are required. Although improvements are observed after 30 sessions, marked change is apparent after 60 sessions. It is also interesting to note that the learning disabled subjects were selected on the basis of poor school grades. About one-third of the subject population satisfied the DSM-III-R criteria for Attention Deficit Disorder with Hyperactivity.

The research cited above on EEG and other physiological activity has been done with supraliminal sounds. The work of Zenhausern and his associates (1973, 1974) and work I have done indicates that effects on the EEG and performance can also occur with subliminal auditory stimuli. Subliminal auditory messages have been shown to affect mood, memory, motor performance, physiological state, problem-solving, and aesthetic judgement (Swingle, 1992). However, of particular relevance to the treatment of attention deficiencies are reports that nonverbal auditory subliminal stimulation can affect performance (Zenhausern et al., 1973; Zenhausern & Hansen, 1974), autonomic arousal (Swingle, August 1992) and brainwave activity (Swingle, March 1993).

One day, several years ago, I was in my laboratory trying to perfect a slow wave modulating supraliminal tone to be used in a recording to guide patients in a paced breathing relaxation exercise. One of my graduate students emerged from his office, which was about 20 feet away, complaining that he had suddenly become quite fatigued and simply could not concentrate. While pondering the cause of his dilemma, it occurred to us that the sound I was working on, although clearly audible to me, might have been subliminal in his office and responsible for his fatigue. We checked this out and indeed the sound could not be heard in the student's office. Extrapolation of the drop in sound pressure over distance from the sound measured at the source indicated that the sound would have been in the effective subliminal range (Swingle, 1992) of approximately - 18 dB(C). We prepared an audio tape of this modulating tone embedded 18 dB(C) below masking white noise, so that the tone would be subliminal, and found the tape clinically useful for many patients with sleep disturbance. The sound was a single sinusoidal tone that modulated between 285 Hz and 315 Hz at about 7 cycles per minute.
This serendipitous episode motivated an investigation of the effects of subliminal presentation of compound sinusoids of the type used in light and sound relaxation devices. The decision to test sounds in the conventional EEG spectrum and to use compound sinusoids was guided by published research, reviewed previously, indicating that such sounds will entrain brainwave activity.

To present stimuli at frequencies within the audible range, two sinusoidal tones, close in frequency, were simultaneously presented at equal amplitudes. When two sinusoids are presented which differ in frequency, beats will be heard. This phenomenon is apparent up to frequency differences of about 60 Hz after which the partials of the compound sound will be heard (Plomp, 1976). The rate of the beating is equal to the difference in frequency of the two sinusoids and the loudness of the beats is maximum when the amplitude of the two sinusoids is equal (Green, 1976).

One might expect the effects of subliminal presentation of the beat frequencies to parallel supraliminal presentation. There is, however, considerable evidence indicating independent processing of supraliminal and subliminal stimuli (Swingle, 1992). Further, the evidence also indicates that subliminal stimuli, the content of which is unavailable to recipients' selective attention, give rise to qualitatively different responses as compared with supraliminal presentation (Swingle, 1992).
The stimulus tapes used in the following laboratory studies and clinical trials were all prepared as described in detail in Swingle (1992). Briefly, two equal amplitude sinusoidal tones, one at 300 Hz and the second up to 25 Hz higher in frequency, are embedded in filtered white noise. The subliminal sounds are presented at a maximum Sound Pressure Level (SPL) of -17 dB(C) relative to the white noise embedding medium. Tapes prepared for clinical use usually have two different embedding SPL sound tracks, one on each side of the tape. On one side, the embedding is at -17 dB(C), whereas the flip side of the tape is embedded at -25 dB(C). The two different SPL tracks coincide with different effective ranges for males and females, and facilitate patient participation in the determination of an effective treatment sound
level (Swingle, 1992).

Several studies with college student volunteers indicate a reliable decrease in heart rate and ratings of subjective arousal with brief presentations of beta frequency subliminal sounds. Presentations of frequencies of 10 Hz and below, on the other hand, give rise to increased heart rate and higher ratings of subjective arousal (Swingle, August 1992).
The heart rate changes associated with the subliminal presentations range from -3.5% to +4.0% over baseline measurements. The average heart rate change associated with tones in the beta range of 15 Hz and 25 Hz were between -1.2% and -3.5%. Heart rate changes for subliminal tones in the slower frequencies of 2 Hz, 5 Hz, 8 Hz, and 10 Hz were between +2.2% and +4.0%.
The beta range tapes have been used clinically with patients who complain of anxiety or stress exacerbated conditions. The effect of the subliminal on the patients' heart rate is determined during standard psychophysiological assessment procedures. Patients who demonstrate heart rate reductions associated with a one-minute presentation of the subliminal are given a treatment cassette for home use. The average heart rate change observed during the one-minute presentation is about a 3% reduction.

