Each type of methodology presents a set of advantages and disadvantages for studying a particular problem in psycholinguistics. Computational modelling, such as the DRC model of reading and word recognition proposed by Max Coltheart and colleagues, is another methodology and refers to the practice of setting up cognitive models in the form of executable computer programs. Such programs are useful because they require theorists to be explicit in their hypotheses and because they can be used to generate accurate predictions for theoretical models that are so complex that they render discursive analysis unreliable. Other examples of computational modelling is McClelland and Elman's TRACE model of speech perception and Franklin Chang's Dual-Path model of sentence production. Psycholinguistics is concerned with the nature of the computations and processes that the brain undergoes to comprehend and produce language. For example, the cohort model seeks to describe how words are retrieved from the mental lexicon when an individual hears or sees linguistic input. Recent research using new non-invasive imaging techniques seeks to shed light on just where certain language processes occur in the brain. There are a number of unanswered questions in psycholinguistics, such as whether the human ability to use syntax is based on innate mental structures or emerges from interaction with other humans, and whether some animals can be taught the syntax of human language. Two other major subfields of psycholinguistics investigate first language acquisition, the process by which infants acquire language, and second language acquisition. In addition, it is much more difficult for adults to acquire second languages than it is for infants to learn their first language (bilingual infants are able to learn both of their native languages easily). Thus, sensitive periods may exist during which language can be learned readily. A great deal of research in psycholinguistics focuses on how this ability develops and diminishes over time. It also seems to be the case that the more languages one knows, the easier it is to learn more. The field of aphasiology deals with language deficits that arise because of brain damage. Studies in aphasiology can both offer advances in therapy for individuals suffering from aphasia, and further insight into how the brain processes language. A 2016 empirical study showed that personal associations are mutually inter-related and that the concepts of self and world are internally connected via direct and mediated dependences, which reflects the structuring of perception and understanding of self and world in people's minds and discusses its implications for psycholinguistics. A short list of books that deal with psycholinguistics, written in language accessible to the non-expert, includes: