Physio CH.5 Vision

Sensation

Process by which NS takes in information from the environment (through activation of sensory receptors).

Perception

Manner in which the brain interprets nerve activity of sensory neurons. Differ qualitatively from the physical properties of a sensory stimulus.

reception

absorption of physical energy by sensory neurons w/ specialized receptors

transduction

conversion of physical energy into electrochemical patterns of activity.

Sensory Neurons

Receptor potential" - local de/hyperpolarization on a sensory neuron. They are graded potentials (not all or none, proportional, decay in time or space). If there's a bigger stimulus, there's a bigger receptor potential.

Coding

how the brain uses activity of the nervous system to discriminate sensory events

- Meaning of the sensation

- Perception of the stimulus

- Coding requires a "one-to-one correspondence"

The Eye

- Light enters through the pupil

- Focused by lens

- Projected to back (retina)

- "Upside down and backwards"

Fovea *

Maximal acuity

Humans

- Directly in the back of the pupil

- Dense receptors

- Less tissue for light to pass through

Neural Pathway:

Begins with retinal stimulation

- The neural signal travels from the inside out

Rods/cones → bipolar cells → ganglion cell

Optic Nerve

(cranial N II)

Axons of ganglion cells

There are no action potentials before the ganglion neurons. Receptors and bipolar do not have action potentials but they do have graded receptor potentials.

Blind Spot

- Optic nerve exits eye

- No receptor cells

- Brain fills in missing sensory information to provide a continuous perceptual experience

Reception

CONES: 4 million cones synapse on 1 million ganglion cells (fewer-to-one)

RODS: 100 million rods synapse on 1 million ganglion cells (many-to-one) provide better low light sensitivity

Transduction

-photoreceptors : rods and cones

- receptor potential

- graded NT release

- no AP until ganglion

photopigments

- Cis-11 retinal is a light-sensitive molecule that changes shape when light hits it

- Opsin is a protein that holds cis-11 and reacts to its shape change

- Together, they trigger a signal to the brain so you can see

coding

- Interface between sensation and perception

- One-to-one correspondence between property of stimulus and neural activity.

- Young-Helmholtz (1800s)

3 kinds of cones:

Each maximally sensitive to different wavelengths.

- short blue

- medium green

- long red

Trichromatic Theory

theory of color vision that proposes three types of cones: red, blue, and green

- EVIDENCE

psychophysical - mixing light

biological - different opsins

Opponent Processing Theory

- Opponent processing theory says colors are seen in opposite pairs (red-green, blue-yellow, black-white)

- When one color in the pair is activated, the other is inhibited

- This is why you can't see certain combinations (like reddish-green) and why afterimages happen

Retinx theory

- Explains: color consistency and Brightness constancy.

Brain compares Current input <--> internal standard

Color constancy

The ability to recognize the color of an object, despite change in lighting.

- Perceptual continuity has significant survival value.

- What we perceive is not always correlated to what is stimulating the receptors.

Compares input ( those differences) to an internal standard.

Then subtracts out the influence of ambient background

So perception of the color remains constant.

color contrast

perceived brightness of a color, varying depending on background

stimulations contrast

perceived brightness varies depending on background.

contrast

perceptual enhancement of color/brightness that depends on background

Contrast of borders

- discriminate boundaries

- foreground, background

- edge from center

lateral inhibition

Neural phenomenon

- Stimulating one cell results in inhibition of neighboring cells

- Enhances contrasts as borders

- Edges of objects appear sharper

Horizontal cells Produce lateral inhibition and have Graded potentials (inhibitory)

inhibition is strongest where?

Inhibition is graded so at the center of

light stimulation there is more excitation so

The inhibition is the strongest. It gets

gradually weaker as it moves away.

hermann grid

A geometrical display that results in the illusion of dark areas at the intersection of two white "corridors." This perception can be explained by lateral inhibition.

recpetive fields

- Part of the retina which causes the target neuron to fire.

critical to discovery that diff parts of cortex resp to diff

types visual info

- Only the center one gets excited, the off-center ones are

inhibited.

Feature detectors

specialized neurons in the brain that respond to specific visual features (like edges, lines, angles, or motion)

- They are mainly found in the visual cortex

- Each detector helps your brain break down and recognize what you're seeing

central visual pathways

Specialization at different levels of these pathways

Retina → ganglion → LGN → occipital lobe

Specialization of Cortical processing

Projections from visual cortex: 2 major pathways called processing streams

Within visual cortex: V1→V2→V3→V4→V5

Dorsal pathway

where/how pathway

- occipital lobe --> parietal lobe

- location, damage: identify = ok

- grasp, walking away, memory for location

Ventral stream

what pathway

- occipital lobe --> ventral formal

- object identity

- damage: location, walk around = ok

- problem: visually identify objects , faces, read

Areas of the visual cortex

V1 primary visual cortex / striate cortex - simple cells

Simple shapes

Bar in particular orientation

edges/borders

Stimulus needs to be in a specific location on the visual field.

V1 & V2 secondary visual cortex cells - complex cells

Bar in particular orientation

Especially bar of light moving in specific direction

Stimulus can be in more than one location of visual field

Large receptor field