30 August 2017
Juntendo University Corporate Communications
(Tokyo, 28 August 2017) Cognitive neuroscience research on non-human primates demonstrated that judgments of whether humans have previously seen or not seen an object are causally determined by neuronal signals generated in the temporal cortex of the brain. The findings are published in Science (August 2017).
“We daily encounter hundreds of visual objects and instantaneously evaluate whether they are safe or dangerous, friendly or hostile, delicious or toxic, or never seen before,” explain Yasushi Miyashita and colleagues at the University of Tokyo School of Medicine and Juntendo University Graduate School of Medicine. While previous studies had shown that visual object memory is related to an area of the brain known as the perirhinal cortex, it was not understood whether and how the neurons in this area causally function in such evaluation (Fig.1).
In early sensory areas, neurons that respond to visual stimuli with similar physical attributes - such as orientation or direction of motion - are found spatially clustered together. In contrast, nearby perirhinal neurons respond to specific memorized objects that can be physically distinct, raising a question whether nearby perirhinal neurons simply convey distinct physical attributes of objects, or convey any non-physical attribute commonly represented among the neurons.
To understand whether and how perirhinal neurons causally function in the recognition of objects, Miyashita and colleagues investigated the behavioral impact of optogenetic and electrical stimulation of the perirhinal cortex in macaque monkeys. In the region where neurons responded to the presentation of previously learnt, that is, OLD objects, both optogenetic and electrical stimulation of neurons increased the probability of the presented objects being deemed OLD (OLD choices). In an adjacent region where neurons did not respond to the OLD objects, electrical stimulation decreased the probability of OLD choices while optogenetic stimulation still increased the probability of OLD choices.
These results suggest that perirhinal neurons do convey a non-physical attribute informing that the encountered object was learned in the past. The researchers propose a possible causal model for the object recognition based on the existence of neurons that selectively respond to specific memorized objects. In that model, the object viewed is deemed old when the activity of these neurons was elevated to reach a certain threshold, while the object is deemed new when the activity failed to reach the threshold.