Ensemble Coding of Hidden Objects in Visual Cortex

Many species exhibit the understanding that visual objects that become hidden by others nonetheless still exist, a property known as object permanence. Previous studies in human and nonhuman primates have provided evidence that neurons within visual cortex encode objects that are remembered but not seen. However, past neurophysiological studies have generally failed to find evidence of visual cortical representations of hidden objects. We measured the activity of large populations of neurons within dorsal extrastriate cortex of macaques trained to monitor the identity of visual objects that moved behind an irrelevant occluder. We found that although the firing rates of neuronal populations signaled the trajectory of hidden objects throughout the occlusion period, coding of object identity in the same activity decayed to chance before the behavioral trial ended. Nevertheless, information about the hidden object was present in the coordinated activity of neuronal populations. Specifically, the strength and presence of pairwise cross-correlations reliably depended on the identity of the hidden object. These results demonstrate that ensembles of visual cortical neurons preserve information about hidden objects independent of single neuron firing rates.