Connection patterns distinguish 3 regions of human parietal cortex

Three regions of the macaque inferior parietal lobule and adjacent lateral intraparietal sulcus (IPS) are distinguished by the relative strengths of their connections with the superior colliculus, parahippocampal gyrus, and ventral premotor cortex. It was hypothesized that connectivity information could therefore be used to identify similar areas in the human parietal cortex using diffusion-weighted imaging and probabilistic tractography. Unusually, the subcortical routes of the 3 projections have been reported in the macaque, so it was possible to compare not only the terminations of connections but also their course. The medial IPS had the highest probability of connection with the superior colliculus. The projection pathway resembled that connecting parietal cortex and superior colliculus in the macaque. The posterior angular gyrus and the adjacent superior occipital gyrus had a high probability of connection with the parahippocampal gyrus. The projection pathway resembled the macaque inferior longitudinal fascicle, which connects these areas. The ventral premotor cortex had a high probability of connection with the supramarginal gyrus and anterior IPS. The connection was mediated by the third branch of the superior longitudinal fascicle, which interconnects similar regions in the macaque. Human parietal areas have anatomical connections resembling those of functionally related macaque parietal areas.     

Functional organization of ferret auditory cortex

We characterized the functional organization of different fields within the auditory cortex of anaesthetized ferrets. As previously reported, the primary auditory cortex, A1, and the anterior auditory field, AAF, are located on the middle ectosylvian gyrus. These areas exhibited a similar tonotopic organization, with high frequencies represented at the dorsal tip of the gyrus and low frequencies more ventrally, but differed in that AAF neurons had shorter response latencies than those in A1. On the basis of differences in frequency selectivity, temporal response properties and thresholds, we identified four more, previously undescribed fields. Two of these are located on the posterior ectosylvian gyrus and were tonotopically organized. Neurons in these areas responded robustly to tones, but had longer latencies, more sustained responses and a higher incidence of non-monotonic rate-level functions than those in the primary fields. Two further auditory fields, which were not tonotopically organized, were found on the anterior ectosylvian gyrus. Neurons in the more dorsal anterior area gave short-latency, transient responses to tones and were generally broadly tuned with a preference for high (>8 kHz) frequencies. Neurons in the other anterior area were frequently unresponsive to tones, but often responded vigorously to broadband noise. The presence of both tonotopic and non-tonotopic auditory cortical fields indicates that the organization of ferret auditory cortex is comparable to that seen in other mammals.     

Cortical input to the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) in macaques: a retrograde tracing study

Using retrograde tracing methods, we investigated the cortical projection to the nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system (NOT-DTN) in macaque monkeys. Tracer injections at electrophysiologically identified recording sites in the NOT-DTN resulted in retrogradely labelled neurons in layer V of various cortical areas. The strongest projection always arose from the middle temporal area (MT) and the adjoining cortex anterior to MT in the superior temporal sulcus. A less dense projection came from the middle superior temporal area (MST). In addition, retrogradely labelled cells were consistently found in areas V1 and V2 at moderate to high density. Furthermore, sparse to moderate labelling occurred in prestriate area V3. These findings were compared with the label resulting from control injections into the superior colliculus in two additional cases. Our results indicate that the cortical input to the NOT-DTN as the sensorimotor interface for the pathway subserving stabilizing eye movements during the optokinetic reflex and smooth pursuit mainly arises from the motion-sensitive areas MT and MST in the superior temporal sulcus, as well as from areas V1 and V2. Clearly the projection to the NOT-DTN does not arise from a single cortical area.     

Thalamic ablations and neocortical development: alterations in thalamic and callosal connectivity.

