Cerebral Lateralizaton

Psy391h - Classes 19-20

(This page last updated 16 April 2006.)
 

 Split-Brain Patients

• With corpus callosum cut, info cannot travel directly from one hemisphere to the other.
• Picture flashed to right visual field.
  • To visual cortex in left hemisphere
  • Left hemisphere language mechanisms can name it.
• Picture flashed in left visual field.
  • Right hemisphere cannot name it.
  • Can identify it by pointing to matchin picture with left hand.
• Simple signals such as emotions can travel from one cortex through subcortical structures to the other.
• The biggest difference between the two hemispheres is in language.
  • In most people, the left hemisphere is capable of understanding and producing language, including complex sentences…
  • While the other hemisphere's language ability is more limited.
    • May be no language ability at all.
    • May be able to use words.
    • Cannot generate syntax.
• For all other cognitive tasks, both hemispheres are capable of performing them, but there are some differences in relative ability.
  • Usually a right hemisphere advantage in visuospatial processing
• chimeric faces
  • Split-brain patients not aware that the two halves of the stimulus do not match.
  • Will name the face in the right visual field.
  • Left hand can point to identify face in the left visual field.

Differences in Global and Local Processing Between Left and Right Hemispheres

• With these hierarchical stimuli, can report either local or global letter.
• In split-brain patients, left hemisphere better at local task, and right hemisphere better at global task.
• More evidence emerges from studying patients with left- and right-hemisphere lesions.
  • With left-hemisphere damage, slower to identify local target.
  • With right-hemisphere damage, slower to identify global targets.
  • Similar problems when left- and right-hemisphere patients draw visual stimuli.
• Even normal subjects show left-hemisphere advantage for location processing and right-hemisphere advantage for global processing.
  • Stimuli presented lateralized, too briefly for eye movement.
  • Searching for one of two targets (H or L) that can be at either local or global level, or both.

Differences in Processing of Spatial Frequencies Between Left and Right Hemispheres

• Spatial frequency in visual stimuli.
  • Gratings with different spatial frequencies can be combined to form more complex patterns.
  • Each visual stimulus can be broken down into a set of gratings such as these, each with a different orientation and spatial frequency
  • A picture with lots of fine detail will contain many high spatial frequencies.
  • A picture in which one broad area is lighter than another broad area will contain low spatial frequencies.
  • Most images hae a combination of high and low spatial frequencies.
  • We can remove just the high spatial frequencies, leaving the low spatial frequencies.
    • The result is like blurring the image.
  • Or we can remove just the low spatial frequencies, leaving the highs.
• Differences in processing spatial frequencies between hemispheres could explain global-local differences.
  • In normal subjects, right hemisphere better than right at distinguishing gratings.
  • More importantly, right advantage is greater for low spatial frequency than for high.
  • Right hemisphere may be better with global stimuli because low spatial frequencies are used to identify them.
• Spatial Frequency Differences Could Also Explain Differences in Face Perception
  • Left hemisphere relatively better at identifying familiar faces.
  • Right hemisphere relatively better at distinguishing males and females.
  • Male/female discrimination can be done with low spatial frequencies.
    • Could explain why this task is well suited for right hemisphere.
  • Face identification requires high spatial frequencies.
    • Could explain why this task is well suited for left hemisphere.

   

Other Differences Between Left and Right Hemisphere

• Categorical vs. Coordinate Processing
  • Right hemisphere advantage for coordinate, or metric, judgments.
    • Is dot near line or far from line?
  • Left hemisphere advantage for categorical judgments.
    • Is dot above or below line?
• Prototypes vs. Examplars
  • Exemplars created by modifying prototypes.
  • Learn category by viewing 4 exemplars.
    • Do not see prototype during learning.
  • Right hemisphere faster at identifying exemplars that were seen previously.
  • Left hemisphere faster at categorizing prototype with correct exemplars.
  • Right hemisphere may work more by recording many exemplars.
  • Left hemisphere may work more by generalizing across exemplars to form prototype.

     

Attention and Recognition in Split Brain Patients

• Either hemisphere can direct attention to either visual field.
• Visual search with split-brains
  • Task: find target shape in array of distractors.
  • For all subjects, response time goes up with display size.
  • Search rate twice as fast for split brains as for normals.
  • Each hemisphere can search its hemifield independently.
• Recognition memory in split-brains.
  • Each hemisphere sees its own series of shapes.
  • At end of series, must decide if new shape has been seen before.
  • Much easier when same sequence is presented to both hemispheres.
  • Not able to attend and record memories for each hemifield independently.

   

Probability Matching

• Predict whether stimulus will appear in upper or lower visual field.
• In upper field on 80% of trials, and lower field on 20%.
• Human subjects usually use a matching strategy.
  • Guess the top location 80% of the time, and the bottom position 20% of the time.
• Rats and goldfish use a maximizing strategy.
  • Always guess the location with the higher probability.
• The maximizing strategy produces higher accuracy.
• In split brain patients, the right hemisphere uses a maximizing strategy.
• The left hemisphere uses a matching strategy.
• One explanation: The left hemisphere seeks a causal explanation for the pattern, while the right hemisphere does not.
  • Usually the causal explanation will lead to better predictions, but not in this task.
  • Left hemisphere seeks to build causal explanations in other circumstances
    • Anecdote: Chicken and shovel (see chapter 16)

next: executive function

Psych 391h: Cognitive Neuroscience Kyle Cave Psychology Dept. U. Mass.