Spectral sensitivity in hemianopic macaque monkeys.

We measured the increment threshold sensitivity to 2 degrees, 200-ms targets presented at a lateral and radial eccentricity of approximately 20-26 degrees in both visual hemifields of three macaque monkeys whose left striate cortex had been removed 5 years earlier, and in one normal control. As in patients with blindsight, sensitivity of the hemianopic field for blue, green and red stimuli was reduced by as little as 0.5 log units. With increasing light adaptation from scotopic to mesopic to photopic levels, there was a progressive increase in the sensitivity to long wavelengths relative to that for short and medium wavelengths. This shift in relative sensitivity ('Purkinje shift') shows that rod and cone mechanisms operate in both the normal and hemianopic fields and that the sensitivity that remains following removal of striate cortex is not mediated exclusively by rods.     

Topographic patterns of V2 cortical connections in macaque monkeys

Patterns of connections of dorsal and ventral portions of the second visual area (V2) were used to evaluate and extend current theories of cortical organization and processing streams in macaque monkeys. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and up to four different fluorochromes in V2 labeled neurons and terminations in V2 and in 1) caudal (DLc) and rostral (DLr) subdivisions of dorsolateral cortex between V2 and the middle temporal area (MT); 2) regions we define as dorsomedial (DM) and dorsointermediate (DI) areas; 3) MT, medial superior temporal area (MST), and fundal superior temporal area (FST); 4) the dorsal part of inferior temporal (TEO) cortex; and 5) two locations in posterior parietal cortex. The largest extrastriate connection zone was DLc, which occupied the caudal one-third to one-half of the fourth visual area (V4) region of other proposals. Based on the connection pattern, foveal vision in DLc is represented adjacent to foveal vision in V2, with the lower quadrant represented dorsally and the upper quadrant ventrally, as in V2, but within a much less extensive region of cortex. The sparser connections of DLr formed a more compressed but parallel visuotopic pattern. A third visuotopic pattern of connections was located in a moderately myelinated region of cortex just rostral to dorsomedial V2. Whereas the region would include parts of dorsal visual area 3 (V3), V3a, and possibly other areas of other proposals, we interpret the connection pattern as reflecting a dorsomedial visual area, DM, with foveal vision represented caudolaterally and other parts of the lower and upper quadrants represented more medially and rostrally. A fourth pattern of label in dorsointermediate cortex suggested the location and organization of another visual area (DI). Most of a fifth connection pattern with MT was congruent with the known visuotopic organization of MT area, but visuotopically mismatched foci of connections were observed as well. Sparser foci of label in MST suggested a rostrodorsal representation of foveal vision, with paracentral vision represented more caudally. Separate dorsal and ventral foci of label in FST were consistent with previous evidence for dorsal (FSTd) and ventral (FSTv) visual areas. Finally, connections with TEO and posterior parietal cortex were sparse. Our results suggest that much of visual cortex organization is similar in New and Old World monkeys. © 1996 Wiley-Liss, Inc.     

Nasotemporal asymmetries in V1: Ocular dominance columns of infant,adult, and strabismic macaque monkeys

