Mirror Illusion and Rubber Hand Illusion Flashcards
Body representation / schema
Our brain needs to keep track of where our limbs are through time and space.
This is no trivial task - the location of our body parts is continually changing with respect to the outside world and each other.
A united percept of the body helps us to
Navigate safely through the world
Interact with objects and other people
Communicate and portray emotion non verbally
A vital requirement for maintaining body representation is multisensory integration.
Reminder – Proprioception
Proprioception is the sense of the position of your body parts relative to one another and their motion.
You can’t always see your body but you still know where it is – proprioception is responsible for this.
When we can see (visual input) and feel (proprioceptive input) the position of our body parts, these inputs are integrated
MSI Example.Hitting a tennis ball
Visual info – ball moving through space, racquet & arm.
Pressure and vibration from racquet through hand and through feet from ground.
Sound as racquet swishes through air and impacts ball.
Proprioceptive feedback from body as its parts change position.
These separate sensory inputs are integrated to give us a unified representation of events, both of the outside world and of our body.
MSI and Body Representation
Multisensory integration of body centred, and body-related environmental inputs enables us to update representation of our bodies continuously.
We are not consciously aware of this process – doesn’t require effortful processing.
- Automatic
Multiple sensory inputs are most likely to be integrated into a single event (or sequence of events) if they correspond in time and space.
Additionally, information each different sense is not equally reliable/ equally weighted sources of perceptual input.
Greater perceptual ‘weight’ is given to more spatially precise sources of information – e.g., relies less on vision and more on proprioception when lighting is very poor and vice versa when lighting is good.
However, we can exploit such perceptual weightings by disrupting the correspondence between sensory inputs using mirrors and fake hands.
MSI in the Brain – Parietal Cortex
There are many areas of the brain involved in MSI (superior colliculus), but the temporo-parietal junction (TPJ) is particularly important for limb representation/ updating.
TPJ damage and dysfunction is associated with out of body experiences(Blanke et al, 2004
Disrupting the TPJ using TMS (transcranial magnetic stimulation) can impair healthy participants’ ability to mentally transform their body (imagine it rotating) but not external objects(Blanke et al, 2005).
Mirror Hand Illusion (MHI)
We can also see interesting misrepresentations of the body using simple, low-tech experimental manipulations such as the mirror hand illusion and rubber hand illusion.
We’ll first look at the mirror hand illusion (MHI).
Demonstration…
WARNING - the demonstration may involve latex gloves and potentially latex-containing plastics so if you are allergic to latex please do not participate in the demonstration.
Holmes & Spence 2005
Holmes and Spence used a similar mirror setup to ours to investigate the mirror hand illusion thoroughly.
We are just going to look at their Experiment 1, which is assessable on the quiz.
We are NOT assessing your knowledge of Experiment 2 from Holmes and Spence.
Holmes & Spence 2005Manipulations
Blue outline and colour = False hand location (mirror image, seen by participants)
Orange outline = Real hand location (behind the mirror, hidden from participants)
Predictions – No-Mirror Condition
First, without the mirror, there should be no systematic pattern of reach errors related to the mirror hand position.
It should not matter where the left hand is sitting because there is no sensory mismatch between the felt and seen hand locations.
This is the control condition.
Prediction – Mirror Condition
Reach errors will be systematic – the more the real and mirror locations differ, the greater the reach error (the further away from the target location the reach should fall).
This pattern of errors will be consistent with the brain representing the hand somewhere between the real and mirror-signalled locations.
This is what they found!
There are 16 conditions all up – we will only look at a few to illustrate the pattern.
Real hand and mirror signal SAME location.
Reach is accurate.
Mirror hand signals a location on the opposite side to the real hand.
Participant reaches relatively straight.
This would be correct if their hand started almost midway between the mirror and real locations (dotted line).
Mirror hand signals a location FURTHER LEFT than the real hand
Reach stops further right.
This would be correct IF the real hand started between its real location and the mirror image (dotted line)
Mirror hand signals a location FURTHER RIGHT than the real hand.
Reach stops further left.
This would be correct IF the real hand started between its real location and the mirror image (dotted line).
Holmes & Spence 2005Experiment 1 Results
- Participants’ reaches showed systematic errors towards the seen location of the hand (visual inputs) rather than what they felt (proprioceptive inputs).
The further the hand in the mirror differed from their real hand, the less accurate they were. - This is consistent with their brain representing their hand as somewhere between its real location and the location signalled in the mirror.
Also provides evidence of visual anchoring of proprioceptive inputs
Proprioceptive drift
The reliability of proprioceptive information is known to degrade over time, with proprioceptive ‘‘drift’’ occurring over very short timescales (Holmes & Spence, 2005).
Proprioceptive drift is a change in the perceived position of the obscured hand.
Without a ‘fake hand (e.g. mirror hand),’ drift tends to occur toward the middle of the body because that’s its ‘default’ position
With a ‘fake hand,’ drift tends to occur toward the fake hand because the brain seems to rely on the visual input more now that the proprioceptive input is degraded
Mirror Box Therapy
For patients with phantom limb syndrome, a limb that has been amputated feels like it is still there, often with painful feelings – e.g., hand clenched so tight it hurts.
The mirror hand illusion can be used to ‘reset’ the missing limb and alleviate the pain.
Also evidence suggesting this can work on pain in intact limbs
Phantom pain in missing limb
Rubber Hand Illusion (RHI)Explanation
One arm is covered and a rubber hand put in its place.
The rubber hand and the hidden ‘real’ hand are stroked to provide tactile input.
The stroking is either synchronous or asynchronous.
RHI & multisensory integration
The synchronous stroking condition provides corresponding tactile and visual input.
Temporal correspondence – sensations occur at the same time
Spatial correspondence – with reference to the hand’s surface, the stroking goes in the direction and at the same location.
This triggers integration of these inputs into a single event
Brain updates body representation accordingly – trying to reconcile the contrast between the location of the touch and the location of the visual info.
In contrast, the integration of the rubber hand into the body representation should be absent or weaker during asynchronous stroking because does not result in multisensory integration.
