Neuroanatomy, circuitry and plasticity of word reading

The use of neuroimaging has provided a basis for suggesting the brain areas active during reading of words and sentences. When combined with high density electrical recording from the scalp, it is possible to obtain information on the time course of activation of these brain areas and compare them with the temporal structure of reading from studies of eye movements. The paper summarizes results in these areas and suggests how acquisition and practice of the skill might alter the circuitry involved.

Attention, self-regulation and consciousness

Consciousness has many aspects. These include awareness of the world, feelings of control over one's behaviour and mental state (volition), and the notion of continuing self. Focal (executive) attention is used to control details of our awareness and is thus closely related to volition. Experiments suggest an integrated network of neural areas involved in executive attention. This network is associated with our voluntary ability to select among competing items, to correct error and to regulate our emotions. Recent neuroimaging studies suggest that these various functions involve separate areas of the anterior cingulate. We have adopted a strategy of using marker tasks, shown to activate the brain area by imaging studies, as a means of tracing the development of attentional networks. Executive attention appears to develop first to regulate distress during the first year of life. During later childhood the ability to regulate conflict among competing stimuli builds upon the earlier cingulate anatomy to provide a means of cognitive control. During childhood the activation of cingulate structures relates both to the child's success on laboratory tasks involving conflict and to parental reports of self-regulation and emotional control. These studies indicate a start in understanding the anatomy, circuitry and development of executive attention networks that serve to regulate both cognition and emotion.

Establishing a time-line of word recognition: evidence from eye movements and event-related potentials

The average duration of eye fixations in reading places constraints on the time for lexical processing. Data from event related potential (ERP) studies of word recognition can illuminate stages of processing within a single fixation on a word. In the present study, high and low frequency regular and exception words were used as targets in an eye movement reading experiment and a high-density electrode ERP lexical decision experiment. Effects of lexicality (words vs pseudowords vs consonant strings), word frequency (high vs low frequency) and word regularity (regular vs exception spelling-sound correspondence) were examined. Results suggest a very early time-course for these aspects of lexical processing within the context of a single eye fixation.

Brain mechanisms of quantity are similar in 5-year-old children and adults

Both 5-year-old children and adults determine the quantity of a number by the use of a similar parietal lobe mechanism. Event related potentials indicate that input from Arabic digits and from dot patterns reach areas involved in determining quantity about 200 ms after input. However, voluntary key presses indicating the relation of the input to the quantity five take almost three times as long in children. The ability to trace the networks of brain areas involved in the learning of school subjects should aid in the design and testing of educational methods.

Mapping the cingulate cortex in response selection and monitoring

Many cognitive tasks have activated areas of the cingulate cortex. These include error detection, divided attention, conflict, and word generation tasks. However, the exact area of the cingulate found to be active has differed. This could be due to difference in subjects, laboratories, data analysis, or task conditions. The current study uses two very different tasks known to activate the cingulate and compares data from the same subjects and same trials to see whether there are temporal and spatial distinctions in cingulate activations. The tasks chosen were generation of the use of a noun and feedback that an error was made in the time window required for generation. High-density electrical recording was used to trace the time course of cingulate activation in the difference waves between correct and error feedback and between generate and repeat. Both tasks produced activity that is consistent with cingulate activation. However, the two tasks produced activity in different areas. These data are consistent with the idea that differences in areas of the cingulate activated differ between cognitive tasks and are not merely due to subject and laboratory differences.

Anatomy of word and sentence meaning

Reading and listening involve complex psychological processes that recruit many brain areas. The anatomy of processing English words has been studied by a variety of imaging methods. Although there is widespread agreement on the general anatomical areas involved in comprehending words, there are still disputes about the computations that go on in these areas. Examination of the time relations (circuitry) among these anatomical areas can aid in understanding their computations. In this paper, we concentrate on tasks that involve obtaining the meaning of a word in isolation or in relation to a sentence. Our current data support a finding in the literature that frontal semantic areas are active well before posterior areas. We use the subject's attention to amplify relevant brain areas involved either in semantic classification or in judging the relation of the word to a sentence to test the hypothesis that frontal areas are concerned with lexical semantics and posterior areas are more involved in comprehension of propositions that involve several words.

