last updated:

April 28, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 34:  Lateralization of Function [Outline]

"Right" versus "Left" Brain?
Why do humans have two sides (hemispheres) to our brain?

What might be an advantage of having two sides to the brain?

Are we the only animals with lateralization of functions? NO:

Consider the following...
Parrots prefer picking up things with their left foot.
Toads tend to attack other toads from the right but go after prey from the left.
Zebra fish are likely to look at new things with their right eye but familiar things with their left.

A. The Right and Left Hemispheres

Lateralization: Refers to those behaviors and cognitive abilities that each hemisphere specializes in.

Contralateral = the other side while ipsilateral = the same side

Left Hemisphere
Right Hemisphere
Contralateral (right side)
Contralateral (left side)
Skin Receptors
Contralateral (right side)
Contralateral (left side)
Vision (Eyes)
Contralateral (Right visual field of both left and right eyes; see diagram below)
Contralateral (Left visual field of both left and right eyes; see diagram below)
Hearing (Ears)
Information from both ears, but stronger from right ear
Information from both ears, but stronger from left ear
Ipsilateral (left side of tongue)
Ispilateral (right side of tongue)
Ipsilateral (left nostril)
Ipsilateral (right side of tongue)
Trunk Muscles & Facial Muscles
Jointly controlled
B. Visual and Auditory Connections

Visual Processing.

[Visual System connections]

Auditory Processing. Information from each ear is transmitted to both sides of the brain.
C. Corpus Callosum and the Split Brain Operation

The corpus callosum is a massive set of axons which allow the two hemispheres to exchange information with one another. There are two other commissures: the anterior commissure and the hippocampal commissure


            Operation1.  Severing the corpus callosum prevents the sharing of most information between the brain hemispheres.

2.  Epilepsy 

YouTube Video. Video of a grand mal (tonic-clonic) seizure (1'18") of a college-age man, Josh. (Note that this is a graphic video.)

Most people with epilepsy (90%) can control their condition with medications that suppress seizure activity. Surgery for epilepsy can take two forms. The earliest form sought to remove any focus of epileptic activity.

3. Commissurotomy (also called corpus callosotomy)

split brain  Roger Sperry

4. Studying Split-Brain Patients.

Split brain

YouTube Video: Split-brain patient 'Joe' being tested with stimuli presented in different visual fields (4'31").

5.  The Right Hemisphere vs. the Left Hemisphere

a.  The right hemisphere is better than the left at perceiving the emotions in people's gestures (non-verbal or paralinguistic information).

b.  People with right hemisphere damage speak with less inflection and expression, plus they often have trouble interpreting the emotions that other people express through their tone of voice (prosody).

c.  Research findings suggest that the right hemisphere is more adept than the left at comprehending spatial relationships.

d.  The left hemisphere is more focused on details and the right hemisphere is better at perceiving overall patterns.

  Role of Left Hemisphere  Role of Right Hemisphere
Speech Production of speech, comprehension of the literal meaning of speech Emotional inflections, understanding jokes & humor, sarcasm, emotional content of speech
Auditory System  Sounds related to speech  Non-language environmental sounds (e.g., rain)
 Emotions Expressions of happiness Expressions of anger, fear, disgust; interpreting the emotional expressions of other people
 Vision Details  Overall configuration;
spatial processing (e.g., arranging pieces of a puzzle or drawing a picture)
 Mode or Style
(How data are processed)
Details, parts, pieces  Gestalt, overall configuration; global form 

D.  Development of Lateralization and Handedness

1. Planum temporale: A section of the temporal cortex that is larger in the left hemisphere in approximately 65% of the population.

2. Corpus Callosum. The corpus callosum matures slowly over the first 5 to 10 years of human life. It contains roughly 200 million fibers that cross from one hemisphere to the other. 

3.  Agenesis of the Corpus Callosum (ACC).

The following two commissures are often larger than normal in people born without a corpus callosum:

a.  Anterior commissure: A very tiny bundle of fibers (about 3.5 million, see image below) that connects the two hemispheres around the anterior parts of the cerebral cortex.

Its major role in people with an intact corpus callosum involves olfaction (smell) and other functions which are not as well understood. In ACC, this commissure seems to convey visual information, a role it typically does NOT play.


b. Hippocampal commissure: Connects the left hippocampus to the right hippocampus. Its general role is not well understood.

handedness vs. language dominanceHandedness

right vs
                left brained?Avoiding Overstatements

What is the answer to the first question at the beginning of this lecture?
Are there "right-brained" and "left-brained" people? NO: This is a myth!

As our text points out, such claims are based on two correct and one incorrect assumptions:
  • each hemisphere specializes in particular functions [TRUE]
  • tasks involving particular functions evoke higher levels of activity in particular hemispheres [TRUE]
  • individual persons rely more heavily and most of the time on one of the hemispheres [FALSE]

We do not have scientific evidence to support that last claim: relying upon either the right or the left hemisphere habitually. 

  • Indeed, the presence of the corpus callosum means that the brain is constantly engaged in interhemispheric communication: one side of the brain is always in conversation with the other side of the brain.
  • Most cognitive processes (including creativity, imagination, seeing details, doing mathematics, understanding language, etc.) actually rely upon functions of both hemispheres.