Sensory Systems

What are the different types of visual receptors?

Rods and Cones

What are the theories of colour vision?

Trichromatic theory

the opponent-process theory

Retinex theory

What is the dorsal path?

• projects through the parietal cortex

  • the how or where pathway

  • important for visually guided movements

What is the ventral path?

• projects through the temporal cortex

  • what pathway

  • identifying and recognising objects

How do we localise sound?

• Time of arrival

• Sound shadow

What are our chemical senses?

Taste and smell

what is the difference between mechanical and chemical senses?

Chemical senses respond to the presence of chemicals, while mechanical senses respond to mechanical displacement of receptors

Cellular Organisation of the Retina

1. Bipolar cells receive input from visual receptors at the back of the eye

2. Bipolar cells then synapse with ganglion cells

   • axons of the ganglion cells form the optic nerve

3. Amacrine cells synapse with bipolar and ganglion cells
   (indirect pathway from bipolar to ganglion cell)

The Fovea

• Central portion of the retina allows acute and detailed vision

• packed tight with receptors

• each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell

  • knows exactly where the light fell

  • high spatial resolution

  • poor low light vision

The periphery of the retina

• everywhere else

• a greater number of receptors converge into bipolar and ganglion cells

  • low spatial resolution

  • convergence of input allows for greater perception of much fainter light in peripheral vision

• better in the dark

Convergence of input

1 con to 1 bipolar cell

Lots of rods to lots of bipolar cells

Photoreceptors

• respond to photos of light

• protein structure that is holing sodium channel open sending inhibition to a bipolar cell when there is no light, when it receives light it changes structure and closes sodium structure and stops sending inhibition and starts sending excitation

• light changes the photopigment

Rods

• most abundant in the periphery of the eye and respond to faint light (120 million)

• 10% of visual experience

Cones

• most abundant in and around the fovea (6 million per retina)

• more light is hitting the fovea so they make up more

• colour and detailed vision

colour vision

The perception of colour is dependent upon the wavelength of the light that the object reflects

Trichromatic theory

• Colour perception occurs through the relative rates of response by three kinds of cones

  • short wavelength (blue)

  • Medium wavelength (green)

  • Long wavelength (red)

• Each cone responds to a broad range of wavelengths, but some more than others

• The ratio of activity across the three types of cones determines the colour

The opponent-process theory

• Suggests that we perceive colour in terms of paired
  opposites

  • Red vs. green

  • Blue vs. yellow

  • Black vs. white

• A possible mechanism for the theory is that bipolar
  cells are excited by one set of wavelengths and
  inhibited by another

Retinex theory

• All of the other 2 theories is happening

• Suggests that the cortex compares information from various parts of the retina to determine the brightness and colour for each area

• Explains how colour perception is consistent despite changes in ambient lighting

  • colour constancy

• Context for colour perception

Retina to thalamus

• Ganglion cell axons form the optic nerve

• the optic chiasm is the place where the two optic nerves leaving the eye meet

• Humans→ half the axons from each eye cross to the other side of the brain (eyes too close together)

• most axons of the optic nerve terminate in the lateral geniculate nucleus in the thalamus

  • some axons project to the superior colliculus → receives visual information

The Path of Visual Input

Retina → optic chiasm → lateral geniculate (thalamus)→ primary visual cortex

Optic chiasm

• split information half left goes to the right, half right goes to the left

• blue meets blue, red meets left

• organised by visual field

Ganglion cells in the pathway to the thalamus

Midget, parasol, bistratified

Midget ganglion cells

• Parvocellular pathway

• receive information from midget bipolar cells

• 1 → 1 →1 (fovea)

• carry it to the lateral geniculate nucleus to parvocellular neurons that reside there

• detailed vision and colour vision

Parasol ganglion cells

• Magnocellular pathway

• large receptive fields and coming from the periphery

• project the magnocellular cells in the lateral geniculate nucleus

• detecting movement

• not so much colour vision

Bistratified ganglion cells

• Koniocellular pathway

• some respond to colour

• some have large receptive fields that allow them to see movement

• super small koniocellular neurons and scattered across the thalamus

Primary visual cortex (V1)

• receives input from the lateral geniculate nucleus

• located in the occipital lobe

• important for conscious visual perception

• Hubel and Weisal distinguished various types of cells in the visual cortex

  • preference for bars of light 3 different types

  • Simple

  • complex

  • end-stopped/hypercomplex cells

Secondary visual cortex (V2)

• receives information area from V1

  • V1 V2 loop

• Also receives input directly from the pulvinar nucleus of the thalamus

  • PN receives information from the superior colliculus

    • attention gaze type of shifts

• Performs additional processing of the visual input

  • important for binocular disparity

    • depth perception

    • comparisons of the eyes

Simple cells

cells that light up in neuro imaging when a specific orientation of a bar of light is presented

