PSYCH 202: Visual Cortex

Visual Cortex Electrophysiology: Hubel & Wiesel

David Hubel and Torsten Wiesel recorded from neurons in the visual cortex of cats during the early 1960s and discovered how visual information is represented in cortical neurons. Their work led to the discovery of orientation-selective cells and earned them the 1981 Nobel Prize (shared with Roger Sperry).

Visual Cortex Electrophysiology: How did Hubel and Wiesel record neuronal activity?

They inserted microelectrodes into the visual cortex to record voltage changes caused by neuronal membrane potentials. The signal was amplified and played through a loudspeaker, where each action potential sounded like a "pop."

Visual Cortex Electrophysiology: How were receptive fields mapped?

Visual stimuli were projected onto a screen while neuronal firing was recorded. Areas that increased firing were identified as excitatory regions, while areas that reduced firing were inhibitory regions.

Visual Cortex Electrophysiology: What happened when a light bar covered both excitatory and inhibitory regions?

Responses often cancelled out, producing little or no change in firing rate

Visual Cortex Electrophysiology: What stimulus properties strongly influenced cortical neuron responses?

Orientation, thickness, and position of light bars within the receptive field.

Visual Cortex Electrophysiology: What happened when a bar was presented at the preferred orientation?

The neuron produced a strong increase in firing.

Visual Cortex Electrophysiology: What happened when the bar was rotated away from the preferred orientation?

Firing decreased and could become inhibitory.

Visual Cortex Electrophysiology: How does population coding explain orientation perception?

• Different orientation-selective neurons respond to different edge orientations.

• The combined activity of many neurons forms a neural representation of object boundaries and shapes.

Simple Cells

Neurons in V1 that respond maximally to line segments of a specific orientation within a specific field location

• responds primarily yo oriented edges and gratings

Simple Cells What are the receptive field properties of simple cells?

They contain separate excitatory and inhibitory zones arranged in elongated patterns

Simple Cells How are simple cell responses generated

Through converging input from multiple LGN neurons aligned RFs

Simple Cells What stimulus produces the strongest response from a simple cell

A line or edge with the correct orientation presented within its RF

Cytochrome Oxidase Blobs

Darkly stained regions in V1 containing neurons that retain colour-opponent response properties

Cytochrome Oxidase Blobs Why do blobs stain darkly

Their neurons are highly metabolically active and accumulate large amounts of cytochrome oxidase

Cytochrome Oxidase Blobs What type of visual information is processed in blobs

Colour information

Cytochrome Oxidase Blobs What is found in interblob regions

Simple cells and orientation-selective neurons involved in form and edge detection

Cytochrome Oxidase Blobs Why are blob regions more metabolically active that interblob neurons

They almost alwats receive colour-related stimulation that generates excitation or inhibition, resulting in sustained activity

Orientation Selectivity

The tendency of neurons to respond maximally to a particular line or edge orientation

Orientation Selectivity what happens during a vertical and horizontal electrode penetration

• Vertical: Neurons preferred the same orientation.

• Horizontal: neurons’ orientations gradually change across the cortex

Orientation-selective neurons are organised into orientation columns.

Modular Organisation of V1 What is modular organisation?

The arrangement of neurons into specialised cortical modules that perform particular computations.

Orientation Selectivity How was modular organisation visualised + resuls

• Using voltage-sensitive dyes that changed colour according to neuronal activity.

• Different cortical regions respond preferentially to different stimulus orientations.

Ocular Dominance Columns

Stripes of cortex containing neurons that respond preferentially to different stimulus orientations

Ocular Dominance Columns What evidence supports ocular dominance columns?

Cats reared with one eye closed showed reduced cortical activity in columns receiving input from the deprived eye.

Ocular Dominance Columns Why are ocular dominance columns important?

They help maintain separate processing of information from each eye.

Complex cells

Cortical neurons that respond to oriented stimuli but are less sensitive to precise stimulus position.

Complex cells What additional properties do they exhibit?

Direction selectivity and contrast insensitivity.

Complex cells What is direction selectivity?

Responding preferentially to movement in a particular direction.

Hypercomplex cells

Cells that respond to specific stimulus features such as line endings or corners.

What visual functions are supported by complex and hypercomplex cells?

Motion detection, contour analysis, shape perception, and object recognition.

How does the cortex analyse complex visual scenes?

Through hierarchical processing in which increasingly complex neurons combine information from simpler neurons

What is the basic progression of visual processing?

LGN → Simple Cells → Complex Cells → Hypercomplex Cells → Higher Visual Areas.

Topographic Organisation

adjacent neurons in the sensory organs (like the retina or skin) connect to adjacent neurons in the brain, reflecting the organisation of the external world

Topographic Organisation Why is cortical space important?

Neurons within the same cortical column participate in the same computation

Topographic Organisation What does cortical topography reflect

The organisation of the external world

Topographic Organisation Example

Neurons within the same cortical column participate in the same computation.

Electrode Penetration

• Vertical: Neurons preferred the same orientation.

• Horizontal: neurons’ orientations gradually change across the cortex

Orientation-selective neurons are organised into orientation columns.

