Lecture 8 Flashcards
Retinal Ganglion Cells
Sensitive to frequency, but not to orientation
Response Properties of V1 Neurons
By combining information from several retinal ganglion cells, it is possible to detect the orientation of lines
Tilt After-Effect
-We have the different populations of neurons that specialise in certain frequencies and orientations
-Adaptation : looking at a pattern of stripes for a certain time will ‘‘tire’’ the neurons and shift the balance in the opposite direction
-Note : this also implies that orientation is encoded by a population of neurons
Primary Visual Cortex : Hypercolumn
A 1mm block of striate cortex containing ‘‘all the machinery necessary to look after everything the visual cortex is responsible for, in a certain small part of the visual world’’
-Each hypercolumn contains cells responding to every possible orientation (0-180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye
Area V1 : Simple Cells
-Respond to light with increasing intensity when the orientation matches their receptive field
-Orientation selectivity represented by tuning curves (firing rate vs. line orientation)
Area V1 : Complex Cells
-Detect motion in their receptive fields
-Selective for line orientation and often for movement direction
Area V1 : Receptive Field Properties
-Larger and less well-defined than simple cells
-Some span a whole hemifield, responding to specific orientations/movements across this area
Area V1 : Specialized Complex Cells ; End-Stopped Cells
-Respond best to moving bars of a specific length that end within their receptive fields
-Do not fire if bars exceed their receptive field
-Involved in detecting angles, corners, and bounderies
Depth Perception
-We naturally percieve it
-Visual information reaching our retina is two-dimensional
-Our mind already knows that space exists before receiving sensory information, and it uses that prior knowledge to interpret sensory information
Monocular Cues
Don’t need 2 eyes to percieve, to make an inference
Binocular Vision
Seeing with 2 eyes
Stereopsis
Perception of depth from binocular vision
Monocular Cues : Pictorial Cues
Depth cues that can be inferred from a static image, using monocular vision
-These include cues like occlusion, relative size, familiar size, linear perspective, texture gradient, and aerial perspective and shading, which help the brain interpret depth in two-dimensional scenes
Pictorial Cue : Occlusion
A depth cue where one object partially blocks another, indicating that the blocked object is farther away
-It is one of the most reliable depth cues
Pictorial Cue : Accidental Viewpoint
A specific line of sight that creates an ambiguous or misleading depth interpretation
Pictorial Cue : Relative Size
A comparison of size between items without knowing the absolute size of either one
-All things being equal, we assume that smaller objects are farther away from us than larger objects
Pictorial Cue : Familiar Size
A comparison of size between items when knowing the absolute size of one of the items
-In this case, we can infer the absolute distance because we know the exact size of one of the objects
Pictorial Cue : Relative Height
For objects touching the ground, those higher in th visual field appear to be farther away (higher up)
-For objects that are floating in the air, shadows can help infer their relative heights
The Reason behind the robust perception of depth through relative height
-Objects that are further away are seen with a wider angle relative to our body
-It is when our sight is parallel to the ground, perpendicular to our body, that we experience the greatest feeling of distance
Pictorial Cue :Texture Gradient
A depth cue based on the geometric fact that items of the same size form smaller, closer spaced images the farther away they get
Pictorial Cue : Linear Perspective
Lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image as they extend into the distance
Pictorial Cue : Vanishing Point
The apparent point at which parallel lines receding in depth converge
Pictorial Cue : Aerial Perspective
A depth cue based on the implicit understanding that light is scattered by the atmosphere
-More light is scattered when we look through more atmosphere
-More distant objects appear fainter, bluer, and less distinct
Pictorial Cue : Shading
Variations in light and shadow on a surface provide information about its shape, depth, and orientation.
-Objects with gradual shading changes appear curved, while sharp contrasts suggest edges or discontinuites in depth
-The brain assumes light comes from above, influencing depth perception
Monocular Cues : Dynamic Cues
Depth information can be extracted from the movement of images on the retina
-When the observer is stationary, depth is perceived through the relative movement of objects at different depths (kinetic depth effect)
-This effect is particularly strong for rotating spheres, where luminance variations further enhance depth perception
Dynamic Cue : Motion Parallax
Images closer to the observer move faster across the visual field than images farther away
-The brain uses this information to calculate the distances of objects in the environment
-Objects beyond their fixation are moving in the same direction as them
Dynamic Cue : Optic Flow
The apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene
-Objects that are closer to the observer will move more rapidly
-Objects in the focus of expansion won’t move
