PSC129: Chapter 3 Spatial Vision

I) Visual Acuity: Oh Say, Can You See?
-Vision scientists talk about acuity in terms of the smallest visual angle of a cycle of the grating that you can perceive.
-A cycle is one repetition of a black and white stripe.
-Visual angle is the angle that would be formed by lines going from the top and bottom of a cycle, through the center of your lens, and on to the retina.
-Visual angle of resolution acuity by dividing the size of they cycle by the viewing distance, then taking the arc tangent of this ratio. Usually = 0.017 degrees.
-Resolution acuity represents the finest high-contrast detail that can be resolved, which is determined by the spacing of photoreceptors in the retina.
-The visual system samples the grating through the array of receptors at back of retina.
-If the receptors are spaced such that the whitest and blackest parts of the grating fall on separate cones. If the entire cycle falls on a single cone, we will see nothing but a gray field or aliasing in which the cycles seem to be longer than it is.
-Cones in fovea are 0.008 degrees apart; two cones are 0.017 degrees apart.
-Rods less packed in periphery so acuity is less in periphery than fovea.

1. A Visit to the Eye Doctor
-Herman Snellen: block letter where the length is five times as long as the stroke.
-Visual acuity = (distance you can identify)/(distance normal vision can identify)
2. Acuity for Low-Contrast Stripes
-Otto Schade - sine wave gratings at different spatial frequencies and adjust contrast.
-Spatial frequency: number of sine wave gratings per unit space; cycles per degree
-Contrast sensitivity function (CSF): is U-shaped; y-axis - reciprocal of contrast threshold; red line the borderline of visibility.

II)Retinal Ganglion Cells and Stripes
-Retinal ganglion cells like spots of light; responds to certain types of stripes.
-ON retinal ganglion cell
a. responds weakly to low spatial frequency because part of grating lands in the inhibitory surround
b. high also weak because both stripes fall within receptive-field center
c. cell responds vigorously when the bright bar fill the center and dark bars in the surround.
d. tuned to specific spatial frequency
-Respond to specific position within the receptive field - its phase.
-Net difference in the light intensity in the receptive field's center and surround.

III) The Lateral Geniculate Nucleus
1. Retinal ganglion cell synapse in the two LGNs which is a relay station from the retina to the cortex.
2. Bottom two layers of LGN is larger than the top four.
-Top four: parvocellular layers; bottom two: magnocellular layers
-Parvocellular receive from midget ganglion cells; magnocellular receive from parasol ganglion cells.
-Magnocellular: large, fast-moving objects; parvocellular: details of stationary objects.
3. The left LGN receives projections from the left sides of both retinas and the right LGN receives from the right sides of the retina.
4. Layers 1,4,6 receives input from the contralateral (other) eye; layers 2,3,5 from ipsilateral (same) eye.
5. Each layer contains organized map of half of visual field.
6. Topographical mapping: ordered mapping of the world onto visual nervous system; know where things are in space.
7. LGN neurons: concentric receptive fields; respond to spots and gratings.
8. LGN is area where feedback from brain modulates input from eyes.

IV) Striate Cortex
-Has six main layers; LGN project to layer 4.
1. Cortical Topography and Cortical Magnification
-2 features of visual cortex
a. Topographical mapping: objects in areas 3,4 of the striate cortex tells visual system that the object is in positions 3,4 of visual field.
b. Scaling of information: objects near fovea are processed in a large part of striate cortex. The distortion of visual map on cortex is cortical magnification.

V) Receptive Fields in Striate Cortex
-receptive fields of striate cortex neurons are not circular as in retina and LGN but are lines.
1. Orientation Sensitivity
-Orientation tuning: cell is tuned to specific orientation; some cells fire vigorously at specific orientation and declines elsewhere; all the neurons as a whole detects all possible orientation.
-Circular receptive fields in LGN transformed into elongated receptive fields in striate cortex: concentric LGN cells that feed into a cortical cell are all in a row

2. Other Receptive-field properties
-Cortical cells respond to gratings (collection of lines) of certain spatial frequency; respond to smaller range of spatial frequency than ganglion cells.
-Narrow tuning of cortical cells: filter for part of image that excites cell.
-Respond well to moving lines, bars, edges and gratings in one direction.
-Receive input from both eyes but have ocular dominance - respond better with input from one eye.