To quantify asymmetries of input from the two eyes into each cerebral hemisphere, we measured ocular dominance column (ODC) widths and areas in the striate visual cortex (area V1) of macaque monkeys. Ocular dominance stripes in layer 4C were labeled by using transneuronal transport of intraocularly injected wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or cytochrome oxidase (CO) histochemistry, after deafferentation of one eye or even by leaving afferent input intact. In infant monkey aged 4 and 8 weeks, ocular dominance stripes labeled by WGA-HRP appeared adultlike with smooth, sharply defined borders. In normal infant and normal adult macaque, ocular dominance stripes driven by the nasal retina (i.e., contralateral eye) were consistently wider than stripes driven by the temporal retina (i.e., ipsilateral eye). Asymmetries in the percentage of area V1 driven by nasal vs. temporal ODCs showed a similar “nasal bias”: in infant macaque, approximately 58% of ODCs in V1 were driven by nasal retina, and in adult macaque approximately 57%. The asymmetries tended to be slightly smaller in opercular V1 and greater in calcarine V1. “Spontaneous” ocular dominance stripes were revealed by CO staining of V1 in a naturally strabismic monkey and in a monkey made strabismic by early postnatal alternating monocular occlusion. In these animals, ocular dominance stripes and CO blobs corresponding to the nasal retina stained more intensely for CO in both the right and left V1. ODC spacing and the nasotemporal asymmetry in ODC width and area were similar in strabismic and normal monkeys. Our results in normal monkeys extend the observations of previous investigators and verify that nasotemporal inputs to opercular and calcarine V1 are unequal, with a consistent bias favoring inputs from the nasal retina. The CO results in strabismic macaque suggest that the nasal ODC bias promotes interocular suppression when activity in neighboring ODCs is decorrelated by abnormal binocular experience in infancy. J. Comp. Neurol. 388:32–46, 1997. © 1997 Wiley-Liss, Inc.     

Anterograde degeneration in the superior colliculus following kainic acid and radiofrequency lesions of the macaque pulvinar

Several studies have reported behavioral deficits following thermocoagulation of the primate pulvinar. However, these deficits may have resulted from damage to corticotectal fibers as they pass through the pulvinar. To evaluate this possibility and to determine whether kainic acid can be used to destroy pulvinar cells without damaging corticotectal fibers, we compared anterograde degeneration in the superior colliculus following kainic acid and radiofrequency lesions of the pulvinar. Kainic acid injections into the pulvinar produced total loss of neuronal perikarya within the inferior and lateral pulvinar. Four to 7 days following the kainic acid lesions, terminal and fiber degeneration within the superior colliculus was no greater than that produced by control injections of saline. By contrast, thermocoagulation lesions of the inferior and lateral pulvinar produced dense fiber and terminal degeneration throughout the superficial and intermediate layers of the superior colliculus. We conclude that whereas thermocoagulation of the pulvinar severely damages the corticotectal tract, kainic acid lesions spare these fibers of passage. Thus kainic acid lesions should provide an effective tool for studying the functional significance of the pulvinar.     

Tangential distribution of cytochrome oxidase-rich blobs in the primary visual cortex of macaque monkeys

We studied the tangential distribution of cytochrome c oxidase (CytOx)-rich blobs in four striate cortices of three normal monkeys (Macaca mulatta). The spatial density and cross-sectional area of blobs were analyzed in CytOx-reacted tangential sections of flat-mounted preparations of the striate cortex (V1). Well-delimited CytOx-rich blobs were found in the middle portion of cortical layer III of the V1. Throughout the binocular field representation, the spatial density of blobs was nearly constant with a mean value of four to five blobs per mm². In the monocular portions of V1, however, blob spatial density diminished. In all cases, the mean cross-sectional area of blobs was constant in the V1. The small variation of CytOx blob topography with visual field eccentricity contrasts with the variation described in previously published material. J. Comp. Neurol. 386:217–228, 1997. © 1997 Wiley-Liss, Inc.     

Topography of the afferent connectivity of area 17 in the macaque monkey: a double-labelling study

