Aphasia is a disruption of language abilities due to injury typically to the left hemisphere of the brain (although a small percentage of individuals house language control in the right hemisphere). Aphasia affects the ability to use language to communicate what we intend to say. Speaking, listening comprehension, reading, and writing may be affected. Aphasia does not affect intelligence.

Many types of aphasia exist. Individuals may experience greater difficulty in one area (for example, speaking) and relative preservation of skill in another area (for example, reading). Aphasia is commonly caused by stroke, but it can also occur following traumatic brain injury, brain tumors, as well as due to neurodegenerative disease (see section discussing Primary Progressive Aphasia). Aphasia may or may not co-occur with speech movement control problems, such as apraxia or dysarthria, however the term aphasia specifically refers to difficulty retrieving and assembling the symbols of language that we use to communicate ideas, not the actual effectiveness of speech articulation movements.

Speech therapy is the current most effective intervention for aphasia. Persons with aphasia (PWAs) may benefit from speech therapy at any point post-onset. Therapies for aphasia range from intensive approaches driving neuroplastic recovery (that is, recovering lost functions to the greatest extent possible and/or "rewiring" activities to recruit other brain regions to pitch in for communication) to adaptive approaches where compensatory strategies are developed for individuals to perform communication life activities with aids and supports. These approaches are often combined. Adaptive interventions may also include developing supplementary or alternative communication systems for the PWA - this may involve "low tech" (non-computerized) or "high tech" (computerized) approaches. Further, interventions for aphasia may include training loved ones and communication partners to interact effectively with the PWA, facilitate communication, and negotiate communication breakdowns. A combination of these approaches, addressing speaking, listening, reading, writing, and communcation as a whole, may evolve over time, as a person's needs change.

Changes in communication and cognition following right hemisphere (RH) stroke are significantly less well-known and understood than aphasia, and may appear subtle, but can significantly affect the lives of the stroke survivor and their loved ones. The right hemisphere has highly complex roles in integrating information and disruption to its circuitry may result in a mismatch between the intended meaning and the means by which information is conveyed. Communication functions including both the expression and comprehension of body language, facial expression, tone of voice and use of pitch, loudness, and rhythm changes to make a point may be disrupted following RH injury. RH stroke survivors also may experience difficulty with subtle nuances in language to communicate or understand sarcasm, abstract language, and communication of emotions. Further, difficulty coordinating information and temporarily holding it in short-term memory to utilize when communicating may cause the RH stroke survivor to struggle to integrate the other person's knowledge, thoughts, and feelings into their communication - this can result in providing too little or too much information or even appearing insensitive. "Discourse" problems may arise from RH injury; this refers to conveying information through conversational speech; individuals with RH injury may be difficult to follow, shift topic suddenly without signaling this effectively, and include information in conversation that is not relevant to their underlying message. Conversely, RH injury can also result in reduced elaboration to the point that not enough information is conveyed to get the message across. Co-occurring problems with self-monitoring can make these communication problems incredibly complex.

Visuospatial deficits following RH stroke also may be present following injury may impact reading, writing, daily math functions (e.g., filling out a deposit slip or a form online). For example, RH stroke survivors experiencing "neglect" syndrome in which the attention to sensory stimuli (visual, auditory, tactile, etc) in the left side of body awareness is diminished, may struggle to understand what is read because the brain does not appreciate words on the left side of the page. Similarly, when writing or typing. the individual with RH injury may not utilize the left side of the page, or if using a computer to write, may lose track of what was just written due to difficulty attending to information on the left.

Cognitive deficits arising from RH injury often affect the complex array of attention processes coordinated by the brain, as well as problems planning, organizing, and prioritizing information. This not only may play a role in the communication disruptions mentioned above, but may also make it difficult for the person to process what is being said to them, due to a reduced attention span or distractibility. Furthermore, underactive attention processes may create problems monitoring the use of language, for example resulting in missing a significant amount of "typos" or incomplete written information in a text or email. Difficulty shifting from an idea (becoming "stuck" on something) may make it impossible for the person to receive incoming information. Lastly, remembering what is said in a conversation may be compromised if there was insufficient attention to the incoming communication in the first place.

