Most of our measurements on the 10 subjects allowed us to record a reliable optical signal in response to brain activation. One exception was Participant 8, in whom the signal-to-noise ratio was low and we were not able to record any reliable hemoglobin response to brain activation in any of the three protocols. We attribute the low signal-to-noise ratio for Participant 8 to thick hair and bad optical coupling. For Participant 4, we lost the data relative to the right hand finger opposition task due to technical reasons. Data from the right and left electrical stimulations on Subject 6 were discarded because of motion artifacts. After discarding the above cases that were affected by technical problems, we were able to analyze the block-averaged oxy-hemoglobin and deoxy-hemoglobin concentration changes induced by the right and left hand stimulation protocols at the contralateral and ipsilateral cortical side (a total of 252 hemoglobin changes for the 9 participants analyzed). We consider a concurrent increase in HbO and a decrease in Hb (equivalent to a positive BOLD signal in fMRI) to be indicative of a hemoglobin response to brain activation. We observed a significant (i.e., above threshold) hemoglobin response to brain activation in 57 out of 63 (90%) measurements. Specifically, there was contralateral hemoglobin response in 10/10 (100%) cases during the right hand finger opposition task, 11/11 (100%) during the left hand finger opposition task, 10/11 (91%) during the right and left hand tactile, 8/9 (89%) during the right and left wrist electrical stimulation. We identified a bilateral hemoglobin response to SMI activation in 9/11 (82%) left hand and 8/10 (80%) right hand finger opposition task measurements. During tactile stimulation, we observed ipsilateral activation above threshold (p< .05) in 3/11 (27%) right hand and 7/11 (64%) left hand tactile measurements. In 8/18 (40%) measurements during median nerve stimulation, we observed a concurrent decrease of HbO and increase of Hb at the ipsilateral SMI, which is indicative of brain deactivation, and in only 1/18 cases (6%) we observed an increase of HbO and decrease in Hb at the ipsilateral SMI. summarizes all the results for the contralateral and ipsilateral oxy-hemoglobin and deoxy-hemoglobin changes induced by the three protocols for the right and left side stimulations.
Summary of All the Results for the Contralateral and Ipsilateral Oxy-Hemoglobin and Deoxy-Hemoglobin Changes Induced by the Three Protocols for the Right and Left Side Stimulations
The spatial maps of the block-averaged oxy-hemoglobin and deoxy-hemoglobin-evoked responses measured on Subject 2,10 s after the onset of stimulation, are reported in . refers to right hand tasks, to left hand tasks. The left images are the block-average images measured during finger opposition, the middle images during tactile stimulation, and the right images during median nerve stimulation. report the oxy-hemoglobin maps, whereas report the deoxy-hemoglobin maps. In the color scales, green indicates no change from baseline, red represents a concentration increase, blue a decrease. The activated area is generally red (increase) for HbO, and blue (decrease) for Hb for the cortex side contralateral to the hand performing the task. During electrical stimulation there is a small decrease of HbO and increase of Hb in the ipsilateral cortex side. This ipsilateral deactivation is more evident in .
Figure 2 Block-averaged hemoglobin maps on Subject 2, 10s after the onset of the stimulation during (a) and (b) right and (c) and (d) left hand stimulation. Top panels (a) and (b): oxy hemoglobin changes. Bottom panels (c) and (d): deoxy-hemoglobin changes. Left (more ...)
Block-averaged deoxy-hemoglobin maps on Subject 10, 10s after the onset of the stimulation during right and left wrist electrical stimulation.
shows the spatial maps of the block averaged deoxy-hemoglobin evoked responses for Subject 10, measured 10 s after the onset of the electrical stimulation.
shows the temporal traces of the block-averaged oxy-hemoglobin and deoxy-hemoglobin evoked responses for Subject 2 at the pixel of maximal response during electrical stimulation for both contralateral and ipsilateral SMI. The top graphs (a) refer to the right hand stimulation, and the bottom graphs (b) to left hand stimulation. The left graphs represent the block averages during finger opposition, the middle graphs during tactile stimulation, and the right graphs during median nerve stimulation. A green horizontal bar indicates the duration of each stimulus.
Figure 4 Temporal traces of the block average hemoglobin responses on the contralateral (red and blue) and ipsilateral (pink and light blue) SMI of Subject 2. Red and pink: oxy-hemoglobin changes; blue and light blue: deoxy-hemoglobin changes. The error bars are (more ...)
In all the stimuli, the typical hemodynamic response pattern in the contralateral SMI is evident, with a task-related increase in HbO concentration and a decrease in the relative concentration of Hb. Again, a smaller change of hemoglobin is visible on the ipsilateral SMI with an opposite behavior for the electrical stimulation (increase of Hb and decrease of HbO).
The grand average across subjects of the temporal traces of the evoked hemoglobin responses on the contralateral and ipsilateral SMI are reported in . reports the results for right hand stimulation; reports the results for the left hand stimulation. The error bars represent the standard errors. For each subject we considered the pixel of maximal activation during electrical stimulation. The same hemodynamic response pattern found in a single subject () is also found in the grand average of all the subjects.
