Visual Information Processing

- Sensation: Process of detecting stimuli in our environment via sense organs & communicating this infomation to the brain.

- Perception: An active process of interpreting the stimulus.

- Bottom-up: When the environment (stimuli) influence our thinking.

- Top-down: When our thinking influences how we see the environment.

Retina-geniculate-striate pathway

- Magnocellular pathway:

     - Movement 

     - Input from rod cells (light and dark photoreceptor cells)

- Parvocellular pathway

      - Colour, fine detail

      - Input from cone cells (bright light photoreceptor cells) 

Two Streams Hypothesis (Milner & Goodale, 1992):

- Dorsal Stream 

     - Known as the 'where stream'

     - e.g., motion processing

     - Links to magnocellular pathway

- Ventral Stream 

     - Known as the 'what stream'

      - e.g., object recognition 

      - Links to parvocellular pathway

Trichromatic Theory (Young-Helmholtz)

- The human retina contains three different recptors for colour

       - One is most sensitive to red 

       - One is most sensitive to blue

       - One is most sensitive to green

Opponent-process Theory (Hering, 1870)

- Ability to see colour is controlled by 3 recptor complexes:

      - Red-green complex

      - Blue-yellow complex

      - Black-white complex

- These cells can only detect one colour of each complex at a time because they oppose one another.

- Provides some explanation for colour-blindness

Dual-process Theory 

- Adopts ideas from both Trichromatic and Opponent-process theory

- Photoreceptors in the retina are trichromatic 

- There are 2 types of retinal ganglion cells (opponent cells)

      - Yellow-blue and Green-red channels 

Dual Process Theory Cont.

- The Yellow-blue channel represents the difference between the sum of medium and long wavelength cone cells on one hand and the short wavelength cone cells on the other. the direction of difference determines whether blue or yellow is seen.

-The Red-green channel represents the difference between the sum of medium and long wavelength cone cells. The direction of difference determines whether red or green is seen.

Monocular Cues:

- Linear Perspective: Parallel line sconverge into vanishing points as they recede into the distance.

- Interposition: Near objects will partially obsure far objects

- Motion Parallax: As we move around, nea robjects move faster and in the opposite direction to objects further away.

Binocular Cues:

- Accomodation  -

                               > Oculomotor cues 

- Convergence    -

- Stereopsis (binocular disparity)

- Perception of motion is a distinct visual process 

- Movement across the retina 

- Foreground movement relative to the background

- Stroboscopic movement (apparent motion) e.g., picture books 

- Uses binocular and monocular depth cues to give the illusion of depth.

- Uses stroboscopic motion to give the illusion of movement 

      - Frame rates must be high enough to give a percept like the external world 

      - Low frame rates can induce disorientation

      - VR developers target 90FPS in their software

- Clinical interventions: treatment for anxiety disorder, PTSD, persecutory delusions

- Researching consiousness

     - Visual hallucinations

     - Body ownership

- Part of the Multi-store Model aka Visual sensory memory

- Briefly holds incoming visual information

- Encoded in memory or vanishes 

- Perceptual phenomenon rather than a memory store

Sperling (1960) - memory recall

- Whole report condition

    - 3-4 items recalled 

- Partial report condition 

    - Tone played after the matrix had disappeared indicates which row to report 

     - 3-4 items still recalled 

     - We can view the entire matrix for a short time, & read out any row before it fades

- Duration of iconic memory

       - immediate recall ~ 80% items recalled

       -  250ms delay ~ 50% items recalled 

       - 500ms delay ~ 3-4 items recalled 

- Sperling suggested iconic memory fades after around 500ms

- Others have provides varying estimates 

      - 300ms (Massaro & Loftus, 1996)

      - 1000ms (Clarke & Mack, 2015)

- Erasure: Loss of the original information when contents of iconic memory are degraded by subsequent stimuli.

- Visual masking now widely used in perceptual experiements 

        - Backward masking: mask is presented after the target stimulus 

        - Forward masking: mask is presented before the target stimulus