3. Spatial Vision
Vision Acuity The smallest spatial detail that can be resolved.
Sine Wave Grating A grating with a sinusoidal luminance profile.
Cycle For a grating, a pair consisting of one dark bar and one bright bar.
Visual Angle The angle subtended by an object at the retina
4. • Visual Acuity: The smallest spatial detail that can be resolved
• Cycle: For a grating, a pair consisting of one dark bar and one bright
bar. http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/channels/chan
nels.html
6. Visual Angle
Under ideal conditions, humans with very good visual acuity can resolve gratings when one cycle subtends an angle of
approximately 1 minute of arc (0.017 degrees).
8. Fundamental Limit of Spatial Vision
• Limit of 0.017 degrees is determined by the spacing of photoreceptors in the
retina.
http://read.uconn.edu/PSYC3501/Lecture04/
11. • Spatial Frequency The number of grating cycles in a given unit of
space. Measured in cycles per degree.
12. How is Visual Acuity affected if Contrast of
Stripes is Reduced?
• Otto Schade showed people sine wave gratings with different spatial
frequencies and had the adjust the contrast of the gratings until they
could just be detected (1956).
13. How is Visual Acuity affected if Contrast of
Stripes is Reduced?
• Otto Schade showed people sine wave gratings with different spatial
frequencies and had the adjust the contrast of the gratings until they
could just be detected (1956).
Intuitively, one might think that the wider the stripes –
the lower the spatial frequency – the easier it is to
distinguish light stripes from dark stripes.
19. First Exploration of Primary Visual Cortex (V1)
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
20. First Exploration of Primary Visual Cortex (V1)
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
22. First Exploration of Primary Visual Cortex (V1)
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
24. First Exploration of Primary Visual Cortex (V1)
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
27. First Exploration of Primary Visual Cortex (V1)
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
29. Summary
• Receptive fields are elongated, as opposed to circular, so they
respond to bars, lines, and edges.
• Orientation tuning Tendency of neurons in V1 to respond optimally to
certain orientation and less to others
• Complex Cells Neuron whose receptive field characteristics cannot be
easily predicted by mapping with spots of light
• Ocular Dominance Neurons in V1 respond to information from both
eyes, but prefer one eye over another
• End Stopped Neuron’s firing rate sensitive to length of stimulus
30. Why Should we Care?
http://www.heightseyecare.com/images/amblyopia.jpg
Eye doctors measure your eye sight using this Snellen chart. If you have 20/20 vision, it means that you can see clearly at 20 feet what a person with normal vision can see clearly at 20 feet. What they are actually doing is calculating the visual angle of your resolution acuity, and comparing it to that of a person with normal vision. So what’s the science behind how this works?
Optometrists describe vision in terms of 20/20, but vision scientists prefer to use these terms.
These are sine wave gratings. Vision scientists describe visual acuity in terms of visual angles.
To determine one’s visual acuity, one asks what is the smallest visual angle of a cycle of the grating that we can perceive.
The ability to resolve an angle of 0.017 degrees is the same as being able to resolve a distance of X to either side of a football post from the length a football field.
Understanding the reason for the limit of spatial vision is important for understanding not just visual acuity but also for Hubel and Wiesel’s work. Cones in the fovea have a center-to-center separation of about 0.008 degrees. Because we need two cones per cycle to be able to perceive the grating accurately, that fits nicely with the observation that 0.017 degrees is the fundamental limit of spatial vision.
The letter as a whole is five times as large as the strokes that formed the letter, where the letter subtends an angle of 0.083 degrees at the eye, and each stroke subtends an angle of 0.017 degrees. Visual acuity is thus defined as (the distance at which a person can just identify the letters) / (distance at which a person with “normal” vision can just identify the letters). Now we need one more piece of background knowledge to be able to talk about spatial vision.
