Perception

• Commonalities among Perceptual Systems

• Vision

• Visual Attention

Commonalities among Perceptual Systems

• Subjective experience depends on the path stimulated

• All paths (except olfactory) have a relay station at the thalamus

• There are Sensory Maps: – A correspondence between external dimension and internal

representation (ex: location, orientation, motion, pitch)

Cortical Maps: External dimension---> internal representation

• Some areas are overrepresented

Somatosensory representation

Motor cortex Representation

Skin surface

Based on skin surface, the hands & lips are overrepresented in cortex, and

the back and arms are underrepresented in cortex

Based on sq footage, New Jersey is overrepresentedWyoming is underrepresented in the house in Congress

USA congressional representationUSA

Primary Visual cortex representationVisual field

Based on visual field, central vision (fovea, in red) is overrepresented in cerebral cortex

peripheral vision (in blue) is underrepresented

Explain what is represented by this picture

Cortical Maps: Stimulus properties need to be ‘recreated’

vision: 2D --> 3D audition: 1D --> 2D

Cortical Maps: • Remapping occurs due to lesion, learning, etc.• Phantom limbs, • Sensory deprivation• Learning

Commonalities among Perceptual Systems• Sensory receptors do transduction

– (ex: mechanical --> electrical)

• Sensory receptors elicit graded potentials – response amplitude is equivalent to stimulus intensity

Perceptual Dimensions

Wave amplitude

Purity of the wave

Wave frequency

photorreceptors are responsive to a subset of frequencies - different wavelengths induce different color perceptions

Vision

“El ojo que ves no es

ojo porque tu lo veas,

es ojo porque te ve”

Antonio Machado

“The eye you see

is not an eye due to you seeing it,

It’s an eye because it sees you”

Vision: Outline

• Eye – Color vision– Receptive Field – Edge Detection

• Visual Path– thalamus (LGN) – primary visual cortex

• Orientation sensitive; Spatial frequency

– Ventral Pathways • Area MT (motion), Object Recognition, Area V4 (color)

• synesthesia

– Dorsal Pathway • Spatial Attention

• Hemispatial Neglect

The eye is a device 'designed' to see

- transparent medium (cornea, aqueous humor, lens, vitreous humor)

- variable-focus lens lens- diaphragm with variable diameter iris - a light-sensitive layer of sensors retina - which lies at the focal plane of the lens - muscles that move the eyes in conjunction - edge detectors

• But the eye isn’t perfect, there is a blindspot where the axons of ganglion cells leave the eye.

Lateral visual field

Medial Retina

Retina: The sensory receptor structure of the visual system

• photorreceptors: cones (color vision) and rods night vision• fovea: center of the retina, high concentration of cones• optic disk (blindspot)• Direct view of arteries (clinical importance)

One Cones --> one ganglion cell high acuity (fovea) Many Rods --> ganglion cells. High sensitivity (periphery)

(e.g, night vision)

Tri-chromatic theory – Blue, red, & green “color”

receptors– But some colors don’t mix!

Peak sensitivities of the three cones

Opponent process theory Red vs. Green; Blue vs. Yellow- Negative afterimage- But there is no ‘yellow’ receptor!

COLOR VISION

(-)

BLUE

(-)

GREEN

(+)

RED

(+)

YELLOW

See example in PsyCog beta (cd) Fig A.3.1 Color Negative Afterimage

Receptive field• is that portion of the visual field in which the presentation

of visual stimuli will produce an alteration in the firing rate of a particular neuron

Edge detection

See example in PsyCog Brightness contrast Fig A.4.1

Hermann Grid

+- -- -

- -

- -

- -- -

+- -- -

- -

- -

- -- -

Inhibition (-)

Excitation (+)+

- -- -

- -

- -

- -- -

VISUAL PATHWAY

LGN thalamic organization

• Magnocellular – M ganglion cells– large receptive fields– motion detection– locating stimulus in space – dorsal cortical stream– parietal lobe

• Parvocellular– P ganglion cells – small receptive fields– Object identity– Color recognition– ventral cortical stream– temporal lobe

Concentric receptive fields (center surround)

Primary visual cortex (V1)

• LGN cells with concentric receptive field provide input to simple cells in V1

• V1 cells respond to lines – of particular orientations – of particular widths.

• There is a topographic map

Neurons respond to specific spatial frequencies

‘Blurry’ As nearsighted

Area MT: motion perception

• Take home message :– Different regions (modules) of

the visual cortex respond to particular features of visual information such as

• orientation, • spatial frequency, • movement,

• color.• retinal disparity, and even

• Objects• faces

Dorsal

Ventral

Object recognition: Ventral Pathway

Lesion of ventral pathwayAgnosia

fMRI: Object recognition

Neuroimaging of face, bird and car experts

“Face Experts”

FusiformGyrus

FusiformGyrus

CarExperts

BirdExperts

FusiformGyrus

Gauthier et al., 2000

Cars-Objects Birds-Objects

ControlGroup

AutismGroup

Hypoactivation of fusiform face area Schultz, et al. 2000

Faces

FusiformGyrus

FusiformGyrus

Children with autism as face “novices”

• synesthesia

Left overs

The creationist claim: ‘the eye, it is perfection, is proof of God’s existence’

The creationist argument:The arrangement of structures in the eye suggests that it has been designed to fulfill a specific function (that is, it cannot have occurred by accident)

The flaws: ??

The evolution argument:The arrangement of structures in the eye suggests that it has been designed to fulfill a specific function (that is, it cannot have occurred by accident).

- However, the ‘design’ occurs not by divine intervention but by a combination of random variation and natural selection

The flaws: ??

Possible criticism:

Circularity: the structure (eye) is said to have evolved to perform a function (vision) that we know in advance, the structure performs very well. It’s like saying that:

- The nose evolved to support glasses- Horses evolved to support saddles, etc.

• Any answer to this criticism?

Answer to the circularity challenge

• Convergent Evolution: species phylogenetically distant develop a similar device

• Fewer functions than biological systems

• Functions can be related to the overall function of survival and reproduction

Evolution by Natural Selection• There are heritable traits• Random Changes: There are random and small

modifications in the heritable traits. Most of them are detrimental, but some few are beneficial.

• Selection Pressure: There is selection pressure so that the (few) random changes in heritable traits that favor survival and reproduction are retained through generations

• natural selection provides a good account of adaptive complexity (any system, composed of many interacting parts, where the details of the parts and arrangements suggest design to fulfill some function. Example: the eye)