Restoration of acoustic orienting into a cortically deaf hemifield by reversible deactivation of the contralesional superior colliculus: the acoustic "Sprague Effect"

Removal of all contiguous visual cortical areas of one hemisphere results in a contralateral hemianopia. Subsequent deactivation of the contralesional superior colliculus (SC) nullifies the effects of the visual cortex ablation and restores visual orienting responses into the cortically blind hemifield. This deficit nullification has become known as the "Sprague Effect." Similarly, in the auditory system, unilateral ablation of auditory cortex results in severe sound localization deficits, as assessed by acoustic orienting, to stimuli in the contralateral hemifield. The purpose of this study was to examine whether auditory orienting responses can be restored into the impaired hemifield during deactivation of the contralesional SC. Three mature cats were trained to orient toward and approach an acoustic stimulus (broadband, white noise burst) that was presented centrally, or at one of 12 peripheral loci, spaced at 15 degrees intervals. After training, a cryoloop was chronically implanted over the dorsal surface of the right SC. During cooling of the cooling loop to temperatures sufficient to deactivate the superficial and intermediate layers (SZ, SGS, SO, SGI), auditory orienting responses were eliminated into the left (contracooled) hemifield while leaving acoustic orienting into the right (ipsicooled) hemifield unimpaired. This deficit was temperature-dependently graded from periphery to center. After the effectiveness of the SC cooling loop was verified, auditory cortex of the middle and posterior ectosylvian and anterior and posterior sylvian gyri was removed from the left hemisphere. As expected, the auditory cortex ablation resulted in a profound deficit in orienting to acoustic stimuli presented at any position in the right (contralesional) hemifield, while leaving acoustic orienting into the left (ipsilesional) hemifield unimpaired. The ablations of auditory cortex did not have any impact on a visual detection and orienting task. The additional deactivation of the contralesional SC to temperatures sufficient to cool the superficial and intermediate layers nullified the deficit caused by the auditory cortex ablation and acoustic orienting responses were restored into the right hemifield. This restoration was temperature-dependently graded from center to periphery. The deactivations were localized and confirmed with reduced uptake of radiolabeled 2-deoxyglucose. Therefore deactivation of the right superior colliculus after the ablation of the left auditory cortex yields a fundamentally different result from that identified during deactivation of the right superior colliculus before the removal of left auditory cortex in the same animal. Thus the "Sprague Effect" is not unique to a particular sensory system and deactivation of the contralesional SC can restore either visual or acoustic orienting responses into an impaired hemifield after cortical damage.     

Cortical control of sound localization in the cat: unilateral cooling deactivation of 19 cerebral areas

We examined the ability of mature cats to accurately orient to, and approach, an acoustic stimulus during unilateral reversible cooling deactivation of primary auditory cortex (AI) or 1 of 18 other cerebral loci. After attending to a central visual stimulus, the cats learned to orient to a 100-ms broad-band, white-noise stimulus emitted from a central speaker or 1 of 12 peripheral sites (at 15 degrees intervals) positioned along the horizontal plane. Twenty-eight cats had two to six cryoloops implanted over multiple cerebral loci. Within auditory cortex, unilateral deactivation of AI, the posterior auditory field (PAF) or the anterior ectosylvian sulcus (AES) resulted in orienting deficits throughout the contralateral field. However, unilateral deactivation of the anterior auditory field, the second auditory cortex, or the ventroposterior auditory field resulted in no deficits on the orienting task. In multisensory cortex, unilateral deactivation of neither ventral or dorsal posterior ectosylvian cortices nor anterior or posterior area 7 resulted in any deficits. No deficits were identified during unilateral cooling of the five visual regions flanking auditory or multisensory cortices: posterior or anterior ii suprasylvian sulcus, posterior suprasylvian sulcus or dorsal or ventral posterior suprasylvian gyrus. In motor cortex, we identified contralateral orienting deficits during unilateral cooling of lateral area 5 (5L) or medial area 6 (6m) but not medial area 5 or lateral area 6. In a control visual-orienting task, areas 5L and 6m also yielded deficits to visual stimuli presented in the contralateral field. Thus the sound-localization deficits identified during unilateral deactivation of area 5L or 6m were not unimodal and are most likely the result of motor rather than perceptual impairments. Overall, three regions in auditory cortex (AI, PAF, AES) are critical for accurate sound localization as assessed by orienting.     

