Anatomical distribution of AM responses within the different auditory areas
The magnitude of the evoked responses (53 trials) was analyzed as a function of the stimulus modulation frequency (Liégeois-Chauvel et al., 2004
). The magnitude values of the spectral components were determined by considering the Fast Fourier Transforms of the responses over 1200 ms. These values are averaged and shows the amplitude of the spectral peaks at the modulation frequency within main auditory areas (PAC, SAC, BA 22 and Post T1) as a function of AM frequency (4, 8,16 and 32 Hz) and patients.
Figure 3 Values of the amplitude spectrum at four modulation frequencies (MF = 4, 8, 16, 32 Hz) for each patient, a) PAC – Case 1, b) SAC – Case 1, c) Post T1 – Case 1, d) PAC – Case 3, e) SAC (HG + PT) – Case 3, f) Area (more ...)
As reported in the previous study, the distribution of MF varies from one contact to another and predominant response of cortical auditory areas to the low AM frequencies (4–8 Hz) is observed in each region. Best synchronization to 16 Hz could be, however, observed as on the three contacts H13-H15 in the PAC of Case 20. The response to 32 Hz AM is greater within the PAC than in the SAC or BA 22 in Case 3.
Propagation within the primary auditory cortex (PAC)
To study the propagation between the different leads located within the PAC, we distinguished the posterior part from the anterior part of PAC in Cases 3 and 20. The corresponding AEPs are taken in the time window between 200 to 1000 ms post-stimulus onset. Results of the propagation expressed by DCOH modules at each frequency f
= MF are reported in for Case 3 and in Table A1 for Case 20 at MF = 8 Hz in Supplementary Material
. These tables follow the reading convention of source (column) to target (row). In and A1 the highlighted values correspond to DCOH modules upper than 0.6.
DCOH modules at the frequency/= MF (MF = 4, 8, 16 and 32 Hz) between all the leads located in the PAC of Case 3.
In , we observe that the auditory flow tends mainly to propagate serially from the medial leads (lead P1C3) to the most lateral ones (lead P4C3). This direction of propagation is found whatever the AM frequency within the posterior part of PAC (electrode PC3). In the anterior part of PAC (electrode HC3), this unidirectionality seems less clear since it exists for H3C3 whose activity is propagated to H4C3 and H5C3, but less for H4C3. Moreover, some variations are seen according to the AM sound. Interestingly, the oscillatory activity was propagated from the posterior to anterior part of PAC (i.e. from electrode PC3 to HC3) except for 4 Hz.
The values in and A1 for MF = 8 Hz are reported on and for Cases 3 and 20 respectively. The arrow width codes interaction strength (0 ≤ no arrow ≤ 0.6 < increasing width solid lines in step of 0.2). For reasons of clarity in , only the directed coherences modules superior to 0.8 are represented.
Figure 4 Figure 4a. Graph of propagation between the leads of electrode P and of electrode H located in the PAC, at the frequency MF = 8 Hz for Case 3. The arrow width codes interaction strength (0 ≤ no arrow ≤ 0.6 < increasing width solid (more ...)
shows that the activity propagates from the posterior part to the anterior part of PAC of Case 3, mainly to H4 and H5. In dealing with Case 20, the location of the electrode HC20 is in the same region as HC3 and the electrode TC20 is more anterior and lateral than HC20. There is a strong tendency of medio-lateral propagation but which is not strictly serial, for instance auditory stream mainly goes from H10C20 and H1C20 to H14C20 and H15C20 and from T12C20 to T15C20 skipping T13C20 and T14C20. In the anterior-lateral part of PACC20 there is a constant interaction inside this area, predominantly from TC20 to HC20.
Between different anatomical areas
Firstly, we will describe the results obtained from Case 3 because the exceptional sampling of the recording sites within the auditory cortex (see Methods) allowed recording of activity in four different anatomical areas of the right auditory cortex (PAC, SAC, PT, BA 22) and in the insular gyrus. In the same manner that we distinguished the posterior part (PAC P: P1-4) from the anterior part of PAC (PAC H: H3-5), we separate evoked activity recorded from posterior PT P (P5 to P9) to anterior PT H (H7-9). We studied the functional connectivity between these seven sub-regions.
