[9] Broca's area is often identified by visual inspection of the topography of the brain either by macrostructural landmarks such as sulci or by the specification of coordinates in a particular reference space.

Further, because of considerable variability across brains in terms of shape, size, and position relative to sulcal and gyral structure, a resulting localization precision is limited.

The PTr and POp are defined by structural landmarks that only probabilistically divide the inferior frontal gyrus into anterior and posterior cytoarchitectonic areas of 45 and 44, respectively, by Brodmann's classification scheme.

Patients with lesions in Broca's area who exhibit agrammatical speech production also show inability to use syntactic information to determine the meaning of sentences.

[13] Also, a number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences.

[14] Further, functional magnetic resonance imaging (fMRI) experiments have shown that highly ambiguous sentences result in a more activated inferior frontal gyrus.

Later work by Nixon et al. (2004)[17] showed that when the pars opercularis (situated in the posterior part of Broca's area) was stimulated under rTMS there was an increase in reaction times in a phonological task.

The results from this experiment conclusively distinguished anatomical specialisation within Broca's area for different components of language comprehension.

[22]: 494–7  This finding, that aspects of gestures are translated in words within Broca's area, also explains language development in terms of evolution.

They also attributed his lack of problems to extensive compensatory mechanisms enabled by neural plasticity in the nearby cerebral cortex and a shift of some functions to the homologous area in the right hemisphere.

[25][26] Aphasia is an acquired language disorder affecting all modalities such as writing, reading, speaking, and listening and results from brain damage.

Other symptoms that may be present include problems with fluency, articulation, word-finding, word repetition, and producing and comprehending complex grammatical sentences, both orally and in writing.

This discovery suggests that Broca's area may be included in some aspect of verbalization or articulation; however, this does not address its part in sentence comprehension.

(There is a wide distribution of Talairach coordinates[39] reported in the functional imaging literature that are referred to as part of Broca's area.)

Newer theories take a more dynamic view of how the brain integrates different linguistic and cognitive components and are examining the time course of these operations.

Since large lesions are typically required to produce a Broca's aphasia, it is likely that these regions may also become compromised in some patients and may contribute to their comprehension deficits for complex morphosyntactic structures.

[42][43][44] Although repeating and reading single words does not engage semantic and syntactic processing, it does require an operation linking phonemic sequences with motor gestures.

[citation needed] In a study published in 2007, the preserved brains of both Leborgne and Lelong (patients of Broca) were reinspected using high-resolution volumetric MRI.

The study also sought to locate the exact site of the lesion in the frontal lobe in relation to what is now called Broca's area with the extent of subcortical involvement.

This study provides further evidence to support the claim that language and cognition are far more complicated than once thought and involve various networks of brain regions.

Human language might have evolved as the "evolutionary refinement of an implicit communication system already present in lower primates, based on a set of hand/mouth goal-directed action representations.

The recent finding that Broca's area is active when people are observing others engaged in meaningful action is evidence in support of this idea.

It was hypothesized that a precursor to the modern Broca's area was involved in translating gestures into abstract ideas by interpreting the movements of others as meaningful action with an intelligent purpose.

Consistent with this idea is that the neural substrate that regulated motor control in the common ancestor of apes and humans was most likely modified to enhance cognitive and linguistic ability.

More recently, the neocortical distribution of activity-dependent gene expression in marmosets provided direct evidence that the ventrolateral prefrontal cortex, which comprises Broca's area in humans and has been associated with auditory processing of species-specific vocalizations and orofacial control in macaques, is engaged during vocal output in a New World monkey.

[50][51] These findings putatively set the origin of vocalization-related neocortical circuits to at least 35 million years ago, when the Old and New World monkey lineages split.