The protocol was approved by the Massachusetts General Hospital (MGH) Institutional Review Board and was conducted in accord with the principles of the Helsinki Declaration. We studied 10 right-handed patients, described in Tables 1, 2, and 3, who were recruited through advertisements posted on the internet and at a substance abuse clinic. Among those with a history of substance abuse, 6 had a past history of opiate dependence, and 1 had a past history of alcohol dependence. Enrollment was made without regard to gender or ethnicity. Inclusion criterion allowed for patients receiving mental health care if they had not altered their treatment during the month preceding the study. At enrollment we asked that they try, but not be required, to maintain their usual treatment until the study's conclusion. All patients complied with this request, and no patient altered their usual treatment for the duration of the 4-week study. We excluded patients who were not right handed, not between the ages of 18 and 60 or failed to meet the structured clinical interview for DSM-IV (SCID) criteria for MDD. We also excluded any person with a past history of a psychotic disorder, a substance abuse disorder that had been active within the 6 months prior to the study, a history of violent behavior, a history of a past suicide gesture or attempt, a history of current suicidal ideation, a history of a neurological condition (e.g. epilepsy, traumatic brain injury, stroke), pregnancy, or a current acute or chronic medical condition. We would have excluded any person whom we judged to have an impaired decision-making capacity. Prior to enrollment, we obtained informed consent according to the existing policies at MGH. No patient, once enrolled, failed to complete the study.

Table 1 Demographics: Age, gender, and SCID diagnosis Full size table

Table 2 Baseline measurements of outcome measures and hemispheric emotional valence. Full size table

Table 3 Unaltered treatments before and during the study period. Full size table

Instruments

Photobiomodulation with near infrared light

The treatment consisted of applying PBM in the form of a light emitting diode (LED) array (Marubeni America Corp, Santa Clara, CA) with a peak wavelength of 810 nm with a full width half maximum of 40 nm, delivering an irradiance of 250 mW/cm2 when applied at 4 mm from the skin. The treatment consisted of exposure to the light for 4 minutes (total delivered fluence per site of 60 J/cm2) at each of 2 sites on the forehead that correspond to the 10-20 EEG sites, F3, and F4. Based on a penetration of 3.7% of the light to the dura, we calculated that 2.1 J/cm2 was delivered to each of the treated areas of the brain. The level of light exposure at the skin was well below the irradiance allowed by the ANSI standard of 320 mW/cm2. Based on that standard, we conclude that the level of light exposure either to the skin (power density of 250 mW/cm2 and total fluence of 60 J/cm2) and to the surface of the brain (power density of 9.5 mW/cm2 and total fluence of 2.1 J/cm2) to each of the 2 treated areas of the forehead poses no significant risk as discussed above. Subjects wore protective eyewear even though the physician administering the PBM was careful to not shine the light in or near the eyes. The output of our device is at least 5 times less than the PhotoThera laser device (personal communication, Luis DeTaboada, PhotoThera Inc, Carlsbad, CA) that was used without observed side-effects in stroke patients [17], and was found in a study of the rat brains exposed to light to cause no observable behavioral or cellular alterations [31]. In the human stroke study, the patients' heads were shaved and they were treated at 20 sites around the entire head. Subjects were not shaved in the present study as light was applied only to the forehead.

The rationale for the optical parameters were as follows: The wavelength of 810-nm is optimum for light penetration of living tissue due to minimization of absorption by all three major tissue chromophores, hemoglobin, melanin and water. Moreover this wavelength has been shown to be effectively absorbed by mitochondria that are believed to be responsible for the biological effects of photobiomodulation. The energy density (60 J/cm2) was chosen with reference to other published studies reporting transcranial laser for stroke in humans and knowledge about the optical properties of human tissue as discussed in the text. The power density (250 mW/cm2) was chosen to be safe and avoid heating of the skin.

Near Infra-red spectroscopy (NIRS) for the measurement of total oxy and deoxy-hemoglobin (cHb) in the left and right frontal poles

We measured cHb in left and right frontal poles by NIRS, using an INVOS system (Somanetics, Troy, MI) http://www.somanetics.com/invos-system, modified by Somanetics to provide cHb, which we believe to be our best reflection of rCBF, in addition to the device's usual oxygen saturation output. The Somanetics device is FDA approved, is commercially available, and is used throughout the world in hospital settings to monitor cerebral perfusion. It poses no harm or discomfort to subjects, yet is convenient, and allows the subject to have relatively free movement. This device can be used to monitor cHb in the left and right frontal poles during PBM. Since our PBM uses continuous wave emission, its light is not detected by this NIRS device because it has a proprietary mechanism for excluding continuous light so that ambient light does not contaminate the device's pulsed photon emitter/detector.

Affect measures

We evaluated the psychological state of patients with the following instruments: Standard Clinical Diagnostic Interview (SCID) [32], a Hamilton Depression Rating Scale 21-item (HAM-D) [33], a Hamilton Anxiety Rating Scale (HAM-A) [34], and a Positive and Negative Affect Scale (PANAS) [35]. We searched for side effects of the treatment with a form we constructed with both open-ended questions and a physical and psychological symptom check list.

Determination of hemispheric emotional valence

Lateral visual field stimulation (LVFS), a simple test, consisting of blocking one visual field so that the patient is looking exclusively out of the left or right lateral visual field, at a photograph of a man or woman with an ambiguous emotional expression. The subject does so for one minute then his affects are rated on an abbreviated PANAS scale [36–39]. LVFS has been shown to alter hemispheric activation by BOLD fMRI [40]. HEV determined by LVFS has predicted in two independent studies the outcomes to a 2-week course of left-sided rTMS for depression [41, 42]. We have suggested that HEV might be used to guide the application lateral treatments to the brain and aid in the evaluation of experimental data [39].

