V2 Detects the Contours of Moving Objects
The figure illustrates how neurons in area V2 can extract contour information of moving objects from optical flow through their unique receptive field properties, thereby aiding object recognition.
In primates, visual motion information is traditionally thought to be processed along the dorsal pathway, while the functional role of V2 in motion processing remains unclear. The lab of Haidong Lu previously discovered that V2 can detect the orientation of motion-defined contours (Chen et al., 2016). To further investigate the mechanisms underlying motion contour detection in V2, they first identified the precise locations of direction-selective neurons in V2 using intrinsic signal optical imaging, then characterized their receptive field properties and responses to motion stimuli using single-electrode recordings.
The results revealed that these direction-selective neurons exhibit distinct receptive field features compared to dorsal pathway neurons, including smaller receptive fields, stronger surround suppression, insensitivity to global motion and speed, but strong responses to motion contrast. Based on these properties, they proposed that V2 processes motion information by extracting motion contrast from optical flow fields, generating a saliency map of motion-defined contours to achieve figure-ground segregation—a critical visual function (Hu et al., 2018).
To further explore V2's role in motion contour perception, the lab conducted multichannel array recordings and found that 10.9% of V2 neurons exhibit orientation selectivity for motion-defined contours (MB neurons). By training macaques to judge the orientation of contours in visual stimuli and analyzing neuronal activity during task performance, they demonstrated that MB neuron responses correlate with the animals' perceptual choices. V2 significantly outperformed primary visual cortex (V1) in these aspects. Finally, by analyzing the functional interactions and response latencies between direction-selective (DS) neurons and MB neurons, they confirmed that DS neurons contribute to motion contour detection in MB neurons (Ma et al., 2021).
This series of studies systematically investigates motion processing in V2 across three levels—single-neuron activity, population activity, and behavior—providing crucial insights into the neural mechanisms underlying visual motion processing.
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