psychology essay

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10/7/2019

Somatosensation / Touch

Touch Mechanoreceptors

• Four types, defined by two categories (2x2) • Slow vs fast adaptation

• Fast – responds when stimulus is applied and again when removed (corpuscles)

• Slow – responds when stimulus is in contact

• Size of receptive field • Small vs. large RF

Two-Point Touch Task

• Receptor density is different throughout body • Some areas are more/less

sensitive

• Cortical magnification of touch • More brain area dedicated to

processing certain regions of the body

• Lips vs. hands vs. legs

• Two-point discrimination task

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Receptive fields of monkey S1 neurons

Neural plasticity • Somatosensory regions can become more

tuned to respond to frequently experienced sensations

• For example, right-handed violin players who use their left fingers to press the strings end up with larger S1 in their right hemisphere devoted to representing their left finger

• Congenitally blind people’s V1 can become involved in tactile discrimination

Perceiving detail • The example of Braille

• 6-dot cells (each dot is raised or not raised)

• Represent the alphabet, as well as other characters

• Experienced Braille readers can read 100 words per minute

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Somatosensory Cortex (S1)

• Parietal-Occipital Junction

• Mapping of body not perfectly topographic but pretty close

• Body magnification

• Lines up nicely with 2- point threshold results

Somatosensory Homunculus

Somatosensory and Motor Organization

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Proprioception

• Sense of body position • Information from muscles

and joints

• Build model of self in space

• Bodily information projects to motor cortex • Dorsal stream

• Integration of where you are and directed goal

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Kinesthetic Receptors (Proprioception)

• Kinesthetic receptors • Mechanoreceptors in

muscles, tendons, and joints.

• Play an important role in sense of where limbs are, what kinds of movements are made

• Muscle spindle • Receptors in tendons signal

tension in muscles attached to tendons

• Receptors in joints react when joint is bent to an extreme angle

Life without Proprioception

https://www.youtube.com/watch?v=pMEROPOK6v8

Body Image

• Proprioceptive information can be used to create model of self in space

• Our body images are systematically distorted towards top-heaviness. • Expanded shoulders and upper

arms • People rate upper half of body to

be larger than lower half. • Consistent with somatotopic

mapping in cortex and sensory homunculus

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Vision vs. Proprioception

• What if visual cues tell us something different than proprioceptive cues?

• Short story – vision > proprioception

Rubber Hand Illusion

https://www.youtube.com/watch?v=sxwn1w7MJvk

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Phantom Limb Treatment

https://www.youtube.com/watch?v=gc3CmS8_vUI

Touch and Proprioception • Rubber hand illusion involves touch, but there are

others

• Pinocchio illusion

• VR and out of body experience induction

Inducing OBE in VR

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“Alice in Wonderland” Syndrome

• Proprioceptive hallucinations

• Parts of the body feel too small or too big

• Sometimes associated with migraine aura

• Lilliputian hallucinations • Extends to visual

perception of body

Vestibulation

Vestibulation • Sense of movement and gravitational upright

• Vestibular organs in inner ear

• Semicircular canals, otolith organs

• Along with proprioception (kinesthesia), allows for posture control and balance

• Spatial orientation

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Spatial Orientation

• A sense consisting of three interacting sensory modalities—perception of linear motion, angular motion, and tilt. 1. Angular motion: Can be sensed when

rotating head from side to side as if to say “no.”

2. Linear motion: Sensed when accelerating or decelerating in a car.

3. Tilt: Sensed as orientation with respect to gravity

Inner Ear

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Semicircular Canals

• Fluid filled chambers

• As head moves, fluid moves with it

• Filled with hair cells (similar to audition)

• Roll, pitch, and yaw • X, Y, and Z planes

• Respond to acceleration • Less responsive when

velocity is constant • Primarily angular

Otolith Organs

• Otolith organs sense acceleration and tilt. • Two otolith organs in each ear:

• Utricle: Contains about 30,000 hair cells.

• Saccule: Contains about 16,000 hair cells.

• Each organ contains a macula: A specialized detector of linear acceleration and gravity.

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Change Detection

• Like other sensory systems, vestibulation primarily tuned to detect change

• Maximum fluid displacement by acceleration, not velocity

• Car example, roller coaster example

Vestibulation and Motor Control

• Vestibulation, along with proprioception (kinesthesia), works alongside motor control • Are we moving, or being moved?

• Am I moving my own head (and should vision be stabilized)?

• Two types of signals • Afferent signals

• Sensory organs to the brain

• Efferent signals • Brain (motor cortex) to muscles

• Provides information about self-generated movement

Vestibulation and Motor Control

• Vestibulo-ocular reflex • Account for head

movement when fixating eyes on target

• Efferent signals from motor cortex move eyes opposite head movement

• Head movement “factored out” – only 10 ms delay between head and eye movement – super impressive!

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Vestibular Problems

• Problems with the vestibular system can lead to peculiar sensations: • Spatial Disorientation: Any impairment of

spatial orientation (i.e., our sense of linear motion, angular motion, or tilt).

• Dizziness: Nonspecific spatial disorientation.

