stimulus recognition and localisation

Question 1

give two weird animal examples of recognition systems

Answer 1

Echolocation - bats use this to detect the smooth surface of water (to drink from), and to detect their prey

Electroreception - some fish have an electric organ, and create an electric field around themselves
They have electric sensors on their body that can detect changes in the electric field due to presence on objects in the field

Question 2

what are two properties of visual recognition?

Answer 2

Orientation invariance - an object is still recognisable in different orientations

Scale invariance - an object is still recognised as the same thing at different sizes

Question 3

how is object recognition hierarchical?

Answer 3

The further along the visual pathway/ventral stream, the neurons detect more complex aspects of an object

detection of edges and contours - object parts - POV - particular person a car - categorisation etc…

Question 4

what increases along the ventral stream?

Answer 4

The receptive field size of neurons also increases along the ventral stream

Question 5

what was seen when there were lesions in the inferior temporal cortex?

Answer 5

Lesions in inferior temporal cortex decrease ability to recognise objects -

removal/damage of this area causes mice to not recognise objects that they could recognise before, and they cannot learn to recognise new objects

Question 6

information from the retina goes to?
describe the structure of it (input of the different layers)

Answer 6

lateral geniculate nucleus

contains 6 layers
the four outer layers receive input from parvocellular cells as these are responsible for fine spatial details and object recognition more so than magnocellular
the inner two layers receive input from magnocellular

contralateral (receives input from eye on the other side) vs ipsilateral
from outer to inner, it goes C, IP, C, IP, IP C

Question 7

the LGN is deep in the brain so is hard to test on and image its activity…

what do we know about how it works?

Answer 7

It acts as a ‘thalamic’ relay station

The ganglion cell axons make 1:1 connections with the neurons here in the LGN - no high computation going on or integration of signals

Receptive fields of the LGN neurons are similar to receptive fields of the retinal ganglion cells - suggests the LGN doesn’t do much

Question 8

in the LGN, the ganglion cell axons make 1:1 connections with the neurons, indicating low levels of computation.

what two things contradicts this?

Answer 8

Presence of local interneurons suggests there is some computation going on, we just don’t know what it is yet

60% of synaptic input at the LGN comes from the cortex (feedback), not the retina - the brain is putting a lot of effort in to regulate the activity of these neurons

Question 9

where do signals go after RGCs - primary visual cortex (V1) - V2?

Answer 9

splits into two

dorsal pathway (where pathway)
to V3 – medial temporal cortex – parietal cortex

ventral pathway (what pathway)
to V4 to inferior temporal cortex

Question 10

what are the dorsal and ventral pathways responsible for?

Answer 10

dorsal = ‘Where’ pathway - responsible for processing info on the object’s position, speed, direction of movement

ventral =
‘what ‘ pathway - object recognition, terminating in the inferior temporal cortex

Question 11

the cortex is arranged in two ways - what are they?

Answer 11

it is organised into layers, don’t need to know them just appreciate the idea

also organised into four different kinds of columns…

Question 12

explain how ocular dominance columns were found?

Answer 12

Injected radioactive proline into one eye, it was uptaken by ganglion cells and transferred to the brain, and then labelled the V1 cortex

saw dark columns showing that these areas receive input from one eye - the eye you injected the dye into, and the areas not showing radioactivity must receive input from the other eye

alternatively inject cortex with radioactive glucose and stimulate only one eye

Question 13

what are possible reasons for having the ocular dominance columns?

Answer 13

May be due to estimating depth perception, though there is minimal proof of this, some papers claims this organisation does not have a particular function

note - LGN has contralateral vs ipsilateral layers, so these columns the columns must receive input from the particular layers of the LGN

Question 14

what are Blobs and how were they found?

Answer 14

Idk like columns within the ocular dominance columns

Cytochrome oxidase was used to stain a sample - it stained certain neurons better than others, resulting in little blobs

Turns out these neurons receive input from the parvocellular cells of the LGN, and respond to/process info on colour

Question 15

what are orientation columns?

Answer 15

so you’ve got the ocular dominance columns, then going alongside the other dimension (like along the length) e.g. a left eye column is split further into columns that then respond to different orientations

these would be horizontal, then slightly tilted etc… to vertical

Question 16

aside from blobs, orientation columns and ocular dominance columns

what is the other?

Answer 16

motion/direction columns

Question 17

what are simple cells of the V1 cortex?

Answer 17

One of the main kinds of cell receiving input from the LGN

they respond to a specific orientation (shown by Hubel and Wiesel measuring activity from these cells in a cat while it views different orientations of a bar)

Question 18

where are simple cells located?

what are their receptive fields like?

Answer 18

located at layers 4 and 6 of the cortex

Their receptive fields are elongated/bar shaped (within a circle)

so like the circle is the area in which an object must be for the cell to fire, but it must be in the orientation of the bar shape (anywhere else in the circle will stop firing)

these simple cells don’t respond when the entire receptive field is lit - it requires an EDGE (same for complex cells

Question 19

what are complex cells in the V1 cortex (input, what they respond to?)

Answer 19

These still respond to orientation like simple cells
In fact it is thought that several simple cells provide the input to a single complex cell. They should respond to the same orientation of a bar, but complex cells should have a larger receptive field (but slightly shifted?)

located in layers 2, 3 and 5

Question 20

what is the key difference between what complex and simple cells respond to?

