Neuro Ch 5

constructive process, take in light and infer what must be present in the world

• we see what we expect
• unaware to things we aren't conscious to 
• we see what we need to know
• hard to see without expectations

sensory transduction: process by which a signal from the world is transformed into electrochemical signals in the nervous system

photoreceptors in the retina transduce a narrow band of the electromagnetic spectrum (“visible light”)

light passes through cornea, iris controls amount of light that enters, goes through pupil and lense and focuses on retina

rods and cons capture light in electrochemical activity through phototransduction

• light strikes pigment molecule in the
• photoreceptor, causing the pigment molecule to break into pieces, which then act on proteins in the cell to change its resting membrane potential

photoreceptors --> bipolar cells ---> ganglion --> optic nerve

amacrine and horizontal cells communicate with other parts of the retina

• LIGHT
• sensitive to light but do not respondselectively to any particular wavelength
• detects degrees of light, good for dim areas

concentrated in periphery

• COLOUR
• less sensitive to light, but the 3 cone types are maximally responsive to different wavelengths
• bright environments

concentrated in fovea (central vision)

• cones > rods b/c
• 1. more light can access photoreceptors directly, smaller ganglion
• 2. each cone connects to its own bipolar cell and then its own ganglion

• each retinal ganglion cell only responds to a specific location of the visual scene (receptive field)
• info of where light originated maintained 
• optimised to detect edges via differences in light levels from one to the next

• on center vs off center
• think circle with circle inside

• work best when centre on and surround off
• weak when all on

• best when centre off and surround on
• weak when all on

L and R visual fields, temporal semiretina signals stay ipsilateral, nasal semiretina move contralaterally, go to visual cortex

• parvocellular: input from cones. Fine details, form, colour
• magnocellular: input from rods. Depth, brightness, movement

• retinal ganglion cells are sent to the lateral
• geniculate nucleus (LGN) in the thalamus

axons travel from the LGN to V1 via the optic radiation

• V1 contains a retinotopic map: each neuron responds to a particular part of the visual field
• v1 has new neurons

• simple cells: preferred orientation at particular place
• complex cells: preferred orientaiton at any place

• each eye gets slightly different view of the world, how we see in 3D
• stereopsis: using difference to infer depth

• direct connections from V1, also retinotopically (topographically) organized but with larger receptive fields
• illusiory contours

• cells respond to more abstract stimuli in their receptive fields (e.g., shapes, textures, movement)
• V5: specialized for motion detection, sends signals to the parietal lobe

• representation of abstract and complex stimuli (e.g., faces, scenes, objects)
• no longer organized retinotopically

ventral and dorsal

• WHAT pathway
• from parvocellular cells in LGN, through V1, V2, V4 and inferotemporal cortex
• tools, animals faces
• position and size invariance

• WHERE pathway
• from magnocellular cells in LGN, through V1, V2, and V5

recognises everything as faces

• prosopagnosia: inability to recognize face
• visual agnosia: an impairment in the ability to recognize objects by sight

small number of clustered neurons that become active in response to a specific visual stimulus

for stimuli that we are expert at

• many neurons distributed over cortical regions are involved, showing varying levels of activity
• the pattern is important

• perception of an opposite direction of motion after prolonged viewing of a moving stimulus
• adaptation to reduce response over time

neurons will be firing below baseline in favour of the opposite direction of motion

• happens after damage to motion detecting areas in the dorsal stream (e.g., V5)
• can't detect motions, just looks like a bunch of still photos getting closer

• directing attention to particular locations in space
• dorsal stream is critical for guiding and adjusting the “spotlight” of attention

• hemineglect: ignores half of the world; caused by damage to the right parietal lobe
• balints syndrome: cannot comprehend the big picture of a visual scene though they can see objects individually
• simultanagnosia: inability to recognize multiple elements in a scene

if different images are presented to each eye, perception shifts back and forth between them

• the brain infers what is likely
• to be out there given incoming signals