From the subjects' left and right-sided LVFS scores on the PANAS we derived the patients' hemispheric emotional valence (HEV). To determine the patient's baseline HEV, before any treatments, we used the PANAS scores recorded one minute after his or her looking out the right visual field (RVF) and that recorded one minute after his looking out the left visual field (LVF). We use this order of testing for all patients. We used the difference between the PANAS scores during the LVF - the RVF to determine the HEV, for which we obtained an individual score. Since the LVF is thought to relate to the right hemisphere, when LVF-RVF was positive (right hemisphere had more positive affect), we assigned a left negative HEV, and when it was negative, a right negative HEV.

Study design and procedures

Each of the 10 patients who met our criteria by a phone interview came to our laboratory and gave written informed consent according to approved protocol. Then each was given a SCID, followed by a baseline HAM-D, HAM-A, PANAS, and LVFS. Subjects were not drug tested, but patients with a history of opiate abuse had been in a stable treatment program for at lease 6-months, and each was believed by their prescribing psychiatrist to have abstained from illicit drugs for this period. Each subject was then connected to our NIRS device by having 5 by 2.5 cm2 adhesive pads containing a photon emitter and detectors attached to each side of the forehead immediately over the eyebrows, as shown in Figure 1.

Figure 1 Near infrared treatment. The NIR LED array is a few millimeters from the skin beneath a heat sink and cooling fan at F3. Somanetics "SomaSensors" with NIR photon emitters and detectors are applied just above each eyebrow to measure left- and right-sided total hemoglobin. Full size image

The NIRS device collected data continuously at 1-second intervals throughout the study. Event marks indicated the beginning and end of each baseline or treatment period. Data were zeroed at the beginning of each period. A researcher, blind to the treatments, administered the PANAS scales at baseline and immediately after each of the 4 treatments. A different researcher administered the HAM-D and the HAM-A at baseline and at 2- and 4-weeks post treatment. He was not blinded because all patients received active treatments during the treatment day. In random order, the patient was given the first of 4 interventions, consisting of: A). NIR "on" for 4 minutes at F3 (of the 10-20 EEG system), left forehead approximately over the left dorsolateral prefrontal cortex, B). NIR "on" for 4 minutes at F4, C). NIR "off" with the NIR device held at F3 for 4 minutes, as shown in Figure 1). The same as intervention #C but at F4. Thus, we had two active treatments and two placebo treatments. A cooling fan and heat sink on the LED prevented detectable heat from reaching the skin of the patient. Patients were asked if they could tell if they had just received a treatment with the light on or off, and all reported that they could not detect any differences between the treatments. After all 4 interventions were completed, the patients were asked about adverse physical or psychological symptoms.

Two weeks and 4-weeks after the treatment day, each patient was given a follow-up HAM-D, HAM-A, PANAS, and the side-effects questionnaire.

Statistics

Our primary outcomes were changes from baseline in HAM-D and HAM-A scores at 2 weeks and 4 weeks post-treatment, and our secondary outcomes were change in PANAS score at 2 weeks and 4 weeks post-treatment, difference in immediate after treatment in PANAS score between NIR on and NIR off, and difference in rCBF between NIR on and NIR off. We also tested for associations between treatment side that was matched or unmatched with the hemisphere with a positive HEV and PANAS changes immediately following treatment. Here we used hierarchical linear models with treatment (NIR on or off), side of treatment (F3 or F4), and their interaction as predictors. HEV and its interaction with the side of treatment were added to these models to test for an association between these factors and immediate treatment benefit.

To test for changes in symptom ratings at 2 weeks and 4 weeks post-treatment, we used repeated measures linear regression models with measurement time as a categorical predictor and unstructured covariance between repeated measurements. Paired comparisons between measurement times were conducted in the presence of an overall difference among mean symptom ratings at baseline, 2 weeks, and 4 weeks significant at the alpha = 0.05 level. To facilitate the clinical interpretation of our findings and comparison with other studies, we also report the number of participants who were "improvers" (20% or more decrease from baseline) and "responders" (50% or more decrease from baseline) based on HAM-D and HAM-A scores, the number who achieved "remission" (a score less than 8 or 11) based on HAM-D and HAM-A scores, and the mean ± standard deviation percentage change at 2 weeks and 4 weeks for the three clinical measures.

To test for associations between treatment and rCBF, paired t-tests compared average rCBF across the left and right hemispheres and rCBF within each hemisphere between NIR on and NIR off. To test whether any treatment effect differed between the left and right hemispheres, an additional paired t-test compared mean differences in rCBF between NIR on and NIR off between the left and right hemispheres. We also considered hierarchical models for the association between treatment and rCBF, but the data did not support their complexity.

Post-hoc tests for associations among hemispheric valence, differences in rCBF, and two-week changes in HAM-A and HAM-D scores were conducted using linear regression with random intercepts for subjects when appropriate. Both point changes and percentage changes in HAM-D and HAM-A were considered as outcomes. These models treated hemispheric valence as a quantitative variable.

Statistical significance required two-tailed p-values less than 0.05. A Bonferroni correction was applied to results from the models for changes in HAM-D and HAM-A scores with treatment to account for our choice of two primary clinical outcomes. Other results were not adjusted for multiple comparisons. Statistical analyses were conducted using R statistical software (version 2.9.2) and the PROC MIXED routine for SAS statistical software (version 9.1.3, Cary, NC).