Editing Face perception (section)

===Traumatic brain injury and neurological illness===
Following brain damage, faces can appear severely distorted. A wide variety of distortions can occur — features can droop, enlarge, become discolored, or the entire face can appear to shift relative to the head. This condition is known as [[prosopometamorphopsia]] (PMO). In half of the reported cases, distortions are restricted to either the left or the right side of the face, and this form of PMO is called hemi-prosopometamorphopsia (hemi-PMO). Hemi-PMO often results from lesions to the splenium, which connects the right and left hemisphere. In the other half of reported cases, features on both sides of the face appear distorted.<ref>{{cite web |last1=Duchaine |first1=Brad |title=Understanding Prosopometamorphopsia (PMO) |url=https://prosopometamorphopsia.faceblind.org/}}</ref>

Perceiving facial expressions can involve many areas of the brain, and damaging certain parts of the brain can cause specific impairments in one's ability to perceive a face. As stated earlier, research on the impairments caused by [[brain injury]] or [[neurological illness]] has helped develop our understanding of cognitive processes. The study of [[prosopagnosia]] (an impairment in recognizing faces that is usually caused by brain injury) has been particularly helpful in understanding how normal face perception might work. Individuals with prosopagnosia may differ in their abilities to understand faces, and it has been the investigation of these differences which has suggested that several stage theories might be correct.

Brain imaging studies typically show a great deal of activity in an area of the [[temporal lobe]] known as the [[fusiform gyrus]], an area also known to cause prosopagnosia when damaged (particularly when damage occurs on both sides). This evidence has led to a particular interest in this area and it is sometimes referred to as the ''[[fusiform face area]]'' (FFA) for that reason.<ref name="Kanwisher N, McDermott J, Chun MM 1997 4302–11">{{cite journal|last1=Kanwisher|first1=Nancy|last2=McDermott|first2=Josh|last3=Chun|first3=Marvin M.|date=1 June 1997|title=The Fusiform Face Area: A Module in Human Extrastriate Cortex Specialized for Face Perception|journal=The Journal of Neuroscience|volume=17|issue=11|pages=4302–11|doi=10.1523/JNEUROSCI.17-11-04302.1997|pmc=6573547|pmid=9151747}}</ref>

It is important to note that while certain areas of the brain respond selectively to faces, facial processing involves many neural networks which include visual and emotional processing systems. For example, prosopagnosia patients demonstrate [[Neuropsychology|neuropsychological]] support for a specialized face perception mechanism as these people (due to brain damage) have deficits in facial perception, but their cognitive perception of objects remains intact. The [[face inversion effect]] provides behavioral support of a specialized mechanism as people tend to have greater deficits in task performance when prompted to react to an inverted face than to an inverted object.{{citation needed|date=April 2021}}

Electrophysiological support comes from the finding that the N170 and M170 responses tend to be face-specific. Neuro-imaging studies, such as those with [[Positron emission tomography|PET]] and [[fMRI]], have shown support for a specialized facial processing mechanism, as they have identified regions of the [[fusiform gyrus]] that have higher activation during face perception tasks than other visual perception tasks.<ref name=":42" /> Theories about the processes involved in adult face perception have largely come from two sources: research on normal adult face perception and the study of impairments in face perception that are caused by brain injury or neurological illness. Novel [[optical illusion]]s such as the [[flashed face distortion effect]], in which scientific [[phenomenology (psychology)|phenomenology]] outpaces neurological theory, also provide areas for research.

Difficulties in facial emotion processing can also be seen in individuals with traumatic brain injury, in both diffuse axonal injury and focal brain injury.<ref>{{cite journal|last1=Yassin|first1=Walid|last2=Callahan|first2=Brandy L.|last3=Ubukata|first3=Shiho|last4=Sugihara|first4=Genichi|last5=Murai|first5=Toshiya|last6=Ueda|first6=Keita|date=16 April 2017|title=Facial emotion recognition in patients with focal and diffuse axonal injury|journal=Brain Injury|volume=31|issue=5|pages=624–630|doi=10.1080/02699052.2017.1285052|pmid=28350176|s2cid=4488184}}</ref>