Lecture 7 - Sensation and perception

what is significant about the stretch reflex circuit?

the strength of the stimulus determines the strength of the response

what is signal divergence

when one neuron sends its signal to multiple other neurons or target cells

what is signal convergence

when many neurons synapse onto a single neuron, which integrates different types of information

what are sensory modalities?

Sensory modalities are the different kinds of senses or types of sensory experiences your body can detect. Each modality refers to a distinct way your nervous system picks up information about the world or your body.

what are some examples of sensory modalities?

vision

smell

taste

touch

thermal sense

pain

hearing

balance

proprioception

what is transduction in sensory and perception

conversion of physical energy from a stimulus into a change in membrane potential in a receptor cell

what are transducers?

receptor cells

what are receptor potentials

local changes in membrane potential

what type of energy do all of the senses use to communicate with the brain?

APs

describe how sensory information processes

each sensory system has a distinct sensory pathway in the brain and passes through stations during processing

where do most sensory pathways pass through?

the thalamus

where does sensory pathways terminate in the brain

cerebral cortex

what is psychophysics

An investigation of the relationship between physical stimuli, sensation, and perception

what is the typical relationship in psychophysics

stronger stimulus → evokes more APs

stronger stimulus → perceived as stronger

what is generator potential

A local, graded electrical change that happens in a sensory receptor cell when it detects a stimulus (like light, pressure, or vibration).

what is the difference between generator potential and action potential

generator potential

• graded, local

• varies with stimulus

action potential

• all-or-non,

• does not vary with stimulus, same strength at all times

what is contrast sensitivity

ability to detect differences in light and dark (contrast) between parts of an image or scene.

what are the low level processing of vision

contrast

colour

disparity

orientation

movement direction

what is contrast

the difference in brightness or color that makes an object stand out from its background.

what is colour

Colour is how your brain interprets different wavelengths of light.

what is disparity

the slight difference between what your left and right eyes see — it’s key for depth perception (stereopsis).

what is orientation

Orientation is the angle or tilt of edges, lines, or patterns.

what is movement direction

This is your ability to see which way something is moving.

what are the intermediate level of visual processing

contour integration

shape discrimination

surface properties

object motion / shape from kinetic cues

surface depth

what is contour integration

This is the brain’s ability to connect separate edge or line segments into a continuous boundary or contour.

what is shape discrimination

The ability to tell apart or recognize different shapes.

what is surface properties

Attributes like texture, color, glossiness, or shading that help identify what a surface is made of.

what is object motion / shape from kinetic cues

Using movement to perceive the 3D structure of an object.

what is surface depth

The perception of how near or far a surface is in 3D space.

what is bottom up processing

perception starts with the raw sensory input, and the brain builds up a mental image step by step from that input — without prior knowledge

what is top down processing

when your brain uses prior knowledge, expectations, context, or experience to interpret and make sense of incoming sensory information.

Describe how visual perception is a constructive process.

• Combining raw data with prior knowledge:
  The brain uses incoming visual signals and what you already know, expect, or assume, to build what you “see.”

• Filling in gaps:
  Your brain completes missing parts of objects and scenes (e.g., you still perceive a whole object when it’s partially hidden).

• Resolving ambiguity:
  It chooses the most likely interpretation when visual input is unclear or conflicting (e.g., optical illusions show this).

• Using context:
  Surrounding cues help determine colors, depth, shapes, and motion — your brain adjusts how you see things based on context.

what is light

a band of electromagnetic radiation; comes in packets of energy called quanta

what is wavelength

distance between two adjacent crests of vibratory activity

what are photons

human visual system responds only to quanta of wavelengths between 400 - 700 nm; quanta in this range are called photons

label the eye

what is refraction

bending of light rays

how is refraction done in the eye

bending of light rays — done by the cornea and lens to form the image on the retina

what is accomodation in terms of the eye

Accommodation is the process by which the ciliary muslces of the eye changes its shape to focus on objects at different distances.

