Visual Acuity: what is it and how can it be tested?
The smallest feature of level of detail that can be resolved
Can be tested by: spatial frequency of lines, size of letters, orientation of letters
Snellen Eye Chart: what type of acuity does it test and how does it test that?
Uses letters of various sizes or orientations to test the minimum recognizable acuity, or the smallest feature that can be identified or named (about the same degree of acuity tested by spatial frequency of lines)
How is visual acuity operationalized or measured by the Snellen eye test?
Operationalizes visual acuity based on distance relative to the “average” person
- 20/20: normal vision
- 20/15: better than average (you can see at 20ft what others see at 15ft/they need to be closer)
- 20/30: worse than average (you can see at 20ft what others can see at 30ft)
Spatial Frequency: what is it and what type of acuity does it test?
The number of cycles (repetition of black stripe and white stripe) per degree of visual angle
- think of it as the distance between the shifts of an oriented grating (smaller distance = higher spatial frequency)
Tests minimum resolvable acuity, or the smallest amount of separation between two objects and still detect them as separate
What are the other kinds of acuity (besides minimum resolvable and recognizable)? How do they relate to thresholds?
Minimum Visible = smallest detail that can be detected (aka absolute threshold)
Minimum Discriminable = smallest change in a feature that can be detected as different (aka just noticeable difference)
Contrast: what is it and how does it relate to oriented gratings?
The difference in luminance between stripes (the sharpness of the shift in a sine wave function or oriented grating)
- lowering contrast will decrease our ability to resolve two features
What is the Contrast Sensitivity Function (CSF) and what is it affected by?
Spatial frequency and contrast are charted together on a graph to see where cycles are visible and invisible (appear grey) on the spectrum
Other than spatial frequency and contrast, the CSF is also affected by:
- overall luminance (see less in scotopic vision than photopic)
- aging
Why does an oriented grating appear grey if we’re too far away from the stimuli or if the spatial frequency is too small? What about when the phase is shifted?
Sine wave gratings are sampled discretely, not continuously, by the visual system (photoreceptors have graded responses)
- If a cycle is small enough, the cone’s receptive field is receiving both light and dark information, which gets combined and perceived as grey
- If things are shifted out of phase (changing the portion of the sine wave seen by the cone’s receptive field), this will also change the cell’s response
After visual information reaches the ganglion cells, where does it travel?
Out of the eye and down the optic nerve –> diverges by visual field at the optic chiasm –> down the optic tract –> reaches the LGN (part of the thalamus) –> V1
What happens to visual information at the optic chiasm?
Information is divided by VISUAL FIELD, where the left VF goes to the right side of the brain
This means visual fields are represented CONTRALATERALLY
How are the eyes divided up by visual field?
Each eye is divided into and inner (called nasal/medial) and outer (called temporal) region
Information taken in by the nasal region of the retina is what crosses over the other side of the brain
NOTE: keep in mind here that the right side of both eyes takes information from the LEFT visual field
Thalamus: what is it for?
The relay center of the brain
- most sensory systems feed information through this structure
- inhibited during sleep, lowering responses to external stimuli
In what ways is the structure and function of the LGN divided?
- Left and right hemisphere of the brain (the right processes the left visual field)
- Each side of the LGN is further divided in half by the Meridian Line, which divides the top half of the visual world from the bottom
- Divided into 6 separate layers of cells, where each layer receives information from EITHER eye, but not both (both hemispheres of the LGN receive info from both eyes, b/c it’s divided by visual field)
How are the layers of the LGN divided? What purpose does each set serve?
Magnocellular Layer (the inner 2 ventral layers): get their information from M cells in the periphery (less representation due to convergence)
Parvocellular Layer (the outer 4 dorsal layers): get their information from P cells in the fovea
What are the different terms used to refer to the areas of the brain and its structures (based on top, bottom, front, and back)?
Top = dorsal or superior
Bottom = ventral or inferior
Front = rostral or anterior
Back = caudal or posterior
Retinotopic Map in the LGN
The LGN preserves location information from the retina because although different layers deal with different eyes, the layers are lined up in a way that maintains retinotopic space
Since visual pathways split at certain points, damage at various locations will create different visual effects/problems… what are they?
Damage at the…
- Optic Nerve = loss of information from one eye
- Optic Chiasm = loss of information from the outer part of the visual field of both eyes
- After the optic chiasm = based on the side of the brain (e.g., damage to the right side means loss of left visual field in both eyes)
Where does visual information go after the LGN? Why does this area have multiple names? What kind of input is the area receiving?
The main subregion of the occipital lobe (located at the posterior of the brain): the Striate Cortex (meaning “striped” due to its layered nature) AKA the Primary Visual Cortex or V1 (meaning first part to receive visual info)
This region receives what we call “bottom-up” input, meaning it’s coming FROM the sensory system
How is information from the visual field represented in V1?
The fovea is represented at the very back of the brain, then as we increase eccentricity (or distance from the fovea, measured in degrees of visual angle), we move more anteriorly in the brain
How does the Striate Cortex represent retinotopic space?
Contralateral visual field
Inverted from reality
Cortical Magnification
It’s at this point that information from BOTH eyes is combined
Cortical Magnification in V1: what is it and why does it occur?
Despite being physically small, the fovea has a larger number of cells in V1 dedicated to its processing (more representation)
This is end result of all the convergence that occurred prior; it’s necessary to maintain the higher acuity of the fovea
What is a consequence of cortical magnification in V1?
The periphery becomes susceptible to Visual Crowding: stimuli becomes harder to perceive when there are other objects around it (vs when it’s observed in isolation)
How are the receptors of V1 cells different than the eye and LGN?
They respond well to lines and bars rather than dots
Input from both eyes is combined
Orientation Selectivity
V1 cells respond to certain orientations of lines (leading to different firing rates)
