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.”
This work was supported by the National Institute of Mental Health, the WVU Rockefeller Neuroscience Institute and The Dana Foundation.
