PSYC 304 - Sensation/Perception

transduction

converting physical energy into membrane potential changes using receptor cells

• receptor cells = transducers

• sensory info → becomes APs

generator potential

local, graded change in membrane potential in receptor cells

• caused by stimulus, eg: stretch, brightness

action potential

• all-or-none

• triggered when a generator potential reaches the threshold

• travels along the axon to CNS

psychophysics

study of the relationship between stimulus, sensation, and perception

• helps measure subjective perception in an objective way

• compares physical inputs with each individual’s perception

perception is built from…

simple to complex features - then, brain fills in gaps using context clues/past experiences

low level processing

orientation, colour, motion

intermediate processing

shape, contours, depth, texture

expectations - effect on perception

• top-down processing

• influences how you perceive

  • eg: assuming depth or recognizing incomplete shapes

bottom-up processing

• starts with raw sensory input

• builds perception from basic features upward

• data-driven

top-down processing

• driven by prior knowledge, expectations, and context

• influences what we perceive

• eg: priming

receptive field

specific patch of photoreceptors on the retina that sends input to one RGC thru bipolar cells

• stimulation in a receptive field (via light) changes the firing rate of this one RGC

size of receptive field

• smaller in fovea: low convergence, high resolution, detects fine detail

• larger in periphery: high convergence, low resolution

shape of receptive field

• “on” or “off” center-surround:

  • ON-center: center = excitatory, surround = inhibitory

  • OFF-center: center = inhibitory, surround = excitatory

• integration of these signals summate from photoreceptors → bipolar cells → RGCs → brain

convergence in rods

high convergence → many rods → few bipolar → 1 RGC

• sensitive to low light, but less spatial detail

convergence in cones

low convergence → 1 cone → 1 bipolar → 1 RGC

• high acuity and colour, but requires daylight

on-center receptive fields

• light in center = excitation

• light in surround = inhibition

  • helps detect brightness, light edges

off-center receptive fields

• light in center = inhibition

• light in surround = excitation

  • helps detect shadows, dark edges

“on” bipolar cells - receptor type

mGluR

“on” bipolar cells in dark

• photoreceptors release glutamate

• glutamate inhibits on bipolar cells using mGluR

• ON cells are hyperpolarized and cannot excite RGCs

“on” bipolar cells in light

• photoreceptors release less glutamate

• less inhibition, so ON bipolar cells depolarize

• send excitatory signals to ON-center RGCs

  • ON bipolar cells activate ON-center cells, when light hits the center of the receptive fields

“off” bipolar cell receptors

ionotropic glutamate receptors

“off” bipolar cell in dark

• photoreceptors release glutamate

• glutamate excites OFF bipolar cells

• OFF bipolar cells are depolarized, which excites OFF-center RGCs

  • Off bipolar cells activate OFF-receptor cells when center = dark, surround = light

“off” bipolar cell in light

• less glutamate → less excitation

• OFF bipolar cells become hyperpolarized

rod morphology

free-floating discs, non-continuous membrane

rod location

within the peripheral retina

rod functionality

• high sensitivity for low-light, grey tones

• for night vision

• low acuity

rods opsins

rhodopsin

cone morphology

discs with continuous membrane

cone location

fovea

cone functionality

• low sensitivity, bright light, colour

• for daylight, colour, detail

• low convergence → high acuity

cone opsins

3 types:

• blue S

• green M

• red L

population coding

• perception based on combined activity pattern of all cone types

• brain compares relative activation of all 3 types to determine the perceived colour

amacrine cells

laterally interconnect bipolar and ganglion cells

horizontal cells

laterally interconnect rods and cones

lateral inhibition

when retinal neurons inhibit their neighbouring neurons via horizontal/amacrine cells

• neurons send inhibitory signals sideways to reduce neighbouring cell activity

lateral inhibition - effect on borders

• when light is evenly distributed, neighbouring cells inhibit each other equally - smooth signals

  • but at edges and borders, light intensity changes sharply

• cells on brighter side = strongly activated, inhibit neighbour cells more

• cells on darker side = receive less inhibition, seem less active compared to neighbours

• this creates contrast enhancement, where bright sides seem brighter and dark sides seem darker

• helps detect edges, borders, object boundaries

RGC types - working in parallel to carry different info

• 20 types of RGCs in retina: each type covers the same visual spot, but processes different info (eg: motion, colour, contrast, timing)

• on-center and off-center respond opposite to light in their receptive fields

RGC parallel info pathway

parallel signals from different RGC types travel together via optic nerve → lateral geniculate nucleus (in thalamus) → visual cortex