Long-latency auditory-evoked potentials: Role of polysensory association cortex in the cat

The objective of this study has been to define the role of polysensory association cortex in the generation of "wave NA" and of "wave C," long-latency auditory-evoked potentials recorded from the vertex of conscious cats as, respectively, a marked negative potential of latency 30-48 msec followed by a broad positive wave of latency 50-75 msec. Wave C may represent the feline analogue of the longer latency human auditory-evoked potential wave P2, insofar as both waveforms are very large amplitude, long duration positivities characterized by long recovery cycles. Based on previous studies of wave C and the generators of other middle-latency evoked potentials, we hypothesized that both wave NA and wave C might reflect, at least in part, the cortical culmination of a nonlemniscal line auditory association system arising in reticulothalamic projections to intralaminar and associated ventral thalamic regions. Relays from these thalamic areas are known to project to polysensory association cortex, including pericruciate gyrus, anterolateral gyrus, and medial suprasylvian gyrus. Therefore we implemented a series of lesion experiments to characterize the role of each of these cortical areas in the production of wave NA and wave C. Our results indicate that all three polysensory association areas contribute significantly to both waves NA and C, although the largest effects followed ablation of the pericruciate area alone. Thus, the generator substrates of waves NA and C appear to involve a long-recovery cycle system which functionally incorporates activation of association cortex.     

The connections of the parietal cortex with the lateral suprasylvian gyrus (the Clare-Bishop field) and the auditory cortex in the cat

It has been shown that a parietal projection to the Clare--Bishop area is moderate and organized in a topographic manner. Associative fibres of area 5 terminate in the anterior part of the Clare--Bishop area, which corresponds to the intermediate and anterior part of the posterior suprasylvian sulcus belt. Area 7 projects to the posterior part of the intermediate and posterior suprasylvian sulcus belt. Area 5 and 7 send a few fibres to the auditory cortex. Associative fibres of area 5 terminate in the middle ectosylvian and sylvian gyri: areas 22, 50. Area 7 is connected only with the superior extremity of the middle ectosylvian gyrus or of areas 22, 50.     

Tonotopic organization in auditory cortex of the cat

Microelectrode mapping techniques were employed in the cat's auditory cortex to relate the best frequencies of a large population of neurons with their spatial loci. Based upon the best-frequency distribution, the auditory region was divided into four complete and orderly tonotopic representations and a surrounding belt of cortex in which the tonotopic organization was more complex. The four auditory fields occupy a crescent-shaped band of tissue which comprises portions of both the exposed gyral surfaces and sulcal banks of the ectosylvian cortex. The anterior auditory field (A) is situated most rostrally upon the anterior ectosylvian gyrus. It extends upon the ventral bank of the suprasylvian sulcus and upon the banks of the anterior ectosylvian sulcus. Adjoining field A caudally is the primary auditory field (AI), which extends across the middle ectosylvian gyrus and portions of both banks of the posterior ectosylvian sulcus. The representations of the highest best frequencies in fields A and AI are contiguous. Caudal and ventral to AI are located the posterior (P) and ventroposterior (VP) auditory fields. They lie mainly upon the caudal bank of the posterior ectosylvian sulcus but also extend upon the rostral bank and upon the posterior ectosylvian gyrus. The low best-frequency representations of fields AI and P are contiguous, whereas the low best-frequency representation of field VP lies near the ventral end of the posterior ectosylvian sulcus. Fields P and VP are joined along their middle and high best-frequency representations. Within each auditory field isofrequency lines defined by the spatial loci of neurons with similar best frequencies are oriented orthogonal to the low-to-high best-frequency gradients.     