The various structures afferent to area 17 (or V1) of the macaque monkey have widely differing retinotopic organizations. It is likely that these differences are reflected in the topographic organizations of the projections from these structures to area V1. We have investigated this issue by placing side-by-side injections of two retrograde fluorescent tracers, fast blue and diamidino yellow, in V1. By examining the extent of mixing of the two populations of singly labelled cells and the presence of doubly labelled cells, in different structures, we have characterized the topography of each projection in terms of the size of its axonal arborization and the amount of convergence and divergence. The afferents from the lateral geniculate nucleus (LGN) and from the pulvinar are organized in a point-to-point fashion. The maximum extent of axonal arborization of these afferents is 0.5 mm and these projections demonstrate little scatter (i.e., neighboring LGN neurons project to adjacent regions of V1). The other two subcortical structures examined, the claustrum and the intralaminar nuclei, demonstrate a much larger scatter and wider axonal arborizations in their projections to V1 than do the LGN and pulvinar. Two-dimensional reconstructions were made of the distribution of labelled neurons in extrastriate cortical areas. Using the separation between patches of labelled cells and transitions in myelin-stained sections, we have identified seven separate cortical regions containing labelled cells. Two of these can be identified as area V2 and the middle temporal visual area (MT). Three other regions correspond to areas V3, V3A and V4t. Finally, two more regions of labelling have been distinguished that belong to area V4. These results demonstrate that, at least within the central 6 degrees of visual field, all the presently known extrastriate visual cortical areas project to V1. This result is interesting in view of the fact that only a few extrastriate cortical areas are reported to receive afferents from V1. Three groups of cortical areas can be distinguished on the basis of the characteristics of their cortical connections to V1. The first group contains area V2, V3, and the posterior region of V4. These areas project to V1 with infra- as well as supragranular layer neurons and show limited axonal arborization and scatter in the projection. The second group consists of two regions of labelling in the superior temporal sulcus corresponding to V4t and MT and another on the annectant gyrus (V3A). These regions contain almost exclusively infragranular labelling and show wide axonal arborization and scatter in their projections to V1.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cortical connections of inferior temporal area TEO in macaque monkeys

In macaque monkeys, lesions involving the posterior portion of the inferior temporal cortex, cytoarchitectonic area TEO, produce a severe impairment in visual pattern discrimination. Recently, this area has been shown to contain a complete, though coarse, representation of the contralateral visual field (Boussaoud, Desimone, and Ungerleider: J. Comp. Neurol. 306:554–575, '91). Because the inputs and outputs of area TEO have not yet been fully described, we injected a variety of retrograde and anterograde tracers into 11 physiologically identified sites within TEO of seven rhesus monkeys and analyzed the areal and laminar distribution of its cortical connections. Our results show that TEO receives feedforward, topographically organized inputs from prestriate areas V2, V3, and V4. Additional sparser feedforward inputs arise from areas V3A, V4t, and MT. Each of these inputs is reciprocated by a feedback projection from TEO. TEO was also found to have reciprocal intermediate-type connections with the fundus of the superior temporal area (area FST), cortex in the most posteromedial portion of the superior temporal sulcus (the posterior parietal sulcal zone [area PP]), cortex in the intraparietal sulcus (including the lateral intraparietal area [area LIP]), the frontal eye field, and area TF on the parahippocampal gyrus. The connections with V3A, V4t, and PP were found only after injections in the peripheral field representations of TEO. Finally, TEO was found to project in a feedforward pattern to area TE and to areas anterior to FST on the lateral bank and floor of the superior temporal sulcus (areas TEm, TEa, and IPa, Seltzer and Pandya: Brain Res. 149:1–24, '78), all of which send feedback projections to TEO. Feedback projections also arise from parahippocampal area TH, and areas TG, 36, and possibly 35. These are complemented by only sparse feedforward projections to TG from central field representations in TEO and to TH from peripheral field representations. The results thus indicate that TEO forms an important link in the occipitotemporal pathway for object recognition, sending visual information forward from V1 and prestriate relays in V2–V4 to anterior inferior temporal area TE. © 1993 Wiley-Liss, Inc.

1 This article is a US Goveriiment work and, as such, is in the public domain in the United States of America.

     

Convergence of parvocellular and magnocellular information channels in the primary visual cortex of the macaque

We investigated whether responses of single cells in the striate cortex of anaesthetized macaque monkeys exhibit signatures of both parvocellular (P) and magnocellular (M) inputs from the dorsal lateral geniculate nucleus (dLGN). We used a palette of 128 isoluminant hues at four different saturation levels to test responses to chromatic stimuli against a white background. Spectral selectivity with these isoluminant stimuli was taken as an indication of P inputs. The presence of magnocellular inputs to a given cortical cell was deduced from its responses to a battery of tests, including assessment of achromatic contrast sensitivity, relative strengths of chromatic and luminance borders in driving the cell at different velocities and conduction velocity of their retino-geniculo-cortical afferents. At least a quarter of the cells in our cortical sample appear to receive convergent P and M inputs. We cannot however, exclude the possibility that some of these cells could be receiving a convergent input from the third parallel channel from the dLGN, namely the koniocellular (K) rather than the P channel. The neurons with convergent P and M inputs were recorded not only from supragranular and infragranular layers but also from the principal geniculate input recipient layer 4. Thus, our results challenge classical ideas of strict parallelism between different information streams at the level of the primate striate cortex.     