A treatment choice protocol (Swingle, 1992) in which patients are required to select the active subliminal from a control sound track, white noise without embedded subliminal tones, revealed that of 7 patients with diverse complaints, all selected the active subliminal as most beneficial.
As Benson (1983) has stated, relaxation is associated with decreased heart rate and increases in cortical Alpha. The 25 Hz subliminal (SUB/B) and the 10 Hz subliminal (SUB/A) tones were presented alternately, every 5 minutes, during 35-minute Alpha brainwave biofeedback sessions for two patients. The mean time over threshold (10 lv) for the 7.5 Hz- 13 Hz Alpha range was 32.2% higher and 51.8% higher when SUB/B was presented as compared to when SUB/A was presented (difference in time over threshold between SUB/A and SUB/B presentations divided by time during SUB/A presentation). The above two averages were for an alcoholic and a manic depressive patient, respectively.

Since subliminal tones can affect arousal and the brainwave spectrum it seemed plausible that a self-administered treatment could be developed for some people with attention deficits. It also seemed quite plausible that the treatment could be administered in-situ with headphones and a portable cassette player. Further, if presented at a low volume, treatment could be self-administered as needed without interference with ongoing activities. In short, patients could self-administer the treatment at work or school and still read, listen to lectures, carry on conversations, and the like.

The most critical feature of the treatment, however, is that the patients must be able to recognize when they are experiencing attention problems. In practice, I have found that many patients are quite capable of making this judgement. As discussed above, some patients report mental "fuzziness" or visual blurring, whereas others simply feel (or are told) that they are not adequately attending to task.

A unifying factor in all of the brainwave treatment protocols for attention deficit is the inhibition of amplitude in the 4-8 Hz band. Hence, the inhibitory effects of the subliminal 10 Hz tone (SUB/A) on 4-8 Hz brainwave frequency band was determined for 11 patients as part of the psychophysiological assessment completed on intake. The testing procedure involved presenting SUB/A or a contrast condition alternately every 2 minutes. The various electrode placements used (all references and ground placements on earlobes) included unipolar Cz; midway between Fz - Cz and Cz - Pz, bipolar; bipolar F3 and F4; and unipolar Fz. Contrast conditions included no sound, white noise only (i.e., no subliminal), a subliminal 25 Hz tone (SUB/B), and a subliminal 300 Hz tone also embedded in white noise.

With each patient, SUB/A was presented, alternately with a contrast condition, for a minimum of six 2-minute presentations. The first effect observable in every case was that white noise, with or without an embedded subliminal signal, reduced Theta. This would appear to be consistent with the treatment procedure developed by Mangina and Beuzeron-Mangina (1992), in which tones were used to enhance arousal during cognitive training. Also in every case, however, SUB/A reduced 4-8 Hz amplitude more than any other condition. The range of Theta amplitude reductions with SUB/A as compared with other noise contrast conditions was from 3.9% to 37.2% In every case, the Theta amplitude reduction was statistically significant. The Theta amplitude reduction during SUB/A presentation relative to no sound contrast conditions ranged from 10.9% to 37.5%

At the time of this writing, 22 patients referred with the diagnosis of Attention Deficit Disorder (ADD), are being treated with SUB/A. The ages of the patients range from 6 to 50. In each case, SUB/A was found to suppress Theta amplitude relative to a no-sound contrast by at least 20%. Patients were instructed to wear headsets with a portable cassette player and listen to the tape as needed. If involved in a task such as school work or other work related activity, the patients were instructed to listen to the sound until they felt that they could continue the activity with adequate attentional focus. They were instructed to keep the volume low enough to be able to converse with others if necessary. They were also told to vary the volume until they found the most effective range. In most cases, the patient could readily determine when attention deficits or feelings of cognitive fuzziness were being experienced. Younger patients were often told by parents or teachers that they were being inattentive or disruptive and instructed to self-administer SUB/A. A note from the therapist explaining the treatment procedure was provided for teachers or employers.

