Visual Cortex Lecture Notes

The visual cortex Outline - Space (depth/distance) perception - Monocular cues - Binocular cues - Non-retinal cues - Motion perception - Observer movement - Object movement - Illusory (apparent) movement Space perception - We perceive three-dimension (3D) space and objec...

The visual cortex Outline - Space (depth/distance) perception - Monocular cues - Binocular cues - Non-retinal cues - Motion perception - Observer movement - Object movement - Illusory (apparent) movement Space perception - We perceive three-dimension (3D) space and objects - I.e., we perceive depth/distance - However, our retinal image is two-dimensional (2D) - How do we reconstruct 3D perception? What cues do we use for this? Depth cues  Monocular pictorial cues - Occlusion - Linear perspective - Size - Texture gradient - Atmospheric perspective - Shading - Hight in the visual field Monocular movement dues - Motion parallax - Objects closer to you move more as compared with objects farther away - The direction of movement changes according to whether objects are closer to or farther away from you relative to your fixation point - Kinetic depth effect - A 2D pattern can be perceived as providing depth information when moving Binocular cues - Binocular disparity - Retinal images are slightly different between the two eyes - The greater the disparity, the farther the object - The direction of the disparity indicates whether objects are closer or farther relative to the fixation - Stereopsis---the process through which depth information is extracted from binocular disparity Non-retinal cues - Accommodation - Muscles that change the thickness of the lens provide some information about distance to a focused object - Convergence - Inward/outward rotations of the eyeballs correspond to the distance to a focused object - These are generally weak cues, but they are among the few that can inform about absolute distance to the object When cues are ambiguous/insufficient - Necker cube - Schroeder stairs - These 2D patterns cause bistable (or multistable) perception Motion perception - Objects in the environment are constantly moving - Even if they are not in motion, you are rarely perfectly still - Thus, our visual perception is based on *dynamic* retinal images - How do we perceive motion out of them? Observer movement - Optic flow - As we move through the 3D environment, different parts of a retinal image move in different directions, depending on how we move and where our eyes are focused - Examples: - - - We are very sensitive to optic flow patterns - e.g., We can estimate the direction of self-motion within 1--2° based on sparse optic flows alone (Warren et al., 1988) object movement - Looming - A pattern on a retinal image gradually increases its size as it approaches an observer - Example: - How can you tell if it is an object moving towards you, or you moving towards the object? - Non-visual self-motion information - Optic flow - Biological motion - - Structured patterns of motion that are unique to animate objects - Point-light displays that only indicate joint movements are sufficient for us to perceive human (and animal) action (Johansson, 1973) - Some of this ability may be innate---2-day-old infants show sensitivity to biological motion (Simion et al., 2008) Illusory 9apparent) movement - How does the visual system extract motion information? - One problem in this model: it does not require continuous motion of a stimulus - However, it may not be a problem at all... In fact, this may well be why we perceive apparent motion from a series of discrete stimuli - e.g., typical TVs and cinema films draw 24 discrete images a second - Stroboscopic effects - - Seeing motion in a series of still images  Research: if you are interested - - Anstis & Rogers (1975) - Backus & Oruç (2005) - Flynn & Shapiro (2018)