Event-related brain potential imaging of semantic encoding during processing single words

Functional brain imaging studies with positron emission tomography (PET) have identified blood flow changes in widely separated areas of brain during the performance of word processing tasks. In the present study we have utilized event-related brain potentials (ERPs) to investigate the temporal relationships among cortical areas previously identified by PET to be differentially activated when performing semantic tasks with visual words. ERPs revealed task-related differences over the central and left inferior frontal regions around 170 and 220 ms, respectively, over a left occipital region around 200 ms, over a large left parietotemporal region around 600 ms, and finally over the right temporal lobe around 800 ms after the word presentation. Analysis of topographic maps and dipole sources as well as PET data allowed relating frontal midline positivity around 170 ms to the anterior cingulate activation, and left inferior frontal positivity around 220 ms to the PET activation of the left inferior prefrontal cortex. The left parieto-temporal positivity around 600 ms seems to reflect the activity of Wernicke's area. The right anterior temporal negativity beginning around 800 ms and peaking around 1100 ms may reflect the activity of the right insula. The left occipital negativity around 200 ms is likely to reflect activation of a visual word-form area in the left occipital lobe. These results provide the time course for parts of the circuitry involved in semantic processing of words and also demonstrate how combining the spatial localization of PET with the temporal resolution of ERPs helps to understand the brain mechanisms involved in human cognition.

Time course of cortical activations in implicit and explicit recall

The distinction between implicit and explicit retrieval of learned material is central to recent thinking about the neural systems underlying memory. Word stem completion is one task in which subjects can be instructed either to make a deliberate recall (explicit instruction) or to be told to complete the stem with any appropriate word (implicit instruction). Positron emission tomography (PET) studies have indicated that during implicit retrieval, there is reduced blood flow in right posterior areas, whereas some tasks of explicit retrieval involve frontal and hippocampal activation. However, there is no information about the timing of these activations or how implicit and explicit retrieval might be related. We used word stem completion tasks similar to those used in the PET studies, but used high-density electrical recording designed to allow localization of the regions involved in the tasks and to provide temporal information. We found reduced activity for primed words in right posterior cortex corresponding to previous PET results. The reduction occurred within the first 200 msec after input, suggesting early interaction with the information stored in this area. Similar reductions observed during explicit recall of the previously presented words indicate that priming is similar under implicit and explicit conditions. In addition, when priming was not an adequate basis for response, then frontal areas were active. Retrieval of unprimed words under implicit instruction elicited right frontal activation, whereas explicit retrieval activated frontal areas bilaterally. Left frontal and hippocampal activations appear to occur only when the retrieval involved use of the words from the list studied previously.

Brain mechanisms of cognitive skills

This article examines the anatomy and circuitry of skills that, like reading, calculating, recognizing, or remembering, are common abilities of humans. While the anatomical areas active are unique to each skill there are features common to all tasks. For example, all skills produce activation of a small number of widely separated neural areas that appear necessary to perform the task. These neural areas relate to internal codes that may not be observed by any external behavior nor be reportable by the performer. There is considerable plasticity to the performance of skills. Task components can be given priority through attention, which serves to increase activation of the relevant brain areas. Attention can also cause reactivation of sensory areas driven by input, but usually only after a delay. The threshold for activation for any area may be temporarily reduced by prior activation (priming or practice). Skill components requiring attention tend to cause interference resulting in the dual tasks effects and unified focus of attention described in many cognitive studies. Practice may change the size or number of brain areas involved and alter the pathways used by the skill. By combining cognitive and anatomical analyses, a more general picture of the nature of skill emerges.

Relating the mechanisms of orienting and alerting

Cues provide two types of information: information about where the target will occur and when it will occur. We hypothesized two underlying processes related to cues, orienting (to location) and alerting. Using a covert orienting task under different conditions of alertness, we found evidence of independence between orienting and alerting (Experiments 3-4). The alerting mechanism is spatially broad and seems common for auditory and visual input (Experiments 1-2). In Experiment 1, visual cues at four locations occur simultaneously to prevent orienting; response facilitation was the same for targets occurring near or far from a cue. In Experiment 2, adding a visual alerting signal to an auditory signal provided no additional benefit. In Experiment 3, an auditory signal was used to modulate the alertness level during a covert orienting task. Orienting, measured by the validity effect, was independent of the level of alertness in this simple reaction task. Experiment 4 extended those results to a choice task. These studies indicate separate mechanisms of alerting and orienting. The global mode of alertness is consistent with the broad axonal distribution of the noradrenergic system. In contrast, human and animal data suggest that the orienting mechanism may be modulated by the basal forebrain cholinergic system.