Figure 5 Grand average of the temporal traces of the evoked hemoglobin responses on the contralateral (red and blue) and ipsilateral (pink and light blue) SMI. Red and pink: oxy-hemoglobin changes; blue and light blue: deoxy-hemoglobin changes. Notice different (more ...)
In we expand the display of the first 10 s of the hemodynamic response reported in . In these graphs we also add the contralateral and ipsilateral total hemoglobin concentration (THC) traces. The evoked change in HbO and THC starts 1.0 ± 0.2 s after the onset of stimulation for finger opposition and finger tactile stimulation. The onset of the rise in HbO and THC precedes that of the decrease in Hb by 1.0 ± 0.3 s. THC peaks at 5.0 ± 0.6 s, HbO peaks at 6.0 ± 0.5 s, and Hb peaks at 7.0 ± 0.4 s. Contralateral and ipsilateral cortices behave similarly. To estimate the significance of the rise time and peak time differences observed for HbO, THC, and Hb, we measured the times at 10% and 90% of the maximum for each subject and for each stimulation condition. These times are representative of the time of response and the time to reach maximum, respectively. The time values reported above are the average and the standard error measured for all subjects during the finger opposition and tactile stimulation. For the median nerve stimulation, the onset of HbO and THC change occurs 2.1 ± 0.4 s after the onset of stimulation, whereas the onset of the Hb change occurs 2.7 ± 0.5 s after the onset of stimulation. The times to reach peak values are the same as for the previous two stimuli to within ± 0.8 s.
Figure 6 Initial contralateral and ipsilateral hemodynamic response to the three tasks for right hand (a) and left hand (b) stimulation. Red and pink: oxy-hemoglobin changes; blue and light blue: deoxy-hemoglobin changes; dark and light green: total hemoglobin (more ...)
shows the bar graphs of the average across subjects of [HbO] and [Hb] changes for the three stimuli performed with left and right hands. For each subject, the hemoglobin changes are calculated on the position of maximum contralateral activation during median nerve stimulation (a), and on the same pixels during the ipsilateral tasks (b). The HbO and Hb amplitudes correspond to the difference between the baseline reading and the average reading 5 to 10 s after the onset of the stimulus. The error bars represent the average value of the standard deviations of the data over the 5–10 s period for all subjects. For the ipsilateral SMI, hemoglobin changes under threshold (p > 0.05) are included in the calculation of the average.
Figure 7 Changes in oxy-hemoglobin (left) and deoxy-hemoglobin (right) at the pixel of maximal contralateral response to stimulation averaged across subjects. The filled bars correspond to right hand stimulations (left SMI); the dotted bars correspond to left (more ...)
To estimate systemic contributions to the oxy-hemoglobin and deoxy-hemoglobin concentration changes, we considered cortical areas that are far from the pixel of maximal hemoglobin response to activation, as described in the data analysis section. shows bar graphs of the grand average across subjects of HbO and Hb changes on a pixel outside the SMI obtained in the same way as in . For each subject, we considered the pixel more posterior and further from the sagittal sinus either on the right or left hemisphere. We discarded the data from 2 subjects (Subjects 2b and 7) because the positioning of the probe on the head for these subjects was more frontal and the maximal activation was too close to the posterior optodes. Left and right hand stimuli were averaged together. In we also show the bar graph of the average heart rate change across subjects (graph on the right). The heart rate change was calculated in the same way as were HbO and Hb, from the block average of the heart rate in each subject.
Figure 8 Oxy-hemoglobin (left), deoxy-hemoglobin (middle), and heart rate (right) changes during stimulation averaged across subjects. Oxy- and deoxy-hemoglobin are measured in the contralateral brain hemisphere outside the SMI cortex at a posterior-medial image (more ...)
We attribute the increase of HbO and decrease of Hb in an area outside the SMI to the heart rate increase during stimulation. We assumed this systemic hemoglobin change to be the same in the whole imaged area, and we subtracted it from the data at each pixel. shows bar graphs of the average across subjects of [HbO] and [Hb] changes at the (a) contralateral and (b) ipsilateral SMI region before and after subtracting the contribution to the signal due to systemic hemoglobin changes. Left and right hand stimuli were averaged together. The bar graph to the right reports the ratio between the average oxy-hemoglobin and deoxy-hemoglobin concentrations with and without subtraction of systemic contributions.
Figure 9 Oxy-hemoglobin (left), deoxy-hemoglobin (middle), and ratio oxy- and deoxy-hemoglobin (right) changes during stimulation. The filled bars correspond to no subtraction of systemic contribution; the dotted bars correspond to the changes after correction (more ...)
We also subtracted the systemic contribution in the images, and compared the size (full width half maximum) of the activated area for HbO, Hb, and THC with and without subtraction (see ).
Figure 10 Oxy-hemoglobin (left), deoxy-hemoglobin (middle), and total hemoglobin (right) activated area during stimulation. The filled bars correspond to no subtraction of systemic hemoglobin contributions; the dotted bars correspond to the activated area after (more ...)