Corticofugal pathways (callosal, intracortical, and subcortical) have initial axon outgrowth to many areas where no adult connections will persist. Corticofugal projections also demonstrate considerable reorganization after early damage. At the level of gross projections from specific thalamic nuclei to cortical cytoarchitectonic areas, early thalamocortical projections appear to show greater specificity for their targets than do corticofugal projections, and their potential for reorganization after early damage is not known. In this article, we explore the nature of the reorganization shown by the thalamocortical system after early thalamic lesions, and contrast it with reorganization of the origin of contralateral visual callosal projections in the same animals. Hamster pups were given electrolytic lesions in the posterior thalamus on the day of birth, damaging principally either the ventrobasal (somatosensory) or the dorsal lateral geniculate (visual) nucleus. After 30 d of age, HRP was implanted in either the somatosensory or the visual cortex, matching the area of implant with the intended thalamic lesion. The thalamus was reconstructed to determine the remaining nuclei, and the distribution of retrogradely labeled cells was plotted. For animals with HRP implants in visual cortex, the location of callosally projecting cells from the contralateral cortex was charted. These animals were compared to a group of normal adult animals with HRP implants approximately matched for size and location. In seven of eight adult animals with neonatal thalamic lesions, the remaining thalamus did not reorganize to innervate the thalamically denervated cortex. In contrast, the callosal projections from the contralateral visual cortex showed a wider tangential origin in the experimental animals compared to the controls. This expanded callosal projection included cells from temporal cortex, a projection not seen in normal animals. Thus, thalamocortical and callosal projection systems differ in both the magnitude and the nature of their reorganization after early damage.     

Listening to narrative speech after aphasic stroke: the role of the left anterior temporal lobe

The dorsal bank of the primate superior temporal sulcus (STS) is a polysensory area with rich connections to unimodal sensory association cortices. These include auditory projections that process complex acoustic information, including conspecific vocalizations. We investigated whether an extensive left posterior temporal (Wernicke's area) lesion, which included destruction of early auditory cortex, may contribute to impaired spoken narrative comprehension as a consequence of reduced function in the anterior STS, a region not included within the boundary of infarction. Listening to narratives in normal subjects activated the posterior-anterior extent of the left STS, as far forward as the temporal pole. The presence of a Wernicke's area lesion was associated with both impaired sentence comprehension and a reduced physiological response to heard narratives in the intact anterior left STS when compared to aphasic patients without temporal lobe damage and normal controls. Thus, in addition to the loss of language function in left posterior temporal cortex as the direct result of infarction, posterior ablation that includes primary and early association auditory cortex impairs language function in the intact anterior left temporal lobe. The implication is that clinical studies of language on stroke patients have underestimated the role of left anterior temporal cortex in comprehension of narrative speech.     

The organization and connections of anterior and posterior parietal cortex in titi monkeys: do New World monkeys have an area 2?

We used multiunit electrophysiological recording techniques to examine the topographic organization of somatosensory area 3b and cortex posterior to area 3b, including area 1 and the presumptive area 5, in the New World titi monkey, Callicebus moloch. We also examined the ipsilateral and contralateral connections of these fields, as well as those in a region of cortex that appeared to be similar to both area 7b and the anterior intraparietal area (7b/AIP) described in macaque monkeys. All data were combined with architectonic analysis to generate comprehensive reconstructions. These studies led to several observations. First, area 1 in titi monkeys is not as precisely organized in terms of topographic order and receptive field size as is area 1 in macaque monkeys and a few New World monkeys. Second, cortex caudal to area 1 in titi monkeys is dominated by the representation of the hand and forelimb, and contains neurons that are often responsive to visual stimulation as well as somatic stimulation. This organization is more like area 5 described in macaque monkeys than like area 2. Third, ipsilateral and contralateral cortical connections become more broadly distributed away from area 3b towards the posterior parietal cortex. Specifically, area 3b has a relatively restricted pattern of connectivity with adjacent somatosensory fields 3a, 1, S2 and PV; area 1 has more broadly distributed connections than area 3b; and the presumptive areas 5 and 7b/AIP have highly diverse connections, including connections with motor and premotor cortex, extrastriate visual areas, auditory areas and somatosensory areas of the lateral sulcus. Fourth, the hand representation of the presumptive area 5 has dense callosal connections. Our results, together with previous studies in other primates, suggest that anterior parietal cortex has expanded in some primate lineages, perhaps in relation to manual abilities, and that the region of cortex we term area 5 is involved in integrating somatic inputs with the motor system and across hemispheres. Such connections could form the substrate for intentional reaching, grasping and intermanual transfer of information necessary for bilateral coordination of the hands.     

