Week 7 Flashcards
Speech in Noise
Vision helps to resolve ambiguity Few “lip readers” but visual speech greatly improves perception in noise Automatic Greater than sum of parts
Warning Signals
Faster response to multimodal stimuli Flashing indicator Beeping indicator Flash/Beep more reliable faster
Flavour
“Flavour is in the brain” Taste Smell Somatosensory Modulators Sight Sounds Smells Expectation
A Unified Perceptual Experience
“… it is surely one of the great remaining scientific puzzles just
how it is that signals from such completely separated and
wholly dissimilar sensory epithelia as the haircells of the
cochlea, the photoreceptors of the retina and the corpuscles
of the skin can be integrated centrally to form such a seamless
unitary perceptual world”.
Molholm and Foxe, 2010 (p 1709)
Overview - Perception
A primary role of the brain
Decide on best/most appropriate action/behaviour
Survival depends on speed and accuracy
with which an organism can evaluate
external events and properly react to
them
gather, interpret and represent information
What are the 4 Multisensory Perceptions?
Light, sound, mechanical and chemicals
what is the role of Multisensory Perception?
evaluate external events and properly react
to them
Different modalities can substitute when individually
compromised (eg vision lost in the dark)
• Different fields of operation – touch/smell/taste for up close and
vision/hearing for distance
• Resolve ambiguities – 2 things may sound the same but look
different; boost signal to noise
Multisensory integration enhances our ability to perceive
and understand our environment, enabling us to better
interact with our surroundings
Redundant Targets Effect
Miller (1982) • Speeded response to either audio (A), visual (V), or audio-visual (AV) target Redundant since additional stimulus doesn’t provide any additional information
what are the two models of redundant target effects?
- Statistical facilitation - independent (parallel)
processing - Neural coactivation - integrated signals
Redundant Targets Effect-
Statistical Facilitation
Both elements of AV stim processed along
independent channels
• One that reaches output stage first triggers
response
• On average, the time of the winner will be less than
the time for either racer
Redundant Targets Effect-
Neural Coactivation
Both components of a redundant signal influence
response on a single trial
• Activation from different channels combine in
satisfying a single criterion for response initiation -
activation builds over time until some criterion is
reached
• Activation builds faster when provided by two
sources rather than one
what is the purpose of redundant target effects?
• responses to redundant signals are too fast to be
explained as the faster of two responses to
individual signals
• The easiest way to explain the speed of responses
to redundant signals is to assume that signals
jointly contribute to the process of producing the
response
• Neural Coactivation - MSI
MSI – Key Issue
Sensory environment is complex – there are
multiple sources in each modality – eg lots of visual
objects and lots of sounds
• Key function of MSI dissociate between stimuli
from different sources and single source
Factors Influencing MSI
- Temporal Coincidence
- Spatial Coincidence
- Temporal patterning
- Crossmodal correspondence
- Stored knowledge
- Context
- Recent experience
- Expectation
- Attention
Factors Influencing MSI- bottom up and subjective
Temporal Coincidence
• Spatial Coincidence
• Temporal patterning
Factors Influencing MSI- top-down and subjective
- Stored knowledge
- Context
- Recent experience
- Expectation
- Attention
Bottom-up Factors
Temporal Coincidence – things that happen at the same
time
• Spatial Coincidence – things that happen at the same
place
• Temporal patterning – things that are correlated over
time
Strong cues that stimuli were caused by the same event
and so belong together
Top-down Factors
Information that is already present in our brain
influences how crossmodal signals are combined
• Stored knowledge
• Recent experience
• Context
• Expectation
• Attention
Top-down: Context
Stream-bounce; 3 blocks of 200 trials each Block 1 – all no sound Block 2 – mixed sound/no sound Block 3 – all no sound
Top-down: Expectation
When are cognitive influences (eg previous response, overall
presence of the tone) impacting
Does expectation lead to an early decision or bias?
Typical S/B – observe entire motion sequence then provide
response – subjective
Want to see how percept evolves
Neuroscience of MSI
• Minimum 32 visual, 15 auditory, 8 somatosensory areas
identified in primate cortex
• Body control - proprioception, vestibular, vision, motor
control
• How are individual senses integrated?
• How is a unified perceptual experience created?
where are multisensory neurons found?
nearly every level in the CNS
Superior Colliculus
Reflexive orienting to stimuli in contralateral space • Produces motor actions that are guided by sensory stimuli • Converging visual, auditory and somatosensory projections from numerous cortical and subcortical sources • Inputs – retina, cortex, IC, spinal cord • Outputs – motor control of eyes, ears and head
Superior Colliculus- Multilayered structure
Superficial layers are visual – optic tectum in non-mammals
• Deeper layers are multisensory
Superior Colliculus- MS cells
- Inputs from 2 or more sensory systems – A/V, V/S, A/V/S
- Overlapping receptive fields
- Can respond to single sensory input – but weakly
- Preferentially (stronger) response to multiple inputs
Superior Colliculus- receptive field
a region of sensory space that a sensory
neuron preferentially responds to stimuli in
what id teh central role of the Superior Colliculus?
