Abstract
Many people have the subjective sense of being able to see more than one object at a time. However, given the large receptive fields of neurons in the later stages of the ventral visual pathway, it is unclear how two similar objects could be perceived without interfering with each other. It has been proposed that the concurrent perception of multiple objects is illusory or only explicable through mechanisms such as neural synchrony (von der Malsburg, 1999). Counter to these proposals, we develop a neural network model of object recognition capable of identifying two objects at a time given only the addition of a dorsal attentional component. This mechanism is consistent with findings from Balint's patients, multi-object tracking, and change detection tasks supporting a role for posterior parietal cortex in the perception of multiple objects.
Our model consists of a ventral pathway, trained to identify objects, and a dorsal pathway, trained to transform visual inputs into potential actions. Dorsal activity emergently represented both object locations and features, consistent with studies that key parietal regions may code for certain object characteristics (e.g. Konen & Kastner, 2008). With the dorsal and ventral pathways connected during training, the network learns to utilize dorsal signals to bias ventral activity towards the correct objects while suppressing errors, allowing the correct simultaneous identification of two objects. We simulate data from illusory conjunction experiments wherein, when two objects are presented briefly, subjects often erroneously report an object which miscombines the features of the actual objects. Simulated dorsal lesions impaired the identification of two objects, with recovery of double-object identification following a similar trajectory as Balint's patient R.M. In contrast, simulated ventral lesions disrupt object identification but not dorsal functions, similar to visual form agnosia patient D.F., whose ventral pathway damage allowed her to manipulate objects she was unable to identify.