The regional cortical basis of achromatopsia: a study on macaque monkeys and an achromatopsic patient

Previous experiments have revealed total loss of colour vision following removal of all inferior temporal cortex, a condition akin to complete cerebral achromatopsia in humans. Whether less extensive ablation genuinely impairs colour perception without abolishing it or retards learning involving coloured stimuli is contested. We therefore tested macaque monkeys, with total removal of temporal areas TEO and TE but sparing rostral and perirhinal temporal cortex and the upper bank of the superior temporal sulcus. Compared with three monkeys with lateral parietal ablations, the monkeys with TEO/TE lesions were impaired at learning and retention of simultaneous two-choice colour discriminations and with a nine-choice oddity discrimination whether the coloured target was embedded among grey distracters of the same luminance or among isoluminant coloured distracters. However, their performance was superior to that of an achromatopsic human subject and to that previously measured in monkeys with much larger temporal lobe ablation. They were only mildly impaired at nine-choice oddity discrimination for grey stimuli where the grey target was brighter than the grey distracters. The impairment could be exacerbated or alleviated by altering the colour of the background of the displays and by static and dynamic luminance masking of the entire display in a manner that indicates that the colour deficit reflects a change in perception rather than a disorder of learning and memory. It resembles central dyschromatopsia in human subjects but falls short of achromatopsia.     

‘Real-motion’ cells in the primary visual cortex of macaque monkeys

Extracellular recordings were carried out in the primary visual cortex of behaving macaque monkeys. Neurons were activated by moving a visual stimulus across their receptive fields during periods of steady fixation and by moving their receptive fields (by visual tracking) across a motionless visual stimulus, taking care that the velocities of stimulus and eye movements were the same. The total cell population (108 neurons) ws divided into 3 groups according to the cell sensitivity to visual stimulus orientation (non-oriented cell and oriented cells) and to the presence or absence of antagonistic areas in in the receptive fields (oriented cells with antagonistic areas). All the non-oriented cells (n = 14) showed almost the same response to visual stimulation both during steady fixation and during visual tracking. Out of a total number of 86 oriented cells, 77 turned out to be activated by the visual stimulation both during fixation and tracking. Eight oriented cells gave a very weak response or no response at all to visual stimulation during smooth pursuit eye movements and one neuron of the same group showed a greater response during visual tracking than during fixation. Six out of 8 oriented cells with antagonistic areas showed almost the same response to the two types of visual stimulation, while the remaining two neurons showed very weak responses during smooth pursuit eye movements. Our results show that a small percentage (about 10%) of striate neurons in macaque monkeys gave very different responses to the same physical stimulation at retinal level, according to the presence or absence of slow eye movements (smooth pursuit eye movements). The activity of these neurons seems to be related to the real movement of something in the visual world, in spite of the retinal image movement per se.     

Distribution of muscarinic cholinergic receptor proteins m1 to m4 in area 17 of normal and monocularly deprived rhesus monkeys