Sound localization during homotopic and heterotopic bilateral cooling deactivation of primary and nonprimary auditory cortical areas in the cat

Although the contributions of primary auditory cortex (AI) to sound localization have been extensively studied in a large number of mammals, little is known of the contributions of nonprimary auditory cortex to sound localization. Therefore the purpose of this study was to examine the contributions of both primary and all the recognized regions of acoustically responsive nonprimary auditory cortex to sound localization during both bilateral and unilateral reversible deactivation. The cats learned to make an orienting response (head movement and approach) to a 100-ms broad-band noise stimulus emitted from a central speaker or one of 12 peripheral sites (located in front of the animal, from left 90 degrees to right 90 degrees , at 15 degrees intervals) along the horizontal plane after attending to a central visual stimulus. Twenty-one cats had one or two bilateral pairs of cryoloops chronically implanted over one of ten regions of auditory cortex. We examined AI [which included the dorsal zone (DZ)], the three other tonotopic fields [anterior auditory field (AAF), posterior auditory field (PAF), ventral posterior auditory field (VPAF)], as well as six nontonotopic regions that included second auditory cortex (AII), the anterior ectosylvian sulcus (AES), the insular (IN) region, the temporal (T) region [which included the ventral auditory field (VAF)], the dorsal posterior ectosylvian (dPE) gyrus [which included the intermediate posterior ectosylvian (iPE) gyrus], and the ventral posterior ectosylvian (vPE) gyrus. In accord with earlier studies, unilateral deactivation of AI/DZ caused sound localization deficits in the contralateral field. Bilateral deactivation of AI/DZ resulted in bilateral sound localization deficits throughout the 180 degrees field examined. Of the three other tonotopically organized fields, only deactivation of PAF resulted in sound localization deficits. These deficits were virtually identical to the unilateral and bilateral deactivation results obtained during AI/DZ deactivation. Of the six nontonotopic regions examined, only deactivation of AES resulted in sound localization deficits in the contralateral hemifield during unilateral deactivation. Although bilateral deactivation of AI/DZ, PAF, or AES resulted in profound sound localization deficits throughout the entire field, the cats were generally able to orient toward the hemifield that contained the acoustic stimulus, but not accurately identify the location of the stimulus. Neither unilateral nor bilateral deactivation of areas AAF, VPAF, AII, IN, T, dPE, nor vPE had any effect on the sound localization task. Finally, bilateral heterotopic deactivations of AI/DZ, PAF, or AES yielded deficits that were as profound as bilateral homotopic cooling of any of these sites. The fact that deactivation of any one region (AI/DZ, PAF, or AES) was sufficient to produce a deficit indicated that normal function of all three regions was necessary for normal sound localization. Neither unilateral nor bilateral deactivation of AI/DZ, PAF, or AES affected the accurate localization of a visual target. The results suggest that hemispheric deactivations contribute independently to sound localization deficits.     

Deficits of visual attention and saccadic eye movements after lesions of parietooccipital cortex in monkeys

