WOW, 3-D! :D

by Michelle T. de los Santos



"Whoaaa! Inception and Despicable Me in 3-D, the best! Awesome okay! :))" -Mitch

We live in a three-dimensional world, but our eyes only receive two-dimensional images. We then thought of the question "How does our brain combine these images into a 3D percept?". We can't deny that most of us have a great appreciation for 3-D structure of scenes, that's why we like, love, and prefer to watch movies in IMAX 3-D right?! That's why nowadays technologies are continuously evolving. They even created 3-D TV's for better appreciation of scenes. Since our current topic in my Psych 135 course, sensation and perception, is about physiology of perception, I was inspired to look at the neurological or physiological structure of this 3D - visual experience -- the physiology of depth perception.

The three-dimensional (3-D) structures that we see have something to do with our judgment of depth. Certainly, our eyes are so incredible that they enable us to see wonderful things in life. Researchers explained that humans could make precise judgment of depth because of motion parallax. Motion parallax is the relative retinal image motion between objects at different distances, that is whether an object is near or far relative to the plane of fixation. See figure below. However, motion parallax alone is not enough to specify the sign of depth, rather the direction of image motion relative to observer motion is important (Nadler, Angelaki, and DeAngelis, 2008).

Our judgments of depth can be referred as depth perception. I agree when the researchers mentioned that perception of depth is a fundamental challenge for the visual system, particularly for observers moving through their environment. I learned in the journal that in order to reconstruct the three-dimensional structure of a scene (3-D), the brain makes use of multiple visual cues. As I mentioned above, one potent cue that they studied was motion parallax, which frequently arises during translation of the observer because the images of objects at different distances move across the retina with different velocities. Under some conditions, the researchers explained that the brain could interpret such ambiguous visual motion by using other cues such as occlusion, size or perspective. Human psychophysical studies have demonstrated that motion parallax can certainly be a powerful depth cue, and as they mentioned, motion parallax seems to be heavily exploited by animal species that lack highly developed binocular vision.

The researchers recorded extracellular single-unit activity from middle temporal area (area MT), the visual area of the brain, by using tungsten microelectrodes (FHC) in two adult rhesus monkeys (Macaca mulatta). The following are the summarized method of the researchers: A custom-made virtual-reality system was used to provide stimuli consisting of sinusoidal translation and/or visual motion. In addition, it displays that simulated objects at different depths, that many neurons in the middle temporal area (area MT) signal the sign of depth (near versus far) from motion parallax in the absence of other depth cues. Animals were trained to maintain fixation on a visual target during translation of the motion platform. Custom-written OpenGL software was used to generate visual stimuli that depicted a random-dot surface at one of several possible depths in a virtual environment. The visual stimulus was viewed monocularly by the animal, and all pictorial depth cues were removed from the stimulus to render the visual motion ambiguous with respect to depth sign. Thus, to compute depth sign based on motion parallax, neurons needed to combine visual motion with extraretinal signals generated by physical translation of the animal (for example, vestibular or eye movement signals).

The experimental design of Nadler, Angelaki, and DeAngelis (2008) compared neural responses in two conditions. 1 – A Motion Parallax (MP) condition in which the combination of physical translation and visual motion specified depth unambiguously. 2 – Retinal Motion (RM) control condition in which the same visual motion stimulus was presented in the absence of extra-retinal signals, thus rendering it ambiguous with respect to depth sign. Neuronal responses were measured as mean firing rates, and the significance of depth-sign selectivity was assessed with permutation tests. Eye position data were filtered (200 Hz lowpass) and analysed to quantify the accuracy of pursuit eye movements.

After reading their method, I felt like whoaaa! Well, I was amazed by their procedures because it seems complicated but they were able to do it as organized and detailed as possible that’s why I like their method. They were able to show the physiology of perception, from stimulus to the brain, and how will the brain process the stimuli/signals. Although, not all can understand their experimental design but no worries that’s expected. In addition, all their procedures were approved by the Institutional Animal Care and Use Committee at Washington University and were in accordance with National Institutes of Health guidelines. Hence, their experimental design will not be in trouble, I hope!:)

The findings of the study come up with the mechanism that generates depth-sign selectivity in area MT. The researchers mentioned that it might possibility be an extra-retinal signal related to head or eye movement simply sums with responses to visual motion, thus enhancing responses to one depth sign (for example, near) and suppressing responses to the opposite depth sign. The data of their study suggest that the mechanism is more complicated, as they have shown that single neurons in area MT carry reliable information about depth sign from motion parallax. Therefore, their findings provide evidence for a neural substrate for this perceptual capacity. Although they mentioned that they cannot yet prove that these signals in area MT are used by the monkey to perceive depth (they might reflect extra-retinal signals used for another purpose), but their findings enable a direct causal test in trained animals. “Such proof notwithstanding, results of the study establish a new potential neural mechanism for processing depth information and suggest that area MT may be involved in integrating multiple cues to depth.” - Nadler, Angelaki, and DeAngelis (2008)

To improve this study, one way is to increase the sample size because they only use two participants, which is not enough to make reliable results. Humans are also better participants so that the study can be more satisfying, although using humans in such experimental design needs considerations. Researchers can also use neuron-imaging (fMRI) to understand the mechanisms underlying visual motion and depth perception better. Researches also need to prove the signals in area MT used in the study if it's really the one used to perceive depth.

Indeed, the study requires to have at least basic knowledge about physiology of perception to somewhat recognize and understand some terms and concepts in the journal. Even I, a Psychology major, cannot understand some of the words and concept in the journal, well, honestly, for me the article was a bit hard to digest and it's complicated. Basically, I just showed a neural representation of depth from motion parallax in middle temporal area or the visual cortex of the brain; the neural circuits underlying the perception of 3D motion. Depth perception are certainly the best! With that, I just want everyone to appreciate and understand how magnificent our brains are and how our brain combine images into a 3-D percept, that enable us to see and enjoy our three-dimensional world to the fullest. Here, see it for yourself! :)

Reference:
Nadler, J.W., Angelaki, D.E., and DeAngelis, G.C. (2008). A neural representation of depth from motion parallax in macaque visual cortex. Nature 452(7187), 642-645.