Antibodies to muscarinic cholinergic receptor proteins m1 to m4 were used in striate cortex tissue of normal rhesus monkeys to determine the laminar distribution of these proteins with special attention to geniculorecipient layers. The normal patterns were compared to those of monkeys whose ocular dominance system had been altered by visual deprivation. In normal monkeys, immunoreactivity of all four proteins was localized in complex laminar patterns; m1 was densest in layers 2, 3, and 6, followed by layer 5. In contrast, m2 reactivity was densest in lower layer 4C and in 4A; the latter exhibited a honeycomb pattern. Layers 2 and 3 displayed alternating dense and light regions; this pattern was complementary to that of cytochrome oxidase (CytOx). Laminar immunoreactivity for the m3 receptor was similar to the CytOx pattern, including a honeycomb in 4A and a pattern of alternating darker and lighter patches in layers 2/3. Antibody to m4 reacted most densely with layers 1, 2, 3, and 5, layers 2 and 3 exhibited alternating dark and light regions, and layer 4A had a faint honeycomb. Layer 4C was the lightest band. The differential distribution of these four muscarinic receptor subtypes suggests distinct roles in cholinergic modulation of visual processing in the primate striate cortex. Furthermore, all four muscarinic receptors appear to be insensitive to elimination of visual input via monocular occlusion from birth, to deprivation of pattern vision in one eye during a specific time period in adulthood, and to long-term retinal injury. J. Comp. Neurol. 388:130–145, 1997. © 1997 Wiley-Liss, Inc.     

Receptive‐field properties of V1 and V2 neurons in mice and macaque monkeys

We report the results of extracellular single‐unit recording experiments where we quantitatively analyzed the receptive‐field (RF) properties of neurons in V1 and an adjacent extrastriate visual area (V2L) of anesthetized mice with emphasis on the RF center‐surround organization. We compared the results with the RF center‐surround organization of V1 and V2 neurons in macaque monkeys. If species differences in spatial scale are taken into consideration, mouse V1 and V2L neurons had remarkably fine stimulus selectivity, and the majority of response properties in V2L were not different from those in V1. The RF center‐surround organization of mouse V1 neurons was qualitatively similar to that for macaque monkeys (i.e., the RF center is surrounded by extended suppressive regions). However, unlike in monkey V2, a significant proportion of cortical neurons, largely complex cells in V2L, did not exhibit quantifiable RF surround suppression. Simple cells had smaller RF centers than complex cells, and the prevalence and strength of surround suppression were greater in simple cells than in complex cells. These findings, particularly on the RF center‐surround organization of visual cortical neurons, give new insights into the principles governing cortical circuits in the mouse visual cortex and should provide further impetus for the use of mice in studies on the genetic and molecular basis of RF development and synaptic plasticity. J. Comp. Neurol. 518:2051–2070, 2010. © 2010 Wiley‐Liss, Inc.     

Microstimulation of V1 input layers disrupts the selection and detection of visual targets by monkeys

Electrical microstimulation delivered to primary visual cortex (V1) concurrently with the presentation of visual targets interferes with the selection of these targets. To determine the source of this interference, we stimulated the visual input layers of V1 as rhesus monkeys generated saccadic eye movements to visual targets presented at and outside the receptive field of the stimulated neurons. Columns of cells in V1 innervated by the left and right eye are segregated according to eye dominance, such that cells within a column respond best to visual stimuli presented to the ocular dominant eye. Interference was maximal when targets were presented to the ocular dominant eye, moderate when presented to the ocular inferior eye, and negligible when presented to both eyes. Thus, electrical microstimulation of the visual input layers of V1 disrupts the flow of visual information along the geniculostriate pathway. Knowing how electrical stimulation of V1 affects visual behaviour is necessary when using monkeys to develop a visual prosthesis for the blind.     

Afferent properties of periarcuate neurons in macaque monkeys. II. Visual responses

The visual responses of single neurons of the periarcuate cortex have been studied in the macaque monkey. Two sets of neurons responding to visual stimuli have been found. The first set, located rostral to the arcuate sulcus, was formed by units that could be activated by stimuli presented far from the animal. These neurons had large receptive fields and were neither orientation nor direction selective. The second set, found predominantly caudal to the arcuate sulcus, was formed by units that were maximally or even exclusively activated by stimuli presented in the space immediately around the animal. These neurons were bimodal, responding also to somatosensory stimuli.According to the location of their visual responding regions the bimodal neurons were subdivided into pericutaneous (54%) and distant peripersonal neurons (46%). The former responded best to stimuli presented a few centimeters from the skin, the latter to stimuli within the animal's reaching distance. The visual responding regions were spatially related to the tactile fields.It is argued that neurons with a receptive field consisting of several responding areas, some in one sensory modality, some in another, have a praxic function and that they are involved in organizing sequences of movements.     