Information Cascade from Primary Auditory Cortex to the Amygdala: Corticocortical and Corticoamygdaloid Projections of Temporal Cortex in the Rat

Corticocortical and corticoamygdaloid connections of temporalcortext and perirhinal cortex (PRh) were examined in the ratwith the anterograde tracer Phaseolus vulgaris leucoagglutinin(PHA-L). lontophoretic injections of PHA-L into area TE1 resultedin columnar axonal terminations in surrounding and contralateralregions of temporal neocortex and in the striatum, but not inthe amygdala. Within temporal neocortex, labeled fibers werepresent locally in adjacent regions of TE1, as well as in TE2d,TE1v, TE3v, and TE2c. Injection of cortical areas TE1v, TE3v,and TE2c, which received projections from TE1, or injectionsof perirhinal periallocortex, which received projections fromTE1v, TE2v, and TE3v, resulted in projections to the amygdala.The pattern of corticocortical and corticoamygdaloid projectionsdiffered among the divisions of auditory cortex. TE1 exhibitedextensive ipsilateral and contralateral projections to temporalcortical regions and no projections to the amygdala. In contrast,areas of temporal neocortex ventral and posterior to TE1, includingTE1v, TE3v, TE2c, and PRh, had more limited ipsi- and contralateralcorticocortical projections but had an increased connectivitywith the subcortical forebrain, especially the lateral nucleusof the amygdala (AL). There was a topographic organization tothe AL afferents. The dorsal subdivision of AL received projectionsfrom TE1v, TE3v, TE2c, and PRh, while the ventrolateral divisionreceived projections from TE3v, TE2c, and PRh. The ventromedialdivision received projections only from PRh, which, unlike othertemporal cortical areas, also projected to the ba-solateraland basomedial nuclei of the amygdala. These findings definethe complete sequence of connections linking primary auditorycortex with the amygdala in the rat. In addition, the findingsindicate that the ventral portion of TE1, designated TE1v, hasconnections that distinguish it from dorsal TE1, namely, denseprojections to AL and a diminished number of corticocorticalprojections ipsilaterally and contralaterally. Finally, theresults suggest a topographic organization to the cortical terminationswithin the amygdala.     

Dynamics of gamma-band activity induced by auditory pattern changes in humans.

Increasing evidence suggests separate auditory pattern and space processing streams. The present paper describes two magnetoencephalogram studies examining gamma-band activity to changes in auditory patterns using consonant-vowel syllables (experiment 1), animal vocalizations and artificial noises (experiment 2). Two samples of each sound type were presented to passively listening subjects in separate oddball paradigms with 80% standards and 20% deviants differing in their spectral composition. Evoked magnetic mismatch fields peaking approximately 190 ms poststimulus showed a trend for a left-hemisphere advantage for syllables, but no hemispheric differences for the other sounds. Frequency analysis and statistical probability mapping of the differences between deviants and standards revealed increased gamma-band activity above 60 Hz over left anterior temporal/ventrolateral prefrontal cortex for all three types of stimuli. This activity peaked simultaneously with the mismatch responses for animal sounds (180 ms) but was delayed for noises (260 ms) and syllables (320 ms). Our results support the hypothesized role of anterior temporal/ventral prefrontal regions in the processing of auditory pattern change. They extend earlier findings of gamma-band activity over posterior parieto-temporal cortex during auditory spatial processing that supported the putative auditory dorsal stream. Furthermore, earlier gamma-band responses to animal vocalizations may suggest faster processing of fear-relevant information.     

Physiological and anatomical evidence for multisensory interactions in auditory cortex

