Topographic organization of projections from the amygdala to the hypothalamus of the rat

Afferent fibers from the amygdala to subdivisions of lateral, ventromedial and dorsomedial hypothalamic nuclei were investigated in rat by retrograde transport of horseradish peroxidase. Small (intranuclear size) peroxidase deposits were placed in hypothalamic nuclei by iontophoresis of a tracer solution containing poly-L-alpha-ornithine which greatly limited diffusion. The medial, central and amygdalo-hippocampal nuclei of the amygdala were found to be the major donors of amygdaloid afferent fibers to the hypothalamus, but there was also substantial labeling of somata in cortical, basomedial, basolateral and lateral amygdaloid nuclei and the intra-amygdaloid bed nucleus of the stria terminalis. No fibers projected from the posterior cortical nucleus of the amygdala to the hypothalamus. Most amygdaloid projections to the lateral hypothalamic area originated in the anterior half of the amygdala, while projections to the ventromedial hypothalamic nucleus arose along the entire length of the amygdala except the posterior cortical nucleus. The amygdalo-hippocampal area projects to the medial hypothalamus. Other amygdaloid nuclei project to both the medial and lateral hypothalamic nuclei. These topographic organizations of amygdaloid afferent fibers to various subdivisions of the hypothalamic nuclei are discussed and compared with other anatomical studies on these connections.     

Scanning laser polarimetry in corneal topographic changes after photorefractive keratectomy

Purpose: To examine whether, or not, corneal topographic changes after excimer laser photorefractive keratectomy (PRK) for myopia and myopic astigmatism have any influence on measurements of the retinal nerve fiber layer (NFL) with scanning laser polarimetry. Methods: Retinal NFL thicknesses were determined by scanning laser polarimetry in 17 eyes of 13 patients with myopia and myopic astigmatism before and after PRK. Total ablation depth ranged from 26 to 71 μm. We used the relative ratios for superior and inferior NFL thicknesses which were calculated by dividing the NFL values of respective regions by the nasal value. Results: The mean superior NFL ratio measured was 3.02 ± 0.92 preoperatively, and 3.00 ± 0.76 postoperatively. The mean inferior NFL ratio was 2.95±0.75 preoperatively, and 2.99±0.66 postoperatively. There was no statistically significant difference between preoperative and postoperative NFL measurements (Wilcoxon signed rank test, p > 0.05). Conclusions: Corneal topographic changes after PRK have no significant influence on NFL measurements by scanning laser polarimetry. Our results suggest that scanning laser polarimetry can be used as a reliable method for retinal NFL thickness measurements even after excimer laser PRK. This revised version was published online in July 2006 with corrections to the Cover Date.     

The topographic distribution of the magnetic P100M to full- and half-field stimulation

Visual evoked magnetic responses were recorded to full-field and left and right half-field stimulation with three check sizes (70, 34 and 22) in five normal subjects. Recordings were made sequentially on a 20-position grid (4 × 5) based on the inion, by means of a single-channel direct current-Superconducting Quantum Interference Device second-order gradiometer. The topographic maps were consistent on the same subjects recorded 2 months apart. The half-field responses produced the strongest signals in the contralateral hemisphere and were consistent with the cruciform model of the calcarine fissure. Right half fields produced upper-left-quadrant outgoing fields and lower-left-quadrant ingoing fields, while the left half field produced the opposite response. The topographic maps also varied with check size, with the larger checks producing positive or negative maximum position more anteriorly than small checks. In addition, with large checks the full-field responses could be explained as the summation of the two half fields, whereas full-field responses to smaller checks were more unpredictable and may be due to sources located at the occipital pole or lateral surface. In addition, dipole sources were located as appropriate with the use of inverse problem solutions. Topographic data will be vital to the clinical use of the visual evoked field but, in addition, provides complementary information to visual evoked potentials, allowing detailed studies of the visual cortex.     