Midlatency auditory evoked responses: differential effects of sleep in the cat

Middle latency responses (MLRs) in the 10-100 msec latency range, evoked by click stimuli, were studied in 8 adult cats during sleep-wakefulness to determine whether such changes in state were reflected by any MLR component. In particular, we wanted to determine whether the 20-22 msec positivity recorded at the vertex, 'wave A,' shown in previous studies to reflect a generator substrate within the ascending reticular formation, was tightly linked to changes in sleep-wakefulness, as reported for single neurons in the ascending reticular activating system. Evoked potentials were collected in 100 trial averages during continuous presentation of 1/sec clicks during initial awake recordings and thereafter during all-night sleep sessions. Continuously recorded EEG, EOG and EMG were scored for wakefulness, slow wave sleep (SWS), and rapid eye movement (REM) sleep during each evoked potential epoch. Recordings were obtained from electrodes implanted at the vertex and overlying the primary auditory cortex referenced to frontal sinus or to neck. In agreement with others, components of the auditory brain-stem response and the 12 msec primary cortical response showed no change in amplitude from wakefulness to either SWS or REM. Only wave A, among the components evaluated in the 1-100 msec range, decreased and disappeared during SWS and dramatically reappeared during REM to an amplitude equal to that during wakefulness. These data lend particular support to a functional relation between wave A and the ascending reticular activating system and suggest that this potential may provide a unique and dynamic probe of tonic brain activity. Moreover, this animal model provides a hypothetical basis for expecting a similar surface recorded potential in the human, a potential which has consequently been discovered.     

Scalp distribution of human auditory evoked potentials. II. Evidence for overlapping sources and involvement of auditory cortex

The scalp distributions of human auditory evoked potentials (AEPs) between 20 and 250 msec were investigated using non-cephalic reference recordings. AEPs to binaural click stimuli were recorded simultaneously from 20 scalp locations over the right hemisphere in 11 subjects. Computer-generated isovoltage topographic maps at high temporal resolution were used to assess the stability of AEP scalp distributions over time and relate them to major peaks in the AEP wave forms. For potentials between 20 and 60 msec, the results demonstrate a stable scalp distribution of dipolar form that is consistent with sources in primary auditory cortex on the superior temporal plant near the temporoparietal junction. For potentials between 60 and 250 msec, the results demonstrate changes in AEP morphology across electrode locations and changes in scalp distribution over time that lead to two major conclusions. First, AEPs in this latency period are generated by multiple sources which partially overlap in time. Second, one or more regions of auditory cortex contribute significantly to AEPs in this period. Additional data are needed to determine the relative contribution of auditory cortex sources on the superior temporal plane and the lateral temporal surface and to identify AEP sources outside the temporal lobe.     

Reversible focal hyperthermic effects on evoked auditory cerebral responses

The click-induced cerebral response recorded as three positive and three negative waves (P₁, P₂, P₃, and N₁, N₂, N₃) from the auditory receiving cortex of cat was selectively modified by focal hyperthermia (FBF). (a) In the posterior ectosylvian gyrus with FBF of 41°C, there was enhanced amplitude of P₂ (24 to 40 ms, latency) and N₂ (44 to 62 ms) waves compared with responses recorded from cortex prior to treatment. (b) Similar enhancement occurred in the middle ectosylvian gyrus with FBF but the effect on the N₂ wave was less prominent. (c) In the anterior ectosylvian gyrus, the amplitude of the N₂ wave was enhanced to a greater extent than the P₂ wave by FBF. (d) In gyrus ansatus, adjacent to the anterior ectosylvian gyrus, FBF resulted in enhancement of the N₁ (16 to 22 ms) wave with smaller increases in the P₂ wave. $\text{P}{}_3\text{N}{}_3$ waves showed increased amplitudes but with less stability than those described above. These observations indicate that focally elevated temperatures of auditory receiving cortex result in increased activities which can be recorded as selective enhancement of components of the click-evoked response. Compatible observations, obtained during FBF of the cerebellar auditory receiving area, show increased amplitudes and frequencies of efferent discharges in the superior cerebellar penduncle.     

The anterior ectosylvian sulcal auditory field in the cat: II. A horseradish peroxidase study of its thalamic and cortical connections.

