Summary: Researchers have identified a new functional type of cell in the medial temporal lobe that may play an essential role in visual search.
Source: West Virginia University.
Every day, people are asked to find something – a familiar face in a crowd, a child in the park, a particular house on a street. While researchers have long-since known that the ability to effectively search and detect goal-relevant targets is controlled by top-down signals from the brain’s frontal area, a researcher from West Virginia University has found evidence that the human medial temporal lobe – or MTL – also plays an essential role in this process.
In a report recently published in “Current Biology,” Shuo Wang, assistant professor of chemical and biomedical engineering, has found that the MTL contains a strikingly functional type of cell never described before in humans: target cells.
Wang and his collaborators – Ueli Rutishauser and Adam Mamelak from Cedars-Sinai Medical Center and Ralph Adolphs from California Institute of Technology – took the very rare opportunity to record single neurons from epilepsy patients, who were undergoing seizure monitoring at Cedars-Sinai. They performed concurrent recordings of eye movements and single neurons in the MTL and medial frontal cortex – or MFC – in human neurosurgical patients performing a memory-guided visual search task.
“During goal-directed visual search, these target cells signal whether the currently fixated item is the target of the current search,” Wang said. “This target signal was behaviorally relevant because it predicted whether a subject detected or missed a fixated target, i.e. failed to abort the search.
Wang was surprised to find that these target cells were not visually tuned; they didn’t care about the content of the target but only whether an item is a target or not. Interestingly, their response to identical items could be different, depending on whether an item was the target of the search or not.
“This type of response is fundamentally different from that observed in upstream areas to the MTL, i.e. the inferior temporal cortex, where cells are visually tuned and are only modulated by target presence or absence on top of this visual tuning,” Wang said. “The discovery of this novel type of cell in the MTL, in humans, shows direct evidence for a specific top-down goal-relevance signal in the MTL.”
The study also found that target cells in the MFC respond significantly earlier relative to target cells in the MTL.
“This latency difference was derived from simultaneously recorded target cells in both areas, which is a rare opportunity to directly investigate the flow of information of top-down signals,” Wang said. “This finding suggests that the MFC may be one specific source of top-down signals that specify stimulus meaning in the MTL.”
Wang and his collaborators are conducting further experiments and analysis on how different brain areas coordinate with each other to give rise to target detection.
“This is particularly important for people with autism, because they show impaired visual search, especially when they search for people and faces,” Wang said. “One possibility is that there is pathology within neurons of the MTL itself. Another is that there is abnormal connectivity between the MTL and other brain areas. Our single-neuron recording experiment permits a temporal resolution that will help distinguish these possibilities.”
Funding: This work was supported by the National Institute of Mental Health, the WVU Rockefeller Neuroscience Institute and The Dana Foundation.
Source: Mary C. Dillon – West Virginia University
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is adapted from the West Virginia University news release.
Original Research: Open access research for “Encoding of Target Detection during Visual Search by Single Neurons in the Human Brain” by Shuo Wang, Adam N. Mamelak, Ralph Adolphs, Ueli Rutishauser in Current Biology. Published June 14 2018
Encoding of Target Detection during Visual Search by Single Neurons in the Human Brain
•A new class of neurons in the human MTL signal target detection during visual search
•Response of target neurons is invariant to visual category
•MFC target neurons precede MTL target neurons by ∼200ms
•Target neurons do not depend on explicitly defined targets
Neurons in the primate medial temporal lobe (MTL) respond selectively to visual categories such as faces, contributing to how the brain represents stimulus meaning. However, it remains unknown whether MTL neurons continue to encode stimulus meaning when it changes flexibly as a function of variable task demands imposed by goal-directed behavior. While classically associated with long-term memory, recent lesion and neuroimaging studies show that the MTL also contributes critically to the online guidance of goal-directed behaviors such as visual search. Do such tasks modulate responses of neurons in the MTL, and if so, do their responses mirror bottom-up input from visual cortices or do they reflect more abstract goal-directed properties? To answer these questions, we performed concurrent recordings of eye movements and single neurons in the MTL and medial frontal cortex (MFC) in human neurosurgical patients performing a memory-guided visual search task. We identified a distinct population of target-selective neurons in both the MTL and MFC whose response signaled whether the currently fixated stimulus was a target or distractor. This target-selective response was invariant to visual category and predicted whether a target was detected or missed behaviorally during a given fixation. The response latencies, relative to fixation onset, of MFC target-selective neurons preceded those in the MTL by ∼200 ms, suggesting a frontal origin for the target signal. The human MTL thus represents not only fixed stimulus identity, but also task-specified stimulus relevance due to top-down goal relevance.