1. Visual attention is often profoundly disturbed in humans after damage to the cortex of the posterior parietal lobe, particularly of the minor hemisphere, with some patients being totally unaware of visual stimuli in the hemifield of extrapersonal space contralateral to the cortical damage. This severe form of visual inattention is usually called contralateral neglect and has occasionally been reported following posterior parietal lesions in monkeys. However, in monkeys, only qualitative observations have been published and those reports are not in agreement concerning the severity of the deficit. The present experiments were designed to measure quantitatively the amount of disruption of selective visual attention which is produced by lesions of posterior parietal and parietooccipital cortical lesions in monkeys. 2. Five monkeys were trained to visually fixate and follow with their gaze a small visual stimulus as it suddenly moved varying distances (8, 16, or 24 degrees) from the midline into the left or right visual hemifields. Two animals then received a unilateral cortical lesion limited to the inferior parietal lobule (IPL). Three animals received unilateral lesions which included both the inferior parietal lobule and a portion of adjacent dorsal prestriate cortex (IPL-PS). 3. Visual inattention is commonly divided into two levels of severity. The more severe form, contralateral neglect, is the complete absence of behavioral response to a stimulus in the visual field contralateral to hemisphere damage. The less severe deficit, usually called visual extinction, is a tendency to ignore the contralateral of two visual stimuli when they appear simultaneously and symmetrically placed with respect to the center of the subject's surroundings. The five monkeys in this study were tested on a stimulus paradigm which simultaneously measured the severity of visual neglect and also the amount and duration of visual extinction which were produced by the cortical lesions. 4. All monkeys displayed contralateral visual extinction after unilateral posterior parietal or parietooccipital lesions. Three of the five monkeys showed a reversal of the visual extinction after a second, symmetrical lesion was placed in the opposite hemisphere. No monkey showed evidence of full-blown contralateral neglect after lesions limited to the parietooccipital cortex, either in the formal testing situation or during informal neurological examinations. The severity of the observed inattention did not appear to be related to the size of the cortical lesions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cancellation of visuoparietal lesion-induced spatial neglect

In humans lesions of right visuoparietal cortex induce a neglect of the contralesional visual field that is characterized in its mild form by inattentiveness to objects and events and, in its more severe form, by a condition that has many features that are indistinguishable from blindness. Here we show that spatial neglect can be induced in cats by lesions of posterior and inferior visuoparietal cortex, and that the lesion-induced neglect can be cancelled by cooling deactivation of the same region in the opposite hemisphere. Electronic Publication     

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions

Summary In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Postoperatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.     

Bilateral parietal and contralateral responses during maintenance of unilaterally encoded objects in visual short-term memory: Evidence from magnetoencephalography

A component of the event-related magnetic field (ERMF) response was observed in magnetoencephalographic signals recorded during the maintenance of information in visual short-term memory (VSTM). This sustained posterior contralateral magnetic (SPCM) field is likely the magnetic equivalent of the sustained posterior contralateral negativity (SPCN) found in electrophysiology. Magnetoencephalography data showed, at the sensor level, a bilateral activation over the parietal cortex that increased in amplitude for higher memory load. Others sensors, also over the parietal cortex, showed an activation pattern similar to the SPCN with higher activation for the hemisphere contralateral to the visual field from which visual information was encoded. These two activation patterns suggest that the SPCN and SPCM are generated by a network of cortical sources that includes bilateral parietal loci, likely intra-parietal/intra-occipital cortex, and contralateral parietal sources.     

Recovery of function following unilateral damage to visuoparietal cortex

Damage to the visuoparietal cortex located in the banks of the middle suprasylvian gyrus of the cat has been shown to produce a deficit in the detection and localization of moving visual cues presented in the contralesional visual hemifield. There is evidence from reversible cooling deactivation studies that the integrity of this orienting function is not completely dependent on the VP cortex and that under the right circumstances, other brain regions may come online and completely take over the processing that subserves this behavior. We examined the recovery of orienting behavior after unilateral damage to the VP cortex. We found that consistent with previous data, VP damage produced an impairment in the capacity to detect and orient to moving visual stimuli in the contralesional visual field. Over a span of days, spontaneous recovery fully occurred. The ability to detect and localize static visual stimuli was tested as a fiducial measure of parietal cortex function, and this function did not recover. We conclude that the detection and localization of moving visual stimuli is not a function that requires VP cortex and argue for the existence of a parallel and redundant subcortical-cortical brain network that serves as the substrate for recovery of function.     

