Chapter 4: Signatures of a conscious thought

Question 1

How does fMRI work?

Answer 1

When brain cells are active, they will use more oxygen, deoxygenating haemoglobin. Deoxyhaemoglobin is paramagnetic (has slight magnetic field) as the Fe is more exposed. The fMRI scanners can detect this change in magnetism in haemoglobin.

Question 2

What is the difference between EEG and MEG?

Answer 2

EEG machines record action potentials in different brain areas
MEG machines record magnetic fields in different brain areas

Question 3

What are the four signatures of consciousness?

Answer 3

• sudden increased activation in parietal and prefrontal lobes
• strong and slow P3 wave present around 300ms after stimulus
• late burst of high-frequency gamma waves
• long-distance bilateral communication between different cortical regions (global ignition)

Question 4

How will P3 wave determine whether a stimulus will be consciously perceived when presented with a distractor mask?

Answer 4

During a masking experiment, there are two P3 waves present. The first wave from the distractor is always consciously perceived and, if long and strong enough, can block the second P3 wave from reaching conscious perception.

Question 5

What is meant by the psychological refractory period?

Answer 5

When there are two P3 waves, there is a delay in when the second one will be perceived
Almost as if there is a funnel and so only one can pass through

Question 6

Delays in information processing and what are some mechanisms used to counteract this

Answer 6

A 300ms delay in conscious perception means that the information is outdated
There are ways of counteracting delays:
- reflex actions can happen unconsciously
- anticipation allows us to perceive events closer to when they actually occur (experiments show anticipated events are perceived earlier than unanticipated ones)

Question 7

Error perception

Answer 7

During first 200ms, the brain reacts the same to conscious and unconscious errors. Then, an automatic system in the cingulate gyrus fires when it realises motor plans did not go as instructed and remains unconscious until we are conscious, then an error related positivity is detected but indistinguishable from P3 wave

Question 8

Global ignition

Answer 8

Self-sustained state of high-level activity occurs when some neurons excite others which in turn return the same excitation

This is nonlinear: happens suddenly and does not improve with an increase in delay between subliminal and conscious stimuli

Question 9

Are initial responses of visual cortex always present?

Answer 9

Yes, P1 and N1 responses are always present and unaffected by length of delay. Afterwards, both left and right visual area waves increase until around 270ms when there are signs of global ignition, unproportional to length of delay

Question 10

What did they find when looking at patients with electrodes placed in areas related to epilepsy?

Answer 10

There was significantly higher influence in these areas from consciously perceived words compared to unconsciously perceived words.
This shows that while unconscious information is limited to certain parts of the brain, conscious information is spread throught.

These electrodes were not placed in areas specifically related to consciousness as it is routinely done in patients with epilepsy.

Question 11

Gamma waves

Answer 11

High frequency waves detected in the first 200ms after word is shown in all perceived words, after which they die in unconsciously perceived words

Question 12

Synchronisation of electromagnetic signals

Answer 12

Occurs at around 300ms after first exposure and helps transmit information more efficiently
- these tend to be either beta or theta frequencies
- invisible images create temporary synchrony but limited to back of the brain

Question 13

Bottom-up and top-down signals

Answer 13

Bottom-up: from sensory receptors to integrative centres
Top-down: from higher-order regions to prediction signals
Both are present throughout cortex

Question 14

When are each type of signals present?
(top-down / bottom-up)

Answer 14

During conscious trials:
bottom-up sending sensory information, top-down checking if input is consistent

During unconscious trials:
top-down signals from anterior areas, no consistent signals from sensory areas

Question 15

Blind spot

Answer 15

Spot in retina where optic nerve goes to brain, no receptors in this area

300ms may be the time needed to adjust sensory input by removing blind spot in interpretation

Question 16

Can we know what someone is consciously perceiving based on brain states?

Answer 16

Each specific stimulus activates just one or two neurons specific to that stimulus in the anterior temporal lobe
- not found in unconscious trials
- needs to show reproducibility over trials, invariance of changes and stability over time

Question 17

Activity in posterior cingulate cortex and parahippocampal gyrus

Answer 17

Posterior cingulate cortex: fires regardless of position of stimulus in visual field

Parahippocampal gyrus: fires based on where it believes it is

Question 18

How does TMS work?

Answer 18

Transcranial magnetic stimulation produces electrical discharges at specific spots
- occasional noise or muscle twitch
- focal discharges can produce conscious perception even in absence of stimulus
- brain has no nociceptors

Question 19

Are all brain circuits equally as important for conscious perception?

Answer 19

Not all brain circuits are equally as important for conscious perception
- peripheral sensory and motor circuits can be activated unconsciously (masking, reflexes)
- temporal, parietal and prefrontal cortex can produce purely subjective hallucinations

Question 20

How is TMS able to induce an earlier P3 wave compared to when using a physical stimulus?

Answer 20

The signal is able to bypass visual processing by 100ms

Question 21

TMS results on conscious perception

Answer 21

Local activity is insufficient for conscious experience and must be sent to other areas for this

Induced activity must first loop back via V1 before conscious perception (recurrent processing)

Question 22

Recurrent processing

Answer 22

Necessary but insufficient for conscious perception
- these must be combined with long-distance connections in front and parietal lobes
- under anaesthesia, small local loops are broken first (needed for wider picture)

Question 23

What does a virtual lobotomy of the dorsolateral prefrontal lobes lead to?

Answer 23

Dissociation between objective behaviour and subjective perception