Attention and normalization circuits in macaque V1

Attention affects neuronal processing and improves behavioural performance. In extrastriate visual cortex these effects have been explained by normalization models, which assume that attention influences the circuit that mediates surround suppression. While normalization models have been able to explain attentional effects, their validity has rarely been tested against alternative models. Here we investigate how attention and surround/mask stimuli affect neuronal firing rates and orientation tuning in macaque V1. Surround/mask stimuli provide an estimate to what extent V1 neurons are affected by normalization, which was compared against effects of spatial top down attention. For some attention/surround effect comparisons, the strength of attentional modulation was correlated with the strength of surround modulation, suggesting that attention and surround/mask stimulation (i.e. normalization) might use a common mechanism. To explore this in detail, we fitted multiplicative and additive models of attention to our data. In one class of models, attention contributed to normalization mechanisms, whereas in a different class of models it did not. Model selection based on Akaike's and on Bayesian information criteria demonstrated that in most cells the effects of attention were best described by models where attention did not contribute to normalization mechanisms. This demonstrates that attentional influences on neuronal responses in primary visual cortex often bypass normalization mechanisms.     

Contribution of striate inputs to the visuospatial functions of parieto-preoccipital cortex in monkeys

In Experiments 1 and 2, monkeys received 3-stage operations intended to serially disconnect parieto-preoccipital from striate cortex. At each stage (unilateral parieto-preoccipital removal, contralateral striate removal and posterior callosal transection) the monkeys were tested for retention of the landmark task, a visuospatial discrimination sensitive to the effects of bilateral parieto-preoccipital damage. To check the effectiveness of the disconnection, the monkeys were also tested after removal of the remaining parieto-preoccipital cortex. The results demonstrated that corticocortical inputs from striate cortex are crucial for the visuospatial functions of parieto-preoccipital cortex, just as they had been shown earlier to be crucial for the pattern discrimination functions of inferior temporal cortex. Relative to inferior temporal cortex, however, parieto-preoccipital cortex was found to be especially dependent on ipsilateral (as compared with contralateral) striate inputs. In Experiment 3, monkeys received bilateral lesions of either lateral or medial striate cortex and were tested on both a pattern discrimination task, to assess residual inferior temporal function, and the landmark task, to assess residual parieto-preoccipital function. The results indicated that the pattern discrimination functions of inferior temporal cortex are especially dependent on inputs from lateral striate cortex, whereas the visuospatial functions of parieto-preoccipital cortex are equally dependent on inputs from lateral and medial striate cortex. The relatively greater contribution to parieto-preoccipital than to inferior temporal cortex made by ipsilateral and medial striate inputs (representing contralateral and peripheral visual fields, respectively) can also be seen in the receptive field properties of parieto-preoccipital and inferior temporal neurons. The differences in the organization of striate inputs to these two cortical association areas presumably reflect differences in the processing required for spatial vs object vision.     

Area V1 in macaque monkeys projects to multiple histochemically defined subdivisions of the inferior pulvinar complex

To investigate how visuotopic connections relate to chemoarchitecture of the inferior pulvinar (PI) complex in macaques, neuroanatomical tracers were placed into known portions of the visual representation in V1. Separate foci of label associated with both the upper and lower visual quadrants occupied neurochemically defined medial, central, lateral, and lateral-shell subdivisions, PI_M, PI_C, PI_L, and PI_L-S. Visuotopic connection patterns thus supported the concept of a larger PI that includes portions of three classically defined ‘nuclei' [C. Gutierrez, A. Yaun and C.G. Cusick, Neurochemical subdivisions of the inferior pulvinar in macaque monkeys, J. Comp. Neurol., 363 (1995) 545–562.], and corresponds to the topographically organized V1 projection zone.     

