Neuronal encoding of associations between visual stimuli during learning

Through experience we acquire an understanding of the relations between objects in our environment. This thesis concerns how brain activity is altered when an association between two objects is learned. Two macaque monkeys were trained to associate pairs of visually unrelated images. Presentation of one stimulus in a pair instructed the monkeys to select the other stimulus in that pair. New stimuli were introduced each day and the monkeys had to determine the correct pairings by trial and error. The behavioral performance of both animals improved over a few hours of training, showing for the first time that monkeys learned to identify that two stimuli were associated in a single day.;As the monkeys learned the new stimulus-stimulus associations, I recorded the spiking activity of neurons in the anterior inferior temporal cortex. Two thirds of the recorded neurons had significant changes in their responses to at least one visual stimulus over the several hours of a typical recording session. This is the first direct observation of changes in neuronal activity related to the formation of associations between stimuli. Thus, the results show that, even in adulthood, neuronal activity is subject to modification by learning.;For about two thirds of the recorded neurons, responses to associated images became more alike over the recording sessions. Initially, neuronal responses to paired stimuli were on average no more similar than responses to unpaired stimuli. By the end of the recording sessions, responses to paired stimuli had become significantly more similar than responses to unpaired stimuli. These similarities in neuronal responses to associated stimuli arose on the same time scale as the stimulus pairings were learned. The development of these similarities was also significantly dependent on successful associative learning.;I conclude, therefore, that associations between visual objects are encoded in the anterior inferior temporal cortex and that the development of similar responses to paired objects provided the neuronal basis for learning these associations. I have demonstrated one way in which the brain, in addition to representing what objects are present and where they are, can represent how these objects are related to one another.