The thalamic and cortical projections to acoustically responsive regions of the anterior ectosylvian sulcus were determined by identifying retrogradely labelled cells after physiologically guided iontophoretic injections of horseradish peroxidase. The medial division of the medial geniculate nucleus, the intermediate division of the posterior nuclear group, the principal division of the ventromedial nucleus, and the lateroposterior complex were consistently labelled after these injections, although each animal showed slightly different patterns of labelling. The suprageniculate nucleus and the lateral and medial divisions of the posterior nuclear group were also labelled in most experiments. The cortex of the suprasylvian sulcus was the most consistently and densely labelled cortical region; each experiment showed a slightly different pattern of labelling throughout the suprasylvian sulcus, with an overall tendency for greater labelling in the ventral (lateral) bank of the middle region of the sulcus. Other cortical regions labelled less consistently included the anterior ectosylvian sulcus itself, the insular cortex of the anterior sylvian gyrus, and the posterior rhinal sulcus. In three experiments the contralateral cortex was examined and a small number of labelled cells was located in the anterior ectosylvian and suprasylvian sulci. Input from extralemniscal auditory thalamus is compatible with previously described auditory response properties of anterior ectosylvian sulcus neurons. The results also confirm the presence of input from visual and multimodal regions of thalamus and cortex, and therefore support claims of overlap of modalities within the sulcus. This overlap, as well as input from motor regions, suggests that the anterior ectosylvian sulcal field serves a sensorimotor role.     

Cerebral areas mediating visual redirection of gaze: cooling deactivation of 15 loci in the cat

In humans, damage to posterior parietal or frontal cortices often induces a severe impairment of the ability to redirect gaze to visual targets introduced into the contralateral field. In cats, unilateral deactivation of the posterior middle suprasylvian (pMS) sulcus in the posterior inferior parietal region also results in an equally severe impairment of visually mediated redirection of gaze. In this study we tested the contributions of the pMS cortex and 14 other cortical regions in mediating redirection of gaze to visual targets in 31 adult cats. Unilateral cooling deactivation of three adjacent regions along the posterior bend of the suprasylvian sulcus (posterior middle suprasylvian sulcus, posterior suprasylvian sulcus, and dorsal posterior ectosylvian gyrus at the confluence of the occipital, parietal, and temporal cortices) eliminated visually mediated redirection of gaze towards stimuli introduced into the contralateral hemifield, while the redirection of gaze toward the ipsilateral hemifield remained highly proficient. Additional cortical loci critical for visually mediated redirection of gaze include the anterior suprasylvian gyrus (lateral area 5, anterior inferior parietal cortex) and medial area 6 in the frontal region. Cooling deactivation of: 1) dorsal or 2) ventral posterior suprasylvian gyrus; 3) ventral posterior ectosylvian gyrus, 4) middle ectosylvian gyrus; 5) anterior or 6) posterior middle suprasylvian gyrus (area 7); 7) anterior middle suprasylvian sulcus; 8) medial area 5; 9) the visual portion of the anterior ectosylvian sulcus (AES); 10) or lateral area 6 were all without impact on the ability to redirect gaze. In summary, we identified a prominent field of cortex at the junction of the temporo-occipito-parietal cortices (regions pMS, dPE, PS), an anterior inferior parietal field (region 5L), and a frontal field (region 6M) that all contribute critically to the ability to redirect gaze to novel stimuli introduced into the visual field during fixation. These loci have several features in common with cortical fields in monkey and human brains that contribute to the visually guided redirection of the head and eyes.     

Origin of the initial negative potential (N15) recorded on the skull by superficial radial nerve stimulation in the cat

The origin of the initial prominent negative potential with a latency of about 15 msec (N15), recorded on the skull by superficial radial nerve stimulation was studied in the cat, and the following results were obtained. In the direct recording from the cortex, SEPs were elicited from the SI and SII areas, as well as the lateral gyrus and anterior suprasylvian gyrus. Among various wave forms recognized in the SI area, diphasic positive-negative (P-N) potential obtained from the postero-lateral part was most distinct, representing the primary evoked potential. Responses recorded on the dura and those directly on the cortex showed similar wave patterns over many sites. Although different forms of evoked potentials were recorded extensively over the skull, the most prominent negative potential was elicited at the site corresponding to the postero-lateral part of the SI area. The latency of this potential was approximately in agreement with that of the negative component of the P-N potential recorded on the cortex. Based on the intracortical laminar analysis of P-N potential, the positive component of this potential was assumed to reflect the activity of cells in the deeper layer of the cortex. The negative component, on the other hand, might represent activities of apical dendrites of the cortex. From the result of functional elimination of the cortex, however, this positive component was thought to contain potentials from far field neural structures. N15 recorded on the skull completely disappeared during cortical spreading depression.(ABSTRACT TRUNCATED AT 250 WORDS)     

