5.4- How do we see? Sensory receptors in the eye transmit visual information to the brain

Question 1

Explain how light is processed by the eyes and the brain

Answer 1

• Light first passes through the cornea, the eye’s thick, transparent outer layer
• The cornea focuses the incoming light, which then enters the lens
• There, the light is bent further inward and focused to form an image on the retina, the thin inner surface of the back of the eyeball
• The retina contains the sensory receptors that transduce light into neural signals

Question 2

Role of the pupil and iris

Answer 2

• The pupil, the dark circle at the center of the eye, is a small opening in the front of the lens
• By contracting or dilating, the pupil determines how much light enters the eye
• The iris, a circular muscle, determines the eye’s color and controls the pupil’s size (the pupil dilates in dim light)

Question 3

Accomodation

Answer 3

• Behind the iris, muscles change the shape of the lens
• They flatten to focus on distant objects and thicken it to focus on closer objects- this process is called accommodation

Question 4

Rods and cones

Answer 4

• The retina has two types of receptor cells: rods and cones

Question 5

Rods

Answer 5

• They respond at extremely low levels of light and are responsible primarily for night vision
• They do not support color vision, and they are poor at fine detail
• This is why, on a moonless night, objects appear in shades of gray

Question 6

Cones

Answer 6

• In contrast to rods, cones are less sensitive to low levels of light
• They are responsible primarily for vision under brighter conditions and for seeing both color and detail

Question 7

More detail about eye?

Question 8

Diagram of eye

Answer 8

Question 9

Transmission from the eye to the brain

Answer 9

• The visual process begins with the generation of electrical signals by sensory receptors in the retina
• These receptors contain photopigments, protein molecules that become unstable and split apart when exposed to light
• Rods and cones do not fire action potentials like other neurons. Instead, decomposition of the photopigments alters the membrane potential of the photoreceptors and triggers action potentials in downstream neurons

Question 10

More info needed?

Question 11

‘What’ and ‘where’ pathways

Answer 11

• One important theory proposes that visual areas beyond the primary visual cortex form two parallel processing streams, or pathways
• The ventral stream projects from the occipital lobe to the temporal lobe and appears to be specialized for the perception and recognition of objects, such as determining their colors and shapes
• The dorsal stream projects from the occipital lobe to the parietal lobe and seems to be specialized for spacial perception - determining where an object is and relating it to other objects in a scene
• These two processing streams are therefore known as the ‘what’ stream and the ‘where’ stream

Question 12

Patient DF

Answer 12

• Damage to certain regions of the visual cortex provides evidence for distinguishing between the ventral and dorsal streams
• Consider the case of D.F. At age 34, she suffered carbon monoxide poisoning that damaged her visual system, particularly in regions involved in the ‘what’ pathway
• D.F. was no longer able to recognize the faces of her friends and family members, common objects, or even drawings of squares or circles
• She could, however, recognize objects if they were placed in her hands
• Her condition - object agnosia, the inability to recognize objects - was striking in what she could and could not do
• For example, if she was to draw an apple, she could do so from memory, but when shown a drawing of an apple, she could not identify or reproduce it
• Nevertheless, D.F. could use visual information about the size, shape, and orientation of the apple to control visually-guided movements
• Because her conscious visual perception of objects - her ‘what’ pathway - was impaired, she was not aware of taking in any visual information about the objects she saw
• Because her ‘where’ pathway appeared to be intact, these regions of her visual cortex allowed her to use information baout he size and location of objects despite her lack of awareness about these objects