Topographic coding of odorant quality is maintained at different concentrations in the salamander olfactory epithelium

In a recent study in the tiger salamander, Ambystoma tigrinum, were demonstrated topographic patterns of responsivity across the olfactory epithelium which were characteristic for each odorant. The present study was initiated to investigate whether these patterns remain constant when odorant concentration is varied. Odorant-induced electro- olfactograms were recorded from at least 12 sites on each epithelium. The odorants used were pinene, amyl acetate and propanol. Each epithelium was tested with one odorant, delivered at 3 concentrations. For comparison between animals, the epithelia were divided into 3 regions with at least 4 recording sites per region. An analysis of variance model was used to study odorants, concentrations, regions and animals. Odorant-induced regional patterns in responsivity were similar across all concentrations. In particular, the region of highest responsivity at one concentration was the region of highest responsivity at all concentrations. It is concluded that topographic patterns of receptor cell responses may reflect an underlying genetic component in the distribution of receptor cells. This distribution is related to two aspects of receptor cell responses: responsivity to particular odorants (Fig. 4) and general responsivity to all odorants (Fig. 5).     

A key role for GAP-43 in the retinotectal topographic organization

To have a proper spatial visual perception, vertebrate retinal ganglion cells connect to their brain targets in a highly ordered fashion. The molecular bases for such topographic retinotectal connection in mammals still remain largely unknown. Using the gene knock-out approach in mice, we report here a key role for the GAP-43 growth cone protein in the development of the visual system. In mice bearing a targeted disruption of GAP-43 exon 1, a high proportion of retinal ganglion cell (RGC) axons was found to grow abnormally into the ipsilateral optic tract and into the hypothalamus. After leaving the optic chiasm during development, the GAP-43-deficient RGC axons generally follow the optic tracts but are unable to form proper terminal zones in the lateral geniculate nucleus. Moreover, in the superior colliculus, RGC axons lacking GAP-43 are intermingled. These results suggest an essential role for GAP-43 in development of the topographic retinotectal connection.     

Topographic tear film trend and new parameters for non-invasive break up time test

AIM: To evaluate the quantitative and qualitative results of the noninvasive tear film break-up time (NI-BUT) test and investigate the predictive ability of the new NI-BUT parameter in discriminating between normal Ocular Surface Disease Index (OSDI; scores ≤12) and abnormal OSDI (scores ≥13). METHODS: A total of 341 eyes of 341 volunteers who applied for routine eye outpatient control were included in the prospective study. All participants’ noninvasive first tear film break-up time (NIF-BUT), noninvasive average tear film break-up time (NIAvg-BUT) and average value of the first three break-up time (A3F-BUT) were analyzed. A3F-BUT, the new NI-BUT parameter, is calculated by adding the NIF-BUT value to the 2nd break-up time value that has a difference of at most 1 second from the NIF-BUT value and to the 3rd break-up time and then dividing the respective sum by 3. Receiver operating characteristic (ROC) curve and forward logistic regression analyses were performed to determine the parameter that had the best predictive ability between the OSDI groups. RESULTS: The NI-BUT values of 255 eyes of 255 volunteers included in the study were analyzed statistically. The mean NIF-BUT, NIAvg-BUT, and A3F-BUT values were calculated as 5.3±3.0, 8±3.1, and 5.8±3.0 seconds, respectively. All three parameters were found to be significantly lower in the abnormal OSDI group (P=0.014, 0.034, and 0.011, respectively). The area under the curve (AUC) of the A3F-BUT to predict abnormal OSDI was AUC=0.625 (0.529-0.720), P=0.011 and NIF-BUT was AUC=0.599 (0.502-0.696), P=0.043. The A3F-BUT parameter and NIF-BUT parameters were found to be significantly efficient in discriminating abnormal OSDI. CONCLUSION: The new parameter for the NI-BUT test has more predictive ability in the discrimination of OSDI groups.     

Hemifield effects of spatial attention in early human visual cortex

Early visual areas (V1, V2, V3/VP, V4v) contain representations of the contralateral hemifield within each hemisphere. Little is known about the role of the visual hemifields along the visuo-spatial attention processing hierarchy. It is hypothesized that attentional information processing is more efficient across the hemifields (known as bilateral field advantage) and that the integration of information is greater within one hemifield as compared with across the hemifields. Using functional magnetic resonance imaging we examined the effect of distance and hemifield on parallel attentional processing in the early visual areas (V1-V4v) at individually mapped retinotopic locations aligned adjacently or separately within or across the hemifields. We found that the bilateral field advantage in parallel attentional processing over separated attended locations can be assigned, at least partly, to differences in distractor position integration in early visual areas. These results provide evidence for a greater integration of locations between two attended locations within one hemifield than across both hemifields. This nicely correlates with behavioral findings of a bilateral field advantage in parallel attentional processing (when distractors in between cannot be excluded) and a unilateral field advantage if attention has to be shifted across separated locations (when locations in between were integrated).     

