Hemispheric asymmetry in the fusiform gyrus distinguishes Homo sapiens from chimpanzees

Abstract

While the neural basis for linguistic communication has been linked to brain structural asymmetries found only in humans (wider connective spacing is found between the minicolumns of neurons in the left hemisphere language areas), it is unknown if the opposite microanatomical asymmetry exists in the fusiform gyrus which typically supports a right hemisphere bias for face processing. Unlike language, face processing is an ability shared with chimpanzees and, as Darwin observed, the widespread use of facial expressions in animal communication suggests a biological basis. We tested the principle that minicolumn asymmetry follows typical functional dominance in humans, and tested its evolutionary continuity, by measuring minicolumn width, neuronal size and density in the mid-fusiform cortex in 14 humans and 14 chimpanzees. We found that microanatomical asymmetry distinguishes humans from chimpanzees although the direction of asymmetry is the same as in language areas-the right hemisphere contained narrower minicolumns and smaller pyramidal neurons, as in auditory language areas. Uniformly narrow minicolumns in chimpanzees and in the human right hemisphere are consistent with mechanistic predictions supporting the apparent bias towards holistic face processing. Wider minicolumns and larger neurons in the human left hemisphere may be consistent with a language function such as word-form processing. Microanatomical asymmetry in the neocortex therefore provides a correlate of hemispheric specialisation.

Independent and Collaborative Contributions of the Cerebral Hemispheres to Emotional Processing

“…In addition to Killgore and Yurgelun-Todd (2007), finding that identification of positive emotions can be accomplished within either hemisphere, they observed that unilateral presentation of sad faces, resulted in bilateral activity. While the depth of processing accomplished by each hemisphere was not directly explored by Killgore and Yurgelun-Todd, it is important to note that the reported LH activity for processing of both happy and sad faces was restricted to regions that are related to language, such as left inferior and medial frontal gyri (syntax, Tyler et al, 2011), medial temporal gyrus (language and semantic memory, Tranel et al, 1997; Chao et al, 1999; Cabeza and Nyberg, 2000; Ashtari et al, 2004), insula (speech, Dronkers, 1996; verbal emotion, Ardila et al, 1997; Ackermann and Riecker, 2004), frontal cortex, fusiform gyrus (face processing in the RH, phoneme/grapheme in the LH, Démonet et al, 1994; Chance et al, 2012), and “lingual areas.” These findings also support the previously mentioned hypothesis that one role of the LH in emotion processing is for verbal encoding and other language-related, higher cognitions such as planning. It is also noteworthy that some of these structures (frontal gyri) are also part of the bilateral DMN, discussed earlier as being important for processing emotion.…”

The cortical microstructural basis of lateralized cognition: a review

“…In humans, cells have become large and less densely packed in the evolution of mid-fusiform cortex compared to the chimpanzee and this is accentuated in the left hemisphere with the result that there is an inter-hemispheric asymmetry that is not found in chimpanzees (Chance et al, 2013). Consequently, in humans, the wider minicolumns and larger neurons are found in the hemisphere opposite to the one that is dominant for face perception.…”

“…Consequently, in humans, the wider minicolumns and larger neurons are found in the hemisphere opposite to the one that is dominant for face perception. Therefore, unlike auditory language processing, it appears that the arrangement of minicolumns that confers dominance for face processing is the thinner, denser spacing that is found in the right hemisphere (Chance et al, 2013). Meanwhile, the absence of asymmetry in chimpanzees may relate to better performance than humans in tasks such as inverted face recognition that have ecological validity for chimpanzees (Matsuzawa, 2007).…”

“…However, inability to read (“pure alexia”) is associated with damage to the left mid-fusiform gyrus (Leff et al, 2006). It has been suggested that the wider minicolumn spacing in this region of the left hemisphere may relate to its role in visual word recognition in humans in addition to its role in face processing (Chance et al, 2013). …”

Glutamate-based magnetic resonance spectroscopy in neuroleptic malignant syndrome

“…[6] In schizophrenia, there is decreased activation of cortico-cerebellar-thalamic circuit in the right hemisphere. [7] Although bilateral affection is biologically plausible in NMS, no evidence has emerged until date.…”