Wednesday, September 19, 2012

Blindsight, disparity, & 3D vision

I came across two very interesting blog posts on 3D Vision. The 1st blog post I came across, “3D Vision and the Brain”, is posted on Psychology Today’s online website. The 2nd blog post, “Reflections on the Growth of 3D”, is the actual blog of Dr. Barry B. Sandrew. Dr. Sandrew is the neuroscientist/3D filmmaker who is actually interviewed in the 1st blog post. (I will forewarn you that the 2nd blog post essentially has the same information that the 1st blog post offers, perhaps in a slightly greater detail.) However, in both blog posts Dr. Sandrew explains how a 3D image is perceived and also talks about a condition called blindsight.

Dr. Sandrew explains, “Some neuroscientists claim that blindsight is simply the result of tiny islands of spared visual cortex that are no longer fully connected as they would be in the intact brain. Others [say]… that there are visual pathways, separate from those projecting to commonly known parts of the brain that mediate a type of vision falling below the radar of conscious visual perception.” In either case, blindsight patients, who have severe damage to their primary visual cortex are said to be able to still see “emotionally charged objects”. Patients claim they can’t see anything but they still display physiological responses to some stimuli, showing they have emotional recognition of objects. In efforts to explain how blindsight patients can still “see” without their visual cortex, research has yielded some findings. “We now know that information from our two eyes [travels] the brain along many different routes including pathways destined for the more primitive parts of the brain called the limbic system that mediate emotional responses evoked by threatening visual stimuli, often in the absence of conscious awareness.” This may explain how we respond to threatening visual stimuli in a survival situation, such as moving out the way from an unexpected fast baseball traveling 90mph.

What does this have to do with 3D vision? Well, it turns out that “emotionally charged visual information travels through the brain via several different pathways, some of which do not include the primary visual cortex and conscious awareness.”  Two of these pathways are called the Parvocellular Pathway and the Magnocellular Pathway. The Parvocellular Pathway is also referred to as the “high road”. The high road is said to send data-rich visual information (color, texture, shape, etc.) that we perceive consciously to other parts of the brain such as the visual cortex, while the Magnocellular Pathway, or the “low road”, sends sparse visual information (location, direction, speed, etc.) about where something is in spatial relation to our body to primal neural structures deep within the brain. The low road is thought to operate in the absence of conscious awareness. Moreover, an area lying deep within the brainstem called the superior colliculus receives low road information. The superior colliculus is known to control gaze, head turning and can subconsciously trigger rapid head & eye movement. “Significantly, the majority of brain cells in the superior colliculus are specifically tuned to react to variations in the disparity coming from our two eyes.”

Dr. Sandrew explains it is this disparity, or “separation of our eyes and our ability to converge them toward an object that gives us a true sense of depth and volume”. “These disparity sensitive neurons in the superior colliculus subsequently send processed disparity information directly to the amygdala in the limbic system as well as to other primitive brain structures that have been well established as modulating emotion.” If you may recall from other psychology courses, the amygdala is associated with emotional responding and is known to “trigger behavioral and autonomic responses (visceral responses below the level of consciousness) that evoke flight or fight reactions such as instantly ducking or moving out of harms way in response to threatening visual stimuli.” According to Dr. Sandrew, disparity, which 2D movies lack, is the “most powerful 3D influence on the brain”. Our mind’s interpretation of 3D visual stimuli due to disparity is what causes us to perceive 3D visual stimuli as 3D and not as 2D visual images.


 "3D Vision and the Brain"-
“Reflections on the Growth of 3D”-

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