Visual process

M.Mohamed Rizwan

• Transduction • the process by which our sensory

systems convert stimulus energy into neural messages

1. Wavelength• The distance from one wave peak to

the next

2. Hue• The color we experience

(determined by wavelength)

3. Amplitude• Height of the wave peak

4. Intensity• Amount of energy in light waves,

influences brightness (determined by amplitude)

*Note: the retina has many receptor cells which convert the inverted image (light energy) into neural impulses. When sent to the brain, these neural impulses are reassembled to create a perceived, upright-seeming image.

Lens – focuses the incoming rays by changing its curvature (called accommodation)

Iris – a colored muscle that surrounds and dilates or constricts the pupil (regulates amount of light entering)

Pupil - small opening which allows light to enter.

Retina – the light-sensitive surface on which the rays focus – the multilayered tissue that lines the inside of the back of the eyeball

• Light travels through outer cells to buried receptor cellscalled rods and cones.

• The rods and cones generate neural signals to alert the next layer of cells called the bipolar cells.

• The bipolar cells activate the ganglion cells, whose axons converge like strands of rope to form an optic nerve that carries information to the brain.

• Where the optic nerve leaves the eye there are no receptor cells, creating a blind spot.

• The fovea is where the cones are clustered, the retina’s area of central focus (no rods).

Rods• Do not have their own bipolar

cells, they share with other rods.

• No color.• Remain sensitive in dim light,

takes about 20 minutes to adjust

• 120 million• Periphery of retina• High sensitivity in dim light –

help see in the dark.

Cones• Many have their own bipolar

cells – aids in precise info and detection of fine detail.

• Color vision• Do not respond in dim light

(can’t see colors in dim light)• 6 million• Center of retina• Low sensitivity in dim light

Visual Information Processing• Feature Detectors

• Certain cortical neurons which receive visual info and respond to only certain features of a scene

• Parallel Processing• The brain’s capability to process visual components

simultaneously (i.e. a face as opposed to an eye, a nose, etc.) – it breaks vision down into sub-dimensions such as color, depth, movement, and form

– We can discriminate 7 million different shades!– 1 in 50 people are “color-deficient”

(esp. males – genetic)

• Young-Hemholz Trichromatic Theory– The retina has 3 types of color receptors – each sensitive to

red, green, or blue. When we combine these, we get all the other colors

• Herring: afterimages – Opponent Process Theory– After leaving the receptor cells, visual information is

analyzed in terms of opponent colors • Red/Green; Blue/Yellow; Black/White

– White contains all colors in spectrum

• Summary– The retina’s red, green, and blue cones respond in

varying degrees to different stimuli; their signals are then processed by the nervous system’s opponent-process cells en route from the thalamus to the visual cortex

• Color Constancy– Perceiving familiar objects as having consistent

color, even if changing illumination alters the wavelengths reflected by the objects• Perception of color is influenced by the context