Imaging resources

The concept of resources has been used in cognitive psychology to refer to computations specific to a given cognitive domain (e.g. orthographic code for words) and those general to many domains (arousal and attention). In this chapter we examine efforts to provide a picture of both kinds of resources with methods designed to explore the anatomy and circuitry of cognitive operations. We conclude that the two types of resources have separable if rather complex anatomical loci and that their common circuitry leads to interactions in which attention and arousal can influence the priority of the task specific cognitive operations. Many situations such as priming, mix the role of automatic activation of specific computations and attention to those same computations. We review some evidence from anatomical and circuit studies that indicate that the anatomy of priming relates closely to the computation involved in the task and suggest methods for separating automatic and attended influences on priming.

Priming reduces input activity in right posterior cortex during stem completion

A recent positron emission tomography (PET) study showed reduced blood flow in the right posterior cortex for primed words during a stem completion task, suggesting reduction in processing demand in this area for primed tasks. The time course of this attenuation was studied using event-related potentials (ERPs) in 40 normal subjects. As compared to the unprimed stems, primed stems elicited significantly less positive ERP in the right posterior channels between 60 and 200 ms after presentation of the stem. The results, while confirming the PET findings, implicate involvement of right posterior cortex in bottom-up processing of primed stimuli. In addition to right posterior channels, the primed stems also had attenuated positivity in the right frontal channels after 250 ms of stem presentation, indicating reduced requirement for explicit recall process.

Scalp electrical potentials reflect regional cerebral blood flow responses during processing of written words

Functional brain imaging studies with positron emission tomography (PET) have identified blood flow changes in widely separated areas of the brain during the performance of word-related tasks. In the present study, we have utilized event-related electrical potentials (ERPs) to investigate the temporal relationships among cortical areas previously identified by PET to be differentially activated when performing a task involving generating the uses of visually presented nouns versus reading aloud. ERPs showed strong task-related differences over left and middle inferior frontal and left parietotemporal regions. Frontal and left parietotemporal channels revealed these differences around 200 and 700 msec, respectively, after word presentation. These results provide the time course for parts of the anatomical circuit involved in generating the meaning of a word. Our results also demonstrate how combining the spatial localization of PET with the temporal resolution of ERPs greatly enhances the capacity to understand the mechanisms involved in human cognition.

Attention: the mechanisms of consciousness

A number of recent papers and books discuss theoretical efforts toward a scientific understanding of consciousness. Progress in imaging networks of brain areas active when people perform simple tasks may provide a useful empirical background for distinguishing conscious and unconscious information processing. Attentional networks include those involved in orienting to sensory stimuli, activating ideas from memory, and maintaining the alert state. This paper reviews recent findings in relation to classical issues in the study of attention and anatomical and physical theories of the nature of consciousness.

Development of orienting to locations and objects in human infants

Two experiments investigated preference for orienting to novel locations and novel objects in young infants. Adults and infants of six months and older show a propensity to orient to locations that have not recently been inspected (inhibition of return). Preference for novel locations undergoes development. We show that, similar to adults, its development is related to the ability to program eye movements to attended locations. This preference appears to emerge as infants gain the ability to program eye movements to target locations. Experiment 1 demonstrates that three-month-olds show inhibition of return for 10 degrees target eccentricities, but not for 30 degrees target eccentricities. In a second experiment, three- and six-month-old infants oriented to 10 degrees targets that varied in location and object identity. Infants of both ages strongly preferred orienting to novel objects at novel locations. At three months, the preference for novel objects was equal to the preference for novel locations, while at six months a tendency to prefer novel objects over novel locations emerged. Overall, the findings support separate development of these two forms of novelty preference, and suggest that novel location preferences (inhibition of return) relates closely to the eye movement system. The findings are discussed in relation to issues concerning development, physiology, and cognition.