Recent studies, conducted almost exclusively in primates, have shown that several cortical areas usually associated with modality-specific sensory processing are subject to influences from other senses. Here we demonstrate using single-unit recordings and estimates of mutual information that visual stimuli can influence the activity of units in the auditory cortex of anesthetized ferrets. In many cases, these units were also acoustically responsive and frequently transmitted more information in their spike discharge patterns in response to paired visual-auditory stimulation than when either modality was presented by itself. For each stimulus, this information was conveyed by a combination of spike count and spike timing. Even in primary auditory areas (primary auditory cortex [A1] and anterior auditory field [AAF]), approximately 15% of recorded units were found to have nonauditory input. This proportion increased in the higher level fields that lie ventral to A1/AAF and was highest in the anterior ventral field, where nearly 50% of the units were found to be responsive to visual stimuli only and a further quarter to both visual and auditory stimuli. Within each field, the pure-tone response properties of neurons sensitive to visual stimuli did not differ in any systematic way from those of visually unresponsive neurons. Neural tracer injections revealed direct inputs from visual cortex into auditory cortex, indicating a potential source of origin for the visual responses. Primary visual cortex projects sparsely to A1, whereas higher visual areas innervate auditory areas in a field-specific manner. These data indicate that multisensory convergence and integration are features common to all auditory cortical areas but are especially prevalent in higher areas.     

Cortical input to the frontal pole of the marmoset monkey

We used fluorescent tracers to map the pattern of cortical afferents to frontal area 10 in marmosets. Dense projections originated in several subdivisions of orbitofrontal cortex, in the medial frontal cortex (particularly areas 14 and 32), and in the dorsolateral frontal cortex (particularly areas 8Ad and 9). Major projections also stemmed, in variable proportions depending on location of the injection site, from both the inferior and superior temporal sensory association areas, suggesting a degree of audiovisual convergence. Other temporal projections included the superior temporal polysensory cortex, temporal pole, and parabelt auditory cortex. Medial area 10 received additional projections from retrosplenial, rostral calcarine, and parahippocampal areas, while lateral area 10 received small projections from the ventral somatosensory and premotor areas. There were no afferents from posterior parietal or occipital areas. Most frontal connections were balanced in terms of laminar origin, giving few indications of an anatomical hierarchy. The pattern of frontopolar afferents suggests an interface between high-order representations of the sensory world and internally generated states, including working memory, which may subserve ongoing evaluation of the consequences of decisions as well as other cognitive functions. The results also suggest the existence of functional differences between subregions of area 10.     

Connectivity patterns of thalamic nuclei implicated in dyskinesia

Thalamic nuclei implicated in the neural mechanisms of dyskinesia (1) have projections to components of the basal ganglia; (2) receive efferents from the corpus striatum, and/or (3) project fibers to regions of the cerebral cortex that generate signals which produce or modulate motor phenomenon. The neostriatum receives a major input from the intralaminar thalamic nuclei (ITN). Thalamostriate fibers projecting to the caudate nucleus (CN) and the putamen (Put) originate from different neuronal populations; clusters of cells in the rostral ITN and in the parafascicular nucleus (PF) project to the CN, while cells in the centromedian nucleus (CM) project to the Put. Smaller numbers of cells in the medial, dorsal and ventral nuclear subdivisions of the thalamus also project to the striatum. The amygdaloid nuclear complex receives afferents from the midline thalamic nuclei. Ventromedial and rostral parts of the subthalamic nucleus receive a small input from the centromedian-parafascicular nuclear complex. Segments of the globus pallidus (GP) and the substantia nigra (SN) do not receive afferents from either the cerebral cortex or the thalamus. Thalamic afferents originate ipsilaterally from the medial segment of the globus pallidus (MPS), and the pars reticulata of the substantia nigra (SNR), and contralaterally from the deep cerebellar nuclei (DCN). Each of these projections to nuclear subdivisions of the thalamus is distinctive without overlap. Projections from the MPS are to the ventral anterior pars principalis (VApc) and ventral lateralis, pars oralis (VLo) thalamic nuclei with collaterals to CM. The SNR provides projections to the ventral anterior, pars magnocellularis, the ventral lateral, pars medialis and the mediodorsal, pars paralaminaris thalamic nuclei. Output from the subthalamic nucleus (STN) projects to both the MPS and the SNR and could modulate influences upon thalamic nuclei. In the monkey the projection of STN to the GP is four times greater than to the SNR. The most massive input to the thalamus arises from the contralateral DCN and terminates in the so-called cell-sparse zone, which consists of the ventral posterolateral nucleus, pars oralis, the ventral lateral nucleus pars caudalis, and pars postrema and area x of Olszewski. Nuclear subdivisions of the thalamus receiving afferents from the MPS and the SNR have gamma-aminobutyric acid (GABA) as their major neurotransmitter; fiber systems originating from the DCN appear to have glutamate as their neurotransmitter.(ABSTRACT TRUNCATED AT 400 WORDS)     

