developing visual system by Torsten Wiesel and David Hubel, it has been suggested that the environment guides the development of language networks in the brain by selecting the neurons and their synapses that are most active during the prolonged early development of humans in a socio-linguistic environment. According to this theory, the neural substrate for language results from a combination of the genetically determined complexity of the human brain and the 'functional validation of synapses' during the long period of postnatal maturation that is unique among primates. Similarly, it has recently been suggested that the relatively slow development of the prefrontal cortex in humans may be one reason that humans are able to learn language, whereas other species are not. Further research has indicated the influence of the FOXP2 gene. Stages 0-1 years of age: An infant mainly uses non-verbal communication (mostly gestures) to communicate. For a newborn, crying is the only means of communication. Infants 1-5 months old have different tones of crying that indicate their emotions. Infants also begin laughing at this stage. At 6-7 months old, infants begin to respond to their own name, yell and squeal, and distinguish emotions based on the tone of voice of the mother and father. Between 7 and 10 months the infant starts putting words together, for example "mama" and "dada", but these words lack meaning and significance. Verbal communication begins at approximately 10-12 months, and the child starts to imitate any sounds they hear, for example animal sounds. The non-verbal communication of infants includes the use of gaze, head orientation and body positioning. Gestures are also widely used as an act of communication. All these stages can be delayed if the parents do not communicate with their infant on a daily basis. Nonverbal communication begins with the comprehension of parents and how they use it effectively in conversation. Infants are able to break down what adults and others are saying to them and use their comprehension of this communication to produce their own. 1–2 years of age: Verbal and nonverbal communication are both used at this stage of development. At 12 months, children start to repeat the words they hear. Adults, especially parents, are used as a point of reference for children in terms of the sound of words and what they mean in context of the conversation. Children learn much of their verbal communication through repetition and observing others.