Attentional networks

Recent brain-imaging and neurophysiological data indicate that attention is neither a property of a single brain area, nor of the entire brain. While attentional effects seem mediated by a relative amplification of blood flow and electrical activity in the cortical areas processing the attended computation, the details of how this is done through enhancement of attended or suppression of unattended items, or both, appear to depend on the task and brain-area studied. The origins of these amplification effects are to be found in specialized cortical areas of the frontal and parietal lobes that have been described as the anterior and posterior attention systems. These results represent substantial progress in the effort to determine how brain activity is regulated through attention. While many philosophical and practical issues remain in developing an understanding of attentional regulation, the new tools available should provide the basis for progress.

Topography of the N400: brain electrical activity reflecting semantic expectancy

When subjects read an semantically unexpected word, the brain electrical activity shows a negative deflection at about 400 msec in comparison with the response to an expected word. In order to study the brain systems related to this effect we mapped it with a dense (64-channel) electrode array and two reference-independent measures, one estimating the average potential gradients and the other radial current density. With these measures, the event-related brain potential (ERP) begins at about 70 msec with the P1, reflecting bilateral current sources over occipitoparietal areas. A strongly left-lateralized N1 then follows, peaking at about 180 msec, accompanied by an anterior positivity, the P2. A separate posterior positive pattern then emerges that seems to repeat the topography of the P1. Next, at about 350 msec, the ERP for the congruous word develops a P300 or LPC, characterized by a diffuse positivity over the superior surface of the head and several negativities over inferior regions. This superior source/inferior sink pattern of the LPC is greater over the left hemisphere. In contrast, the ERP for the incongruous word in this interval displays the N400 as a period in which topographic features are absent. At about 400 msec the ERP for the incongruous word begins to develop an LPC, which then remains relatively symmetric over the two hemispheres.

The neurobiology of selective attention

Research in the field of selective visual attention has recently seen substantial progress in several areas. Neuroimaging and electrical recording results have indicated that selective attention amplifies neural activity in prestriate areas concerned with basic visual processing. Imaging and cellular studies are delineating the networks of anatomical areas that serve as the source of attentional modulation and have suggested that these networks are anatomically distinct from the sites of the resulting amplifications. Cognitive studies of visual search have explored the role of these amplified computations in the integration of visual features into objects. Attentional effects in normal subjects, and their disruption following brain injury, have revealed the mental representations upon which attention operates.

Components of neglect from right-hemisphere damage: an analysis of line bisection

Two versions of a line bisection task were given to patients with posterior right-hemisphere damage and normal control subjects. One, which we refer to as the directed-manual task, was the traditional bisection task in which lines were transected with a pen held in the right hand. In the other task, referred to as the directed-visual task, subjects observed the experimenter move a pen along a line from right-to-left (the left-scan task) or from left-to-right (the right-scan task) and they verbally indicated the subjective midpoint. Patients showed significant left neglect in the manual and the left-scan tasks only. Controls showed no consistent biases and no influence of scanning direction. Right and left cues biased bisection for both groups. The results indicate that when the directed manual response is eliminated, scan direction determined the presence or absence of neglect on bisection. The findings are discussed in terms of the efficiency of visual orienting.

Covert attention and eye movements during reading

Eye movements were monitored during the reading of spatially transformed text in order to examine covert attentional processes in reading. In some conditions, the sequence of letters within a word was congruent with (i.e. in the same direction as) the sequence of words in the sentence; in other conditions the direction of letters within words and the direction of words in the sentence were incongruent. In addition, the window of visible text was varied so that in some conditions only the fixated word (and all preceding words) were visible, whereas in other conditions the fixated word and the succeeding word were both visible. Readers were able to extract more parafoveal information from text when the words themselves were normal than when the letters within the words were transformed. However, with practice, readers were able to use some parafoveal information even when the words were transformed. The most important finding was that the congruity of the word and letter order had no reliable effect on the ability to extract parafoveal information and influenced reading performance only when the words themselves were normal. We conclude that covert attention in reading is not a letter-by-letter scan that sweeps across the page, but either an asymmetric spotlight held constant on each fixation or a shifting of an attentional spotlight extending across multiletter units (possibly words) with the direction of shifts of attention closely coupled to the direction of eye movements.