Figure illustrates the basic method that we used to examine and compare the absorbance patterns evoked by different amplitudes of flutter stimulation. Panel A shows the cortical field (SI cortex of squirrel monkey) that was imaged. Panels B and C are the light absorbance images evoked in this subject by low-amplitude (50 μm) and high-amplitude (400 μm) flutter stimulation of the same spot on the thenar eminence. The responses to both stimuli occupy the same approximately circular, 2 mm-diameter cortical region. The stimulus-evoked activity in the central 2 × 2 mm sector of this responding region was plotted as a 3-dimensional surface map (Panels D and E) to facilitate the view of the evoked patterns of response. In order to quantify this pattern, the pattern was segmented along the cortical anterior-posterior axis and the medial-lateral axis (segmentation orientations indicated by Panels F and G) and generated spatial histograms (Panels H, I, L and M). These histograms hint at a spatial pattern that could have some underlying spatial frequency. To observe their spatial frequency composition, the power spectra of these spatial histograms are plotted in Panels J, K, N and O. The periodograms in these panels reveal major differences in the spatial organization of the SI responses to the 50 μm and 400 μm stimuli when segmented in the anterior-posterior dimension. That is, the SI response to the higher-amplitude stimulus is greatly dominated by lower spatial frequencies between 1.5–2.5 cycles/mm, whereas in the SI response to the weaker stimulus the relative power in these frequencies is greatly reduced and greater power is present at spatial frequencies between 5.5–9 cycles/mm. In contrast, when segmented in the medial-lateral dimension (Panels N and O), there is no shift in the power spectra between the low and high amplitude responses. The medial-lateral power spectra are clearly dominated by the low spatial frequencies around 1.5–2.5 cycles/mm.
Figure 1 Comparison of SI cortical responses evoked by low- and high-amplitude 25 Hz flutter stimuli. A: View of the somatosensory cortex with the lateral end of the central sulcus (CS) at the top. Stimulus location is shown on the hand figurine. B: Optical response (more ...)
Comparable results were obtained from the 4 other subjects, (3 subjects shown Fig. ). Viewing the 3-D activity maps (Figure , second-left column), it is evident that the low amplitude stimulus evokes a pattern with a much higher spatial frequency than that evoked by the higher amplitude stimulus. The third column also shows that in each subject there was a shift of the most prominent frequency band of the OIS power spectrum from ~7.5 cycles/mm to ~2 cycles/mm as the stimulus amplitude was increased from 50 μm to 400 μm, when the stimulus-evoked activity was sampled in the anterior-posterior dimension. In the medial-lateral dimension (Figure , right column), no such shift in the power spectra was observed. Low frequencies around 2–2.5 cycles/mm dominate all of these spectra.
Figure 2 Comparison of SI cortical responses evoked by low- and high-amplitude 25 Hz flutter stimuli in three additional subjects. Shown for each subject (A-C) are: (1) the stimulus location on the hand, (2) the average light absorbance images of the cortical (more ...)
Figure illustrates results obtained in a more detailed examination of the effect of increased amplitude of flutter stimulation on the spatial organization of the OIS response pattern. Figure shows the SI optical responses, obtained in the same experiment, to five different stimulus amplitudes ranging from 0 (control) to 400 μm. 3-D surface maps of the central 2 × 2 mm activated cortical region are displayed in Figure . The power spectra of the responses sampled in the anterior-posterior dimension (Fig. ) show that in the absence of flutter (0 μm stimulus amplitude) the distribution of power at different spatial frequencies is relatively uniform, with the most dominant frequency at 6.5 cycles/mm. With increments in stimulus amplitude, however, the relative power in the spectral distribution shifts towards lower spatial frequencies. In the case of the medial-lateral dimension (Figure ), the power spectra remain essentially constant, with an exception of the no-stimulus control, and there does not appear to be a shift in the distribution of the power with increasing the amplitude of stimulation.
Figure 3 Comparison of SI optical responses evoked at different stimulus amplitudes (0, 50, 100, 200, and 400 μm) in an exemplary experiment. A: Average light absorbance images for each of the stimulus amplitudes (20 trials, 3 poststimulus images per trial). (more ...)