Convergent and complementary projections of the caudal paralaminar thalamic nuclei to rat temporal and insular cortex

Thalamic nuclei adjacent to the medial geniculate body play a pivotal role in processing of sensory stimuli during emotional situations. These nuclei, which include the suprageniculate nucleus (SG), the posterior intralaminar nucleus (PIN), the peripeduncular nucleus (PP) and the medial division of the medial geniculate body (MGm), project to both cortex and amygdala, but target areas and the extent of the projection of individual nuclei are not known yet. The aim of the present study was to analyze the contribution of individual nuclei to the cortical projection with modern sensitive tracing techniques. Small injections of Miniruby or PHA-L were made into single thalamic nuclei. All thalamic nuclei have in common a projection into the upper portion of layer I of the temporal aspect of the cortical mantle. Furthermore, SG, PIN, MGm and PP each demonstrated a convergent projection to lower layer III and to layer IV of the ectorhinal and visceral cortex. Only MGm projects to layer VI of primary auditory and temporal association cortices. Within the perirhinal cortex zones of convergence and divergence exist. The present results demonstrate a differential thalamocortical projection of single thalamic nuclei to those cortical areas which are involved in the transmission of sensory signals to the amygdala via the thalamocortico-cortical pathway and to the hippocampus via the entorhinal cortex. The thalamic nuclei are thus in a position to activate the amygdala and to modulate the information flow of the thalamocortico-cortical pathway to both amygdala and hippocampus.     

Concurrent tonotopic processing streams in auditory cortex

The basis for multiple representations of equivalent frequency ranges in auditory cortex was studied with physiological and anatomical methods. Our goal was to trace the convergence of thalamic, commissural, and corticocortical information upon two tonotopic fields in the cat, the primary auditory cortex (AI) and the anterior auditory field (AAF). Both fields are among the first cortical levels of processing. After neurophysiological mapping of characteristic frequency, we injected different retrograde tracers at separate, frequency-matched loci in AI and AAF. We found differences in their projections that support the notion of largely segregated parallel processing streams in the auditory thalamus and cerebral cortex. In each field, ipsilateral cortical input amounts to approximately 70% of the number of cells projecting to an isofrequency domain, while commissural and thalamic sources are each approximately 15%. Labeled thalamic and cortical neurons were concentrated in tonotopically predicted regions and in smaller loci far from their spectrally predicted positions. The few double-labeled thalamic neurons (<2%) are consistent with the hypothesis that information to AI and AAF travels along independent processing streams despite widespread regional overlap of thalamic input sources. Double labeling is also sparse in both the corticocortical and commissural systems ( approximately 1%), confirming their independence. The segregation of frequency-specific channels within thalamic and cortical systems is consistent with a model of parallel processing in auditory cortex. The global convergence of cells outside the targeted frequency domain in AI and AAF could contribute to context-dependent processing and to intracortical plasticity and reorganization.     

The use of brain positron emission tomography to identify sites of postoperative pain processing with and without epidural analgesia

It is not known how different analgesic regimes affect the brain when reducing postoperative pain. We performed positron emission tomography (PET) scans on a 69-yr-old woman in the presence of moderate postoperative pain and then with epidural analgesia producing complete analgesia, during the first 2 days after total knee arthroplasty. Day 2 postsurgery PET scan data (no pain with epidural analgesia) were subtracted from Day 1 postsurgery PET scan data (time of moderate pain without epidural analgesia) to determine the brain regions activated. Postsurgical pain was associated with increased activity in the contralateral primary somatosensory cortex. Other brain regions showing increased postsurgical activity were the contralateral parietal cortex, bilateral pulvinar and ipsilateral medial dorsal nucleus of the thalamus, contralateral putamen, contralateral superior temporal gyrus, ipsilateral fusiform gyrus, ipsilateral posterior lobe, and contralateral anterior cerebellar lobe. This study demonstrates the feasibility of evaluating the central processing of acute postoperative pain using PET.     