Answer 20

complex cells will respond to a bar in any part of their receptive field (simple cells only respond to a bar in the centre of the receptive field)

but complex cells are direction selective

Question 21

what are hypercomplex cells of the V1 cortex?

Answer 21

responds to a bar in a certain orientation, as long as it is in the receptive field - if the bar is longer and enters the inhibitory surround area the cell stops responding

UNLESS

The bar changes orientation between the inhibitory surround and the centre of the receptive field - in this situation the neurons fire again (so responds combination of orientations)

Question 22

downstream of V1…

Answer 22

Increase in complexity of the stimulus
Receptive fields increase in size

Question 23

what does the temporal lobe do? talk about how this was shown

Answer 23

Detects more complex objects - the study tested this with familiar complex objects - faces

Even very basic faces still got the same neurons responding

Progressive removal of hallmark features (e.g. eyes) reduces neuronal response

Question 24

explain the experiment behind the Jennifer Anniston neuron

Answer 24

Showed humans a large number of images of faces
Found one neuron responding to loads of different images of jennifer anniston, but not other faces

Whether other neurons would work with it, wasn’t confirmed, but it did show that neurons can be super specific in the object they recognise

Question 25

what are some problems/some things not explained by the hierarchical model of progressive complexity along the visual pathway?

Answer 25

Still poor in scale- and orientation invariance
Do not take into account feedbacks from higher cortical areas (“top-down regulation”)

Need experimental validation which is difficult, because one needs to record pre- and postsynaptic responses - this isn’t as hard nowadays, e.g. GCaMP method - use a red one, using a virus that can transfer this from pre to postsynaptic neurons etc… IDK

Question 26

what is the orienting reflex?

Answer 26

orientating the head/eyes to focus a salient stimulus - e.g. something moves, or we hear a sound - we want the image to fall on the fovea (highest acuity/spatial resolution)

Question 27

evidence for which brain area controls the orienting reflex?

Answer 27

Ablation of the optic tectum (superior colliculus in lower vertebrates) leads to disappearance of orienting reflex

Question 28

smooth pursuit?

Answer 28

following a moving object

Question 29

motion prediction/anticipation - why do we need it?

Answer 29

Phototransduction cascade takes around 60 ms - what you see happened 60 ms ago, it’s a small delay but it means we need a motion anticipation mechanism for a lot of things (hunting prey, running from predators etc…)

Question 30

saccadic movements - when do we use this and why?

Answer 30

during object inspection - very small movements of the eyes to focus different parts of the object on the fovea - e.g. looking at a face we spend a lot of time looking at the two eyes, mouth, nose

Question 31

what areas are involved in stimulus localisation and motion processing?
include cell type where you can

Answer 31

Retina (direction selective ganglion cells, show motion anticipation, if a GC receives input from moving spot, it responds as if the motion happened 60 ms before)

Dorsal stream in the cortex

Superior and inferior colliculus

Question 32

what odes the superior colliculus do?

Answer 32

Integrates information from sensory systems (auditory, retinal ganglion cells so visual, somatosensory)

Main function = regulation of saccadic movements (moving the eye toward the stimulus)

Question 33

what experiment proved/showed the function of the superior colliculus?

Answer 33

Cat sees stimulus
Cat orients body and eyes toward the stimulus

Cat with an ablated Sup. col cannot do this/doesn’t respond this way

Question 34

what is the composition and organisation of the superior colliculus like?

Answer 34

Has different layers receiving input from different sensory systems

The retinal input in the SC is organised into a retinotopic map (a specific area of the retina when activated, activates a specific area of the SC, which often causes a specific downstream response)

Question 35

retinotopic map in zebra fish - how is it used?

Answer 35

Zebra fish show a retinotopic map also (don’t have SC but have a homologous optic tectum)
The retinotopic map is preserved/determines which motor neurons are activated in order to instruct the fish body on how much it needs to turn

Question 36

how do we have motion detection and direction selectivity?

Answer 36

some neurons - I think its complex cells? - are specific to both orientation and motion, i.e. a bar has to be in the correct orientation AND moving in the correct direction in the receptive field in order to elicit a response

Question 37

what pathway is responsible for motion detection and stimulus localisation?

Answer 37

Where pathway/dorsal stream, V1 → V2 → V3 → medial temporal cortex → parietal cortex
responsible for localisation of objects, also analysing the speed of
motion, the direction of motion and so on

Patient - stoke in the MT cortex, could recognise objects and colour but couldn’t perceive motion in certain directions

Question 38

where does direction selectivity occur?

Answer 38

Direction selectivity actually occurs at the retina in the ganglion cells, responding to movement in specific directions

As they are ON/OFF cells they respond to both the stimulus initiation and termination

Question 39

what is the morphology of direction selective RGCs like?

what could it possibly mean?

Answer 39

DS (direction selective) cells show dendritic trees in one plane, not all over the place → these likely respond to movement in the vertical plane

Question 40

how exactly can RGCs be direction selective?

Answer 40

they have a preferred direction - the direction they respond to
and a null direction - the opposite one

Ganglion cells receive excitatory input from bipolar cell, and inhibitory input from amacrine cell (inner plexiform layer)

Excitatory input (from BC) in blue - in preferred direction it has larger input to ganglion cell
The amacrine inhibitory input in red - in preferred direction it has less input

So more excitation + less inhibition in preferred direction = more spiking

Reverse is the case for the null direction

note - You can record the bipolar Exc. input and amacrine Inhib. input separately