what is the role of the pupil

control the amount of light entering the eye by controlling the iris

what controls pupil dilation

Sympathetic nervous system

what controls pupil constriction

parasympathetic nervous system

what controls eye movement

extraocular muscles

what does the nuclear layers contain

cell bodies/nuclei of retinal neurons

what does the plexiform layers contain

synapses and axons

how the structure (anatomy) of the retina relates to its function

The retina’s anatomy is perfectly organized to convert light into neural signals and start visual processing.

what is the visual field

whole area one can see without moving head or eyes

what is visual acuity

sharpness of vision; falls off towards the periphery of the visual field

where is visual acuity the greatest and why

fovea

because there’s a high concentration of cones

what is optic disc

where blood vessels enter and leave the eye; no photoreceptors here results in a blind spot

summarize retina anatomy

Photoreceptors (rods and cones) are at the very back of the retina circuitry

• Rods and cones connect to bipolar cells

• Bipolar cells connect to retinal ganglion cells (RGCs)

• RGC axons leave the eyeball at the optic disc forming the optic nerve – creates a

blind spot

• Horizontal cells laterally interconnect rods and cones

• Amacrine cells laterally interconnect bipolar and ganglion cells

• Ganglion cell bodies are pushed to the side at the fovea (less light dispersion)

describe the structure of rods and cones

an outer segment, inner segment, cell body, synaptic terminal

what neurotransmitter does rods and cones release?

glutamate

what are opsins

light sensitive proteins

what is rods’ role in vision

better for night/low-light vision

what is cones’ role in vision

better for color and sharp central vision

what do opsins do

absorb photon and undergo conformational change, causing signal cascade, leading to hyperpolarization of the photoreceptor

which is more sensitive, rods or cones

rods

where are rods located the most in

periphery

where are cones located the most in

fovea

use the concept of convergence to distinguish between rod and cone neural circuitry

rods

• high convergence

• more sensitive in the dark but blurry image

• rods pool signals to detect dim light, but can’t distinguish fine detail

cones

• low convergence

• less sensitive but sharp, detailed vision

what is the receptive field (RF)

the region of the retina that elicits changes in the firing of a given sensory neuron (ganglion cell)

describe the receptive field in fovea

small, supporting sharp vision

describe the receptive field in periphery

large, increases sensitivity but reduces detail

Distinguish between on-center and off-center receptive fields

On-center cells are activated (excited) when light hits the center of their receptive field and are inhibited when light hits the surround.

Off-center cells are the opposite: they are inhibited by light in the center and excited by light in the surround.

Describe how different bipolar cells might contribute to each type of receptive field

“on” bipolar cells

• metabotropic glutamate receptors

• in dark: continuous glutamate release from photoreceptor hyperpolarizes the cell

• add light: less glutamate release from photoreceptors → depolarization

“off” bipolar cells

• ligand gated ionotropic glutamate receptors

• in dark: glutamate binds - opens cation channels → depolarizes cell

• add light: less glutamate release from photoreceptors — less bound ionotropic receptors, cell becomes more hyperpolarized

what is lateral inhibition

Lateral inhibition is a process in the retina (and other sensory systems) where an excited neuron reduces the activity of its neighbors.

how does lateral inhibition connect to contrast enhancement?

Lateral inhibition helps the retina sharpen edges by increasing contrast at boundaries between light and dark.

• Lateral inhibition is always happening, but its effect is most noticeable at borders where there is a sharp contrast, because it exaggerates differences and makes edges clearer.

what is population coding

information about a stimulus (like its location, intensity, or type) is represented by the pattern of activity across a whole group of neurons, not just by a single neuron acting alone.

what are retinal ganglion cells

neurons in the retina that collect visual information from the bipolar and amacrine cells and send that information to the brain.

They are the final output cells of the retina.

what are photoreceptors

sensory cells in the retina that detect light and convert it into electrical signals — this process is called transduction.

describe the process when light hits photoreceptors

light → hyperpolarization → less glutamate release to bipolar cells

what does the magnitude of the hyperpolarization in photoreceptors determine

determines the reduction in neurotransmitter release

what are horizontal cells

make contacts photoreceptors and bipolar cells

what are amacrine cells

contaaact bipolar and ganglion cells

which cells generate only graded local potential and which can fire APs

all cell types except ganglion generate only graded local potentials

ganglion cells fire APs