Lecture 6 Flashcards
Receptive Field
The region on the retina in which stimuli influence a neuron’s firing rate
ON-Center/OFF-Surround Ganglion Cells
Activated by light in the center and inhibited by light in the surround
-Doesn’t like light in peripheral (surrounding)
OFF-Center/ON-Surround Ganglion Cells
Activated by light in the surround and inhibited by light in the center
-Doesn’t like light in the center
Steps of Lateral Inhibition
- Light hyperpolarizes (renders polarity more negative) the center cone
- On-center bipolar cells reverse the sign of the cone (Off-center bipolar cell would keep the same sign)
- Dark depolarizes (renders polarity more positive) surround cones
- This activates horizontal cells which in turn inhibits all the cones.
-Because the centre cone is placed in the middle of 2 horizontal cells, it receives more inhibition than surround cones - This amplifies the bipolar On cell activity, and subsequently retinal ganglion cell activity
ON-Center Bipolar Cells
Reverse the sign of the photoreceptor
OFF-Center Bipolar Cells
Don’t reverse the sign of the photoreceptor
Visual Acuity
Smallest spatial detail that can be resolved
-Seems to depend on frequency and contrast
-For most people it corresponds to one minute of arc, 1/60 of one degree of visual angle
Methods to measure visual aquity
Optemetrists use distance to characterize visual acuity, as in ‘‘20/20’’ vision
-Your distance/normal vision distance
-The patient is typically placed at 20 feet from the letters
Vision Scientists
Smallest visual angle of a cycle of grating
Visual Angle
Vision scientists measure the size of visual stimuli by how large an image appears on the retina, not by how large the object is. The visual angle of an object is a function of both its actual size and distance from the observer and it corresponds to the size of the object on the retina
Cycle
If you consider a grating of alternating black and whie stripes, a cycle is one black and one white stripe
Better Vision
Smaller visual angle required to identify a grating cycle
Spacing of photoreceptors in the retina
Visual aquity is determined by this
-Cones in the fovea have an average center-to-center separation of 0.5 arc minute, 1/120 degree
-But with 3 cones, the distance between the leftmost and the rightmost cones is 2x 0.5 = 1 arc minute, 1/60 degree!
In Low Contrast Environments
Visual aquity is strongly dependent on spatial frequency and is optimal at 7 cycles/degree
The frequency of a certain pattern
Defined by the number of cycles within one degree of visual angle
Sine Waves
The simplest pattern that can repeat
Square Waves
Can be further decomposed into sine waves
Low Frequencies
Allow us to see the general picture
High Frequencies
Allow the perception of details