This effect of stimulus amplitude on the distribution of spatial frequencies in the OIS response along the anterior-posterior dimension is highly reproducible across all subjects. Figure shows the average across-subject (n = 5) power spectra, obtained in response to flutter stimuli delivered at 5 different amplitudes. The 6–9.5 cycles/mm frequency band, which is most prominent in the absence of stimulation (control; 0 μm amplitude), loses relative power as stimulus amplitude is increased (Fig. top). In contrast, relative power at 1.5–3 cycles/mm grows as stimulus amplitude is increased, and at the highest amplitude used (400 μm) it greatly dominates the power spectrum (Fig. bottom). Overall, the OIS power spectrum appears to respond to an increase in stimulus amplitude by a shift of the relative power towards lower frequencies. This tendency is expressed more clearly in Figure , where the highest-power frequency (Fig. ; or, inversely, in Fig. , the highest-power period) is plotted as a function of the stimulus amplitude.
Figure 4 Across-subject reproducibility of the effect of flutter stimulus amplitude on the power spectra of the data sampled in the A-P dimension. A: Average periodograms of the OIS responses of 5 subjects to 5 different stimulus amplitudes. B: Average relative (more ...)
Figure 5 Dependence of the position of the largest spectral peak in OIS periodograms on the stimulus amplitude. The frequency (A) and the period (B) of the largest spectral peak (average of 5 subjects – see Figure 4) are plotted as a function of the stimulus (more ...)
Stimulus duration appears to alter the spatial organization of SI optical response to flutter in a manner similar to the alteration that accompanies an increase in stimulus amplitude. Figure displays the temporal evolution of the responses evoked by four different amplitudes. In each case, the pattern of absorbance evoked by the flutter stimulus appears to become more organized and periodic with time after the stimulus onset. In other words, with increasing stimulus duration, the local aggregates of above background absorbance tend to form larger clusters – which would lead to higher periodic values (lower spatial frequencies). The spatial frequency changes with stimulus duration were quantified in a manner similar to those that were used to quantify the spatial frequency characteristics that changed with stimulus amplitude. To give a representative example, Figure shows the temporal evolution of the SI response of a subject to a 400 μm-amplitude flutter stimulus. The images in Figure were obtained 1 sec prior to stimulus onset (control), as well as at 1, 3, 5, and 7 sec after the onset of continuous skin flutter stimulation. The power spectra obtained from these images (Fig. ) show a systematic leftward shift of the dominant frequencies with increasing time after stimulus onset, from ~6.5 cycles/mm prior to stimulation, to ~2 cycles/mm after 7 sec of continuous stimulation. The plots in Figure show that the 6–9.5 cycles/mm frequency band, which is dominant in the resting state, loses relative power after onset of stimulation, and during this same time relative power within 1.5–3 cycles/mm frequency band also becomes maximal.
Figure 6 Comparison of temporal development of cortical patterns of absorbance evoked by different amplitudes of flutter vibration. 3-D activity maps of the same cortical region (of the same subject) are plotted at -1 (control), 1, 3 and 5 secs after the stimulus (more ...)
Figure 7 Temporal development of power spectra in response to a 400 μm-amplitude flutter stimulus. A: Light absorbance images obtained at selected times before and during flutter stimulation. B: Power spectra of OIS activity sampled in the anterior-posterior (more ...)
Figure summarizes the temporal development of OIS response patterns by plotting the highest-power spatial frequency in the anterior-posterior dimension (averaged across 5 subjects) at different times after stimulus onset. Two plots are shown for comparison: the first for 400 μm-amplitude stimuli, the second for 50 μm-amplitude stimuli. These plots make it evident that during continuous flutter stimulation: (1) the spectral power of the response migrates towards lower frequencies with increasing time after stimulus onset – indicating that SI optical response pattern undergoes a gradual spatial reorganization; and (2) the higher the stimulus amplitude, the faster the shift of spectral power towards lower frequencies.
Figure 8 Temporal shift of the position of the largest spectral peak in the anterior-posterior OIS periodograms and its dependence on stimulus amplitude. The frequency (A) and the period (B) of the largest spectral peak (average of 5 subjects) are plotted as a (more ...)
Figure plots the magnitude of OIS response to the 50 μm- and 400 μm-amplitude stimuli as a function of time after the stimulus onset, showing that during the time when the dominant spatial frequency migrates across the power spectrum, the OIS also grows overall in its magnitude. The concurrency of these changes raises a parsimonious possibility that OIS periodicity is a direct function of the OIS magnitude (and thereby only an indirect function – via their control of the OIS magnitude – of the stimulus strength and duration). To evaluate this possibility, Figure plots the highest-power spatial frequency at any given time (taken from Figure plot) as a function of the overall OIS magnitude at that time (taken from the Figure plot). As the Figure plot shows, the relationship between OIS magnitude and periodicity obtained with the 50 μm-amplitude stimuli appears to be different from the relationship obtained with the 400 μm stimuli, suggesting that OIS periodicity cannot be explained simply by the overall OIS magnitude.