4.5 Functional Roles of Adjacent Areas

Outline 4.5 Functional Roles of Adjacent Areas

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4.5 Functional Roles of Adjacent Areas

4.5.3 Specificity Validation: FEF vs. 55b

A parallel validation compared FEF with its immediate premotor neighbour, area 55b, using partial correlation to rule out nonspecific regional signals. Despite their direct anatomical neighbourhood, the two seed regions exhibit clearly dissociable connectivity patterns. In both hemispheres, the FEF maintains dominant coupling with parietal and motor areas (PF: left $z=9.73$, $\overline{r}=0.02$; right $z=17.84$, $\overline{r}=0.03$; PFcm: left $z=16.94$, $\overline{r}=0.02$; MST: left $z=9.63$, $\overline{r}=0.01$). In contrast, area 55b dominates language-related connectivity, with the highest mean connectivity values in the entire analysis: PSL (left $z=23.24$, $\overline{r}=0.13$; right $z=17.53$, $\overline{r}=0.08$), STSdp (left $z=23.58$, $\overline{r}=0.07$; right $z=23.23$, $\overline{r}=0.04$), and Broca’s areas 44 (left $z=23.33$, $\overline{r}=0.07$; right $z=18.45$, $\overline{r}=0.03$) and 45 (left $z=23.60$, $\overline{r}=0.06$; right $z=23.79$, $\overline{r}=0.07$). The FEF profile shows no equivalent coupling with temporal or language regions in either hemisphere.

Figure 4.X: Functional connectivity heatmap comparing FEF and area 55b partial corre- lation z-scores across all auditory target ROIs. Despite anatomical adjacency, FEF (upper rows) couples preferentially with parietal and motor regions, while 55b (lower rows) couples predominantly with temporal, opercular, and language areas. This dissociation confirms that the spatial connectivity profile is specific to the FEF and does not reflect a general dorso-prefrontal signal.

Figure 4.X: FEF versus 55b partial correlation circular diagrams, left (A) and right (B) hemisphere. Area 55b (green) exhibits dominant connectivity to superior temporal regions, Broca’s complex (Area 44, Area 45), and early auditory areas (A4, A5), while FEF (red) emphasises inferior parietal and premotor areas. The opposing profiles confirm that 55b functions as a language relay rather than a spatial control hub.

Figure 4.X: FEF versus 55b partial correlation projected onto the cortical surface, left (A) and right (B) hemisphere. Red shading indicates preferential FEF coupling; green shading indicates preferential 55b coupling. The spatial contrast between parietal (FEF-dominant) and temporal (55b-dominant) cortex visually confirms the functional dissociation between the spatial control hub and the language relay.

Interestingly, 55b shows no significant coupling to spatial areas in the parietal lobe, but maintains strong direct coupling with auditory association areas A5 (left: $z=13.27$, right $z=19.42$) and A4 (right: $z=7.51$). The latter connections are mostly absent in the FEF profile (only in the left hemisphere to A4, $z=4.06$, $\overline{r}=0.01$).

4.5.4 Results of the Broca-Seed Validation

Next, we evaluate the proposed functional subdivision of Broca’s area within the auditory dual-stream architecture (Rolls et al. (2023) - Cerebral Cortex). The connectivity patterns of Areas 44 and 45 were independently assessed using partial correlation to isolate direct functional pathways.

The comparative analysis reveals that Area 45 couples more strongly with semantic temporal areas than Area 44; in the right hemisphere, this extends to a stronger coupling with 55b. Area 44, by contrast, shows stronger coupling with 55b in the left hemisphere.

Area 45: The Ventral Anchor
Area 45 exhibits broad integration into the ventral what-stream, with its connectivity profile extending extensively into the temporal lobe (e.g., STSdp: left $z=11.68$, $\overline{r}=0.06$; right $z=20.12$, $\overline{r}=0.05$) and demonstrating strong coupling with PSL (left $z=5.36$, $\overline{r}=0.06$; right $z=16.91$, $\overline{r}=0.06$). Although the visualisation may suggest coupling with superior parietal (7AL, 7Am, 7PC) and premotor areas (FOP3), the corresponding mean connectivity values are near-zero or negative ($\overline{r}\le 0.00$), indicating that these statistically detected associations do not reflect meaningful functional coupling strength. This robust temporal-prefrontal coupling firmly anchors Area 45 as a core semantic node within the Broca complex. In contrast to Area 44, Area 45 alone shows direct coupling with early auditory areas A4 and A5 (e.g., A5: left $z=9.38$, $\overline{r}=0.01$), indicating that the gateway between early acoustic processing and the Broca complex runs specifically through Area 45.

Area 44: The Motor-Articulatory Interface
Area 44 reveals a fundamentally different architecture. Rather than coupling with the temporal-semantic network, its significant connectivity is concentrated in the anterior ventral insula (AVI: left $z=19.18$, $\overline{r}=0.03$; right $z=23.91$, $\overline{r}=0.05$), the premotor language node 55b (left $z=5.89$, $\overline{r}=0.07$), and IFJa (left $z=11.15$, $\overline{r}=0.04$; right $z=8.93$, $\overline{r}=0.04$). Additionally, Area 44 shows a direct coupling with FEF (right: $z=6.52$, $\overline{r}=0.02$), providing a direct link to the dorsal spatial control hub. Regions such as MT, MST, and PFop show near-zero or negative partial correlation values ($\overline{r}\le 0.00$), confirming the absence of integration with parietal and motion-sensitive areas. This suggests that 44 plays a role in connecting semantics from ‘what’-stream to articulatory output via 55b.

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