A group of 36 healthy human participants (24 female, 12 male, mean age 24 years) were recruited for this study (Table 2) and were compensated for their participation. All participants were pre-screened prior to admission into the study against a preclusionary criteria of personal or family history of epileptiform disorders, metallic implants, and neuroleptic medications85. Informed consent was obtained from all participants. Consenting participants reported no history of neurological or psychiatric disorders and had normal or corrected-to-normal vision. The research protocol implemented in the present study was approved by the Boston University School of Medicine Institutional Review Board and was carried out in accordance with the Code of Ethics of the World Medical Association.

Table 2 Demographic information of the experimental groups (9 participants per group) that took part in the current study. Independent groups of participants completed visual performance experiments involving the stimulation of the right PPC or the scalp vertex. Visual detection tasks were carried out in separate experiments with low (3 cpd) or high (12 cpd) spatial frequency Gabors. Full size table

Visual paradigm

Briefly, contrast sensitivity was measured with a standard two alternative forced choice paradigm (2AFC) task using single vertically-oriented circular Gabor stimuli subtending 3° of visual angle40,48. programmed in-house with C# using Microsoft’s Visual Studio 2010 (Microsoft, Redmond, WA, USA). Participants observed stimuli with their eyes’ canthi positioned 57 cm from the computer monitor so that 1° of visual angle measured 1 cm along the horizontal axis of the computer monitor. Each trial consisted of two sequentially presented visual stimuli, presented for 100 ms and separated by 400 ms. Stimuli consisted of a centrally located black border (subtending 13° of visual angle (400 × 400 pixel grid, border width = 1 pixel). Gabor stimuli were flashed for 100 ms with equal likelihood during either the first (T1) or the second (T2) time interval. Participants were instructed to respond as quickly and accurately as possible signaling whether the Gabor appeared in either the first (T1) or the second (T2) interval using the keys “1” for T1 and “2” for T2 on a numeric keypad with the index and middle fingers of the right hand. RT was calculated as the duration between the end of the second interval (T2) and the time at which a finger response was executed (Fig. 1A). When participants were unsure which interval contained the stimuli, they were encouraged to make their best guess. Participants were given 1500 ms to provide an answer before the next trial was automatically started. In the case of an absent response an audible warning indicated an error and the trial was repeated using the same contrast value.

Contrast sensitivity measures were obtained using an adaptive staircase procedure). Gabor stimuli were initially presented at 5.0% Michelson contrast: C = (L max − L min )/(L max + L min ). Stimulus contrast was subsequently increased or decreased according to task performance. The first two reversals used a 1-down/1-up rule, i.e., Gabor contrast was either increased or decreased a step value of 0.6% Michelson contrast in response to an incorrect or a correct response, respectively. At the initiation of the third reversal a 3-down/1-up rule was implemented with a step value of 0.2%; i.e., following 3 correct responses, contrast value was decreased by 0.2% and following each incorrect response it was incremented by 0.2%. Trials were continually presented until twenty reversals were completed following the 3-down/1-up rule with a step value of 0.2% contrast40. A reversal was counted when there was a shift of sign between contrast values for the current and the just previously presented trial. Individual contrast threshold values were calculated from the final twenty reversal values from each evaluation time point. Contrast sensitivity was then obtained by calculating the inverse of the contrast threshold; CS = 1/C (reported in log units). Reaction time values for all correct responses completed during the 3-down/1-up rule were included in our analyses. Reaction time values included in the analysis were restricted to responses between 50 ms and 1000 ms86.

Transcranial magnetic stimulation

Repetitive TMS was performed using an air-cooled 70 mm figure-eight coil attached to TMS machine (Superapid2, Magstim, Withland, UK) and fixed in position with the aid of a ‘magic-arm’ (Manfrotto Bassano del Grappa, Italy). Patterns of 1 Hz rTMS were delivered to the middle portion of the right PPC, as well as to an additional control location (scalp vertex, Cz) for a total of 15 minutes (900 pulses total). Low-frequency rTMS was chosen due to its minimal seizure risk and because similar rTMS interventions to parietal and occipital locations have been used to investigate visual perception27,28,69. Stimulation intensity was initially set up at 80% of each participant’s motor threshold and occasionally, adjusted for individual comfort. The final stimulation (± Standard Deviation) intensities applied to participants averaged 62 ± 10% of the maximum rTMS machine output.