3. Simple and Complex Cells
-Simple cells: neurons that have clearly defined excitatory and inhibitory regions; phase-sensitive
-Complex cells: tuned to particular orientation, spatial frequency, and ocular preference; will respond if stripe is in receptive field regardless of location; phase-insensitive

4. Further complications
-End stopping: a property of some cells in striate cortex; increase firing rate when bar elongated to fill receptive field but decreases when the bar stretches out of field.
-Hypercomplex cells: cells with end stopping
-End stopping play an important role to detect luminance boundaries and discontinuities.

VI) Columns and Hypercolumns
-Neurons with similar orientation preferences are arranged into columns that extended vertically through cortex
-All the orientations were 0.5mm apart
-Hypercolumn: a 1mm block of striate cortex containing two sets of column with one preferring right eye and the other left.

VII) Selective Adaptation: The Psychologist's Electrode
-Expose cells to certain orientation; stimulus will fatigue cells with that orientation preference

1. The Site of Selective Adaptation Effects
-Transfer of adaptation effects from one eye to another indicate that selective adaptation occurs in cortical neurons and not retinas or LGNs.

2. Combing Spatial frequency and orientation selectivity
-Cell respond to specific orientation and specific spatial frequency.

3. Spatial Frequency-Tuned Pattern Analyzers in Human Vision
-Fergus Campbell and John Robson: human contrast sensitivity function reflects sensitivity of multiple individual pattern analyzers
-Spatial frequency channels: patter analyzers, implemented by ensembles of cortical neurons, with each set of neurons tuned to a limited range of spatial frequencies.
-Multiple spatial frequency model: spatial freq that stimulate different pattern analyzers will be detected independently even if the different freq are combined in the same image
-Visual system use spatial freq filters to analyze images because different spatial freq emphasize different information.
-Zebra: how many - low freq channels; details - high freq channels
-At near-threshold contrasts, pattern analyzers operating at different scales of analysis are independent; at high contrasts, pattern analyzers interact

VIII) The Girl Who Almost Couldn't See Stripes
-During critical period, normal binocular visual stimulation is required for normal cortical development.
-Cortical neurons are being wired up to their inputs from the two eyes.
-Strabismus: a misalignment of the two eyes, so that a single object in space is imaged on the fovea of one eye, and on a nonfoveal area of the other eye.

Vocabulary:
1. Contrast: The difference in illumination between a object and the background, or between brighter and dimmer parts of the same object.
2. Acuity: the smallest spatial detail that can be resolved.
3. Visual Angle: the angle subtended by an object at the retina.
4. Cycle: for a grating, a pair consisting of one dark bar and one bright bar.
5. Sine wave grating: a grating with a sinusoidal luminance profile.

Summary:
1. In this chapter we followed the path of image processing from the eyeball to the brain. As we saw, neurons in the cerebral cortex translate the array of stars perceived by retinal ganglion cells into the beginnings of forms and patterns. We learned that the primary visual cortex is organized into thousands of tiny computers, each responsible for determining the orientation, width, color, and other characteristics of the stripes in one small portion of the visual field. In Chapter 4, we will continue this story by seeing how other parts of the brain combine the outputs from these minicomputers to produce a coherent representation.
2. Perhaps the most important feature of image processing is the remarkable transformation of information from the circular receptive fields of retinal ganglion cells to the elongated receptive fields of the cortex.
3. Cortical neurons are highly selective along a number of dimensions, including stimulus orientation, size, direction of motion, and eye of origin.
4. Neurons with similar preferences are often arranged in columns in primary visual cortex.
5. Selective adaptation provides a powerful, noninvasive tool for learning about stimulus specificity in human vision.
6. The human visual cortex contains pattern analyzers that are specific to spatial frequency and orientation.
7. Normal visual development requires normal visual experience. Abnormal visual experience early in life can cause massive changes in cortical physiology that result in a devastating and permanent loss of spatial vision.