Show a baby a toy, then cover the toy with a blanket. How does the baby know it’s still there? For that matter, how do any of us remember where things are without gazing directly at them? Psychologists call this cognitive feat object permanence, and this year a group at Princeton got a look at part of the answer, in monkeys at least. They found that some neurons not only encode information about an object’s location but can even retain a short-term memory of it.
Neuroscientist Michael Graziano and his colleagues monitored the electrical activity of individual neurons in the pre-motor cortex, a brain region involved in planning and executing movement. In an earlier experiment they had learned that individual neurons in a particular neighborhood of the premotor cortex respond to both visual and tactile stimuli: they fire rapidly when an object is looming within ten inches or so of a monkey’s face and also when it touches the face, even if it is out of sight. These neurons’ overarching job, we think, is to say, ‘There is an object on or near this particular part of the body,’ says Graziano. This past year Graziano and his colleagues showed that those neurons, even individual ones, can briefly store information about an object’s whereabouts. The researchers put a monkey in the dark, placed a bar within ten inches of the left side of its face, and illuminated the bar for a split second. As expected, a particular premotor cortex neuron began responding when the bar first came into view—but it continued firing rapidly for five seconds after the bar disappeared in darkness. If during that interval Graziano turned the monkey’s head, putting the bar near the right side of the face, the neuron stopped responding—only to resume its rapid firing when the monkey’s face was turned back to the starting point. The neuron was in effect remembering not only the presence but the location of the bar in relation to the face. It was responsible only for the left side.
Humans and monkeys alike also form longer-term memories of familiar terrain. But Graziano’s experiments suggest where we get our crucial ability to grab a cup of coffee without looking and without sending it flying across the desk: we owe it to individual neurons in our pre-motor cortex, which form short-lived maps of particular chunks of nearby space. When we look around, Graziano says, we know where objects are located, and we feel that there must be one map in our brain that keeps track of everything. But what’s coming out is that there are a lot of subregions and modules keeping track of parts of space. Spatial processing seems to be broken up in a kind of divide-and-conquer method.