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Figure 7b generally consistent with the classic homunculus described by Penfield, based on intraoperative electrical stimulation.

2. Auditory Evoked Responses

Auditory evoked responses are typically elicited by clicks or tone bursts, which may be delivered to one or both ears. With appropriate electrode placement, it is possible to noninvasively measure electrical responses of the cochlea, including receptor potentials, and signals that reflect from neural encoding. From 1 to 12 msec poststimulus auditory brain stem responses can be measured. These responses consist of a series of mostly discrete waves labeled I-VII. Although the sources of these components are still a matter of some debate, there is general agreement that wave I is due to a compound action potential or graded dendritic potentials at the dictal (cochlear) end of the acoustic nerve. Waves III-V are generated in the brain stem. Waves V and VII are associated with higher brain stem structures—perhaps the medial geniculate body. Auditory brain stem responses are useful for hearing assessment in infants, uncooperative adults, and cases of functional deafness, as well as for evaluating brain stem function in suspected multiple sclerosis. The general temporal structure of auditory evoked responses is illustrated in Fig. 9.

Figure 7c-d

Middle latency auditory evoked responses are typically observed from 12 to 50 msec poststimulus and are considered to represent subcortical activation.

Late auditory evoked responses (50 msec or more after the stimulus) are generally a product of neocor-tex. Such responses are best evoked by tone bursts and in EEG recordings show the highest amplitude over the vertex. MEG studies lave localized these responses to primary sensory areas along the Sylvian fissure and to nearby association areas. Such studies have also demonstrated clear tonotopic organization in primary cortical areas. With dipole localization techniques, very fine-grained discrimination of relative locations is possible. Auditory stimuli are often used for language studies, and in this role they may elicit an interesting array of endogenous responses that reflect the neural processing of language. However, at least one endogenous response appears purely acoustic: the mismatch negativity is observed when a repetitive auditory stimulus is briefly altered and may serve as an orienting response to cause a shift in the focus of attention.

Figure 8 Integrated analysis of fMRI and MEG. (a) Time courses of fMRI equivalent sources estimated from MEG data. fMRI visual data were acquired using blocked steady-state stimulation, using the same video display from a previous MEG experiment. Currents were assumed to vary within the source according to the distribution of functional MRI activation. Currents were constrained to lie normal to cortex as indicated by anatomical MRI. Topographies were derived for each of the assumed sources and used as basis functions for a linear decomposition of the time-varying field maps. Estimated time courses for 11 areas are coded in color. (Figure courtesy of Dale et al.) (b) MEG time courses of fMRI sources using a weighted minimum norm procedure. Areas of activation from a visual fMRI experiment (involving visual motion) are shown on an unfolded cortex. A 0.9:0.1 weighted pseudo-inverse procedure was applied to field maps at each time point. Estimated time courses for activation in four identified visual areas are illustrated. Differences as a function of stimulus type are coded in color. (Figure courtesy of Dale et al.)

Figure 9 Component structure of the auditory event-related potential. The trace schematically represents the averaged evoked response of the auditory system to a brief stimulus such as a click or a tone. A logarithmic time scale allows visualization of the major response component peaks in a single trace. Components include auditory brain stem responses (I-VI), early positive (P) and negative (N) cortical components (Na, Nb, Pa, P1), and late cortical components (N1, P2). Other components that vary as a function of cognitive or attentive states are shown with dashed lines. (Figure courtesy of Hillyard and colleagues)

Figure 9 Component structure of the auditory event-related potential. The trace schematically represents the averaged evoked response of the auditory system to a brief stimulus such as a click or a tone. A logarithmic time scale allows visualization of the major response component peaks in a single trace. Components include auditory brain stem responses (I-VI), early positive (P) and negative (N) cortical components (Na, Nb, Pa, P1), and late cortical components (N1, P2). Other components that vary as a function of cognitive or attentive states are shown with dashed lines. (Figure courtesy of Hillyard and colleagues)

3. Visual Evoked Responses

In primates, the visual system is the largest and most distributed of the sensory modalities, consisting of over three dozen discrete areas and spanning at least one-third of the neocortical surface area. The system has been studied extensively with invasive electrophysiological techniques, as well as with MEG and EEG (and fMRI). The comparatively small signals and their dynamic complexity make this system a major challenge for sensory evoked response studies.

As in the auditory system, it is possible to measure the electrical response of the sensory organ—the eye. The electroretinogram (ERG) is typically measured using a contact lens electrode referred to a reference on the head surface. The response consists of receptor and neural components. Because the retina is a relatively accessible outpost of the brain, it has been the target of a number of studies of information processing by neural networks. In the future, optical techniques may allow noninvasive characterization of retinal network dynamics.

The visual evoked response observed with surface sensor arrays is dominated by primary cortical areas. The initial cortical activation in layer 4 of striate cortex probably occurs around 70-80 msec poststimulus, although this component is often small and difficult to detect. Other cortical responses are observed in striate and nearby areas from 90 to 120 msec poststimulus, and evoked activity often lasts through 250 msec. Source localization studies have demonstrated the anticipated retinotopic organization of primary visual cortex as well as extrastriate areas. The visual field is systematically projected onto striate cortex, mostly buried in the fissure between the hemispheres along the calcarine fissure. The central field representation is found near the posterior pole of occipital cortex and may extend onto the posterior surface. The lower quadrants of the visual field are mapped onto the upper banks of the contralateral calcarine and inter-hemispheric fissure; the upper field projects to the lower banks, and the horizontal meridian projects to the depths of the calacrine in a scheme summarized by the cruciform model (due to its appearance in coronal section). Noninvasive studies have confirmed the outlines of this model, although individual departures appear common. Such studies also support the idea of the cortical projection factor: the cortical area devoted to a given size patch of the visual field systematically decreases from the center to the periphery of the visual field.

Noninvasive techniques so far have largely been used to confirm in humans results suggested by invasive studies in animals. Thus, a number of specialized areas have been identified in humans analogous to those identified in electrophysiological studies in nonhuman primates, including areas specialized for processing visual motion, color, texture, and even faces. The visual system appears to be organized into two major processing chains or streams. The dorsal stream, arrayed mainly across occipital cortex and the upper surface of the parietal lobe, operates in low contrast and is involved in processing visual motion. The system is probably involved in orientation and allocation of attention and may interact with motor control. The ventral stream flows along the base of the occipital and temporal cortices, and is involved in the processing of color, texture, and other detailed attributes of visual information. This system probably interacts with language processing centers. Some investigators have dubbed these streams the what and where systems.

B. Motor Control

The control of voluntary movement can also be studied with event-related response techniques. In a typical experiment, the subject is instructed to perform a series of self-paced voluntary movements, and the signal is averaged relative to the movement as registered by a button press or an electrical response recorded from muscle. The response appears as a slowly developing negative potential shift somatoto-pically arrayed along the central sulcus, starting approximately 1 sec before movement. This response is called the readiness potential and is taken as an index of motor preparation; the amplitude of the response is correlated with the complexity of the subsequent movement as well as the force and speed developed. The readiness potential preceding a lateralized response (such as a hand movement) is maximal over the contralateral hemisphere. Some investigators use signal subtraction techniques to remove the ipsilateral contribution to the signal. In addition to the readiness potential, other movement-related responses can be resolved, as well as somatosensory and proprioceptive feedback generated as a consequence ofthe movement.

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