• Vertigo: A sensation of rotation or spinning. • Imbalance

• Blurred vision

Vision and Vestibulation • Information about acceleration from vestibular

system combined with vision

• “If a large part of my visual field is in motion, then it must be ME that is moving!” • Induced self-motion illusions

• Riding on BART (or whatever)

• Vection

Optical Flow

• Forward and backward motion

• Focus of expansion • Target location (impact)

• No relative motion

• Motion parallax (lateral motion)

• Provides information about heading and direction

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https://www.youtube.com/watch?v=nrok5as9dYE

Tumbling Room Experiments • Person strapped into a chair in a fake room • Room and chair can “flip” independently • Chair upright, room upside down

• Feels like you are upside down (even without seatbelt!) • Goes away if eyes are shut

• Chair AND room upside down • Feels like you aren’t upside down • Things are “pulling” you up (gravity)

Simulator Sickness

• Bad news – providing visual input without vestibular input means visual/vestibular mismatch • Can lead to motion sickness – specifically “simulator

sickness” when in virtual reality

• Vection effects also strong – additional mismatch

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Overcoming Simulator Sickness

• Some ways to fight simulator sickness: • Don’t let people move using free-walking

• Teleportation-based movement • Embed person in chair or cart that moves around (think

rollercoasters)

• Fade peripheries while moving • Google earth does this – peripheries go gray while movement is

occurring, only fovea moves

• Put a nose on it • Adding a virtual nose can reduce sickness • Most people don’t even notice it

• Power through it • Earn your “VR legs”

Anti-gravity hills

How important is the “mythology”?

https://www.youtube.com/watch?v=M2lAqYZ6N5o

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Anti-gravity Hills

• Clear view of true horizon not visible

• More than one surface plane in sight

• Have to judge slopes based on misleading visual cues

• Slope of middle plane influenced by other two

This looks mostly flat

This looks like it’s sloping downhill

Mystery Spots

How do they work?

• Similar to anti-gravity hills – no clear view of horizon to “calibrate” vision

• No surfaces actually horizontal or vertical (everything is slanted) • Assume that slight tilt is not actually there! • All angles are slightly more or less than 90 degrees

(non-rectilinear)

• Changing heights – Ames room

• Rolling balls – anti-gravity hills

• Tilt illusion

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Tilt illusion

Illusion is greatest for backgrounds tilted around 15 degrees

Average difference from right angles in mystery spots

Enns, 2006

Hanging ball at Mystery Spot

• Ball appears to be hanging at an angle

• Pushing ball towards what looks like vertical is harder

• Expectations about gravity

• It should be easier to push something if working with gravity

Bridgeman, 2005

Dual Visual System Theory

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Dual Visual System Theory

• Separate systems for conscious and unconscious visual processes

• Divide between PERCEPTION and ACTION

• What is the evidence here?

Blindsight

• Caused by damage to/removal of V1 • One or both hemispheres

• Visual field deficits related to location of damage

• No subjective perception of “blind” region(s)

• However…

Blindsight

https://www.youtube.com/watch?v=GwGmWqX0MnM

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More about Blindsight

• People with blindsight respond emotionally to stimuli presented to blind area • Galvanic skin response

• “Guessing”

• Facial expressions are processed, but not identity

• Pathway to amygdala intact (emotional evaluation?)

• Implications for consciousness? • Consciousness not strictly necessary for action

Early Warning and the Amygdala • Early processing routed through amygdala and thalamus

• Highly responsive to potentially threatening and emotionally relevant stimuli

• “Early warning” system • This thing you are about to “see” might be dangerous • Brace yourself!

• Fear response can then either be acknowledged or suppressed

Unilateral Neglect • No subjective “blindness” like in blindsight but…

• Inability to attend to left visual field

• Unilateral neglect

• Damage to temporal-parietal junction (usually)

• Similar to blindsight, some degree of unconscious awareness

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Unilateral Neglect

Disorders of Object Recognition

• Object agnosia • Difficulty or inability to engage in object recognition

• Two forms

• Apperceptive agnosia • Cannot identify objects based on vision

• Problem with scale (only pays attention to fragments)

• Associative agnosia • Can see and even reproduce objects

• But can’t recognize them as what they are

• No access to name, usage, or meaning

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Double Dissociation: Associative Agnosia and Optic Ataxia

Associative object agnosia – can’t identify, can act Damage to ventral stream

Optic ataxia – can’t act, can identify Damage to dorsal stream (doesn’t always include simultagnosia)

https://www.youtube.com/watch?v=rwQpaHQ0hYw

Action and Perception • In certain situations, perceptual and motor

judgements differ

• Grasping in Ebbinghaus illusion

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Action vs. Perception

Duellmer, Franz, & Schwarzer, 2007

Ventral Dorsal

“what” an object is “where” an object is

what an object does how to interact with an object

Slow – mostly parvo stream Fast – mostly magno stream

Damaged in agnosia, blindsight, and neglect

Damaged in ataxia

Consciousness needed? Consciousness not needed?

Perception Action

Dual Visual System Theory