👁️Perception Unit 7 – Depth perception and spatial cognition

Key Concepts and Definitions

• Depth perception involves interpreting visual cues to perceive the world in three dimensions and estimate the distance of objects
• Stereopsis refers to the perception of depth and 3D structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision
• Spatial cognition encompasses how we acquire, organize, utilize, and revise knowledge about spatial environments
• Accommodation is the process by which the eye changes optical power to maintain a clear image or focus on an object as its distance varies
• Convergence involves the inward movement of both eyes toward each other to maintain single binocular vision when viewing an object
  • Allows the eyes to fixate on the same point in space to form a single image
• Monocular cues are depth cues that can be processed by using only one eye and do not depend on binocular vision
• Binocular cues are depth cues that depend on the combination of the images from the two eyes and require binocular vision

Visual Cues for Depth Perception

• Monocular cues include relative size, texture gradient, interposition, linear perspective, and motion parallax
  • Relative size cue states that if two objects are known to be the same size, the object that appears smaller is farther away
  • Texture gradient refers to the fact that the texture of an object appears more detailed and crisp when it is closer to the viewer
  • Interposition or occlusion occurs when one object partially blocks the view of another object, indicating that the blocked object is farther away
• Binocular cues include retinal disparity and convergence
  • Retinal disparity is the difference between the images on the left and right retinas due to the horizontal separation of the eyes
  • Convergence is the inward turning of the eyes to fixate on a single point, with greater convergence required for closer objects
• Pictorial cues are monocular cues that can be depicted in a two-dimensional image, such as a painting or photograph (linear perspective, relative size, occlusion)
• Oculomotor cues involve the sense of muscle tension in the eyes (accommodation, convergence)
• Motion-based cues include motion parallax and optic flow
  • Motion parallax refers to the relative motion of objects against a background due to the motion of the observer
  • Optic flow is the pattern of apparent motion of objects in a visual scene caused by the relative motion between the observer and the scene

Binocular vs. Monocular Depth Cues

• Binocular depth cues require the use of both eyes and include stereopsis, convergence, and shadow stereopsis
  • Stereopsis is the perception of depth from binocular disparity, which is the difference in the images projected onto the left and right retinas
• Monocular depth cues can be used with only one eye and include pictorial cues and motion-based cues
• Monocular cues are generally less effective than binocular cues for judging absolute distances but can still provide useful information about relative distances
• The combination of multiple depth cues, both monocular and binocular, enhances the accuracy and robustness of depth perception
• In some cases, monocular and binocular cues may provide conflicting information, leading to perceptual illusions or ambiguities (Ames room)

Neural Mechanisms of Depth Perception

• The primary visual cortex (V1) contains neurons that are sensitive to binocular disparity, which forms the basis for stereopsis
  • Binocular disparity refers to the difference in the position of an object's image on the left and right retinas due to the horizontal separation of the eyes
• Disparity-sensitive neurons in V1 have receptive fields that are slightly offset in the left and right eyes, allowing them to detect differences in the position of an object's image on the two retinas
• Higher-level visual areas, such as V2 and V3, further process disparity information to extract more complex depth relationships and 3D shapes
• The dorsal stream, which includes areas like the middle temporal (MT) and medial superior temporal (MST) areas, processes motion-based depth cues and integrates depth information with other visual features
• The ventral stream, which includes areas like the inferior temporal (IT) cortex, is involved in object recognition and may use depth information to help segment objects from the background
• The parietal cortex integrates depth information with other sensory modalities (touch, proprioception) to create a unified representation of space

