Wednesday, September 11, 2024
9/11/2024
In natural vision, objects change appearance over time as they translate, rotate, become occluded or undergo complex transformations, e.g., during biological motion. In these dynamic environments, the visual cortex integrates information over multiple spatial and temporal scales to compute motion trajectories and represent the shape of objects. To understand how form and motion percepts are derived from such dynamic visual input, we will investigate how neuronal responses in area V4—an intermediate stage along the ventral visual pathway—are shaped by the spatiotemporal integration of time-varying visual stimuli. We will test the hypothesis that the spatiotemporal characteristics of V4 neurons are suited for tracking dynamic objects: specifically, V4 motion signals arise from object-tracking over longer spatiotemporal windows than comparable dorsal-stream areas and that V4 signals reflect form transformations at an object level rather than at the level of the local retinal image. We will leverage the percept of long-range apparent motion to probe the role of V4 neurons in motion perception (Aim 1). When a stimulus intermittently skips across the visual field with large spatial and temporal steps, it induces a strong illusory motion percept but neurons in V1 and MT of the dorsal visual stream are strikingly insensitive to the direction of the perceived motion. Psychophysical studies have argued that long-range apparent motion relies not on the dorsal stream but on higher order object tracking processes with large spatiotemporal windows in the ventral visual stream. We will conduct the first neurophysiological investigations in the awake monkey to ascertain the role of V4 in the perception of long- range apparent motion. Next (Aim 2), we will use dynamic stimuli that rotate in the fronto-parallel plane, and translate and rotate in depth, to determine whether V4 neurons encode other common dynamic object transformations (beyond long-range translation), and whether the encoding is based on a sequence of static poses, as in the inferotemporal (IT) cortex, or dynamic transformations. Finally, we will examine the encoding and perception of partially occluded dynamic objects (Aim 3). When an occluded object moves, different parts of the object are revealed over time and integration across time and multiple neuronal receptive fields is required to build an entire object representation. As animals discriminate moving occluded objects, we will study 50-100 neurons with high-density Neuropixels probes. We will use single trial population decoding methods to determine how dynamic stimulus information is integrated across the V4 network to extract object shape and the motion trajectory and how V4 contributes to psychophysical behavior. We anticipate that our results will reveal an important role for V4 in the processing of dynamic stimuli that is complementary to those of MT and IT cortex and will establish the level of internal visual representation operating in V4. Our studies will provide a deeper understanding of the neuronal basis of global motion perception and the tracking of dynamic objects—processes that are impaired in aging populations, especially those with Alzheimer’s disease.
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