3. Color Deficiences and Depth Perception

V4

the most color-specific cortical area

Neural background of color vision

Cells are selectively sensitive to relatively narrow wavelength ranges, not sensitive to white

Cells that are sensitive to different wavelength ranges cluster in cell columns

trichromacy

normal sighted people, can distinguish all three colors

dichromacy

they can recognize colors but they can make errors with respect to certain wavelengths. Most common is green and red, less common is blue-yellow

monochromatic

color blind, unable to distinguish wavelengths

Red-Green color vision deficiency

1. deuteranomaly

2. protanomaly

blue-yellow color vision deficiency

1. tritanomaly

Deuteranomaly

most common, Sensitivity to M cones shifts towards L

Protanomaly

makes certain shades of red look more like green and less bright. This type is mild and usually doesnt get in the way of normal activities. The sensitivity of L cones shifts towards M

protanopia and deuteranopia

unable to tell the difference between red and green at all

Tritanomaly

sensitivity of S cones shifts towards longer wavelengths

tritanopia

makes someone unable to tell the difference between the blue and green, purple and red, and yellow and pink

complete color vision deficiency

monochromacy or achromatopsia

stereopsis

info from both eyes is needed to perceive space and depth

allows estimation of distance

stereo-blindness

no binocular depth perception, 5% of the population, linked to the childhood visual impairment (strabismus)

strabismus

the two eyes cannot focus in the same place, so binocular competition occurs (info from one eye suppresses the other)

Two types of spacial cues aiding depth perception

1. Monocular cues 2. Binocular cues

Monocular spatial cues

1. Relative size- > Objects further away are percieved as smaller

2. Occultation -> We usually percieve the overlapping object as closer than the one it overlaps

3. Elevation -> objects higher off the ground (closer to the horizon) are percieved as farther

4. perspective -> parallel line seem to converge in the distance. Narrower parts are percieved as further away

5. Motion parallax -> objects closer to us seem to be moving faster, further objects seem to move slower (looking out the window of a moving train)

6. Blue hue makes objects seem further away

7. Fine textures

Binocular parallex

each eye views a slightly different angle of an object (seen by the left and right eyes)

binocular disparity

the difference of retinal images in the left and right eyes. If an object is far away, the disparity of that image falling on both retinas will be small. If the object is close or near, the disparity will be large.

v5 (in MT)

the central brain region of motion perception

damage to it causes cortical motion blindness (the world is a series of still images, one after the other like a slideshow)

types of motion

1.True motion: actual physical movement
2. Apparent/stroboscopic motion: No physical movement, yet motion is percieved ( sequence of sill images elicit the perception )
3. Motion aftereffect e.g. waterfall illusion
4. Induced movement- if a larger object surrounding a smaller one moves, we see it as if the smaller one were moving, even though the smaller one is actually stationary e.g.: the moon

Motion illusion

special category, artificial evocation of the sensation of movement, neural background not entirely known

Neural basis of motion perception

• Direction of movement is detected by direction-selective cells

• in lower-level species (e.g. frogs, flies) these are located in the retina, in higher species the line direction sensitive cells of V1 are involved

• After Werner Reichardt, we call these circuits Reichardt detectors

• Reichardt detectors can only detect local movement, but cannot provide information about larger objects, more complex movements

Johansson's experiment proved the existence of biological motion perception

• film of 10-12 points of light in motion, observers could identify the sex, age, type of activity

• Even babies can distinguish biological from nonbiological movement

• They looked more at white dots moving against a black background that followed the movement of the joints of a running human figure than at dots moving randomly in all directions

These neurons are specialised for the identification of biological motion, presumably located in the superior temporal sulcus

Optic ataxia (visual motor coordination disorder):

visual movement disorder, e.g. difficulty grasping objects with eyes open, but being able to button clothes with eyes closed