Vision : part 2 Flashcards
How vision scientists measure visual acuity
Smallest visual angle of a cycle of grating.
visual angle
Unit used to measure the size of an object in your visual field (regardless of its absolute distance or size).
- Since there is a direct one-to-one correspondance between the size of the object in your visual field and on your retina, the visual angle is the unit used to measure the size of an object on the retina.
Cycle
If you consider a grating of alternating black and white stripes, a cycle is one black and one white stripe
Better vision = ____smaller/larger visual angle required to identify a grating cycle.
Smaller
For most people, visual acuity corresponds to…
One minute of arc, or 1/60 of one degree of visual angle.
The visual angle is _____dependent/independent of the actual size of the object in the world.
Independent
A large object in the distance, may take up the same degree of visual angle as a _____ small object
Close
~10% of retinal outputs bypass the LGN and project to the …
Superior colliculus (SC)
The superior colliculus (SC) has a _______ layout of the contralateral visual field.
Retinotopic
Blindsight Phenomenon
- proves you need cortical processing for conscious vision
- Seen in patients with damage to the primary visual cortex.
Unconscious visual perception in blindsight
- Patients respond to visual stimuli without conscious awareness
- Can accurately guess line orientation, object movement, and spatial location.
Blindsight is due to…
SC projections bypassing the primary visual cortex.
Primary visual cortex / Area V1/ Striate cortex
- Essential for conscious vision
- In occipital lobe
Secondary visual cortex is also called…
- Area V2
- Extrastriate cortex
Retinotopic Mapping
The spatial organization of the visual field is preserved in the LGN, with neighboring areas of the retina mapping onto neighboring areas of the primary visual cortex (V1) in a point-by-point manner.
More cortical space in V1 is dedicated to ____central/peripheral vision compared
Central
The fovea has a higher density of _____ cells than the peripheral retina.
Ganglion
The LGN has twice as many ______ layers as magnocellular layers.
Parvocellular
Cortical Magnification
V1 further amplifies the total number of neurons devoted to central vs. peripheral vision where the central 10° of the visual field (~1% of the total visual field) occupies 50% of V1’s surface
Retinal ganglion cells are very good at extracting the spatial frequency of the visual input, but not the ______
Orientation
By combining information from several _____ cells, it is possible to detect the orientation of lines
retinal ganglion cells
Retinal ganglion cells with a ON center OFF surround receptive field diagonal line organization
If they are presented with a line that touches all the on centers of all the retinal ganglion cells : they will fire at the same time, transmit the info to the LGN then the V1 neurons
Neurons in the V1 have a preferred ______ .
Orientation. They will respond less and less as it gets farther from this preferred orientation
The response to stimulus is ______ if only one retinal ganglion receptive field is touched in its on-center
Moderate
Adaptation
Looking at a pattern of stripes for a certain time will “tire” the V1 neurons and shift the balance in the opposite direction
Tilt after-effect
- Habituate a diagonal line
- Present vertical lines again.
- The pattern of a population of V1 neurons activity that are preferentially tuned to 45° will exhibit a decreased firing rate compared to before the habituation occurred.
- The ones that are not close to the 45 degree pattern won’t change their response pattern because they haven’t habituated. As a result, the whole pattern of activity across the population of V1 neurons is tilted to the right.
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.
In an area of __mm, you will have columns that have a preferential response for all possible orientations, that preferentially respond to input from the left eye and other columns that preferentially respond to input from the right eye
1
Each hyper column as all of the tools necessary,
to perceive all ___ and ______ in any area of your visual field
Lines and orientations
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).
Complex cells
- Detect motion in their receptive fields.
- Selective for line orientation and often for movement direction.
Receptive Field Properties of complex cells
- Larger and less well-defined than simple cells
- Some span a whole hemifield, responding to specific orientations/movements across this area.
End-Stopped Cells (specializied complex cells)
- Respond best to moving bars of a specific length
- Do not fire if bars exceed their receptive field.
- Involved in detecting angles, corners, and boundaries.
the visual information reaching our retina is __2D/3D
2D
We do not get depth directly from our ____ : the visual system is prearranged for this
Senses
Pictorial Cues
Depth cues that can be inferred from a 2D static image, using monocular vision.
- occlusion, relative size, familiar size, linear perspective, texture gradient, aerial perspective and shading
Occlusion
A depth cue where one object partially blocks another, indicating that the blocked object is farther away.
- one of the most reliable depth cues.
Accidental Viewpoint
A specific line of sight that creates an ambiguous or misleading depth interpretation
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.
Familiar size
A comparison of size between items when knowing the absolute size one of the items. In this case, we can infer the absolute distance because we know the exact size of one of the objects.
Relative height
For objects touching the ground, those higher in the visual field appear to be farther away.
Shadows
For objects that are floating in the air, shadows can help infer their relative heights.
Objects that are further away are seen with a ____ angle relative to our body
Wider
It is when our sight is parallel to the ground, i.e. perpendicular to our body, that we experience the greatest feeling of ______.
Distance
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.
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
Vanishing point
Apparent point at which parallel lines receding in depth converge.
Aerial perspective
Implicit understanding that light is scattered by the atmosphere. More light is scattered when we look through more atmosphere. Thus, more distant objects appear fainter, bluer, and less distinct.
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 discontinuities in depth. The brain assumes light comes from above, influencing depth perception.
Dynamic cues of depth
Depth information can be extracted from the movement of images on the retina.
Kinetic depth effect
When the observer is stationary, depth is perceived through the relative movement of objects at different depths , particularly strong for rotating spheres, where luminance variations further enhance depth perception
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
When we move our eyes to track a particular object, it seems that objects beyond the fixation point are moving in the ____ direction
Same
Optic flow
Apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.
Objects in the _____ ___ ______ won’t move when we look at them
focus of expansion (right in front, parallel to our eyes)