Topographic representations of object size and relationships with numerosity reveal generalized quantity processing in human parietal cortex

Humans and many animals analyze sensory information to estimate quantities that guide behavior and decisions. These quantities include numerosity (object number) and object size. Having recently demonstrated topographic maps of numerosity, we ask whether the brain also contains maps of object size. Using ultra-high-field (7T) functional MRI and population receptive field modeling, we describe tuned responses to visual object size in bilateral human posterior parietal cortex. Tuning follows linear Gaussian functions and shows surround suppression, and tuning width narrows with increasing preferred object size. Object size-tuned responses are organized in bilateral topographic maps, with similar cortical extents responding to large and small objects. These properties of object size tuning and map organization all differ from the numerosity representation, suggesting that object size and numerosity tuning result from distinct mechanisms. However, their maps largely overlap and object size preferences correlate with numerosity preferences, suggesting associated representations of these two quantities. Object size preferences here show no discernable relation to visual position preferences found in visuospatial receptive fields. As such, object size maps (much like numerosity maps) do not reflect sensory organ structure but instead emerge within the brain. We speculate that, as in sensory processing, optimization of cognitive processing using topographic maps may be a common organizing principle in association cortex. Interactions between object size and numerosity maps may associate cognitive representations of these related features, potentially allowing consideration of both quantities together when making decisions.Humans and animals share a sense of numerosity (object number) that guides behavior and decisions (1, 2), for example choosing numerous objects when foraging or shopping. As such, numbers and numerical processing are fundamental to cognitive neuroscience and are linked to mathematics, value judgments, and economics (1, 3). Because aspects of numerosity perception mirror primary sensory perception, it has been referred to as a “number sense” (4). However, another theory (5) sees numerosity as one aspect of a more generalized quantity system. Here we investigate the representation of another quantity: object size.Behaviorally, object size and numerosity perception interfere with each other (6). At the neural level, single neurons in macaque parietal cortex can be tuned to numerosity (7), line length (a measure of object size), or both (8). However, it is unclear whether numerosity and object size preferences are related, either in the same neurons or in nearby neurons (8). Using human neuroimaging, we have shown that numerosity-tuned neural populations in human posterior parietal lobe are topographically organized (9): Similar numerosity preferences are grouped together, changing gradually across the cortical surface. Visual features of the presented stimuli affect numerosity preferences, which may reflect preferences for particular object sizes (9, 10).Here we ask whether object size-tuned responses are found in the same area, whether these are topographically organized, and how tuning and organization relate to representations of numerosity and visual space in the same area. We find topographically organized object size-tuned responses that largely overlap with numerosity maps and show correlated tuning preferences. However, many differences between object size and numerosity tuning and map organization suggest that responses arise from distinct mechanisms.These intermingled neuronal representations of object number and size may allow generalization and abstraction in quantity processing and consideration of related quantities when making decisions. Optimization of cognitive processing using topographic maps may be a common organizing principle in association cortex, particularly in quantity processing, as it is in sensory processing.     