Remapping the remembered target location for anti-saccades in human posterior parietal cortex

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in anti-saccades. To do so, we exploited the laterality of a subregion of the PPC for remembered target location. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the location of a flashed target, then were instructed to plan either an anti- or pro-saccade to that location, and finally were instructed to execute the movement. At first, the region responded preferentially to the memory of a target location presented in the contralateral visual field. However, when an anti-cue specified a saccadic response into the opposite visual field, we observed a dynamic shift in cortical activity from one hemisphere to the other. This shows that this region within the human posterior parietal cortex codes the target location for an upcoming saccade, rather than the location of the remembered visual stimulus in an anti-saccade task.     

Double dissociation of 'what' and 'where' processing in auditory cortex

Studies of cortical connections or neuronal function in different cerebral areas support the hypothesis that parallel cortical processing streams, similar to those identified in visual cortex, may exist in the auditory system. However, this model has not yet been behaviorally tested. We used reversible cooling deactivation to investigate whether the individual regions in cat nonprimary auditory cortex that are responsible for processing the pattern of an acoustic stimulus or localizing a sound in space could be doubly dissociated in the same animal. We found that bilateral deactivation of the posterior auditory field resulted in deficits in a sound-localization task, whereas bilateral deactivation of the anterior auditory field resulted in deficits in a pattern-discrimination task, but not vice versa. These findings support a model of cortical organization that proposes that identifying an acoustic stimulus ('what') and its spatial location ('where') are processed in separate streams in auditory cortex.     

Neuroplasticity in the cat’s visual system: test of the role of the expanded retino-geniculo-parietal pathway in behavioral sparing following early lesions of visual cortex

Sparing of the ability to redirect head and eyes to new stimuli and expansion of the retino-geniculo-parietal pathway are both robust aspects of the repercussions of early lesions of occipital visual areas in cats. The purpose of the present work was to test the proposition that the pathway expansions and spared behaviors are causally linked. The proposition was tested by deactivating either the dorsal lateral geniculate nucleus (dLGN) and thereby uncoupling the primary and secondary limbs of the retino-geniculo-parietal pathway, or silencing the terminus of the pathway, and then testing the ability of cats to detect and orient head and eyes to visual targets. Six cats sustained experimental unilateral lesions of occipital areas 17 and 18 and variable amounts of area 19 on postnatal days 1-2 or 26-30 to induce rewiring and expansion of visual pathways from retina through the dLGN onto a critical region of visuoparietal (VP) cortex. Unilateral lesions ensured that we could use the orienting performance of the intact hemisphere as a fiduciary marker of performance against which performance of the experimental hemisphere could be gauged. When the cats were adult, a secondary test lesion was made on the damaged side by injecting, under electrophysiological guidance, ibotenic acid into either dLGN of four cats or into VP cortex of two cats. Prior to injection of ibotenic acid, all cats oriented head and eyes with high proficiency throughout the contralesional field, and performance was indistinguishable from orienting to stimuli presented in the ipsilesional field; sparing of the orienting behavior was complete. Ibotenic acid lesions of both dLGN and VP cortex induced a profound neglect of stimuli introduced into the contralesional hemifield. Orienting into the ipsilesional field remained high throughout. Subsequently, there was restoration of orienting behavior over the next 4-6 (dLGN deactivation) and 9-12 (VP deactivation) days. The test results demonstrate the essential contribution made by the retino-geniculo-parietal pathway to the ability to detect and redirect head and eyes to look at visual stimuli following early lesions of occipital visual cortices. The subsequent post-test lesion restoration of high orienting proficiency shows that in the absence of dLGN, or the critical region of VP cortex, other regions of cerebral cortex, or other structures such as the superior colliculus, can emerge and make important contributions to orienting behavior. These results reveal a maintained residual, beneficial adaptive plasticity of mature neural circuits even in brains compromised by early lesions of occipital visual areas.     

