Visual perception plays a critical role in navigating 3D space and extracting semantic information crucial to survival. Even though visual stimulation on the retina is fundamentally 2D, we seem to perceive the world around us in vivid 3D effortlessly. This reconstructed 3D space is allocentric and faithfully represents the external 3D world. How can we recreate stable 3D visual space so promptly and reliably? To solve this mystery, we have developed new concepts MePMoS (Memory-Prediction-Motion-Sensing) and NHT (Neural Holography Tomography). These models state that visual signal processing must be primarily top-down, starting from memory and prediction. Our brains predict and construct the expected 3D space holographically using traveling alpha brainwaves. Thus, 3D space is represented by the three time signals in three directions. To test this hypothesis, we designed reaction time (RT) experiments to observe predicted space-to-time conversion, especially as a function of distance. We placed LED strips on a horizontal plane to cover distances from close up to 2.5 m or 5 m, either using a 1D or a 2D lattice. Participants were instructed to promptly report observed LED patterns at various distances. As expected, stimulation at the fixation cue location always gave the fastest RT. Additional RT delays were proportional to the distance from the cue. Furthermore, both covert attention (without eye movements) and overt attention (with eye movements) created the same RT delays, and both binocular and monocular views resulted in the same RTs. These findings strongly support our predictions, in which the observed RT-depth dependence is indicative of the spatiotemporal conversion required for constructing allocentric 3D space. After all, we perceive and measure 3D space by time as Einstein postulated a century ago.

Human vision has a remarkable ability to recognize complex 3D objects such as faces that appear with any size and 3D orientations at any 3D location. If we initially memorize a face only with a normalized size and viewed from directly head on, the direct comparison between the one-sized memory and a new incoming image would demand tremendous mental frame translations in 7D. How can we perform such a demanding task so promptly and reliably as we experience the objects in the world around us?Intriguingly, our primary visual cortex exhibits a 2D retinotopy with a log-polar coordinate system, where scaling up/down of shape is converted to linear frame translation. As a result, mental scaling can be performed by linearly translating the memory or the perceptual image until they overlap with each other. According to our new model of NHT (Neural Holography Tomography), alpha brainwaves traveling at a constant speed can conduct this linear translation. With this scheme, every scaling up/down by a factor of two should take the same amount of extra mental time to recognize a smaller/larger face.To test this hypothesis, we designed a reaction time (RT) experiment, where participants were first asked to memorize sets of unfamiliar faces with a given specific size (4° or 8°). Following the memorization phase, similar stimuli with a wide range of sizes (from 1° to 32°) were presented, and RTs were recorded. As predicted, the increase in RT was proportional to the scaling factor in the log scale. Furthermore, we observed that RTs were fastest for 8° faces even if the memorized face was 4°. This supports our hypothesis that we always memorize faces at the exact size of ~8 °. To our surprise, the increases in RT were also consistent with the mentally-estimated depth sensation, which indicates that the apparent size of the recognized face can create a proper depth sensation.

Visual perception plays a critical role in navigating space and extracting useful semantic information crucial to survival. To identify distant landmarks, we constantly shift gaze vectors through saccades, while still maintaining the visual perception of stable allocentric space. How can we sustain stable allocentric space so effortlessly? To solve this question, we have developed a new concept of NHT (Neural Holography Tomography). This model states that retinotopy is invisible (not available to consciousness) and must be converted to a time code by traveling alpha brainwaves to perceive objects consciously. According to this framework, if identical alpha phases are continually assigned to a landmark, we perceive its exact and consistent allocentric location.To test this hypothesis, we designed reaction time (RT) experiments to observe evidence of the predicted space-to-time conversion. Various visual stimuli were generated at a wide range of eccentricities either on a large TV (up to 40°) or by LED strips on a hemispherical dome (up to 60°). Participants were instructed to report the observed patterns promptly under either covert (no eye movement) or overt (with eye movement) conditions. As predicted, stimuli presented at the center of fixation always produced the fastest RTs. The additional RT delay was precisely proportional to the eccentricity of the peripheral stimulus presentation. Furthermore, both covert and overt attention protocols created the same RT delays, and trajectories of saccadic eye motions were in parallel to the overt RT vs. eccentricity. These findings strongly support our NHT model, in which the observed RT-eccentricity dependence is indicative of the spatiotemporal conversion required for maintaining a stable allocentric frame of reference. That is, we perceive space by time.