The directed coherence algorithm was then applied to each couple constituted of AEPs relative to leads located in the different sub-regions. The corresponding AEPs are taken in the time window between 200 to 1000 ms post-stimulus onset. The means of all the DCOH modules over all the frequencies MF = 4, 8, 16 and 32 Hz from the different couplings were summarized in Table A2 (see Supplementary Material
) and displayed in for Case 3.
Figure 5 Graph of propagation between 7 auditory sub-regions of Case 3, averaged over all the modulation frequencies MF = 4, 8, 16 and 32 Hz. The arrow width codes interaction strength (0 ≤ no arrow ≤ 0.6 < increasing width solid lines (more ...)
This graph shows the major propagations of the oscillations between the seven auditory sub-regions of Case 3 independently of the AM frequency. The main source of information is the PAC P, which directs auditory information to the PAC H, confirming the posterior-to-anterior influence within the primary auditory cortex, and to the SAC T showing a primary-to-secondary auditory areas connectivity. In the same way, the PAC H is linked to the PT H (medial-to-lateral connection) and to the SAC T. This figure presents examples of strong unidirectional influence of primary auditory areas (PAC P and PAC H) on secondary ones (SAC T and A22 T) and on planum temporale (PT P and PT H). The insular gyrus (Insula H) also seems to send information to other areas (PT H, SAC T and A22 T) and receives information only from PAC P.
For Case 3, the modules of DCOH calculated for 8, 16 and 32 Hz are given in Tables A3, A4 and A5, presented as Supplementary Material
. We considered the Heschl’s sulcus (lead H6) as a single area.
Modules of DCOH of Cases 3, 1 and 12 calculated for 8 Hz are summarized on the graphs shown on respectively. Only modules of directed coherences superior to 0.6 are displayed.
Figure 6a. Graph of propagation between the 8 sub-regions of Case 3 at the frequency MF = 8 Hz. The arrow width codes interaction strength (0 ≤ no arrow ≤ 0.6 < increasing width solid lines in step of 0.2).
In , the main connections are similar to those given in , which correspond to the principal directed connections present between the 8 zones whatever the modulation frequency. But one can readily see that the HS dispatches information to several zones including PAC P, PT P, Insula H, PT H, SAC T and A22 T. The other main source of information is the PAC P which propagates information to the PT P, Insula H, PAC H, PT H, SAC T and A22 T. In general, information flows from posterior to anterior zones and from medial to lateral zones.
In and , auditory information propagates from primary and secondary auditory areas (PAC and SAC) to the posterior part of STG (Post T1). As previously observed, HS sends information to PAC but it receives information from Post T1 in Case 12.
shows the information propagation at MF = 16 Hz in Case 3. As in the previous case, PAC P sends information to PT P, PAC H, PT H, SAC T and A22 T. Mainly, information flows from posterior to anterior zones and from medial to lateral zones. We observe the striking influence of HS, which directs information to the PAC, SAC and BA 22 and even to the Insula.
Figure 7a. Graph of propagation between the 8 auditory sub-regions of Case 3 at the frequency MF = 16 Hz. The arrow width codes interaction strength (0 ≤ no arrow ≤ 0.6 < increasing width solid lines in step of 0.2
Interestingly, we notice that the connectivity is frequency dependent as shown in for 32 Hz. One can readily see that the interaction strengths are very high, particularly from PAC P, Insula H and PAC H to the other zones. Contrary to the two other MFs, the Sulcus H here almost only receives activity from the other zones and a strong connectivity is observed between Insula H and Sulcus H, SAC T and A22 T. More importantly than at other MFs, information flows from posterior to anterior zones and from medial to lateral zones.
summarizes all the results of Case 3 represented in terms of its anatomy. Two main streams emerge, one from the postero-medial part of HG to the anterior part of HG and STG, and another from medial to lateral part of HG. The role of Heschl’s sulcus dispatching auditory stream to the nearby regions is revealed. The connectivity of insula is strongly modulated by the AM frequency and seems maximal for the 32 Hz-MF. Data from the three other patients confirm the medio-lateral stream in HG and show that Post T1 receives information from PAC and SAC, sending in return to PAC via HS.
Figure 8 Cortico-cortical connections within the auditory areas of Case 3. Arrows indicate the direction of auditory stream. Localisation of intracerebral electrodes in auditory cortex is superimposed on the patient’s MRI slices. Each dashed line (labeled (more ...)