Are you wondering why part 3 was relatively brief? Yes, I’ll admit that I had premature publication – got so excited about the topic I hit “Publish” instead of “Preview”! In any event, let’s carry on where we left off, with a beautiful block diagram from the work of Miles’ group published in the Journal of Neuophysiology, Short-Latency Vergence Eye Movements Induced by Radial Optic Flow in Humans: Dependence on Ambient Vergence Level. I’m indebted to Sue Barry for bringing the significance of his work to my attention.
This is a proposed linkage between the translational vestibulo-ocular reflex (TVOR) and the radial-flow vergence mechanism. “HT” represents input from translational velocity of the head and “WT” represents input from translational velocity of the world, that is the visual surroundings. OTO are the otolith organs in the semicircular canals. “d” represents viewing distance, and “k” represents the gain of the VOR. “EV” is the vergence velocity, “HV” is the vergence velocity required to offset the head translation, and “GV” is the gaze vergence velocity error. Although not specified by Miles’ group, the influence of lenses, prisms or optical filters would likely influence input at the level of optical geometry or spatial filters, the latter postulated to be radial flow signals extracted by dorsal stream components of the cortex.
So when our visual system is integrating effortlessly with our vestibular system, we are ideally unaware of the component pieces. However when we are overloaded by either internal factors such as head and body movement, or external factors such as the complexity or clutter of the environment, these process become fragmented. There is improper or inadequate modulation in the transmission of information through sensorimotor pathways. This accentuates the literal and figurative meaning of “going with the flow” – and we’ll have more to say about that later on. Suffice it say that many patients experience significant difficulties with driving at higher speeds, particularly on busy roads, or navigating in crowded malls or down supermarket aisles because of integrative difficulties in managing what Gibson has termed the ambient optic array.
Here is how those of use with effortless eco-optics would perceive the view of a supermarket aisle:
Here’s the experience for someone having difficulty managing optic flow:
To review, here is how Miles would explain it. Centrifugal (expanding) flow, which signals forward motion of the observer, increases the vergence angle between the two eyes, and centripetal (contracting) flow has the converse effect. Such eye movements are appropriate insofar as the vergence angle must change to keep both eyes aligned on the object of regard in the scene ahead. However, the geometry of the situation dictates that vergence angle for binocular alignment is inversely proportional to the viewing distance. As the observer moves forwards or backwards, this vergence angle must change at a rate inversely proportional to the square of the viewing distance. This raises a potential problem when the observer moves through a cluttered environment: any convergence resulting from the optic flow created by nearby items (or people in crowded space) would be inappropriate if the observer is trying to fixate something far ahead. Under normal conditions vergence induced by radial flow would be attenuated with distant viewing, an effect that would help to reduce the impact of nearby clutter in the scene as we look ahead. This attenuation results from the modulation of transmission likely within central pathways that are shared by translational vestibulo-ocular reflexes operating in synergy with radial-flow vergence.
Would it be surprising if patients with unstable vergence and/or poor integration in visual-vestibular pathways had difficulty managing their way? And if they do, what can we do to help?