Broca’s area
Pierre Paul Broca, in the 1860s, observed that damage to the posterior portion of the left frontal lobe, specifically the posterior inferior frontal gyrus, now known as Broca’s area, resulted in a specific language deficit. His most famous patient, Leborgne (nicknamed “Tan”), could understand spoken language but was unable to produce more than a single syllable, despite otherwise normal cognitive abilities. Broca found similar deficits in other patients with lesions in the same region.
Broca concluded that this area was crucial for the production of articulated language. The deficit, later termed “Broca’s aphasia,” is characterized by non-fluent, effortful speech with relatively preserved comprehension, although understanding complex sentences can be impaired. Broca’s systematic clinical and autopsy studies established that language production is typically lateralized to the left hemisphere and localized to this specific frontal region.
Wernicke’s area
Carl Wernicke (1874): Wernicke observed that damage to the posterior part of the left superior temporal gyrus (Wernicke’s area) resulted in fluent but nonsensical speech and poor comprehension. This complemented Broca’s findings by showing that different brain regions control different aspects of language.
The medial temporal lobe
The case of H.M. is historically famous. Due to uncontrollable seizures, he had large portions of the medial temporal lobe removed in surgery. The portions included all the hippocampal formation, the amygdala, and another area which I do not know. H.M. had perfectly intact working memory (seconds to minutes), but as soon as he was distracted, he lost the memories. He could not form new long-term memories after the surgery. The long-term memories he had before were intact.
Similar but milder effects were observed in R.B., which had only a destruction of pyrmadial cells in the CA1 region of the hippocampus.
Enthorinal Cortex (EC)
The EC has many projections, one of which is to the hippocampal dentate gyrus (DG) via the so called perforant path (PP). PP axons can be divided in two distinct bundles, based on the area they origin: the medial PP starts from the medial EC and lateral PP starts from the lateral EC. The two paths also innervates two different regions of the dentate gyrus. Noteworthy, they are not only are anatomically segregated but also deliver inputs of different nature. LPP transmits olfactory, auditory, and visual information while MPP conveys spatial information.
The evidence for a differential contribution by the two pathways to learning and memory processes was reinforced by behavioral studies. In these studies, animals subjected to selective lesions in either MEC or LEC showed distinct types of behavioral impairment: MEC injury caused defects mainly in place learning paradigms, whereas LEC injury mainly in contextual learning ones (Burwell et al., 2004; Ferbinteanu et al., 1999; Hunsaker et al., 2007).
References
- Di Castro, Maria Amalia, and Andrea Volterra. “Astrocyte control of the entorhinal cortex‐dentate gyrus circuit: Relevance to cognitive processing and impairment in pathology.” Glia 70.8 (2022): 1536-1553.