Question

What kinds of perceptual and cognitive processing is needed for understanding spoken and written language? What neural structures are involved?

Answer 1

THE HUMAN BRAIN:

The brain is divided into two halves, a left hemisphere and a right hemisphere. This is called lateralization.In human beings, it is the left hemisphere that usually contains the specialized language areas. While this holds true for 97% of right-handed people, about 19% of left-handed people have their language areas in the right hemisphere and as many as 68% of them have some language abilities in both the left and the right hemispheres.

WHAT IS LANGUAGE PROCESSING?

Language processing refers to the way humans use words to communicate ideas and feelings, and how such communications are processed and understood. Thus it is how the brain creates and understands language.

UNDERSTANDING SPOKEN LANGUAGE :
Sound waves received by the ear are turned into neural activity by a complex mechanism involving the eardrum, the bones in the middle ear, and the hair cells within the cochlea. The auditory nerve carries the signal from the ears to the brainstem, from where it passes via the thalamus to the auditory areas of the cerebral cortex. In the cortex, speech sounds are extracted from the incoming signal. There are neural circuits in the auditory cortex that are specialized for speech and language as opposed to other types of sound.

Speech production and speech perception takes place predominantly in one hemisphere of the brain, usually the left. Several areas within the left hemisphere are involved. Broca’s area, in the frontal lobe, seems to be crucial for syntactic operations in both production and perception of speech. Wernicke’s area, in the temporal lobe, seems to be crucial for accessing the concrete meanings of words. The evidence for the distinction in function between posterior and anterior language areas comes from the study of aphasia, that is, difficulties with language resulting from brain injury.

PERCEPTUAL PROCESSES INVOLVED
Speech perception
Acoustic stimuli are received by the auditive organ and are converted to bioelectric signals on the organ of Corti. These electric impulses are then transported through scarpa's ganglion (vestibulocochlear nerve) to the primary auditory cortex, on both hemispheres. Each hemisphere treats it differently, nevertheless: while the left side recognizes distinctive parts such as phonemes, the right side takes over prosodic characteristics and melodic information.
The signal is then transported to Wernicke's area on the left hemisphere (the information that was being processed on the right hemisphere is able to cross through inter-hemispheric axons), where the already noted analysis takes part.
During speech comprehension, activations are focused in and around Wernicke's area. A large body of evidence supports a role for the posterior superior temporal gyrus in acoustic–phonetic aspects of speech processing, whereas more ventral sites such as the posterior middle temporal gyrus (pMTG) are thought to play a higher linguistic role linking the auditory word form to broadly distributed semantic knowledge.

Speech production
From Wernicke's area, the signal is taken to Broca's area through the arcuate fasciculus. Speech production activations begin prior to verbal response in the peri-Rolandic cortices (pre- and postcentral gyri). The role of ventral peri-Rolandic cortices in speech motor functions has long been appreciated (Broca's area). The superior portion of the ventral premotor cortex also exhibited auditory responses preferential to speech stimuli and are part of the dorsal stream.
Involvement of Wernicke's area in speech production has been suggested and recent studies document the participation of traditional Wernicke's area (mid-to posterior superior temporal gyrus) only in post-response auditory feedback, while demonstrating a clear pre-response activation from the nearby temporal-parietal junction (TPJ).
It is believed that the common route to speech production is through verbal and phonological working memory using the same dorsal stream areas (temporal-parietal junction, sPMv) implicated in speech perception and phonological working memory. The observed pre-response activations at these dorsal stream sites are suggested to subserve phonological encoding and its translation to the articulatory score for speech. Post-response Wernicke's activations, on the other hand, are involved strictly in auditory self-monitoring.

COGNITIVE PROCESSES INVOLVED

Listening
The acoustic speech signal provides a nearly-continuous stream of spectral information about what the speaker’s mouth is doing (and indirectly, about the speaker’s intentions). The signal is broken by brief periods of silence that correspond to closures of the mouth (during stop consonants) and not generally to breaks between words. A great deal is known about how the acoustic properties of the speech waveform are mapped into the perception of phonological segments and about the recognition of spoken words.The recognition of words in normal speech has to take place quickly and efficiently. Estimates of normal speaking rate range from 120 to 200 words per minute, or 120 to 600 syllables, or in some estimates, as many as 20 to 30 segments per second.One of the important tasks for a listener is to segment the continuous speech stream into words. While there are some signals to word beginnings in the speech streammuch of the work of segmentation must be based on knowledge of the words that exist in a language. A listener is generally able to take a string of segments that could be broken at different points into possible words, and parse it into a coherent string of words in which every segment is assigned to a word .

