As visual information flows into the brain through the retina; the visual cortex transforms the sensory input into coherent perceptions. Neuroscientists have long hypothesized that a part of the visual cortex called the inferotemporal (IT) cortex is necessary for the key task of recognizing individual objects, but the evidence has been inconclusive. In a new study, MIT neuroscientists have found clear evidence that the IT cortex is indeed required for object recognition; they also found that subsets of this region are responsible for distinguishing different objects.
The neurons transform
In addition, the researchers have developed computational models that describe how these neurons transform visual input into a mental representation of an object. They hope such models will eventually help guide the development of brain-machine interfaces (BMIs); that could used for applications such as generating images in the mind of a blind person.
In addition to its hypothesized role in object recognition; the IT cortex also contains “patches” of neurons that respond preferentially to faces. Beginning in the 1960s, neuroscientists discovered that damage to the IT cortex could produce impairments in recognizing non-face objects, but it has been difficult to determine precisely how important the IT cortex is for this task.
The IT cortex’s role
The MIT team set out to find more definitive evidence for the IT cortex’s role in object recognition; by selectively shutting off neural activity in very small areas of the cortex and then measuring how the disruption affected an object discrimination task. In animals that had trained to distinguish between objects such as elephants; bears, and chairs, they used drug muscimol to temporarily turn off subregions about 2 millimeters in diameter. Each of these subregions represents about 5 % of the entire IT cortex.
The researchers found that silencing each of these tiny patches produced distinctive impairments in the animals’ ability to distinguish between certain objects. For example, one subregion might involved in distinguishing chairs from cars; but not chairs from dogs. Each region was involved in 25 to 30 % of the tasks that the researchers tested; regions that closer to each other tended to have more overlap between their functions; while regions far away from each other had little overlap.
“It would incorrect to say that because they observed a deficit in distinguishing cars when a certain neuron was inhibited; this is a ‘car neuron. “Instead, the cell is responding to a feature that they can not explain that is useful for car discriminations. There has work in this lab and others that suggest that the neurons are responding to complicated nonlinear features of the input image.
Neural activity in the IT
Bevil Conway, a principal investigator at the National Eye Institute; says the new study makes significant progress toward answering the critical question of how neural activity in the IT cortex produces behavior. “The paper makes a major step in advancing our understanding of this connection; by showing that blocking activity in different small local regions of IT has a different selective deficit on visual discrimination.
This work advances our knowledge not only of the causal link between neural activity and behavior but also of the functional organization of IT: How this bit of brain is laid out,” says Conway, who was not involved in the research. The experimental results were consistent with computational models that DiCarlo; Rajalingham, and others in their lab have created to try to explain how IT cortex neuron activity produces specific behaviors.
The researchers plan to continue refining their models; incorporating new experimental data from even smaller populations of neurons, in hopes of developing ways to generate visual perception; in a person’s brain by activating a specific sequence of neuronal activity. Technology to deliver this kind of input to a person’s brain could lead to new strategies to help blind people see certain objects.