Speech-language pathology services can play a vital role in addressing communication and cognitive problems following RH injury. While research in the area of cognitive and communicative neurorehabilitation following RH injury is relatively scarce compared to aphasia literature, significant advances in our understanding of RH functions in communication have occurred in the last 2 decades. RH injury survivors and their loved ones can significantly benefit from interventions that can help to gain specific awareness of the nature of the complex communication problem, to learn targeted strategies to improve functioning or to adapt to the problem, and to capitalize upon healthy brain regions to participate in the target activity. Survivors can greatly benefit from using principles of neuroplasticity (the brain's ability to change) to improve communication and cognitive problems that are important to them and relative to their personal goals.

Strokes and other insults to the brain may cause problems with the execution of speech movements, resulting in speech sounding "slurred", "mumbling", quiet, or with altered rhythm from normal. Strokes in certain brain areas may make it difficult to initiate speech movements at all. The area of the brain that is affected, the size of the stroke, and the status of the rest of the brain are primary determining factors in the type of speech movement problem. Common motor speech disorders arising from stroke include "dysarthria" and "apraxia", although less common speech movement problems may occur as well. Speech movement problems may or may not co-occur with problems with language (aphasia) or cognition; speech movement problems themselves may co-occur as well (for example, an individual could experience both apraxia and dysarthria from the same stroke).

Dysarthria refers to a collection of motor speech disorders in which the strength, range of motion, speed, coordination, or direction of speech movements is disrupted from habitual speech patterns established in childhood development. Dysarthria may be very mild (for example, resulting in a slight problem with a specific sound such as "s") ranging to very severe (total loss of speech movement, typically from very rare forms of brainstem stroke). Apraxia of speech refers an acquired difficulty of the brain's ability to plan and/or program speech movements. It typically occurs from damage to a specific area in the left hemisphere. It affects the articulation of speech movements as well as speech rate and often, the rhythm and length of speech sounds. Apraxia of speech can also range from very mild, with minimal impact upon intelligibility, to very severe, making it incredibly difficult for the stroke survivor to create the speech movements needed to transmit the words that are in their mind.

Individuals with motor speech disorders may experience significant frustration and daily challenges, resulting from others having difficulty understanding them as well as from stigma resulting from lack of public knowledge understanding that speech disorders are not directly associated with any decrease in intelligence. Speech therapy services are a vital intervention to maximize outcomes following the onset of a motor speech disorder. Interventions may include teaching individuals to alter tongue, lip, jaw, voice, and breathing movement patterns to achieve clear and understandable speech; use of compensatory strategies such as rate or pause techniques; use of prosthetic devices; and in the case of severe motor speech disorders, supplementary or alternative speech communication systems. Interventions also include teaching clients and loved ones interaction strategies to navigate breakdowns in communication as well as how to optimize communication settings.

Cognition refers to mental activity, and encompasses brain processes that describe how we gain, store, use, and retrieve knowledge. Cognition includes, but is not limited to, the ideas of attention, memory, visuospatial processing, and executive functioning. Language is also considered a special type of cognitive process. Attention includes the ideas of focusing, concentrating, filtering out internal and external distractions, multitasking, and shifting between ideas or activities. Memory refers to the temporary or long-term storage of knowledge as well as the ability to retrieve knowledge. Visuospatial functions are the brain's work of processing and making sense of the information that comes in through the eyes. Executive functioning is an "umbrella" term for high-level brain processes of coordinating the activities of the rest of the brain; planning, organizing, putting things in order, prioritizing, decision-making, problem solving, analysis, and similar processes.

Although frontal lobe injury is a relatively well-known brain area associated with cognition, cognitive problems from stroke or other brain problems can occur across multiple brain regions. Deeper parts of the brain (for example, the subcortex) play important roles in cognition, and even the cerebellum, known mostly for movement control, has recently been found to play subtle roles in cognitive processing, possibly related to its connectedness to other brain regions.

Rehabilitation of acquired cognitive problems is a vital role in the field of medical speech-language pathology. Interventions may include empowering individuals with education as to the specific nature of the cognitive disorder to begin the path toward self-management, directly working on problematic areas of cognition to achieve maximum neuroplastic benefit, development and training of self-cuing and compensatory strategies, the use of adaptive aids and technologies to help cognition, modifying the person's settings to improve cognition, and training caregivers to set the individual up for success.