It is important to note that most of the patients were involved in some other treatment in addition to the self-administration of SUB/A. These other treatments included various biofeedback modalities including EEG, EEG disentrainment, psychotherapy and hypnosis. Further these other treatments were, in some cases, administered by therapists other than the present author. Patients for whom SUB/A was prescribed were those found to show Theta inhibition with SUB/A testing during intake. The results reported on the effects of as needed application of SUB/A are based on the reports from patients, teachers and parents.

Unsolicited school progress reports of children using SUB/A indicate increased attentional focus in a variety of areas including reading, listening, independent study, science, interdisciplinary studies, social-emotional development and sports.

Initial concern that children may become dependent on SUB/A, much like dependence on stimulants such as Ritalin, arouse from teachers' statements such as "...needs (SUB/A) to stay focused.", "...good work if using (SUB/A).", "...interacts well (with other children) when using (SUB/A).", "...remarkable improvement (in sport activity) with (SUB/A)."
However decreased use of SUB/A has been reported by patients as they progress in other neuronal regulation treatments.
Two patients requested SUB/A treatment alone, although they had been referred for EEG training. Both of these patients, high school students, reported improved attention and reduced time required to complete homework. Neither has requested further treatment and both have discontinued use of SUB/A in school but both continue to use SUB/A, at times, while doing homework.
Every patient has noted improvement in attentional focus and each reported that attentional deficiency improved immediately upon self-administration of SUB/A. One patient had lost several jobs and she was, at the time of referral, at risk of losing her present job. Her employer reported marked improvement as does the patient. Parental report of one of the patients indicates reduced self-stimulating behavior and improved "attitude" toward school work. Finally, a patient who experienced visual blurring and "spaciness" reported that the tape markedly improves her condition to the point where she can engage in activities previously precluded by her symptoms.
A major feature of SUB/A is that it provides immediate benefit resulting from EEG Theta suppression without any observed negative side effects. SUB/A is also very inexpensive, particularly relative to CNS stimulants such as Ritalin, and appears to facilitate neurofeedback treatment for ADD. Further, since patients experience immediate relief with the use of SUB/A, neurofeedback sessions can be less frequent and therefore affordable to more families.
Given the in-situ self-administered protocol of SUB/A treatment, it may well be found to potentiate EEG biofeedback training for the remediation of ADD/ADHD. Ideally, for that group of clients who can discriminate states of adequate attention from states of inadequate attention, the self-application of SUB/A may facilitate learned control of Theta inhibition in a manner similar to that accomplished with EEG Theta inhibition training. Such learning does appear to occur with some clients exposed to EEG biofeedback training for ADD. Steve Stockdale (personal communication) described cases in which after biofeedback training, children learn to switch on adequate focus. In one case that he described, the child would say to himself "get BIG", an acronym for Brain-in-Gear to help access focused attention.
The effects of SUB/A and SUB/B on autonomic arousal and the EEG spectrum appear robust but are nonetheless counterintuitive. Supraliminal tones, and flashing lights, in the Alpha and Beta ranges have been found to produce autonomic and EEG changes opposite to those found with subliminal auditory presentations. If the compound sinusoids of SUB/A (i.e., blended 300Hz and 310Hz tones) are presented above auditory threshold one hears the 10Hz beat frequency. However, the blended sinusoids have other acoustic properties that are not readily available to phenomenal representation. Even at a supraliminal level, there are harmonics of the blended sinusoids that may be subliminal. When the carrier and the beat frequencies are subliminal there are presumably many other acoustic properties of the sounds that are also subliminal. Hence, it is possible that the properties of the subliminal signal that are influencing the autonomic and central nervous systems are those other than the fundamental beat frequencies.
The clinical application of SUB/A is data driven in the sense that EEG Theta suppression was observed. Fundamental work on the effects of manipulating the many acoustic properties of subliminal signals will facilitate an understanding of the apparent clinical effects of subliminal auditory stimulation. Finally given that SUB/A suppresses EEG Theta, it is possible that SUB/A presentations during neurofeedback may facilitate treatment efficacy. This possibility is presently being investigated.