Frontal cortex stimulation evoked neostriatal potentials in rats: intracellular and extracellular analysis

Evoked potentials, action potentials and intracellular events were recorded in the neostriatum of urethane anesthetized rats to electrical stimulation of frontal cortex white matter, motor cortex and pre-limbic cortex. Five major waves of the evoked potential were identified. Wave N1 (3.9 msec latency) was small, preceded cellular events and probably represents activation of corticostriate terminals. Wave P1 (10.8 msec latency to peak following white matter stimulation) coincided with an EPSP and neuronal firing. Both wave N2 (38.0 msec latency to peak) and P2 (approximately 110 msec duration) overlapped the intracellularly recorded hyperpolarization and inhibition of cell firing. Based upon this correspondence and upon the behavior of waves N2 and P2 with changing current and during conditioning-test paired pulse stimulation, it was concluded that the waves represent different processes contributing to the cellular hyperpolarization. A late wave, N3 (175 msec onset latency) corresponded to a late rebound firing and cellular depolarization. This late wave was eliminated from the neostriatum, but not from the overlying sensorimotor cortex, by kainic acid lesions that destroyed medial thalamus but left thalamic lateral nuclei and reticular nucleus intact.     

Auditory cortical projections to the superior colliculus in the cat

The auditory cortical projections to the superior colliculus were studied in the cat with silver impregnation methods.The auditory area AII and the suprasylvian fringe auditory cortex (SF) project bilaterally to the superficial layers of the superior colliculus. The projection is heavier ipsilaterally. The ectosylvian posterior auditory region (EP) projects to the deep layers of the ipsilateral superior colliculus. No evidence was found of fibres to the superior colliculus arising from the auditory cortex AI or from the anterior and posterior sylvian gyri (insulo-temporal auditory cortex). The dorso-caudal turn of the ectosylvian gyrus has apparently only scanty projections to the deep layers of the ipsilateral superior colliculus but projects heavily to pretectal areas.These findings are compared with earlier evidence of auditory cortical projections to the superior colliculus. The possible functions of this pathway are briefly discussed, and it is suggested that the pathway may be related to complex auditory-visual behavioural relationships.     

Auditory evoked potentials from the primary auditory cortex of the cat: topographic and pharmacological studies.

Wave VI (8.4 msec) of the brain-stem auditory evoked potential (BAEP) was maximal in a discrete region of primary auditory cortex (AI) of the anesthetized cat. Wave VI underwent rapid amplitude decrease over millimeter distances in the AI region and followed high stimulation rates. Wave VI did not show intracortical polarity inversion nor was it abolished by epicortical or intracortical GABA administration. The data are compatible with a wave VI source in the terminal axons of the thalamo-cortical radiations. Middle latency auditory responses (MAEPs) generated 10-40 msec after auditory stimulation were also recorded in a circumscribed area of AI. In contrast to wave VI, these primary auditory cortex potentials (Pa 18.3 msec; Nb 31.9 msec) underwent transcortical polarity inversion, correlated with intracortical multi-unit activity in the AI region and were reversibly altered or abolished by epicortical or intracortical GABA administration to the AI region. The data suggest that the Pa and Nb components of the cat MAEP are intracortically generated by neuronal elements in the AI region.     

Visual and auditory association areas of the cat's posterior ectosylvian gyrus: cortical afferents

In a preceding report, we described patterns of thalamic retrograde labeling following 17 tracer deposits on the cat's posterior ectosylvian gyrus and concluded, on the basis of patterns of thalamic connectivity, that the posterior ectosylvian gyrus is composed of three major divisions: a tonotopic auditory zone located anteriorly, a belt of auditory association cortex occupying the gyral crown, and a visual belt located posteriorly. We describe here patterns of transcortical retrograde labeling obtained from tracer deposits in the three zones so defined. Our results indicate that the tonotopic auditory strip is innervated primarily by axons from low-order auditory areas (AAF, AI, P, VP, and V), that the auditory belt receives its strongest input from nontonotopic auditory fields (AII, temporal cortex, and other parts of the auditory belt), and that projections to the visual belt derive primarily from extrastriate visual areas (ALLS, PLLS, DLS, 19, 20, and 21) and from association areas affiliated with the visual system (insular cortex, posterior cingulate gyrus, area 7p, and frontal cortex). We discuss the results in relation to previous systems for parcellating the posterior ectosylvian gyrus of the cat and consider the possibility that divisions of the feline posterior ectosylvian gyrus correspond directly to areas making up the superior temporal gyrus in primates.     

