<P> The cerebrum has a contralateral organisation with each hemisphere of the brain interacting primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa . The developmental cause for this is uncertain . Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross sides in the brainstem . Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibres from each nerve split off to join the other . The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain . Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa . Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field . </P> <P> The left and right sides of the brain appear symmetrical, but they function asymmetrically . For example, the counterpart of the left - hemisphere motor area controlling the right hand is the right - hemisphere area controlling the left hand . There are, however, several important exceptions, involving language and spatial cognition . The left frontal lobe is dominant for language . If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills . </P> <P> A substantial part of current understanding of the interactions between the two hemispheres has come from the study of "split - brain patients"--people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures . These patients do not show unusual behaviour that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it . These patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown . </P> <P> Emotions are generally defined as two - step multicomponent processes involving elicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies . Attempts to localize basic emotions to certain brain regions have been controversial, with some research finding no evidence for specific locations corresponding to emotions, and instead circuitry involved in general emotional processes . The amygdala, orbitofrontal cortex, mid and anterior insula cortex and lateral prefrontal cortex, appeared to be involved in generating the emotions, while weaker evidence was found for the ventral tegmental area, ventral pallidum and nucleus accumbens in incentive salience . Others, however, have found evidence of activation of specific regions, such as the basal ganglia in happiness, the subcallosal cingulate cortex in sadness, and amygdala in fear . </P>

Which part of the brain contains the most neurons