We’ve gone over the fact that high-contrast sine wave gratings can be resolved as long as adjacent pairs of light or dark stripes are separated by at least 1 art minute of visual angle. But what happens if the contrast of the stripes is reduced? This is an important question because the real world has much more low contrast than high-contrast stimuli.
Draw one degree angle on both graphs, show that there are more cycles for high spatial frequency graph per 1 degree.
Do you think people doing Schade’s experiment increased the contrast more for the high SF or the low SF gratings? In other words, does either high SF or low SF gratings require more contrast than the other in order to resolve them?
Contrast sensitivity = 1/contrast threshold
For example, a 1-cycle/degree grating to be just distinguishable from uniform gray, the dark stripes must be about 1% darker than the light stripes (that is, if a tiny patch of a light stripe reflects 1000 photos, a dark stripe should reflect 990 photos). The reciprocal of this threshold is 1/.01 = 100; so that is the point plotted on the CSF line for this particular spatial frequency.
So, as one increases in frequency, the contrast sensitivity increases, which means that the difference between dark and light stripes is decreasing. This happens until you get to about this area, and then for greater spatial frequencies, one needs to increase the contrast between dark and light stripes in order to distinguish them.
This point here is equal to 60 cycles per degree, which corresponds to a cycle width of 0.017 degrees, which is the human resolution limit.
Shared the 1981 Nobel Prize for their work on the primary visual cortex.
The bar-shaped stimulus on the screen causes nerve cells in the cortex to fire, and a recording electrode picks up the signals generated by these nerve cells. In an actual experiment, the cat was anesthetized and its head is held in place for accurate positioning.
Huebel and Wiesel hypothesized that LGN cells were lined up in a arrow, feeding into the elongated arrangement of the striate cortex receptive field.
Orientation tuning of a simple cortical cell. This cell responds best to a vertical bar and responds less well as the bar is tilted in either direction.
Later research has shown that, in addition of the arrangement of LGN inputs for establishing the orientation selectivity of striate cortex cells, neural interactions within the cortex also play an important role in orientation tuning.
Hubel and Wiesel further made a distinction between simple and complex cells. Complex cells, like simple cells, prefer oriented visual stimuli. But the most obvious difference is that simple cells are picky about the position of a visual stimulus within the receptive field, while complex cells are not. In other words, a simple cell might only respond is a stripe is presented in the center of its receptive field, whereas a complex cell will respond regardless of where the stripe is presented, as long as it is somewhere within the cell’s receptive field.
Given hat we see a single, unified world, it makes sense that information entering the two eyes should be brought together at some point. Up until Hubel and Wiesel’s discoveries, there were heated debates about how and where this process occurs. The argument can be traced all the way back to 1664, where Rene Descartes described a fusion center of the brain.
Hubel and Wiesel were the first to show that neurons in the primary visual cortex respond to input from both the left and the right eyes. However, V1 neurons often have a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other. This property of V1 receptive fields is called “ocular dominance”.
When Hubel and Wiesel tested response of cells to ever increasing bar lengths, they found that different cells responded in different manners to the length of the bar. For some cells, the response rate first increased as the bar filled up its receptive field, and then decreased markedly as the bar was lengthened further (beyond the receptive fields). Such cells they termed “end-stopped cells”, because once the bar extends beyond the receptive field of an end-stopped neuron, that neuron stops firing. Some cells continue to fire even when the stimulus length reaches beyond the cell’s boundaries.
Studies dating back to Hubel and Wiesel have shown that depriving one eye of sight can cause permanent consequences for seeing patterns. In other words, there is a critical period for development of visual development. For humans, that critical period is from 3 to 8 years. If, for example, a child is born with cataracts – which is a clouding of a lens – and the cataracts is left untreated, then the child may grow up with amblyopia. That is an inability to perceive binocular depth perception. The photo represents the vision of someone with amblyopia. This is especially relevant for much of the countries in developing world, where often children are born with cataracts in both eyes that go untreated. These children grow up essentially blind.
