Outline: Mental Imagery

• When do we use Imagery?

• Subcomponents of Imagery .

• Relation to perception

• Types of Imagery: Spatial vs. Visual

• Oral Presentations: Info

Mental Imagery

– Which one is bigger, a tennis ball or a 100-watt light bulb?

– Do monkey’s ears protrude from their head?

– Do robins lay eggs? (no imagery needed)

When do we use imagery?

• when the following three conditions are met:– The information is a subtle visual property

– The property has no verbal label (has not been considered before)

– The property cannot be inferred from other information

• We also use imagery to anticipate trajectories – Would the bilard go into the socket?

• These two functions (memory recognition, visual tracking) are also the primary functions of visual perception

Sub-components of Visual Imagery• Generating • Scanning (inspection) • Retaining • Mental Rotation (transformation)

• Zoom (transformation)

Generate

• Imagine an elephant skating

• This occurs in a piecemeal fashion:

• First, an ‘image frame’ is generated (global shape)

• Next, parts and details are added to the the global shape, based on needs and imager preference

• Example: Imagine a letter F

Mental Scanning

• Map is displayed

• Subjects memorize it

• Map is removed (imagery)

• Task: “Imagine black speck moving from straw hut to well. Press the key when you arrive”

• Independent variable: The distance

• Dependent variable: Response Time

• Main Finding: Distance scanned in a mental image is directly related to the time it takes to complete the scan

Image Retention

• Note that in visual perception the very rapid decay is a virtue (but it is a burden for imagery)

Mental Rotation

• Stimulus: A pair of 3-D objects

• Task: Are the two shapes the same but viewed from a different perspective? Or are they different?

• Independent variables:

– Rotation type (in 2-D, in depth)

– Same/different

– Angle of rotation (0 --> 180º)

• Dependent variable: Response Time

• Main Findings:

– Imagined movement resembles actual movement (linear relation between RT and angle of rotation)

– This is true for both plane and depth rotations

2-D yes

no

yes

Are they the same shape?Rotation

depth

depth

Mental Rotation: What is the nature of mental imagery?

• Hypothesis:

• - ‘Moving’ mental images is like moving real objects.

• Assumptions:– Obviously, real objects move in a continuous fashion

– to judge whether the letters are normal or mirror reversed requires rotating image to “upright” positions

• Prediction:

RT: fast slowFast RT

Slower RTSlowest RT

Results

The greater the angle the letter had to be rotated to get back toupright…the longer it took subjects to judge whether they weremirror reversed or not.

Zoom-in

• Instructions: • “Imagine a mouse next to

an elephant…”

• “Do the mouse’s whiskers point up or down?

• Inspection of whiskers requires zooming-in

• Such zoom-in process is time consuming (slow RT)

Zoom-in (cont’d)

• Instructions: • “Imagine a mouse next

to a paper clip…”

• “Do the mouse’s whiskers point up or down?

• Inspection of whiskers does NOT require zooming-in

• Faster RT than when paired to the elephant

Overlap of Imagery and Perception

• Hypothesis:– Imagery uses the same machinery as perception

• Evidence– Behavioral (interference)

– Neuroimaging

– Brain Lesion

•

• Primary Task: – Detect a weak perceptual signal

• Secondary Task: – Imagery

• Independent variable: – Modality across tasks

• Same (high interference): – visual-visual, – auditory-auditory

• Different (low interference): – visual-auditory, – auditory-visual

• Dependent variable: Perceptual Sensitivity (hit - FA)

• Finding: There is interference• Thought question:Predict result when

perceptual and imaged stimuli are the same

Behavioral Evidence (‘inspection’ interference)

Neuroimaging Evidence: Similar Brain Areas Activated by Imagery and Perception

Neuroimaging Evidence: Topographic correspondence between perception and Imagery

Background:- Primary visual cortex has a retinotopic map, that is, - peripheral visual field is represented in anterior part of BA17 - fovea is represented in posterior part of BA17

Task: Letter Imagery Independent Variable: Size of imageResult: Image of small letters activates more posterior parts of primary visual cortex than image large letters.

Neuropsychological Evidence

• Subject: Patient with occipital lesion and reduced visual field (vs. Normal controls)

• Instructions: • “Imagine an elephant, start

walking toward it until it covers all visual field. How far away is it?”

Normal Visual field

ReducedVisual field

Neuropsychological Evidence

Visual vs. Spatial Imagery• Visuo-spatial imagery can be divided into:

– Visual Imagery proper• Unavailable to blind people

• Possibly related to introspective ‘vividness’ of image

• Disrupted by secondary visual task (but not by spatial task)

– Spatial Imagery

• Evident in blind people

• Unrelated to ‘vividness’ of image

• Disrupted by non-visual spatial task

• Probably more dependent on parietal cortex

• Individual differences data argue for independence among subcomponents (also between visual and spatial aspects)

Imagery: Conclusions

• Mental imagery can be studied in the lab

• Visual Imagery is not a monolithic concept.

• Instead, many sub-components exist (rotate, zoom, scan, generate)

• Mental images are a little bit like pictures (analogical)

• Imagery seems to rely in perceptual neural substrates