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Areas of high acuity and sensitivity like lips and fingers occupy relatively large areas of somatosensory cortex , while areas of low sensitivity and acuity like back and leg occupy relatively small areas of cortex . Humans have a relatively huge cortex , massive frontal lobe , but only rudimenary olfactory region . Because serotonin plays an important role in sensory function , with serotonin axons densely innervating visual cortex , we chose to use the functional magnetic resonance imaging ( fMRI ) BOLD method to study the effects of prior MDMA use on visual cortical activation during photic stimulation . By five weeks , the organ that looks like a lumpy inchworm has already embarked on the most spectacular feat of human development : the creation of the deeply creased and convoluted cerebral cortex , the part of the brain that will eventually allow the growing person to move , think , speak , plan , and create in a human way . Wernicke 's area is at the boundary of the temporal and parietal lobes , near the parietal lobe association cortex , where cross-modality integration is said to take place , and is adjacent to the auditory association cortex in the temporal lobe .