Spatial arrangement drastically changes the interaction between visual stimuli that compete in multiple feature domains in extrastriate area MT

Natural scenes often contain multiple objects. However, how neurons in the visual cortex represent multiple visual stimuli within their receptive fields (RFs) is not well understood. Previous studies have shown that, when multiple stimuli compete in one feature domain, the evoked neuronal response is dominated by the stimulus component that has a stronger signal strength, which can be explained by response normalization. Here we investigate how neurons in middle-temporal (MT) cortex of the macaque monkey represent multiple stimuli that compete in more than one feature domain within their RFs. Visual stimuli were two random-dot patches moving in different directions. One stimulus moved at high coherence with low luminance contrast, whereas the other moved at low coherence with high contrast. We found that, although the peak MT response elicited by the "low contrast & high coherence" stimulus alone was stronger than by the "high contrast & low coherence" stimulus, MT response to both stimuli when they were overlapping was almost completely dominated by the high-contrast stimulus. When two stimuli were spatially separated within the RF, the contrast dominance was abolished. We found the same results when using contrast to compete with motion speed. Computer simulations using a V1-MT model suggest that the contrast dominance is due to normalization occurring at input stage fed to MT and MT neurons cannot overturn it according to their own feature selectivity. Our results reveal new rules on stimulus competition and highlight the importance of hierarchical processing on the neural representation of multiple visual stimuli in the extrastriate cortex.