Human middle-latency auditory evoked potentials: vertex and temporal components.

We recorded middle-latency (20-70 msec) auditory evoked potentials (MLAEPs) to monaural and binaural clicks in 30 normal adults (ages 20-49 years) at 32 scalp locations all referred to a balanced non-cephalic reference. Our goal was to define the MLAEP components that were present at comparable latencies and comparable locations across the subject population. Group and individual data were evaluated both as topographic maps and as MLAEPs at selected electrode locations. Three major components occurred between 20 and 70 msec, two well-known peaks centered at the vertex, and one previously undefined peak focused over the posterior temporal area. Pa is a 29 msec positive peak centered at the vertex and present with both monaural and binaural stimulation. Pb is a 53 msec positive peak also centered at the vertex but seen consistently only with binaural and right ear stimulation. TP41 is a 41 msec positive peak focused over both temporal areas. TP41 has not been identified in previous MLAEP studies that concentrated on central scalp locations and/or used active reference electrode sites such as ears or mastoids. Available topographic, intracranial, pharmacologic, and lesion studies indicate that Pa, Pb and TP41 are of neural origin. Whether Pa and/or Pb are produced in Heschl's gyrus, primary auditory cortex, remains unclear. TP41 is probably produced by auditory cortex on the posterior lateral surface of the temporal lobe. It should prove of considerable value in experimental and clinical evaluation of higher level auditory function in particular and of cortical function in general.     

Electrophysiology of the frontal granular cortex. III. The cingulate-prefrontal relation in primate

An investigation of the nature of laminar potentials and impulse discharges of units of dorsolateral prefrontal cortex (FC) evoked by stimulations of cingulate gyrus was made in rhesus monkey.Anterior cingulate stimulations evoked 1.0–1.5 msec latency waves, positive on pial surface and negative at FC depths of about 500–2000 μm with about the same time scale. Following the positive wave there was also a negative wave response.The amplitude and duration of the surface positive wave and negative wave of response varied according to the state of background EEG of the area. During steady EEG the positive wave was well developed and the negative wave was incipient but during EEG oscillations, the positive wave was markedly diminished and the negative wave enlarged. The alteration of responsiveness was also confirmed in depths by changes of reversed polarity potentials corresponding temporally to surface potentials.Discrete responses of FC were evoked at stimulus frequencies of up to 20 Hz. Higher frequencies resulted in depression of responsiveness. Alternate responses were depressed between 20 and 40 Hz and all responses were progressively depressed with still higher frequencies.Impulse discharges of FC units were evoked either singly or in bursts by the cingulate stimulations. Single impulse responses were commonly found. Presumably monosynaptic responses were in the latency range of 1.2–3.0 msec. They were found at depths of about 500 μm and 1000 μm. Long latency (7 msec) responses were also observed in the upper levels of cortex. The latency of the unitary response of a multi-impulse burst was in the range of 6.5–16 msec and the frequency of impulses in burst was in the order of 500–750 Hz. Such burst responses occurred on the declining phase of the depth negative wave evoked by the cingulate stimulation.The anterior cingulate effects were found to be distributed on the prefrontal cortex from arcuate sulcus to two-thirds distance rostrally. The unit responses were more commonly found in laminae between 0.4 and 1.0 mm depth than at other depths. The cingulate effects on FC were not affected by lesion of nucleus medialis dorsalis thalami or by ablation of the pre-motor cortex.The study discloses a basis by which functional states of limbic cortex can powerfully modulate the functioning of prefrontal neocortex of primate.     

