This study provides evidence that regions of vFOC are comparably functionally engaged in monkeys and humans by sequences that violate the ordering relationships of an auditory artificial grammar8,9. We used a comparative sequence processing paradigm and fMRI to provide a first test of a prominent neurobiological hypothesis about the evolution of human brain regions involved in initial syntax-related processes between words in a sentence or evaluating adjacent sequence elements2,4,7,9. Since non-human animals lack human language abilities, the results of this comparative human and monkey fMRI experiment inform us on the domain-general processes, not specific to language, which macaques and humans both possess and whose neural substrates are identified. The results of these comparative analyses provide evidence that regions of vFOC support evolutionarily conserved sequence processing functions. Our human fMRI results also link to the larger body of evidence that the frontal opercular cortex is associated with certain forms of human language processing and domain-general processes, as we consider.

vFOC is argued to represent an initial stage in neurobiological processes related to human syntax4. This region is involved in processing relationships within and between adjacent phrases in natural language4,32,33,34. Sequences that violate an artificial grammar designed to model similar adjacent syntactic relationships in natural language22 also engage these ventral frontal and opercular human brain areas2,28. Thus, prior human neuroimaging studies have suggested that the vFOC , which lies adjacent to but does not include areas 44/45, is involved in ‘local structure building’, thought to represent a key initial stage of human language processing2,4,7. By contrast, sequences that violate more complex sequence relationships engage this region and also areas 44/45 (refs 2, 27, 28, 29, 30, 31). Areas 44/45 are particularly engaged by syntactically and semantically complex natural language processes4,5,6,42,43 as well as other complex domain-general sequencing processes3,44. In parallel, non-human primates have been shown to be able to detect illegal, violation transitions between at least adjacent sequence elements8,11,22,24. There is also some evidence that New World monkeys and apes are sensitive to non-adjacent relationships45,46,47, although there is a paucity of evidence that any non-human animals are able to recognize sequences which model more of the complexity of human language. These studies led to the novel hypothesis, tested here, that local sequence processing abilities, and the ventral frontal opercular regions that support them in humans, might be functionally conserved in certain extant non-human primates2,4,7, reflecting a common evolutionary origin.

Our fMRI results reveal that ventral and opercular regions of frontal cortex are comparably involved in auditory sequence processing operations in humans and macaques. This vFOC activation was statistically robust, and was consistently observed across all three macaques and in the human participants at the whole brain and ROI levels, including with individual monkey and human ROIs (Figs 2, 3, 4, 5, Supplementary Figs 4 and 5). These results suggest that the sequence processing function of the vFOC is shared by humans and macaques. The role of areas 44/45 in this task was variable both within and between species. Activation in this region was observed in two out of the three macaques (Fig. 3e,f). No significant activation was seen at the group level in humans (Fig. 4c), however, individual analyses revealed that areas 44/45 were engaged in 6 of the 12 human participants (Fig. 5). Furthermore, in the human RM-ANOVA group results, the vFOC ROI was significantly more responsive to the violation sequences than the areas 44/45 ROI, and the across species RM-ANOVA analyses showed an interaction indicative of significantly greater areas 44/45 ROI engagement in the macaques relative to the humans in response to violations of the sequencing relationships. It remains possible that if more human participants were tested significant activation might have been observed in human areas 44/45 at the group level. However, we are careful not to over interpret the variable activation pattern in areas 44/45 in the current experiments. Therefore, the primary observation in this study is the statistically robust activation in the vFOC in both the humans and monkeys. These results provide evidence supporting the hypothesis that vFOC is comparably involved in sequence processing in macaques and humans. This region appears to be specifically involved in evaluating the transitions between sequence elements, in the context of the regularities learned during the prior exposure.