Barlow, J. S. (1960). Rhythmic activity induced by photic stimulation in relation to intrinsic Alpha activity in the brain of man. Electroencephalography and Clinical Neurophysiology, 12, 317-326.
Benson, H. (1983). The relaxation response: Its subjective and objective historical precedents and physiology. Trends in Neurosciences, 6, 281-284.
Braverman, E. R., Blum, K., & Smayda, R. J. (1990). A commentary on brain mapping in 60 substance abusers: Can the potential for drug abuse be predicted and prevented by treatment? Current Therapy Research, 48, 569.
Carter, J. L., & Russell, H. L. (1981). Changes in verbal performance IQ discrepancy scores after left hemisphere EEG frequency control training: A pilot report. American Journal of Clinical Biofeedback, 4, 66-67.
Cox, A., & Heath, R. (1975). Neurotone therapy: Preliminary report on electrical activity. Diseases of the Nervous System, 36, 245-247.
Delmonte, M. M. (1984). Electrocortical activity and related phenomena associated with meditation practice: A literature review. International Journal of Neuroscience, 24, 217-231.
Fried, R. (1993, March). Normalizing the spectral composition of the EEG in seizure sufferers with breath control and PCO2 biofeedback. Paper presented at the meeting of the Association of Psychophysiology and Biofeedback, Los Angeles, CA.
Green, D. M. (1976). An introduction to hearing. Hillsdale, NJ: Erlbaum.

Hardt, J. V. (1993, March). Alpha EEG feedback: Closer parallel with Zen than Yoga. Paper presented at the meeting of the Association of Applied Psychophysiology and Biofeedback, Los Angeles, CA.
Hardt, J. V., & Gale, R. (1993). Creativity increases in scientists through Alpha EEG feedback training. Paper presented at the meeting of the Association for Applied Psychophysiology and Biofeedback, Los Angeles, CA.
Jarzembski, N. W. (1985). Electrical stimulation and substance abuse treatment. Neurobehavioral Toxicology and Teratology, 1, 119-123.
John, E. R., Prichep, L. S., Friedman, J., & Easton, P. (1989). Neurometric topographic mapping of EEG and evoked potential features: Application to clinical diagnosis and cognitive function. In B. Maurer (Ed.), Topographic brain mapping of EEG and evoked potential. Heidelberg: Springer-Verlag.
Lukas, S. E. (1991). Brain electrical activity as a tool for studying drugs of abuse. Advances in Substance Abuse, 4, 1-88.
Mangina, C. A., & Beuzeron-Mangina, J. H. (1992). Psychophysiological treatment for learning disabilities: Controlled research and evidence. International Journal of Psychophysiology, 12, 243-250.
Meissner, J., & Pirot, M. (1983). Unbiasing the brain: The effects of meditation upon the cerebral hemispheres. Social Behaviour and Personality, 11, 65-76.
Melzack, R., & Perry, C. (1975). Self-regulation of pain: The use of alpha-feedback and hypnotic training for the control of chronic pain. Experimental Neurology, 46, 452- 469.
Ohatrian, G. E., Peterson, M. C., & Lazarte, J. A. (1960). Responses to clicks from the human brain: Some depth electrographic observations. Electroencephalography and Clinical Neurophysiology, 12, 479-489.
Plomp, R. (1976). Aspects of tone sensation: A psychophysical study. New York: Academic Press.
Post, R. M., Trimble, M. R., & Pippenger, C. E. (1989). Clinical use of anticonvulsants in psychiatric disorders. New York: Demos Publications.
Potashkin, B. D., & Beckles, N. (1990). Relative efficacy of ritalin and biofeedback treatments in the management of hyperactivity. Biofeedback and Self-Regulation, 15, 305-315.
Schatzberg, A., Elliot, G., Lerbinser, J., Marcel, B., & Duffy, F. (1986). Topographic mapping in depressed patients. In F. Duffy (Ed.), Topographic mapping of brain electrical activity. Boston, MA. Butterworth.
Schwartz, J., Crawford, D. G., & Schwartz, G. E. (1993, March). Hypnosis induction and EEG brainmapping. Paper presented at the meeting of the Association of Applied Psychophysiology and Biofeedback, Los Angeles, CA.
Swingle, P. G. (1992). Subliminal treatment procedures: A clinician's guide. Sarasota, FL: Professional Resource Press.
Swingle, P. G. (1992, August). Subliminal treatment procedures. In symposium entitled "Subliminal influences: For better or for naught?". American Psychological Association meeting, Washington, DC.

Swingle, P. G. (1993, March). Treatment of attention deficiency with a Theta suppressing subliminal auditory signal. Case study reported at the meeting of the Society for the Study of Neuronal Regulation, Avalon, CA.
Zenhausern, R., Ciaiola, M., & Pompo, C. (1973). Subliminal and supraliminal accessory stimulation and two trapezoid illusions. Perceptual and Motor Skills, 37, 251-256.
Zenhausern, R., & Hansen, K. (1974). Differential effects of subliminal and supraliminal accessory stimulation on task components in problem-solving. Perceptual and Motor Skills, 38, 375-378.