The laminar pattern of connections between prefrontal and anterior temporal cortices in the Rhesus monkey is related to cortical structure and function

The laminar pattern of axonal termination from prefrontal (caudal orbitofrontal, rostral orbitofrontal and lateral areas) to anterior temporal areas (entorhinal cortex, perirhinal cortex and area TE) and from temporal to prefrontal areas was investigated with the aid of anterograde tracers. Both regions are characterized by structural heterogeneity, and include agranular, dysgranular and granular cortical types, denoting, respectively, the absence, incipience and presence of granular layer 4. In addition, both the prefrontal and anterior temporal cortices are composed of areas that have related though specialized functions. The pattern of cortical axonal termination was associated with both the structural type of the cortex of origin and the structure of the destination cortex. Thus, efferent fibers from a single origin in either prefrontal or anterior temporal cortex terminated in different patterns depending on their target area. Conversely, axons terminated in different patterns in a single target area, prefrontal or anterior temporal, depending on their area of origin. Projections from agranular or dysgranular type cortices (e.g. medial temporal areas and caudal orbitofrontal areas) terminated mostly in the upper layers of granular cortices (e.g. area TE and lateral prefrontal areas), and projections from granular cortices terminated mostly in the deep layers of agranular or dys- granular cortices. A robust projection from dysgranular orbitofrontal areas terminated in the deep layers of the agranular entorhinal cortex. Projections from prefrontal areas to area TE terminated in the upper layers, and may facilitate focused attention on behaviorally relevant stimuli processed through reciprocal pathways between prefrontal and temporal cortices.     

Function and connectivity in human primary auditory cortex: a combined fMRI and DTI study at 3 Tesla

Human primary auditory cortex (PAC) is functionally organized in a tonotopic manner. Past studies have used neuroimaging to characterize tonotopic organization in PAC and found similar organization as that described in mammals. In contrast to what is known about PAC in primates and nonprimates, in humans, the structural connectivity within PAC has not been defined. In this study, stroboscopic event-related functional magnetic resonance imaging (fMRI) was utilized to reveal mirror symmetric tonotopic organization consisting of a high-low-high frequency gradient in PAC. Furthermore, diffusion tensor tractography and probabilistic mapping was used to study projection patterns within tonotopic areas. Based on earlier physiological and histological work in nonhuman PAC, we hypothesized the existence of cross-field isofrequency (homotopic) and within-field non-isofrequency (heterotopic)-specific axonal projections in human PAC. The presence of both projections types was found in all subjects. Specifically, the number of diffusion tensor imaging (DTI) reconstructed fibers projecting between high- and low-frequency regions was greater than those fibers projecting between 2 high-frequency areas, the latter of which are located in distinct auditory fields. The fMRI and DTI results indicate that functional and structural properties within early stages of the auditory processing stream are preserved across multiple mammalian species at distinct evolutionary levels.     

Effects of tectal grafts on sound detection deficits induced by inferior colliculus lesions in hooded rats

The midbrain inferior colliculus (IC), a major integrating center for auditory processing, provides a model for structural, functional, and behavioral recovery. The present study examined the role of IC in spatial sound detection, and the effects of neural transplantation in sparing of behavioral performance. Hooded rats were presented noise bursts at ambient noise levels and 15 dB above this level randomly at one of eight locations in the horizontal plane, and rats were required to suppress licking upon detecting signal presentations. Following training, rats received bilateral IC lesions, bilateral lesions followed in 1 wk by bilateral tectal grafts (embryonic Day 18), or were sham operated. During repeated testing 15 to 30 days and 40 to 45 days following surgical procedures, rats with lesions showed impaired detection task performance when compared to grafted or sham animals. Detection ratios were statistically higher for grafted rats than for rats with lesions at repeated testing times. Performance of grafted animals was not statistically separable from the sham control group; however, grafted rats never achieved preoperative detection rates nor rates as high as sham rats. Postmortem implantation of diI crystals unilaterally into grafts demonstrated fiber labeling in the ipsilateral IC and lateral lemniscus, and retrograde labeling of neurons in the remaining host IC and dorsal nucleus of lateral lemniscus. Combined with results of previous studies, the results of this study suggest that sparing of detection performance for sounds occuring at random spatial locations may be attributable to partial integration of host-graft pathways.     

