EEG and symmetry

EEG

• Test that measures electrical activity in the brain. Uses electrodes attached to the scalp

• measures voltage between each active electrode

• most affordable neuroimaging technique

• poor spatial resolution

• good temporal resolution

ERPs

event-related potentials-signal you get from an EEG: brain activity changes in response to a stimulus

Spatial resolution

accuracy with which a technique can identify the exact location of brain activity

Temporal resolution

measure of how accurately a scanner can detect changes in brain activity in relation to time

claims about symmetry perception in EEG research

• Network of symmetry sensitive brain areas in the extrastriate cortex. This generates the SPN

• Extrastriate network responds to symmetry automatically

• Amplitude of symmetry response scales with the perceptual goodness of regularities of configuration

• Network always responds to symmetry- sometimes recovers symmetry in object

• Network can code symmetry independently in each hemisphere

• Response to symmetry stronger than anti-symmetry

• Indifferent to colour

• Network can integrate patterns info presented at different points in time

• Attending to negative superimposed words blocks symmetry processing

•  Symmetry processing is robust to changes in visual working memory load

• The symmetry responses increases with repeated presentation (SPN priming)

• SPN priming generalizes between regularity representations

•  We can learn a lot about symmetry perception by pooling data from all experiments into a big Catalogue

• Symmetry processing is enhanced during active regularity discrimination tasks

• The brain response to symmetry is right lateralized, but the laws of perceptual organization that determine SPN amplitude, are similar in both hemispheres.

• Hallucinogenic drugs activate the extrastriate symmetry network

Network of symmetry sensitive brain areas in the extrastriate cortex: evidence

Sasaki et al:

• people put in scanner and shown both symmetrical/random dot patterns

• Symmetrical- back of head activated (extrastriate areas) not Primary visual cortex (V1)

• only the extrastriate cortex sensitive to symmetry

• Amplitude lower for reflection patterns than random- called the Sustained posterior negativity (SPN)

Extrastriate network responds to symmetry automatically: evidence

• Study: varied proportion of symmetry in the dot pattern- some random, some symmetrical

• Also varied colour of the dots (light or dark)

• one group discriminated regularity other discriminated colour

• Results: when attend to regularity SPN scales with the proportion of symmetry, when attend to colour still goes down but not as strong

Amplitude of symmetry response scales with the perceptual goodness of regularities of configuration: evidence

correlation between W load and SPN amplitude- higher the W load (obvious symmetry) larger (more negative) SPN

perceptual goodness

refers to the strength or salience of visual regularity

Network always responds to symmetry

• either had 2 axis in image and 2 axes in object or 1 axis on retina, 2 axis in object

• findings: whether people are attending to regularity or colour

  • Regularity: same SPN for flat presentation as for the slanted: get a view invariant representation

  • Colour: large SPN for slant and small for slanted: colour matters most

view invariance

the ability to recognise an object despite variations in viewpoint, lighting, retinal size or background

Network can code symmetry independently in each hemisphere: evidence

• Study-fixate in the centre of screen- random pattern on the left, symmetrical on the right or vice versa

• Here the SPN just over the contralateral hemisphere

• SPN is similar whether attending to random or symmetrical patterns

 

Network can integrate patterns info presented at different points in time: evidence

• Experiment- pattern hidden behind a door, then moves across and reveals other side- people then judge whether the half seen pattern was a reflection or random

• Get SPN generated by second half of a half occluded pattern

• Brain has got info, stored in memory, and used it for new info- noted reflection

Attending to negative superimposed words blocks symmetry processing: evidence

- Say if the word above the image has positive or negative valance

- When positive brain responds to symmetry and generates an SPN but not when negative

- Seeing negative words grabs attention blocking symmetry processing

Symmetry processing is robust to changes in visual working memory load: evidence

• Visual working memory capacity is 4 objects

• Found the SPN was enhanced when people's memory is full of shapes- thus enhancing visual processing

The symmetry responses increases with repeated presentation (SPN priming): evidence

• Brain is sensitive to what has just come before- no 'pure' visual responses

• Habituation and priming-Can get both in the visual cortex

• SPN subject to priming

SPN priming generalizes between regularity representations: evidence

• Black and white symmetry representations overlap

• SPN priming transfers from black to white

We can learn a lot about symmetry perception by pooling data from all experiments into a big Catalogue

• The complete Liverpool SPN catalogue since COVID

• Move towards open science, to help with the reproducibility crisis

• 40 projects, 249 grand average SPNs, 6674 SPNs from 2215 pts, over 1 million trials

Symmetry processing is enhanced during active regularity discrimination tasks: evidence

- Attention to regularity increases SPN amplitude

- R2- Tells us the proportion of variance explained by the regression line

The brain response to symmetry is right lateralized, but the laws of perceptual organization that determine SPN amplitude, are similar in both hemispheres: evidence

- Significant right lateralisation- SPN goes further down in the right than the left-right cares more about symmetry, left is language

 Hallucinogenic drugs activate the extrastriate symmetry network: evidence

• DMT is a psychedelic

• DMT is an endogenous neurotransmitter- It is already used for something in your brain.

• All psychedelics bind to serotonin receptors- DMT is a 5-HT2AR agonist

Andres Gomez Emilsson: Six levels of the DMT trip:

1. Threshold (4MG dose)- highly aesthetic visual fascination- like in HD- everything is crisp and vivid

2. The Chrysanthemum (4-8MG)- like the best screensaver- patterned surfaces will 'symetrify'

3. The magic eye- overflow where surfaces begin to fold in semi-symmetrical ways

4. The waiting room- finding an element in the scene and using it as a window to another dimension- feels like hyperspace

5. The breakthrough- the new universe feels palpably panopsychic- symmetrical religious epiphany

6. Amnesia- impossible to retain representations of these geometries and describe them mathematically so you forget it all

Perceptual constancy

tendency to perceive objects as having a standard size, shape, colour or location even when the sensory stimulus changes. Possible as the brain corrects for variations

Contralateral SPN

ERP that occurs when visual symmetry is presented to either the left or right of central fixation. Is similar when symmetry is in the attended or unattended visual hemifield (only slightly enhanced with attended)

Visual working memory

limited capacity of our mind to temporarily hold and manipulate visual information from our environment

Habituation

a decrease in response to stimulus after repeated presentations

Priming

how exposure to one stimulus can influence a person's response to subsequent stimulus, without conscious awareness

R²

statistical measures in a regression model that determines the proportion of variance in the dependent variable that can be explained by the independent variable

open science

promotes the transparency, reproducibility and accessibility of research