Coloured Magnetic Resonance Imaging (MRI) scan in axial section through the head of a patient, showing a large arachnoid cyst (at left) in the Sylvian fissure of the brain. This cyst is an abnormal collection of cerebral spinal fluid (CSF) which is benign. But the size and location of the cyst can cause many symptoms, and the Sylvian fissure is associated with language and speech, so it may cause speech disturbances.