Lecture 2: Illusions and Lesions

Question 1

Explain the mechanisms behind the illusion in which you focus on a cross in the centre and the purple dots surrounding it slowly fade

Answer 1

This illusion is due to the adaptation of neurons when focused on the same visual stimuli, the strength of the signal in the neuron is weakened through repetition

Question 2

What does the white and gold dress demonstrate?

Answer 2

the effects of colour constancy and how our expectations and knowledge of the world shape our perception. People can have different perceptions of the same input.

Question 3

What does the circular illusion which creates a perception of motion demonstrate?

Answer 3

The microsaccades we make as we look at this complex visual stimuli likely creates the illusion of motion.

Question 4

As a whole what do these illusions demonstrate

Answer 4

All in all these demonstrate that your conscious perception is not just a 1 to 1 reflection of the outside world!

Question 5

Give another word for blindsight

Answer 5

cortical blindness

Question 6

What is usually the cause of blindsight?

Answer 6

early visual cortex damage

Question 7

What is the phenomena of blindsight?

Answer 7

The amount of damage on the visual cortex scales with the amount of damage to the visual field. This type of cortically blind patients insist that they don’t see anything. But when they are forced to make a choice (to guess), they can:

=>Can discriminate shapes, orientation, colour when forced to make a choice

=>Can accurately reach for objects

=>Accurate hand positioning for grasping etc

=>Direction of motor perception

Question 8

What is likely reason for this phenomena of blindsight?

Answer 8

This is due to blindsight utilising alternative routes through the subcortex, 90% of the information goes to the visual cortex, the last 10% of this information, travelling through the dorsal stream in the subcortex is responsible. Retina - Thalamus - Superior Colliculus - Dorsal Stream. So it bypasses the V1 and directly goes to the Dorsal Stream and the information is processed there. Dorsal Stream is the “Where” pathway so it processes the information about localisation and motion behavior. So the information can be used to guide behavior through the Dorsal stream, although it doesn’t go though the Ventral pathway leading to an accurate perceptual representation of the world.

Question 9

Describe a study which manipulated blindsight in an experimental setting

Answer 9

Even when the visual cortical areas were lesioned, monkeys could still dodge objects etc- use their sight for action. They removed bilaterally her visual cortex. She is put in a maze to move around. She is supposed to be ‘blind’ as the whole cortex is removed, a human would report not seeing anything. However, Helene is able to move around the objects without touching them. (But monkeys have great auditory skills so there may be a multiprocessing going on).

Question 10

What findings have their been in a clinical setting which is similar to these lesioned blindsight monkeys

Answer 10

Patients who can manouver arounnd objects while walking down a hallway etc

Question 11

Describe a similar condition to blindsight and how it is different

Answer 11

visual neglect: the patient has lateral damage on parietal context (often on the Temporal Parietal Junction) , and neglects the left side of the world. She is asked to draw a daisy and she only draws the right side of it, even in her memory it’s a complete daisy for her. So they neglect both while copying and while spontaneous drawing. She realises this only after it’s pointed out to her.

However neglect is concerning attention: Visual information is received in the visual areas and processed, but not attended to.

Question 12

Damage to where is associated with visual neglect?

Answer 12

Damage to the right tempo-parietal junction and other parietal areas is more pronounced than damage to the left tempo-parietal junction.

Question 13

Name and describe a disorder which demonstrates a dissociation between perception and action

Answer 13

Visual form agnosia- Patient DF for example was unable to identify the shape of objects (recognised objects by colour) and was unable to copy line drawings of objects. She was able to produce line drawings of objects from memory (not due to loss of knowledge about objects.)

There was a difference in ability when it came to identify the shape of an object and utilising the shape of an object through an action- matching the orientation of the slot vs sliding the object into the slot.

Question 14

What brain damage resulted in visual form agnosia in patient DF?

Answer 14

damage to the lateral occipital cortex due to carbon monoxide poisoning (brain cells die in the absence of oxygen)

Question 15

How is there a dissociation in the brain between perception and action?

Answer 15

Ventral stream deficit related to problems in perception (e.g. matching), but not (or much less so) for action (e.g. posting)

Question 16

How can this dissociation be observed in healthy humans? Describe a study

Answer 16

Illusions - Participants see an Ebbinghaus illusion where the central circle and surrounding circles were perceptually identical but physically different or perceptually different and physically identical. They then had to grab a central object and the distance was measured between their fingers. There was a difference found when the circles were perceptually identical but physically different but not when perceptually different and physically identical, demonstrating that perception suffers from illusions but action does not.

Question 17

How are these two streams related to consciousness?

Answer 17

Ventral stream activation is related to consciousness (perception, suffers from illusions)

Dorsal stream activation is unconscious (action, is automatic)

Question 18

Give an example of how conscious perception is not an exact replica of the outside world

Answer 18

Our eyes make saccades, very fast eye movements. We are blind during the saccadic eye movements. We don’t process information during those saccades and they take about 100 to 150ms. But we experience the world as if it’s one smooth movie. We don’t sense the blind saccade moments

Question 19

What does the necker cube illusion demonstrate?

Answer 19

Necker cube shows that conscious perception is serial and we can only perceive one interpretation at a time. Brain decides which interpretation to see.

Question 20

Describe how visual neglect is demonstrated in a study through report and through objective measure

Answer 20

When we present them two house images with one of them on fire (on the part they ‘neglect’), they choose the house without the fire (although they don’t ‘see’ the fire).

This is also seen in eye-movement ‘heat maps’. Patients has a bias towards the right hemifield.