Retinotopic Organisation

Preservation of retinal spatial relationships throughout the visual system

Retinotopic Organisation evidence

Radioactive glucose studies produced cortical activation maps that mirrored visual stimuli presented to the retina

Retinotopic Organisation method

• Given dose of radioactive glucose --> used in metabolism --> accumulate in active cells

• Eyes fixed on fixation point "F"

• Screen displayed dartboard stimulus

  • Colours periodically inverse colours

• End of experiment: visual cortex placed in X-ray film --> replica of stimulus on primary visual cortex

Retinotopic Organisation

•  Perfect correspondence between soace in outside world & the brain

  • Axons preserve space in such a way that neighbouring regions of visual space map onto neighbouring regions of brain space

Retinotopic Organisation importance

It preserves spatial relationships needed for visual processing

Cortical Magnification

The overrepresentation of central vision within the visual cortex.

Cortical Magnification Why does cortical magnification occur?

Central vision has the highest visual acuity and therefore requires more cortical processing resources.

How is visual information separated in V1?

Into colour, form, and motion pathways.

Pathway	Location
Colour	Blobs
Form/Edges	Interblobs
Motion	Layer 4B

Does pathway separation continue beyond V1?

Yes. Separate pathways remain partially segregated in V2 and beyond.

V2 Processing How are pathways organised in V2

Into cytochrome oxidase stripes

V2 Processing Which V1 regions project to V2 thin stripes

Blobs (colour pathway)

V2 Processing Which V1 regions project to V2 pale stripes

Interblobs (form pathway)

V2 Processing Which V1 regions project to V2 thick stripes

Magnocellular motion-related pathways

V2 Processing What information is processed in thick stripes

Motion and stereoscopic depth

Area MT (V5)

A highly specialised extrastirate region involved in motion and depth perception

Area MT (V5)Where does MT receive most of its input?

• Layer 4B of V1

• thick stripes of V2

Area MT (V5)What happens following MT damage

Motion blindness (akinetopsia), where motion appears are series of static images

What does damage to V4 cause

Loss of colour perception (cortical achromatopsia)

What principle is demonstrated by V4 and MT

Functional localisation - different cortical regions specialise for different aspects of vision

Extrastriate Visual Pathways What are the two major visual streams

• dorsal

• ventral

Extrastriate Visual Pathways dorsal

where/how stream

Extrastriate Visual Pathways ventral stream

what stream

Extrastriate Visual Pathways What inputs primarily feed the dorsal stream

magnocellular inputs

Extrastriate Visual Pathways What does the dorsal stream process

• spatial location

• movement

• body position

• visually guided actions

Extrastriate Visual Pathways which cortical lobe is associated with the dorsal stream

Parietal lobe

Extrastriate Visual Pathways Why is the dorsal stream called the “how” pathway

it helps guide interactions between the body and objects in space

Extrastriate Visual Pathways What other sensory systems interact strongly with the dorsal stream

somatosensory and motor systems

Extrastriate Visual Pathways What inputs primarily feed the ventral stream

parvocellular and koniocellular inputs

Extrastriate Visual Pathways what does the ventral stream process

object identification and recognition

Extrastriate Visual Pathways Which cortical lobe is associated with the ventral stream

temporal lobe

Extrastriate Visual Pathways Why do RF change in the ventral stream

they become larger and less dependent on exact location

Extrastriate Visual Pathways why is location less important in the ventral stream

the goal is to identify what an object is regardless of where it appears

How do receptive fields change as processing moves away from V1?

They become larger and respond to increasingly complex stimulus features.

What happens to response properties in higher visual areas?

They become more specialised and abstract.

What do fMRI studies show about extrastriate cortex, and what principle does this illustrate

Different regions respond preferentially to faces, objects, places, motion, colour, and words.

• Parallel processing.

Synaesthesia

An automatic and consistent blending of one sensory or cognitive experience with another

Synaesthesia key characteristics

it is involuntary, consistent, and does not reflect properties of the external world

Synaesthesia grapheme-colour synaesthesia

seeing colours when viewing letters of numbers

Synaesthesia affect on visual search tasks

Synaesthetes can detect patterns more rapidly becaise numbers of letters evoke distinct colours

• Serial search task: looking through items one by one (5's and 2's)

 

V4

area specialised for colour vision

Synaesthesia Visual Word Form Area (VWFA)

a specialised cortical region involved in processing letters, words, and digits

Synaesthesia VWFA location

adjacent to area V4

Synaesthesia How does VWFA develop

through experience and exposure to written language

Synaesthesia what neural explanation has been proposed for grapheme-colour synaesthesia

stronger than normal structural connections between VWFA and area V4

Synaesthesia neural explanation for grapheme-colour synaesthesia evidence

MEG = records the tiny magnetic fields produced when large groups of neurons change their electrical activity at the same time.

• uncoloured words shown on screen

• VWFA: control group and the synaesthesia group showed a peak of activity

• V4: control group showed little/no response; synathaesthesia group showed strong activation even though no colour was physically present

Cortical Receptive Fields: Dorsal vs Ventral Streams What characterises dorsal stream receptive fields?

motion-sensitive neurons with strong connections to motor ans somatosensory systems

Cortical Receptive Fields: Dorsal vs Ventral Streams Which area contains many motion-selective neurons

MT/MST (V5)

Cortical Receptive Fields: Dorsal vs Ventral Streams What characterises ventral stream receptive fields?

large RFs specialised for object recognition

Cortical Receptive Fields: Dorsal vs Ventral Streams Which ventral stream region contains face-selective neurons?

Inferotemporal cortex (IT), particularly the fusiform gyrus.

Cortical Receptive Fields: Dorsal vs Ventral Streams How do receptive fields change from V1 to IT?

They progressively increase in size and complexity, supporting object and face recognition.