Sound localization deficits during reversible deactivation of primary auditory cortex and/or the dorsal zone

We examined the contributions of primary auditory cortex (A1) and the dorsal zone of auditory cortex (DZ) to sound localization behavior during separate and combined unilateral and bilateral deactivation. From a central visual fixation point, cats learned to make an orienting response (head movement and approach) to a 100-ms broadband noise burst emitted from a central speaker or one of 12 peripheral sites (located in front of the animal, from left 90 degrees to right 90 degrees, at 15 degrees intervals) along the horizontal plane. Following training, each cat was implanted with separate cryoloops over A1 and DZ bilaterally. Unilateral deactivation of A1 or DZ or simultaneous unilateral deactivation of A1 and DZ (A1/DZ) resulted in spatial localization deficits confined to the contralateral hemifield, whereas sound localization to positions in the ipsilateral hemifield remained unaffected. Simultaneous bilateral deactivation of both A1 and DZ resulted in sound localization performance dropping from near-perfect to chance (7.7% correct) across the entire field. Errors made during bilateral deactivation of A1/DZ tended to be confined to the same hemifield as the target. However, unlike the profound sound localization deficit that occurs when A1 and DZ are deactivated together, deactivation of either A1 or DZ alone produced partial and field-specific deficits. For A1, bilateral deactivation resulted in higher error rates (performance dropping to approximately 45%) but relatively small errors (mostly within 30 degrees of the target). In contrast, bilateral deactivation of DZ produced somewhat fewer errors (performance dropping to only approximately 60% correct), but the errors tended to be larger, often into the incorrect hemifield. Therefore individual deactivation of either A1 or DZ produced specific and unique sound localization deficits. The results of the present study reveal that DZ plays a role in sound localization. Along with previous anatomical and physiological data, these behavioral data support the view that A1 and DZ are distinct cortical areas. Finally, the findings that deactivation of either A1 or DZ alone produces partial sound localization deficits, whereas deactivation of either posterior auditory field (PAF) or anterior ectosylvian sulcus (AES) produces profound sound localization deficits, suggests that PAF and AES make more significant contributions to sound localization than either A1 or DZ.     

Binocular responses of cortical cells and the callosal projection in the albino rat

Four hundred and fifteen cells were recorded in the binocular segment of the visual cortex in the albino rat. Cells encountered were mainly dominated by the contralateral eye. The percentage of binocularly-driven cells increased as the electrode was moved towards the border between areas 17 and 18a. Ninety percent of the cells studied in the region of the border could be driven by electrical stimulation applied at the corresponding site in the opposite hemisphere. Within area 17, however, there were only about 30% of such cells. Through the combined use of electrical stimulation and reversible cortical cooling, two types of contributions by callosal fibres were revealed. One is that the callosal fibres constitute the only inputs from the ipsilateral eye to a cell. The other is that the callosal input provides ipsilateral reinforcement to a binocular cell. These results are compatible with neuroanatomical findings and show that binocularity of visual cortical cells in this animal depends, to a great degree, on the function of callosal fibres.     

Activation and deactivation in response to visual stimulation in the occipital cortex of 6-month-old human infants

In an infant's developing cortex, the explanation for the mechanisms underlying the activations and deactivations in response to visual stimuli remains controversial. While previous near-infrared spectroscopy (NIRS) studies in awake infants have demonstrated cortical activations in response to meaningful/attractive visual stimuli, functional magnetic resonance imaging (fMRI) studies performed on sleeping infants showed negative blood oxygenation level-dependent (BOLD) responses to high-luminance unpatterned stimulations, such as a photic stimulation. To examine the effect of the characteristics of visual stimuli on cortical processing in awake infants, we measured cortical hemodynamic responses in 6-month-old infants during the presentation of a high-luminance unpatterned stimulus by using a NIRS system with 94 measurement channels. Results from 35 infants showed dissociated cortical responses between the occipital region and the other parts of the cortex, including the temporal and prefrontal regions. Although the visual stimulus produced sustained increases in oxygenated hemoglobin (oxy-Hb) signals in the temporal and prefrontal regions, it produced a transient increase in oxy-Hb signals followed by a salient decrease in oxy-Hb signals during a trial in a focal region of the occipital visual region. This suggests that the deactivation of the occipital visual region in response to visual stimulation is not a phenomenon that occurs only in the sleeping state, but that a high-luminance unpatterned stimulus can induce deactivation even in the awake infants.     

