Connecting structure and function in early visual circuits
How does the brain interpret signals from the outside world? Walking through a park, you might take for granted the ease with which you can understand what you see. Rather than seeing a series of still snapshots, you are able to see simple, fluid movement — of dogs running, squirrels foraging, or kids playing basketball. You can track their paths and know where they are headed without much thought. “How does this process take place?” asks Rudy Behnia, PhD, a principal investigator at Columbia’s Mortimer B. Zuckerman Mind Brain Behavior Institute. “For most of us, it’s hard to imagine a world where we can’t see motion, shapes, and color; where we can’t have a representation of the physical world in our head.” And yet this representation does not happen automatically — our brain has no direct connection with the outside world. Instead, it interprets information taken in by our senses. Dr. Behnia is studying how the brain builds these representations. As a starting point, she focuses on how we see motion
Looking and listening while moving
In this talk I’ll discuss our recent work on how visual and auditory cues to space are integrated as we move. There are at least 3 reasons why this turns out to be a difficult problem for the brain to solve (and us to understand!). First, vision and hearing start off in different coordinates (eye-centred vs head-centred), so they need a common reference frame in which to communicate. By preventing eye and head movements, this problem has been neatly sidestepped in the literature, yet self-movement is the norm. Second, self-movement creates visual and auditory image motion. Correct interpretation therefore requires some form of compensation. Third, vision and hearing encode motion in very different ways: vision contains dedicated motion detectors sensitive to speed, whereas hearing does not. We propose that some (all?) of these problems could be solved by considering the perception of audiovisual space as the integration of separate body-centred visual and auditory cues, the latter formed by integrating image motion with motor system signals and vestibular information. To test this claim, we use a classic cue integration framework, modified to account for cues that are biased and partially correlated. We find good evidence for the model based on simple judgements of audiovisual motion within a circular array of speakers and LEDs that surround the participant while they execute self-controlled head movement.
The role of motion in localizing objects
Everything we see has a location. We know where things are before we know what they are. But how do we know where things are? Receptive fields in the visual system specify location but neural delays lead to serious errors whenever targets or eyes are moving. Motion may be the problem here but motion can also be the solution, correcting for the effects of delays and eye movements. To demonstrate this, I will present results from three motion illusions where perceived location differs radically from physical location. These help understand how and where position is coded. We first look at the effects of a target’s simple forward motion on its perceived location. Second, we look at perceived location of a target that has internal motion as well as forward motion. The two directions combine to produce an illusory path. This “double-drift” illusion strongly affects perceived position but, surprisingly, not eye movements or attention. Even more surprising, fMRI shows that the shifted percept does not emerge in the visual cortex but is seen instead in the frontal lobes. Finally, we report that a moving frame also shifts the perceived positions of dots flashed within it. Participants report the dot positions relative to the frame, as if the frame were not moving. These frame-induced position effects suggest a link to visual stability where we see a steady world despite massive displacements during saccades. These motion-based effects on perceived location lead to new insights concerning how and where position is coded in the brain.
The developing visual brain – answers and questions
We will start our talk with a short video of our research, illustrating methods (some old and new) and findings that have provided our current understanding of how visual capabilities develop in infancy and early childhood. However, our research poses some outstanding questions. We will briefly discuss three issues, which are linked by a common focus on the development of visual attentional processing: (1) How do recurrent cortical loops contribute to development? Cortical selectivity (e.g., to orientation, motion, and binocular disparity) develops in the early months of life. However, these systems are not purely feedforward but depend on parallel pathways, with recurrent feedback loops playing a critical role. The development of diverse networks, particularly for motion processing, may explain changes in dynamic responses and resolve developmental data obtained with different methodologies. One possible role for these loops is in top-down attentional control of visual processing. (2) Why do hyperopic infants become strabismic (cross-eyes)? Binocular interaction is a particularly sensitive area of development. Standard clinical accounts suppose that long-sighted (hyperopic) refractive errors require accommodative effort, putting stress on the accommodation-convergence link that leads to its breakdown and strabismus. Our large-scale population screening studies of 9-month infants question this: hyperopic infants are at higher risk of strabismus and impaired vision (amblyopia and impaired attention) but these hyperopic infants often under- rather than over-accommodate. This poor accommodation may reflect poor early attention processing, possibly a ‘soft sign’ of subtle cerebral dysfunction. (3) What do many neurodevelopmental disorders have in common? Despite similar cognitive demands, global motion perception is much more impaired than global static form across diverse neurodevelopmental disorders including Down and Williams Syndromes, Fragile-X, Autism, children with premature birth and infants with perinatal brain injury. These deficits in motion processing are associated with deficits in other dorsal stream functions such as visuo-motor co-ordination and attentional control, a cluster we have called ‘dorsal stream vulnerability’. However, our neuroimaging measures related to motion coherence in typically developing children suggest that the critical areas for individual differences in global motion sensitivity are not early motion-processing areas such as V5/MT, but downstream parietal and frontal areas for decision processes on motion signals. Although these brain networks may also underlie attentional and visuo-motor deficits , we still do not know when and how these deficits differ across different disorders and between individual children. Answering these questions provide necessary steps, not only increasing our scientific understanding of human visual brain development, but also in designing appropriate interventions to help each child achieve their full potential.