From across a crowded room, a familiar face leaps out instantaneously. This lightening-fast recognition may come courtesy of a small patch of neurons in the temporal pole—at least in monkeys. According to a paper in today’s Science, these neurons fire when a monkey views a face of another with which they’ve shared personal encounters, but not when viewing a “famous monkey” they have merely seen on screen. Led by Winrich Freiwald at The Rockefeller University, New York City, the study uncovers specialized circuitry that may explain the uncanny speed with which familiar faces are recognized.
- Neurons in the temporal pole fire fast when a monkey sees a picture of a familiar monkey’s face.
- These neurons stay mum in response to pictures of unfamiliar monkey faces—or human faces.
- The cells fire with surprising speed, suggesting a specialized pathway for familiar face recognition.
It also could shed light on prosopagnosia, a socially debilitating disorder in which a person cannot recognize faces of people they know. This symptom crops up in people with neurodegenerative diseases, including frontotemporal degeneration and Alzheimer’s. No longer being recognized by a parent with dementia is among the saddest experiences for caregivers.
The process of noticing a familiar face involves the rapid integration of sensory perception with stored memories associated with the face. A network of “face areas” is scattered throughout the superior temporal sulcus and inferotemporal (IT) cortex (Moeller et al., 2008; Freiwald and Tsao, 2010). Within the IT, the anteromedial (AM) face area is thought to be the crown jewel of this core face processing network.
This network has been well-described for encoding unfamiliar faces. However, Freiwald’s group previously singled out groups of neurons in two other regions that fired only in response to well-known faces. Using functional magnetic resonance imaging to observe which regions fired as a monkey observed images, the researchers reported that while regions such as the AM lit up in response to any face, neurons in the temporal pole and the perirhinal cortex fired only in response to familiar faces (Landi and Freiwald, 2017). Could these neurons handle familiar face recognition?
For the current study, first author Sofia Landi and colleagues took a closer look at how neurons in the temporal pole (TP) might facilitate rapid recognition of a familiar face. They used electrophysiological probes to monitor individual neuronal responses in the TP and the AM in two rhesus macaques, which they showed a series of 205 images of familiar or unfamiliar faces, bodies, and objects. They flashed 30 familiar and 30 unfamiliar human faces; 12 familiar and 72 unfamiliar monkey faces; as well as the monkey’s own face. The monkeys had met the familiar monkeys and humans in person many times, while they had seen images of the unfamiliar monkeys and people on screen before. In this way, the unfamiliar monkeys were more akin to faces of famous people commonly used in face-name recognition tasks. As the monkeys looked at the images, the researchers monitored the activity of 98 neurons in their TP and 130 neurons in their AM.
Neurons in the TP responded with remarkable specificity to familiar monkey faces. While some TP neurons lit up in only response to a single familiar face, others responded to multiple familiar faces. Conversely, most TP neurons remained silent when the monkey gazed upon a human face, regardless of personal familiarity. Familiar monkey faces evoked triple the response as unfamiliar faces in the TP population. In contrast, neurons in the AM fired in response to all faces, known or unknown, monkey or person.
Zeroing in on the particulars, the researchers found that the temporal pole neurons responded holistically to the face, rather than ramping up their responses linearly as more of the face was revealed. The inner face—eyes, nose, and mouth—was sufficient to evoke the response, and viewing the entire face or the rest of body did little to enhance it.
Finally, the researchers clocked the neurons’ responses. To their surprise, they found that temporal pole neurons spiked in response to familiar faces within 100 milliseconds, just as fast as AM neurons reacted to any face. This suggested that rather than lying downstream of the AM within the core facial recognition network, the TP neurons fired in response to familiar faces in parallel—and with astonishing speed. The findings cast these specialized cells as a new class of face memory neuron.
The authors propose two parallel pathways of face memory. The first one—involving the AM, perirhinal face area, entorhinal cortex, and hippocampus—creates new associations between fresh faces and person identities. The second—involving neurons in the temporal pole—would grant quick, direct access to familiar face information. “Our findings might explain the rush of recognition that comes with seeing a familiar face,” Landi said. Notably, regions involved in both pathways are hit hard by Alzheimer's disease.
How many times must a face be seen, and in what context, before these fast-track temporal pole neurons encode it? This is one of many questions Landi, now a postdoc at the University of Washington in Seattle, is investigating. She also wants to know how this initial burst of recognition links up with other associations about the person behind the face. For example, person concept cells—dubbed “Jennifer Aniston neurons”—react to famous faces and even to familiar objects such as the Eiffel Tower (Quiroga et al., 2005). These cells react on a much slower timescale than the temporal pole neurons, and Landi wonders if the temporal pole neurons influence their activity.
How might these familiar face neurons relate to symptoms that surface in neurodegenerative disease? In people with frontotemporal dementia, prosopagnosia has been tied to atrophy in the right temporal lobe, particularly the medial temporal lobe, fusiform gyrus, and anterior temporal pole (Josephs et al., 2008; Erkoyun et al., 2020). People with AD also lose their ability to recognize people they know, a deficit ascribed to loss of both memory and visual perception (Mazzi et al., 2020). These different manifestations of facial recognition problems could reflect which parts of the brain are damaged in the two disorders, Landi said.
Landi’s study drew a distinction between the recognition of personally familiar faces versus those only viewed on a screen. Face-name recognition tests are widely used in Alzheimer’s research, but they tend to use famous faces as proxies for familiar ones, and hence might miss dysfunction of these temporal pole neurons, she said. To tap these cells, perhaps these tests could be personally tailored to include pictures of people a person knows in real life, she said.
Besides face cells, other specialized neurons such as hippocampal “place cells,” which are crucial for spatial memory, may also degenerate in AD (May 2008 news).—Jessica Shugart
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No Available Further Reading
- Landi SM, Viswanathan P, Serene S, Freiwald WA. A fast link between face perception and memory in the temporal pole. Science. 2021 Jul 30;373(6554):581-585. Epub 2021 Jul 1 PubMed.