Complex cells

• respond more to a bar of light across movement

• larger visual fields

• detect motion

• responds to moving bars of light

End-stopped/hypercomplex cells

• clear inhibitory and clear excitatory zones

• good for detecting things

Facial recognition

• Complex

  • identification, emotion, social cues, communication

• 2 systems

  • Core

    • Inferior occipital gyri, superior temporal sulcus, lateral fusiform gyrus

    • that is a face

  • Extended

    • Using that in some way, recognition of a person, emotion, social cues

    • Intraparietal sulcus, auditory cortex, limbic system, anterior temporal lobe

Motor perception

• The middle temporal cortex contains cells with directional selectivity

  • a preference for the movement of an object in a particular direction

• Medial superior temporal cortex

  • optic flow

  • live streaming

  • everything moves relative to you

  • I am moving they are static

• Both receive input from the magnocellular pathway

Audition

• hearing alerts us to useful information

• hearing is the detection of sound waves

  • sound waves are transduced

Process of transduction

• Outer ear

  • Pinna

• Middle ear

  • tympanic membrane vibrates at the same frequency as the sound

  • vibrates the malleus, incus, stapes to mechanical displacement

  • gets smaller and smaller down to the Oval window intensity is going up

    • window into the inner ear

• Inner ear

  • Cochlea

    • three fluid filled cavities

    • displacement of fluid

    • basilar membrane (has the hair cells) and tectorial membrane press against each other which bends hear cells that have mechanically gated channels and opens sodium channels

Theories of pitch perception

Place theory

frequency theory

Pitch theory

Place theory

• each are along the basilar membrane has hair cells sensitive to only one specific frequency of sound wave

• is it close to the oval window or far away

• the biggest displacement of a high frequency is close the start of the basilar membrane

• low frequency the back

frequency theory

the basilar membrane vibrates in synchrony with the sound and causes auditory nerve axons to produce action potentials at the same frequency

Pitch theory

• combines modified versions of both

  • low frequency sounds best explained by the frequency theory

  • for high frequency sounds best explained by place theory

Auditory cortex

• The primary auditory cortex (A1) is the destination for most information from the auditory system

  • located in the superior temporal cortex

  • Mostly embedded within the Sylvian fissure

  • Each hemisphere receives information from both ears

  • Cochlear nucleus → superior olive → Medial geniculate

function of the auditory cortex

• Primary sight for processing auditory information

• Tonotopic map

  • cells are more responsive to preferred tones

sound localisation

• Sound localisation does not rely on activity of particular sensory receptors

  • location must be computed within the CNS

• 2 cues sound shadow and time of arrival

sound shadow

• intensity of the sound

• sound from the right will be louder in my right ear than left ear (head sound barrier)

time of arrival

right sound comes to right ear first

vestibular sensation

• vestibular organ is composed of several structures which detect the position and movement of the head in the ear

• the saccule and utricle contain otoliths and hair cells and detect linear acceleration

• three semi-circular canals contain a jellylike substance and hair cells detects rotational acceleration

  • 90* angles from each other

Touch sensation

• Mechanoreceptors transduce touch/pressure

  • touch displaces the receptors and opens sodium channels

• Include a range of receptor types

  • Meissner’s corpuscles

    • light pressure and fast adapting

  • Merkel’s disks

    • light pressure and slow adapting

    • watch on wrist

  • Ruffini endings

    • deep pressure and slow adapting

    • knee movement

  • Pacinian corpuscles

    • deep pressure and fast adapting

      • respond selectively to vibration on skin

• Touch sensation to the CNS

  • Information from touch receptors in the head enters the CNS through the cranial nerves

  • information from receptors below the head enters the spinal chord via spinal nerves

Taste

• Organisation of taste receptors

  • papillae are structures on the surface of the tongue that contain the taste buds

  • each papillae may contain up to ten or more taste buds

  • each taste bud contains approx. 50 million receptors

5 basic tastes

• Sweet

• sour

• bitter

• salty

• Unami

Labelled-line principle taste

• each receptor responds to a limited range of stimuli and ends a direct line to the brain

• taste depends on which receptor is activated

• we do have a sweet receptor

• problem taste experience is so complex there is not a labelled line for each taste

Across-fibre pattern principle taste

• each receptor responds to a wider range of stimuli and contributes to the perception of each of them

• pattern of activity dictates taste perception

Taste transduction

• The saltiness receptor permits sodium ions to cross the membrance

  • results in depolarisation

  • sodium from that salt enters into the receptors

• Sour receptors detect the presence of acids

  • hydrogen ions enter into the cell and act on the depolarisation of those cells

• Sweetness, bitterness, and umami receptors activate a second messenger system

  • dislodges something that then send something to the cell

Taste coding in the brain

• three cranial nerves are responsible for carrying taste information from different parts of the tongue

• cranial nerves project to a structure in the medulla → the nucleus of tractus solitarius

• Projections from the NTS reach the thalamus, which then relays taste information to the primary gustatory cortex (anterior insula)

smell

Perception of the activation of olfactory receptors

Olfactory receptors

• Olfactory cells line the olfactory epithelium in the rear of the nasal passage

• Vertebrates have hundreds of olfactory receptor types, which are highly responsive to some related chemicals and unresponsive to others

• transduction occurs via a second messenger system

• doesn’t travel very far

Olfaction in the brain

• axons from olfactory receptors carry information to the olfactory bulb

  • similar chemicals excite neighbouring areas

  • coding is determined by which part of the olfactory bulb is excited

• Olfactory information then projects to a range of other regions

  • collectively referred to as the olfactory cortex

  • research continues to profile the regions involved in olfaction