Brain stem auditory evoked response development in the kitten

The development of brain stem auditory evoked responses (BAERs), recorded from a surface electrode as short-latency, volume-conducted potentials, was studied in a series of kittens over a postnatal period ranging from birth to 60 days. Repeated, longitudinal observations on particular kittens were supplemented with observations on additional kittens during the first and second postnatal week to determine age of onset of the BAERs. The position of the animal and sound source within the recording chamber were held constant across recording sessions, as was click intensity except during recordings in which intensity effects were specifically studied. Click rates of 1, 10, 50 and 100/sec were routinely presented. Reference electrodes at the tongue, pinna and neck showed volume-conducted responses to the click stimuli and resulted in considerable distortion of the activity recorded by the vertex electrod; the forepaw, in contrast, showed no activity and a vertex-forepaw electrode configuration provided good resolution of the BAERs across development.A number of new observations were made. BAERs were first observed at 4 days of age, approximately the same age at which depth evoked potentials are first recorded in brain stem auditory nuclei. Initially the BAERs were diffuse, high threshold and fatigued rapidly, characteristics shared with depth evoked potentials in the early postnatal period. Over the first two weeks, the potentials showed marked decrease in threshold, increased resistance to fast click rates, and better definition of wave forms. All BAER components showed exponential decreases in latency. Because all of the brain stem evoked potentials could be recorded concurrently and longitudinally in the same subject a number of developmental comparisons were possible among the BAER components. Wave 1, related to the acoustic nerve in the adult cat, showed a developmental time course and adult latency similar to that reported for N₁. Wave 2, related to the cochlear nucleus in the adult, showed a marked bimodality over the first month; wave 2a was a large amplitude clearly separated wave which gradually fused as an inconspicuous conspicuous leading shoulder on wave 2b. Wave 2b developed with a time course and adult latency similar to that reported for the ventral cochlear nucleus. Wave 3, related to the region of the superior olivary complex in the adult, showed a clear but transient bimodality during the third week of development. Wave 5, related to the inferior colliculus in the adult, appeared later than waves 1–4 and showed a significantly slower rate of development than waves 1–4. These data indicate that differential developmental changes occur within the brain stem auditory pathway and that the BAERs provide a dynamic probe of concurrent maturational interactions.     

Midbrain evoked potentials and oculomotor activity produced by optic nerve stimulation

Evoked potentials, produced by electrical stimulation of the optic nerve, were recorded from the superior colliculus and the midbrain close to the oculomotor nucleus. The experiments were done on 23 adult cats immobilized by gallamine triethiodide after hemidecerebration by prethalamic transection. The evoked potentials consisted of three components, namely, an early wave, a series of recurrent waves with latency of about 6 msec, and a late slow wave with latency of about 16–23 msec. It was concluded that the early wave was due to the arrival of optic nerve impulses, the recurrent wave to repetitive firing of collicular neurons, and the late slow wave to the collicular outflow onto the oculomotor nucleus. In the majority of instances in which the late slow wave was observed, the preceding optic nerve volley revealed facilitation of the antidromic spike of oculomotor nucleus. In a few instances in which the late slow wave was not obtained, either facilitation or inhibition of spontaneous oculomotor discharge was observed, indicating the dual facilitatory and inhibitory components of colliculofugal outflow to the oculomotor nucleus.     

Presynaptic modulation of synaptic effectiveness of afferent and ventrolateral tract fibers in the frog spinal cord

In the isolated frog spinal cord, antidromic stimulation of motor nerves produces intraspinal field potentials with a characteristic spatial distribution. When recording from the ventral horn, there is a short latency (1–2 msec) response corresponding to activity generated by antidromic activation of motoneuron cell bodies and proximal dendrites. In addition, in the dorsal horn, a delayed wave (12–13 msec latency) corresponding in time with the negative dorsal root potential is also recorded. This wave (VR-SFP) is positive at the dorsal surface of the cord and inverts to negativity at more ventral regions. The negative VR-SFP is maximum between 300–500 μm depth from the dorsal surface and decays with increasing depth towards the motor nucleus. Six days after chronic section of the dorsal roots L7 to L9 in one side of the spinal cord, stimulation of the motor nerves on the deafferented side produces only the early response attributable to antidromic activation of motoneurons. No distinctive VR-SFPs are recorded at any depth within the cord. These findings are consistent with the interpretation that afferent fiber terminals are the current generators of the VR-SFP. The presynaptic and postsynaptic focal potentials recorded in the motor nucleus after stimulation of the ventrolateral tract, as well as the corresponding synaptic potentials electrotonically recorded from the ventral roots, are not depressed during conditioning stimulations which produce primary afferent depolarization. This contrasts with the depression of the presynaptic and post-synaptic focal potentials and synaptic potentials produced by stimulation of sensory fibers. It is concluded that, unlike the afferent fiber terminals, the terminals of the ventrolateral tract are not subjected to a presynaptic modulation of the type involving primary afferent depolarization.     