Discontinuity of cortical gradients reflects sensory impairment

Topographic maps and their continuity constitute a fundamental principle of brain organization. In the somatosensory system, whole-body sensory impairment may be reflected either in cortical signal reduction or disorganization of the somatotopic map, such as disturbed continuity. Here we investigated the role of continuity in pathological states. We studied whole-body cortical representations in response to continuous sensory stimulation under functional MRI (fMRI) in two unique patient populations—patients with cervical sensory Brown-Séquard syndrome (injury to one side of the spinal cord) and patients before and after surgical repair of cervical disk protrusion—enabling us to compare whole-body representations in the same study subjects. We quantified the spatial gradient of cortical activation and evaluated the divergence from a continuous pattern. Gradient continuity was found to be disturbed at the primary somatosensory cortex (S1) and the supplementary motor area (SMA), in both patient populations: contralateral to the disturbed body side in the Brown-Séquard group and before repair in the surgical group, which was further improved after intervention. Results corresponding to the nondisturbed body side and after surgical repair were comparable with control subjects. No difference was found in the fMRI signal power between the different conditions in the two groups, as well as with respect to control subjects. These results suggest that decreased sensation in our patients is related to gradient discontinuity rather than signal reduction. Gradient continuity may be crucial for somatotopic and other topographical organization, and its disruption may characterize pathological processing.The somatotopic “homunculus” representation in the human cortex is one of the most important discoveries of modern neuroscience (1, 2). The early electrophysiological findings of Penfield and coworkers (1, 2) have been confirmed and extended in several neuroimaging studies in healthy individuals (3–7) and have been further explored in patients and nonhuman primates with different pathologies using both electrophysiology and neuroimaging (8–14). The latter elicited changes in the cortical pattern of activity after a damage to the somatosensory system, manifested as functional cortical reorganization. Kaas, Merzenich, and Killackey suggested that there may be “several types of cortical reorganization, including (i) the somatotopic expansion of previously existing representations of body parts, (ii) the development of ‘new’ representations, (iii) the activation of large regions of the cortex from a very limited region of a receptive field surface, and (iv) a ‘nonsomatotopic’ activation of the cortex from scattered receptive fields” (9). The first three reorganization options were established whereas non-somatotopic representation remains understudied and unclear. A potential explanation is that most previous studies focused on the reorganization of single organ representation or changes in limited cortical area without exploring large scale topographical changes. We hypothesized that large scale nonsomatotopic reorganization may be associated to the relationship between the representation of the disturbed body part and the whole-body representation. In fact, the whole-body representation may have particular importance because continuous somatotopic organization reflects not only adjacency of different body parts, but also the general principle that neural populations that are involved in similar computational tasks are located in close spatial proximity (15, 16). Nonsomatotopic discontinuous whole-body representation may thus reflect a general pathological principle.To examine the role of continuity and discontinuity in somatosensory processing, we searched for a model that would enable us to compare processing of physiological and pathological whole-body continuous signals in the same study subject. One such model is the cervical partial (sensory) Brown-Séquard syndrome. This syndrome is characterized by injury to one half of the spinal cord, which disturbs sensory signal conduction from half of the body below the lesion to the contralateral hemisphere (17, 18). Patients with Brown-Séquard syndrome experience a reduction in sensation of one side of their body (hemihypoesthesia). Cervical Brown-Séquard syndrome is unique, involving a unilateral representation of the body, but in patients without brain pathology, thus serving as an ideal model to compare physiological and pathological cortical patterns from the disturbed and nondisturbed body sides in each individual patient. Another model that may causally demonstrate the role of continuity and discontinuity in somatosensory processing is one involving patients before and after surgical repair of cervical disk protrusion. This model enables examination of continuity in the same study subjects before and after intervention. Notably, whereas studies in nonhuman primates compare response to lesion before and after induction, no such study, to our knowledge, has examined response to surgical repair. We therefore used functional MRI (fMRI) in these two patient populations to compare responses in the most prominent somatosensory homunculi—the primary somatosensory cortex (S1) and supplementary motor area (SMA) (1, 19)—with continuous sensory stimulation of the whole body.To describe a sequential change in cortical activation coinciding with continuous sensory stimulation, we use the term “gradient” (20, 21). We quantified the continuity of gradients by analyzing a one-dimensional series of functional cluster geometric centroids that represent responses to sensory input from different body parts. Signal power was measured as well because somatosensory deficit that results in hypoesthesia may be reflected in signal reduction. We hypothesize that somatotopic representation in the hemisphere contralateral to the sensory deficit exhibits functional reorganization manifested as gradient discontinuity.     

Seasonal affective disorder: spatial organization of EEG power and coherence in the depressive state and in light-induced and summer remission

The present study analyzed EEG power and coherence in subjects with seasonal affective disorder (SAD) during depressive episodes and during light-induced and summer remission. Baseline EEG activity was recorded during the winter period before light treatment (31 SAD patients, 30 control subjects); after 10 days of 2-h morning light treatment (10 SAD subjects); and during the summer period (14 SAD subjects, 27 control subjects). EEG power and coherence were calculated for the delta, theta-1, theta-2, alpha, beta-1 and beta-2 frequency bands. Compared with control subjects, SAD subjects had lower than normal EEG power in most frequency bands; asymmetrical distribution of delta, theta-1, theta-2 and alpha activity in parietal and temporal regions due to increased EEG power over the left electrode sites; and beta activity in the lateral frontal region due to increased beta power over the right electrode site. The foci of decreased EEG coherence were mainly in the right and left frontal and the right posterior regions. Remitted SAD subjects showed normalization of inter-hemispheric asymmetry in lateral frontal areas; increases of delta, theta-2, and alpha activity compared with control values; theta-1 activity in excess of control values; and disappearance of the foci of decreased coherence in anterior areas of the left hemisphere.