Spatial Cognition and Mental Maps

• Spatial cognition involves the acquisition, organization, utilization, and revision of knowledge about spatial environments
• Mental maps, also known as cognitive maps, are internal representations of spatial information, such as the layout of a familiar environment or the relative locations of objects
• Mental maps can be used for navigation, spatial reasoning, and planning routes
• The hippocampus plays a crucial role in the formation and use of mental maps, particularly for allocentric (world-centered) representations of space
  • Place cells in the hippocampus fire when an animal is in a specific location within an environment, forming the basis for a cognitive map
• The entorhinal cortex, particularly the grid cells found in the medial entorhinal cortex, provides a metric representation of space that supports navigation and spatial memory
• The parietal cortex is involved in egocentric (self-centered) representations of space and the integration of spatial information from different sensory modalities
• Mental rotation, the ability to mentally represent and manipulate 2D and 3D objects, is an important aspect of spatial cognition and involves regions like the parietal cortex and the motor cortex

Development of Depth Perception

• Depth perception develops gradually during infancy and early childhood as the visual system matures and the infant gains experience with the environment
• Newborns have limited depth perception due to the immaturity of their visual system and lack of experience
  • However, they show some sensitivity to depth cues such as motion parallax and size constancy
• Stereopsis, the ability to perceive depth from binocular disparity, emerges around 3-5 months of age as the infant's eyes become more coordinated and the visual cortex matures
• Infants begin to show evidence of using pictorial depth cues, such as occlusion and relative size, by around 7-8 months of age
• The development of crawling and walking provides infants with important experience in navigating and understanding the spatial layout of their environment
• Depth perception continues to refine throughout childhood as the visual system matures and the child gains more experience with the environment
  • This includes improvements in the ability to use multiple depth cues, judge absolute distances, and perform complex spatial tasks

Disorders and Abnormalities

• Amblyopia, also known as "lazy eye," is a developmental disorder characterized by reduced visual acuity in one eye due to abnormal visual experience during the critical period of visual development
  • Amblyopia can result from strabismus (misaligned eyes), anisometropia (unequal refractive error between the eyes), or visual deprivation (e.g., congenital cataract)
  • Amblyopia can lead to impaired depth perception, particularly for tasks requiring fine stereopsis
• Strabismus is a condition in which the eyes are misaligned and point in different directions, leading to impaired binocular vision and depth perception
  • Strabismus can be caused by abnormalities in the muscles that control eye movements or in the neural control of these muscles
• Stereoblindness is the inability to perceive depth from binocular disparity, despite having normal visual acuity in both eyes
  • Stereoblindness can be caused by strabismus, amblyopia, or other abnormalities in the development of binocular vision
• Akinetopsia, also known as motion blindness, is a rare neurological disorder characterized by the inability to perceive visual motion
  • Akinetopsia can result from damage to the middle temporal (MT) area of the visual cortex, which is critical for processing motion information
  • Individuals with akinetopsia may have difficulty perceiving motion-based depth cues, such as motion parallax
• Topographical disorientation is a disorder characterized by the inability to orient oneself in familiar environments or to learn the layout of new environments
  • Topographical disorientation can result from damage to the hippocampus, entorhinal cortex, or parietal cortex, which are involved in spatial cognition and navigation

Real-World Applications and Research

• Virtual reality (VR) technology relies on the principles of depth perception to create immersive 3D environments
  • VR systems use stereoscopic displays and motion tracking to provide binocular and motion-based depth cues
  • VR is used in various applications, such as gaming, education, training, and therapy
• Augmented reality (AR) overlays digital information onto the real world, requiring accurate depth perception to align virtual objects with real-world surfaces
• Robotics and computer vision systems use depth perception techniques to navigate and interact with the environment
  • Techniques such as stereo vision, structured light, and time-of-flight cameras are used to estimate depth in robotic systems
• Depth perception research has implications for the design of 3D displays, such as autostereoscopic displays that provide depth cues without the need for special glasses
• Studies on the development of depth perception in infants and children provide insights into the plasticity of the visual system and the role of experience in shaping perception
• Research on disorders of depth perception, such as amblyopia and stereoblindness, contributes to the understanding of the neural mechanisms underlying binocular vision and the development of new treatment approaches
• Investigations into the neural basis of spatial cognition and navigation, including the role of the hippocampus and entorhinal cortex, have implications for understanding spatial memory, learning, and disorders such as Alzheimer's disease