Intermodal selective attention. I. Effects on event-related potentials to lateralized auditory and visual stimuli.

The effects of intermodal selective attention on event-related brain potentials (ERPs) were examined in 2 experiments. In experiment 1, auditory ERPs were compared (1) when subjects responded to easy and difficult-to-detect target tones in sequences of tone bursts; and (2) when they ignored the same auditory sequences and played a demanding video game. In experiment 2, auditory ERPs to tone bursts and visual ERPs to vertical line gratings were compared as subjects responded to difficult-to-detect targets in one modality or the other. Attention to auditory stimuli resulted in biphasic enhancements in auditory ERPs, the Nda (negative auditory difference wave, latency 120-160 msec) and the Pda (positive auditory difference wave, latency 200-240 msec) waves. These had longer latencies and somewhat different scalp distributions than N1 and P2 components evoked by non-attended tones. The Nda and Pda could be contrasted with the monophasic processing negativities typically found in dichotic selective attention tasks. Nda amplitudes were larger for difficult-to-detect targets (closely resembling standards) than for standards themselves, but no Ndas were recorded to highly deviant targets. Deviant auditory stimuli evoked mismatch negativities (MMNs) that persisted during visual attention. MMN amplitudes to difficult-to-detect deviants were enlarged with attention, but no change was found in MMN amplitudes to easy-to-detect deviants. In experiment 2 intermodal attention was associated with biphasic changes in visual ERPs over the posterior scalp: the occipital Pdv (100-130 msec), and contralateral-temporal Ndv (120-320 msec) deflections. Deviant visual stimuli also elicited mismatch negativity/N2b components, largest over the inferotemporal cortex contralateral to the stimulated visual field. Like the auditory MMN, the MMN increased in amplitude with attention, but it was also evident during attend auditory conditions. The results suggest that sustained, intermodal attention depends primarily in processing modulations in modality-specific cortex. We found no evidence of the participation of modality non-specific cortex. This excludes the possibility that intermodal attention depends on a single, supramodal attention system. The relatively long latency of intermodal effects suggests that they may depend on the reafferent (top down) modulation, and do not index "template matching" operations.     

Differentiation between cortical and subcortical lesions following focal ischemia in cats by multimodality evoked potentials

Regional ischemia was induced in cats by occluding the middle cerebral artery. Evoked and spontaneous electrical activity as well as regional cerebral blood flow (rCBF) were recorded with platinum depth macroelectrodes in three primary cortical areas: the auditory cortex (A, middle ectosylvian gyrus) and the front and hind limb somatosensory cortex (SF and SH, lateral and medial posterior sigmoid gyrus). To distinguish among the various evoked potentials after click, median or tibial nerve stimulation, electrical field interactions had to be eliminated using a multiplex stimulation and analysis system. Spontaneous electrocortical activity was evaluated by power spectral analysis. In all areas, evoked potentials were abolished 10 min after arterial occlusion. However, rCBF behaved differently in these regions: it was severely reduced in A, decreased moderately in SF and remained unchanged in SH. The graded reduction of rCBF in the three cortical areas was related to changes in electrophysiological activity during the first minutes of ischemia. In A, auditory potentials were abolished within 3 min after occlusion, whereas in SH, the decrease of somatosensory responses started after about 5 min. In SF, two components of the EP changes were found: an early decrease immediately and a later decrease about 5 min after occlusion. The different rates of EP impairment possibly correspond to two types of ischemia. The fast EP abolishment seems to be caused by local cortical damage whereas the delayed EP decrease probably reflects impairment of subcortical white matter structures. Thus, this method may be useful for distinguishing between gray and white matter ischemia.