Cortical connections of area V4 in the macaque

To determine the locus, full extent, and topographic organization of cortical connections of area V4 (visual area 4), we injected anterograde and retrograde tracers under electrophysiological guidance into 21 sites in 9 macaques. Injection sites included representations ranging from central to far peripheral eccentricities in the upper and lower fields. Our results indicated that all parts of V4 are connected with occipital areas V2 (visual area 2), V3 (visual area 3), and V3A (visual complex V3, part A), superior temporal areas V4t (V4 transition zone), MT (medial temporal area), and FST (fundus of the superior temporal sulcus [STS] area), inferior temporal areas TEO (cytoarchitectonic area TEO in posterior inferior temporal cortex) and TE (cytoarchitectonic area TE in anterior temporal cortex), and the frontal eye field (FEF). By contrast, mainly peripheral field representations of V4 are connected with occipitoparietal areas DP (dorsal prelunate area), VIP (ventral intraparietal area), LIP (lateral intraparietal area), PIP (posterior intraparietal area), parieto-occipital area, and MST (medial STS area), and parahippocampal area TF (cytoarchitectonic area TF on the parahippocampal gyrus). Based on the distribution of labeled cells and terminals, projections from V4 to V2 and V3 are feedback, those to V3A, V4t, MT, DP, VIP, PIP, and FEF are the intermediate type, and those to FST, MST, LIP, TEO, TE, and TF are feedforward. Peripheral field projections from V4 to parietal areas could provide a direct route for rapid activation of circuits serving spatial vision and spatial attention. By contrast, the predominance of central field projections from V4 to inferior temporal areas is consistent with the need for detailed form analysis for object vision.     

Hearing lips: gamma-band activity during audiovisual speech perception

Auditory pattern changes have been shown to elicit increases in magnetoencephalographic gamma-band activity (GBA) over left inferior frontal cortex, forming part of the putative auditory ventral "what" processing stream. The present study employed a McGurk-type paradigm to assess whether GBA would be associated with subjectively perceived changes even when auditory stimuli remain unchanged. Magnetoencephalograms were recorded in 16 human subjects during audiovisual mismatch perception. Both infrequent visual (auditory /ta/ + visual /pa/) and acoustic deviants (auditory/pa/ + visual /ta/) were compared with frequent audiovisual standards (auditory /ta/ and visual /ta/). Statistical probability mapping revealed spectral amplitude increases at approximately 75 and approximately 78 Hz to visual deviants. GBA to visual deviants peaked 160 ms after auditory stimulus onset over posterior parietal cortex, at 270 ms over occipital areas and at 320 ms over left inferior frontal cortex. The latter GBA enhancement was consistent with the increase observed previously to pure acoustic mismatch, supporting a role of left inferior frontal cortex for the representation of perceived auditory pattern change. The preceding gamma-band changes over posterior areas may reflect processing of incongruent lip movements in visual motion areas and back-projections to earlier visual cortex.     

Orbitofrontal cortex pathology in Alzheimer's disease

The orbitofrontal cortex has been examined in Alzheimer's disease (AD) from the viewpoint of neurofibrillary tangle (NFT) pathology, its laminar distribution and topography. NFT pathology in the orbitofrontal cortex is extensive in AD. In cases with extensive cortical pathology, NFTs extend from the pole of the frontal lobe to the orbitoinsular junction. In lesser affected cases, the anterior granular part of the orbital cortex is less invested by NFTs. Layers III and V contain the greatest density of NFTs and these are most dense in the dysgranular areas, posterior to the transverse orbital sulcus. Posterior and medial orbitofrontal areas, forming area 13 and the posterior tip of the paraolfactory gyrus, are the most severely damaged, as are the smaller agranular fields that surround the olfactory tract and cortex. The widespread orbitofrontal damage in AD affecting projection neurons suggests that this pathology may contribute heavily to the many non-memory-related behavior changes observed in this disorder.