To test the frontal operculum evolutionary hypothesis4 it was necessary to select a sequence processing task with an appropriate level of complexity, which both humans and macaques learn48. In this study, we used a moderately complex sequencing paradigm that can generate a range of legal sequences of different lengths, containing a wide range of transitions between elements (Fig. 1a, ref. 8). Our human fMRI results show that these sequencing operations are complex enough to activate ventral frontal opercular regions, consistent with earlier human fMRI studies of adjacent sequencing2,28. Furthermore, our behavioural results demonstrate a striking correspondence between the responses of the macaques and the humans (Fig. 1b,c, Supplementary Figs 2 and 3), suggesting that (unlike the behavioural pattern we previously observed in another species of monkey, common marmosets8) both macaques and humans produce a similar pattern of responses. Neither the responses of the humans or the macaques can plausibly be attributed to reliance on simple cues (such as the novelty of the testing sequences, Supplementary Fig. 2), or by trivial acoustical differences between the consistent and violation sequences (Supplementary Fig. 5). Furthermore, we demonstrated that both the humans and macaques respond more strongly to sequences containing less predictable transitions, suggesting that both species track the statistical properties of the transitions throughout the sequences (Supplementary Fig. 3).

The form of sequence processing required by this paradigm is markedly different to ‘deviance detection’ or ‘oddball’ responses elicited by the appearance of an unexpected sound or pattern of sounds in a stream of auditory stimuli49. In these tasks detecting the deviant stimuli involves noticing a perceptual difference in the stream. By comparison, our paradigm contains no ‘oddball’ sounds in any of our sequences: the same five nonsense word elements appear in both the violation and consistent sequences. Therefore none of the individual elements represent ‘deviants’ in the same way as in oddball tasks; only the ordering relationships between the same set of elements is regulated or violated. Furthermore, the two types of paradigms appear to engage different brain networks49 (Supplementary Note 6).

A number of studies highlight the correspondences that can be drawn between the prefrontal cortex in humans and non-human primates, as well as current areas of uncertainty. The earlier comparative neuroanatomical studies by Brodmann and more recent efforts have identified anatomical correspondences between many prefrontal regions, such as between areas 44 and 45 in humans and non-human primates50,51. Other active fields of research involve understanding the comparative connectivity of frontal and other areas that in humans are known to be involved in language-related processes39,52. Some authors have predicted that during human language evolution the interconnectivity between frontal and other areas increased53, which also predicts functional differences in otherwise cytoarchitectonically corresponding areas, such as area 44 (ref. 54). Our comparative behavioural and fMRI results in humans and macaques contribute to these efforts by providing functional insights on ventral frontal cortex regions, highlighting the role of the frontal operculum, which is a less well studied region of the frontal cortex. We also present results using both anatomically defined ROI analyses (Figs 3 and 4) and whole-brain analyses, unconstrained by any particular parcellation scheme (Figs 2 and 4, Table 1), highlighting the correspondences in activation patterns at particular locations in the brains of the monkeys and humans across the species. This information could now be used as seed regions to determine the cross-species correspondences in anatomical pathways that are currently under question37,55,56.

Although many language-related processes are left-lateralized in humans (especially those involving complex syntax), these experiments revealed no significantly left-lateralized effects in either species. While some of the results are suggestive of potential right lateralization, statistical testing failed to provide consistent evidence that sequence processing in the key frontal areas is significantly lateralized in either macaques or humans. For a summary of the observations on lateralisation effects see Supplementary Note 3. Previous hypotheses about the evolutionary origins of the functions supported by the ventral frontal cortex have primarily focussed on the left hemisphere2,4,7. However, the human and macaque results reported here suggest that the right hemisphere also plays an important role in sequence processing. More generally, these observations of a lack of strong lateralization provide the first direct comparative evidence in support of a ‘dual neurobiological language systems’ hypothesis6,57,58, but extend it in important ways. This hypothesis proposes that in modern humans, specializations for core syntactic language functions depend on a left-lateralized ventral fronto-temporal system, and that this left-lateralized system is functionally integrated with a more bilaterally distributed network, that has more general language-related functions, such as those that combine semantic and syntactic operations in natural language6. Given that our results are based on an artificial grammar or sequence learning paradigm8,9, the current findings suggest that the hypothesized bi-hemispheric system also supports sequence processing in both humans and monkeys, forming an important part of the ancestral systems that underpin aspects of language function in the modern human.