UNDERSTANDING WRITTEN LANGUAGE:

PERCEPTUAL PROCESSES INVOLVED

The primary visual cortex decodes visual information from printed words, sentences, and text. Neuroimaging studies of lexical tasks have also revealed the importance of the fusiform gyrus for lexical processing in writing systems of several languages, including alphabets and logographs.The VWFA is largely implicated in mapping visual information to meaning or retrieval of meaning ,especially in reading ideograms .It processes information of fine-grained visual form such as (but not exclusively) information required for discriminating between words and for combining visual and verbal linguistic information. Lesions to the VWFA are associated with impairments in oral reading and oral naming tasks

COGNITIVE PROCESSES INVOLVED
Reading
During reading, the visual properties of the text are encoded via a series of eye movements generally from left-to-right across the line of text. The visual information is encoded during fixations, which typically last about 200-250 ms (though the range is from 50 ms to over 500 ms). The movements of the eyes between the fixations, saccades, typically last 20-30 ms; no new information is obtained during these movements. Indeed, vision is suppressed during saccades . On average, the eyes move 7-8 letter spaces (though the range is from a single letter space to over 20 spaces) for readers of English and other alphabetic writing systems; letter spaces, rather than visual angle, are the appropriate measure for indexing how far the eyes move during reading On about 10-15% of the saccades, regressions, readers move their eyes backwards in the text to look at material that has received some prior processing. As text difficulty increases, readers tend to increase fixation durations, decrease saccade size, and increase regressions.

The reason readers need to make eye movements during reading is very much related to the anatomy of the retina.
The cognitive operations involved in writing may be divided for convenience into central and peripheral processes.
Central processes include semantic, syntactic and other sentence-level operations, along with those processes responsible for either retrieving from memory the spelling of a familiar word or assembling from sound a plausible spelling for an unfamiliar word or nonword. The end-product of these central processes is an abstract graphemic representation of words as letter strings.
Peripheral writing processes translate that abstract graphemic representation into a range of possible output modes, including handwriting, typing, and spelling aloud. A model of peripheral writing processes is presented, concentrating on the processes responsible for creating handwritten output. A number of different peripheral acquired dysgraphias including mirror writing are then reviewed and interpreted in terms of impairment at different levels in the conversion of abstract graphemes into written letters and words.

NEURAL STRUCTURES INVOLVED IN LANGUAGE PROCESSING :
Much of the language function is processed in several association areas, and there are two well-identified areas that are considered vital for human communication: Wernicke's area and Broca's area. These areas are usually located in the dominant hemisphere (the left hemisphere in 97% of people) and are considered the most important areas for language processing. This is why language is considered a localized and lateralized function.the non-dominant hemisphere also participates in this cognitive function, and there is ongoing debate on the level of participation of the less-dominant areas. The non-dominant hemisphere may be particularly involved in processing the prosody of spoken language.

There are several areas of the brain that play a critical role in speech and language.

Broca’s area, located in the left hemisphere, is associated with speech production and articulation. Our ability to articulate ideas, as well as use words accurately in spoken and written language, has been attributed to this crucial area.

Wernicke’s area is a critical language area in the posterior superior temporal lobe connects to Broca’s area via a neural pathway. Wernicke's area is primarily involved in the comprehension. Historically, this area has been associated with language processing, whether it is written or spoken.

The angular gyrus allows us to associate multiple types of language-related information whether auditory, visual or sensory. It is located in close proximity to other critical brain regions such as the parietal lobe which processes tactile sensation, the occipital lobe which is involved in visual analyses and the temporal lobe which processes sounds. The angular gyrus allows us to associate a perceived word with different images, sensations and ideas.
Other Areas
Despite the fact that Broca's and Wernicke's Areas are in different lobes, they are actually quite near each other and intimately connected by a tract of nerves called the arcuate fascilicus. There are also people who have damage to the arcuate fascilicus, which results in an aphasia known as conduction aphasia. These people have it a bit better than other aphasias: They can understand speech, and they can (although with difficulty) produce coherent speech, they cannot repeat words or sentences that they hear.