Effects of rearranged vision on event-related lateralizations of the EEG during pointing

We used event-related lateralizations of the EEG (ERLs) and reversed vision to study visuomotor processing with conflicting proprioceptive and visual information during pointing. Reversed vision decreased arm-related lateralization, probably reflecting the simultaneous activity of left and right arm specific neurons: neurons in the hemisphere contralateral to the observed action were probably activated by visual feedback, neurons in the hemisphere contralateral to the response side by the somatomotor feedback. Lateralization related to the target in parietal cortex increased, indicating that visual to motor transformation in parietal cortex required additional time and resources with reversed vision. A short period of adaptation to an additional lateral displacement of the visual field increased arm-contralateral activity in parietal cortex during the movement. This is in agreement with the, which showed that adaptation to a lateral displacement of the visual field is reflected in increased parietal involvement during pointing.     

fMRI signal increases and decreases in cortical areas during small-field optokinetic stimulation and central fixation

Small-field optokinetic nystagmus (OKN) was performed in seven healthy volunteers in order to analyze the activation and deactivation patterns of visual motion, ocular motor, and multisensory vestibular cortex areas by means of fMRI during coherent visual motion stimulation. BOLD signal decreases (deactivations) were found in the first and second long insular gyri and retroinsular areas (the human homologue of the parietoinsular vestibular cortex and the visual posterior sylvian area in the monkey) of both hemispheres, extending into the transverse temporal gyrus and inferior-anterior parts of the superior temporal gyrus (BA 22), and the precentral gyri at two separate sites (BA 4 and 6). Further deactivations were found in cranioposterior parts of the superior temporal gyrus (BA 22) and the adjacent inferior parietal lobule (BA 40), anterior cingulate gyrus, hippocampus, and corpus callosum. Most of these BOLD signal decreases involved parts of the "multisensory vestibular cortical circuit". These findings support the concept of a reciprocally inhibitory visual–vestibular interaction that has now been demonstrated not only for large-field visual motion stimulation that induces vection (without eye movements) but also for optokinetically induced eye movements (without vection). The functional significance of this concept may be related to the perception of self-motion, since both large-field visual motion stimulation and optokinetic nystagmus are linked to the visual control of self-motion. With respect to activation of the cortical ocular motor system two separate and distinct areas of activations were delineated in the precentral sulcus of both hemispheres, one ventrolaterally (in BA 9) and the other dorsomedially at the junction of the superior frontal sulcus with the precentral sulcus (in BA 6). Both probably correspond to different subregions of the frontal eye field and the premotor cortex for the ocular motor performance of OKN. Electronic Publication     

Bilateral impact of unilateral visual cortex lesions on the superior colliculus

We examined the functional impact of a long-standing, unilateral primary visual cortex lesion on the superior colliculus (SC) using radiolabeled 2-deoxyglucose (2DG) as a marker of neural activity. In accord with known corticotectal connectivity and functional influence, 2DG uptake in the superficial layers of the ipsilesional SC was decreased. We also found a decrease in the superficial layers of the contralesional SC. These data suggest that modifications in activity in one SC can have a substantial influence on activity in its contralateral partner, and that processing in one visual hemifield does not occur independently of processing of signals in the opposite hemifield. The effects are not mediated by the contralateral hemisphere but are probably mediated by intercollicular circuitry.     