The neurobiological hypotheses that we tested here focussed on vFOC and areas 44/45. However, we also observed activation in broadly comparable perisylvian regions, including parietal regions, in the humans (Fig. 4) and two out of three of the monkeys (Fig. 3). See Supplementary Fig. 7 for ROI results from this region in humans and monkeys. The focus of the parietal clusters that we observed appears to involve BA7 in macaques and BA39 in humans, both of which lie below the intraparietal sulcus. There has been considerable uncertainty as to which dorsal parietal regions are functionally homologous59, given that human BA7 lies above the intraparietal sulcus. Our results suggest a level of functional correspondence between BA7 in macaques and BA39. Relatedly, MRI-based estimates of connections in the inferior parietal lobule in humans and macaques have shown similarities between different aspects of the inferior parietal lobule in the two species60. It is interesting to note that in the human brain this parietal region is engaged by a wide range of language tasks4,32,61,62. However, neuropsychological and neuroimaging evidence (for example, ref. 41) suggests that the human inferior parietal lobule, although providing a substantial role in sentence comprehension, is not as critical to the core syntactic language functions as the perisylvian inferior/ventral frontal regions. Therefore, the precise role of the parietal region in our task might well differ in relation to the role of the vFOC in both species, and would need to be further studied in the future.

It is also important to consider the impact of differences in the way the species were tested on the results, since it is not possible to conduct all measurement in the same way with both species. The behavioural results show that despite different testing methods after the exposure period, both species responded similarly to the statistical properties of the test sequences (Fig. 1, Supplementary Figs 2 and 3). Furthermore, fMRI data obtained from human participants scanned using the same fixation task as the macaques produced activation in the ventral frontal cortex that is very similar to the effects observed using the button press task in the main human experiment (compare Fig. 4 with Supplementary Fig. 8), and also to the macaque experiment (Figs 2 and 3). While it is possible that these or other testing differences (for example, the macaques and humans, respectively, received either juice or money as reward for participating) could obscure potential similarities between the species, the highly consistent results we observe in vFOC, in both species and in all of the monkeys, are remarkable given the inevitable methodological differences in how the species were tested (Supplementary Note 7). Our data suggest that the engagement of vFOC in both species is a key property of a common neural response to the violation of the sequencing relationships, and that this is present independent of the responses required by different tasks.

The extent to which language-specific and cognitive-domain-general processes overlap and share common processes in the human brain is an active area of research that informs us on the function of the frontal cortex. For example, Fedorenko et al.3, have suggested that domain-general subregions of area 44 lie next to regions that are more involved in language-specific functions3. Other studies have shown that the human inferior frontal gyrus and operculum are involved in cognitive processes such as attention and task control44,63,64. The type of comparative study that we conducted can help to link these disparate strands of research (Supplementary Fig. 1) and provide important inputs into future developments. For example, human neuroimaging studies aiming to distinguish domain-general from language-specific processes often use hierarchically organized natural language material. Our study points to the greater need for such studies to compare sequence processing tasks of different levels of complexity, some of which, as we show, can now be linked to a primate model system. Sequence processing paradigms can also readily incorporate visual or auditory stimuli and thus inform us on sequencing processes that are sensory modality independent. Moreover, although we found little evidence for non-specific deviance-detection responses in any of our results (Supplementary Note 6), such processes may well engage the human and monkey vFOC once transitional regularities are encoded. Future studies might tease apart the contributions of different cognitive processes involved in various forms of sequence processing. Statistical or sequence learning paradigms remain appealing because they can create specific prediction errors (violations) in sequencing relationships (Supplementary Fig. 6; also see ref. 65) that can be studied at the neuronal level in animal models and, as we show, linked to processes in humans by way of comparative behaviour and neuroimaging (fMRI or electroencephalography65).

In summary, the results of these experiments provide the first evidence supporting the hypothesis that functions of key regions of vFOC are evolutionarily conserved in macaques and humans. These regions play an important role in sequence processing, and, as shown in a number of prior human neuroimaging studies, are also involved in analysing local grammatical relationships in the human brain. We conclude that vFOC in both species supports neuro-computational functions which include evaluating sensory sequences in relation to the probabilities with which the transitions between elements previously occurred. This raises the possibility that language-related processes in modern humans are functionally integrated with highly conserved, originally non-linguistic processes shared with our extant primate relatives.