integration of info from different
modalities and generation of spatial orienting
responses
Important as a MS structure, but also good as a
general model of MSI
Multisensory Enhancement
MS neurons – response to appropriate multisensory
stimuli exceeds the response to individual unisensory
inputs and can even exceed the sum of the
unisensory inputs (super-additivity)
But – get suppression of inappropriate (incoherent or
misaligned) stimuli (sub-additivity)
3 drivers of Multi sensory (MS) enhancement or super-additivity
- Spatial rule
MS stimuli must occur at the same region of space - Temporal rule
MS stimuli must reach the MS cell at the same time - Principle of Inverse effectiveness
Enhancement is greater for weak stimuli than strong
Spatial rule- unimodal
Visual or auditory alone
Weak response
Spatial rule- multimodal
Large spatial offset Depressed response – less than just visual (spatial separation of AR40V and AR30V) Smaller spatial offset Unimodal response – just the visual (AR20V) Spatial Coincidence Strong multisensory response (AR10V)
Temporal Rule- unimodal
Visual or auditory alone Weak response (V and A)
Temporal Rule- multimodal
Asynchronous AV Unimodal response of whichever stimulus comes first (A200V and V200A) Simultaneous AV Strong multisensory response (AV0)
Principle of Inverse Effectiveness
strong and weak stimuli
Superior Temporal Sulcus (STS)
• STS as a region involved in audio–visual speech
processing
• STS may be generally involved in binding auditory
and visual inputs, regardless of whether they
contain speech or biological motion
where does visual processing occur?
occipital lobe and ventral visual to inferior temporal gyrus
where does auditory occur?
superior temporal gyrus
where is Wernicke’s area?
superior temporal gyrus
where is the angular gyrus?
inferior parietal lobe
McGurk Effect
vision and speech perception
research in STS and the McGurk Effect
• Beauchamp et al. (2010)
• Use fMRI to identify MS area – left posterior STS
responded to both auditory and visual speech
• Show mismatched AV speech (McGurk) with and
without TMS of Left STS
• Control 1 – auditory only with and without TMS –
check if effect is speech in general or MSI
• Control 2 – TMS of second site
STS and the McGurk Effect
- No TMS – mostly illusion
- TMS of left STS - illusion down to 50% of trials
- Control 1 – TMS and auditory only – no effect on
speech perception - Control 2 – TMS of second site – no effect on
McGurk susceptibility
STS and the McGurk Effect-
Plasticity
• Stroke patient with ablated left posterior STS
• Recovery and rehab reported good understanding
of speech but it was effortful
• Report experiencing McGurk effect
• Right STS activation and larger volume than age
matched controls
Posterior Parietal Cortex (PPC)
• PPC plays a critical role in functions related to attention
allocation for unimodal and multisensory processing
• Reference frame remapping by the PPC may be critical for
aligning inputs to facilitate integration
• PPC is also ideally situated to mediate interactions between
the sensory systems by shaping processing in primary
sensory areas via feedback projections
• anodal tDCS speeds reaction times for detecting auditory,
visual and bimodal auditory–visual targets when applied
over right PPC
PPC and the Flash/Beep Illusion
Flash/Beep (Shams et al. (2002):
• single flash of light
with multiple beeps
• Sound induced illusory flashing - Report 2 or more
flashes when 2 or more beeps (fission)
PPC and the Flash/Beep Illusion
• Kamke et al (2012)
• Illusory flash perception is typically only reported
on a proportion of trials suggesting that stimulus
characteristics alone do not determine the illusory percept
TMS to one of 2 PPC areas (angular gyrus and
supramarginal gyrus) or S1 (control)
PPC and Stream-Bounce
Stream-bounce with TMS over left or right PPC
Unisensory Cortex and MSI
• Also modulation of low-level sensory cortex by other modalities
• Human fMRI studies by Calvert et al. found that the substantial improvement of auditory speech perception when the speaker’s face was visible was accompanied by a significant response enhancement in auditory cortex
• Romei et al. demonstrated that auditory stimulation
can decrease the threshold of perceived phosphene
induced by a single pulse TMS applied over the occipital
pole
Unisensory Cortex and MSI- studies
• Single flash being misperceived as two with two beeps
accompanies an enhancement of visual activity in V1
(Watkins et al., 2006)
• Double flash being misperceived as a single flash due to
a single beep relates to a decrease in V1 activity
(Watkins et al., 2007)
• ERP work showing interactions between auditory and
visual stimuli at very short latency (46ms to 150ms)
Multisensory Integration model
former model- Hierarchical functionally Homogeneous Serial
receptors-> Thalamus-> Primary Sensory Cortex-> Secondary Sensory Cortex-> Association Cortex
Multisensory Integration
Key Learnings
• MSI – increase precision, amplifies weak signals
and resolve ambiguities
• Improved orienting towards, detection and
identification of relevant events
• Greater than the sum of parts
• RTE – neural coactivation = MSI
• Key issue – what to bind
• Range of bottom-up and top-down factors
• Top-down factors and stream-bounce
• Multisensory neurons are found at nearly every
level in the CNS
• SC - Deeper layers are multisensory – A, V, S
• MS enhancement – spatial rule, temporal rule,
principle of inverse effectiveness
• Superior Temporal Sulcus (McGurk)
• Posterior Parietal Cortex (Flash-Beep and StreamBounce)
• Cross-modal modulations of low level unisensory
• Integrated, interconnected, feedforward, feedback