Contralateral connections of the dog's frontal association cortex

We studied the topography of contralateral connections of both prefrontal and premotor regions of the dog's frontal association cortex (FAC) by charting distributions of retrogradely labeled cells following unilateral HRP injections to various areas of this cortex. Generally, in the contralateral hemisphere the labeled cells were most numerous in the FAC areas localized homotopically to the injection sites, less numerous in FAC areas heterotopic to injections, and the least numerous in cortical areas situated outside the frontal lobe. The nonfrontal areas which project to the dorsal and ventral FAC differ from one another. Dorso-caudal parts of the cingular and insular areas, as well as the auditory, somatosensory and visual association cortices project to the dorsal FAC, while the ventro-rostral parts of the cingular and insular areas, together with the prepiriform and periamygdaloid areas of the olfactory cortex as well as the subcallosal area send their axons to the ventral FAC. Thus, the dorsal and ventral FAC areas are supplied by contralateral afferents originating from different cortical areas. Similar organization of ipsilateral FAC connections was described previously.     

Patterns of interhemispheric and striate-peristriate connections in visual cortex of the South American marsupial Marmosa elegans (mouse opossum)

Summary We have analyzed the distributions of inter-hemispheric and striate-peristriate connections in the South American marsupial, Marmosa elegans (mouse opossum). Following multiple injections of horseradish peroxidase (HRP) into one hemisphere, we found that anterogradely labeled terminations and retrogradely labeled perikarya are distributed unevenly in the contralateral hemisphere, forming a distinct tangential pattern in striate and peristriate cortex. This pattern delineates as many as eight peristriate areas relatively poor in commissural connections in lateral peristriate cortex, and in lateral and anterolateral portions of peristriate cortex. Single injections of HRP conjugated with wheat germ agglutinin into anterior or posterior regions of striate cortex produced as many as nine discrete ipsilateral fields of labeled perikarya, and terminations distributed over a broad cortical area in lateral and anterolateral peristriate cortex. Our observations of multiple areas with little or no HRP labeling in the interhemisferic pattern, and of multiple ipsilateral striate projection fields, indicate that the topography of visual cortex in Marmosa is highly elaborate, and suggest that extrastriate cortex is subdivided into several visual areas. Furthermore, by showing that the organization of visual cortex in this marsupial is as complex as in many placental mammals, our data support the view that a basic cortical plan, consisting of multiple visual areas, appeared early in mammalian evolution.     

Effects of cooling somatosensory cortex on response properties of tactile cells in the superior colliculus

The corticotectal influences of somatosensory cortex were investigated by using reversible deactivation of cortex by cooling. More than half of the somatosensory superior colliculus (SC) cells studied exhibited a response depression (often not apparent qualitatively) or an elimination of responses to somatosensory stimuli during the period in which cortex was rendered inactive. Responses were restored to their initial levels by cortical rewarming. Hyperresponsiveness was never observed as a consequence of cortical cooling. Susceptibility to cooling-induced depression was not invariably linked to a specific cell type, location in the SC, or receptive-field size. Yet cells that had small receptive fields and were activated by hair displacement had the highest probability of being affected by this procedure. In some cells a contraction of the receptive field was induced by cortical cooling. This observation is consistent with previous experiments that showed that SC somatosensory receptive fields are constructed by the convergence of ascending and descending inputs and indicates that the responsiveness of specific receptive-field regions may depend on the functional integrity of cortex. Two cortical regions were found to produce cooling-induced effects in somatosensory SC cells: 1) SIV (and para-SIV), located in the anterior ectosylvian sulcus, and 2) the cortex within the rostral suprasylvian sulcus. These results indicate that somatosensory cortex, like visual cortex, plays a critical role in modulating the responses of SC cells. Apparently, the ability of both somatosensory and visual SC cells to code the presence of peripheral stimuli depends largely on the functional influences of their respective cortices. However, in contrast to previous observations on visual corticotectal influences, no specific receptive-field properties could be shown to be impressed on SC cells by somatosensory cortex.