As done elsewhere27,28,69, for the right PPC stimulation condition, the TMS coil was placed over a scalp location corresponding to site P4 of the 10–20 EEG system and the middle portion of the IPS overlaying the right Angular Gyrus (AG)87. The site of vertex stimulation was located at the intersection between a line connecting the left and right preauricular points and the nasion-inion line (Cz on the international 10–20 EEG system). The correct locations of our targets for the PPC and vertex sites were verified using individual structural MRI images with vitamin E high contrast markers placed on the targeted regions at the P4 site and Cz sites indicated above. To this end, high-resolution T1-weighted images were acquired on a 3 Tesla Philips MRI scanner using a 3D-turbo field echo (TFE) T1-weighted sequence (equivalent to MP-RAGE). Parameters included a field of view (FOV) 240 mm (RL) × 256 mm (VD) × 192 mm (AP); Fold-over-axis: RL, data matrix: 160 × 160 × 144 zero-filled to 256 in all directions (approx 1 mm iso-voxel native data), TR/TE = 9 ms/6 ms, flip angle = 8°. The pial surface of the brain was visualized using FSL’s88,89 Brain Extraction Tool90 then superimposed on a whole head image. As expected87, in all cases the vitamin E marker was located above the level of the middle portion of the right IPS and 0.5–0.7 cm ventral to the sulcus in the lateral bank corresponding to the angular gyrus. In this location, the coil was oriented in a lateral-to-medial and caudal-to-rostral orientation at approximately 45° from the interhemispheric longitudinal midline.

Detailed inspection also confirmed that vitamin E capsules placed over the vertex location overlaid the longitudinal interhemispheric fissure, at which point the midsagittal and interaural lines intersect. This area closely corresponds to the medial origin of the postcentral gyrus83. The TMS coil was oriented in a lateral-to-medial and caudal-to-rostral orientation.

Experimental sessions

Participants were assigned to two independent groups, each associated with one of two stimulation conditions: An experimental condition receiving rTMS to the right PPC and a control condition receiving rTMS to the scalp vertex. For the right PPC rTMS and vertex rTMS conditions, participants were randomly assigned to one of two groups (n = 9 participants each). One group was tested with high spatial frequency (12 cpd) stimuli while the other was tested using low spatial frequency (3 cpd) achromatic Gabor stimuli. Each experimental session required approximately 120 minutes (including preparation, performance measurements and stimulation time). Participants were first fitted with a tight fitting Lycra swim cap. The location of the targeted region of cortex was determined and the site of stimulation and coil orientation marked. At the start of each experiment, a baseline visual performance measure was obtained (pre-rTMS). Repetitive TMS was then applied for 15 minutes at 1 Hz. Immediately following rTMS, visual performance was re-measured (post-rTMS). After a one hour ‘washout’ period, contrast sensitivity was obtained (1h-post-rTMS) for the last time during the session (Fig. 1B). Prior to the baseline (pre-TMS) time point, participants completed a practice performance task. The purpose of this practice session was to limit immediate learning effects and to ensure that participants fully understood the task. This practice condition was not included in the analyses.

Statistical analysis and data presentation

Data were analyzed using JMP 13.0 (SAS, Cary, NC USA). Contrast sensitivity values and reaction times were evaluated using linear mixed models, which accounts for the inter-subject variability in the repeated measure by crossing random effects with independent variables. These models were fit for restricted maximum likelihood. Performance to low and high spatial frequency Gabor stimuli were analyzed separately. For the reaction time analysis, we added an additional factor which was whether the stimulus was flashed in the first (T1) or the second (T2) 100 ms interval and tested the possibility of an order effect interacting with stimulation site (PPC, vertex) or TMS time point (pre-rTMS, post-rTMS, 1h-post-rTMS). Significant main effects were followed up with planned two-tailed Student’s t-tests, and Bonferroni correction was used to modify the p value when multiple comparisons were used. Significance was set at p = 0.05.