Transient subcortical connections of inferior temporal areas TE and TEO in infant macaque monkeys

Whereas in mammals postnatal neurogenesis, gliogenesis, and angiogenesis appear to be kept at low rates, in fish the capability for the production of new brain cells during adulthood is very pronounced. Many of the newly generated cells originate from germinal layers that maintain their proliferative activity during adulthood. By employing incorporation of the thymidine analogue 5-bromo-2′-deoxyuridine (BrdU) into mitotic active cells, we have quantitatively mapped such proliferation zones in the brain of adult Apteronotus leptorhynchus (Gymnotiformes, Teleostei). In the telencephalon, diencephalon, mesencephalon, and rhombencephalon, the total number of BrdU-labelled cells was low, making up approximately 25% of all mitotic active cells in the brain. Many of these cells were scattered over wide areas. Otherwise, zones of high proliferative activity were typically located at or near the surface of ventricular, paraventricular, and cisternal systems. Approximately 75% of all BrdU-labelled cells found in the brain of adult Apteronotus leptorhynchus were situated in the cerebellum. Zones displaying proliferative activity were restricted to small areas, such as narrow stripes around the midline of corpus cerebelli and valvula cerebelli, the boundary between corpus and valvula, and a large portion of the area covered by the eminentia granularis medialis. Counts indicate that, on average, 100,000 cells, corresponding to approximately 0.2% of the total population of cells in the brain of adult Apteronotus leptorhynchus, are in S-phase within a period of 2 hours. At least part of these newly generated cells is added to the population of already existing cells. This leads to a permanent growth of the brain with increasing size of the fish, a process that appears to slow down only in individuals of relatively advanced age. © 1995 Wiley-Liss, Inc.     

Sensory and premotor connections of the orbital and medial prefrontal cortex of macaque monkeys

Sensory and premotor inputs to the orbital and medial prefrontal cortex (OMPFC) were studied with retrograde axonal tracers. Restricted areas of the lateral and posterior orbital cortex had specific connections with visual-, somatosensory-, olfactory-, gustatory-, and visceral-related structures. More medial areas received few direct sensory inputs. Within the lateral and posterior orbital cortex, area 121 received a substantial projection from visual areas in the inferior temporal cortex (TE). Area 12m received somatosensory input from face, digit, or forelimb regions in the opercular part of area 1–2, in area 7b, in the second somatosensory area (SII), and in the anterior infraparietal area (AIP). Areas 13m and 131 also received a projection from the opercular part of areas 1–2 and 3b. The posteromedial and lateral agranular insular areas (Iapm and Ial, respectively) received fibers from the ventral part of the parvicellular division of the ventroposterior medial nucleus of the thalamus (VPMpc) that may represent a visceral afferent system. The dorsal part of VPMpc projected to the adjacent gustatory cortex. These restricted inputs from several sensory modalities and the convergent corticocortical connections to orbital areas 13l and 13m suggest a network related to feeding. The OMPFC was also connected to premotor cortex in ventral area 6 (areas 6va and 6vb), in cingulate area 24c, and probably in the supplementary eye field. Area 6va projected to area 12m, whereas a region of area 6vb projected to area 131. The region of the supplementary eye field projected to areas 121, 120, and 12r. Area lal received fibers from area 24c. Lighter and more diffuse projections also reached wider areas of the OMPFC. For example, injections in several orbital areas labeled a few cells scattered through the anterior part of area TE and the superior temporalrus. There was also a projection to the intermediate agranular insular area (Iai) and to areas 13a and 12o from the apparently multimodal areas in the superior temporal sulcus and gyrus. © 1995 Wiley-Liss, Inc.     

BOLD human responses to chromatic spatial features

Animal physiological and human psychophysical studies suggest that an early step in visual processing involves the detection and identification of features such as lines and edges, by neural mechanisms with even‐ and odd‐symmetric receptive fields. Functional imaging studies also demonstrate mechanisms with even‐ and odd‐receptive fields in early visual areas, in response to luminance‐modulated stimuli. In this study we measured fMRI BOLD responses to 2‐D stimuli composed of only even or only odd symmetric features, and to an amplitude‐matched random noise control, modulated in red–green equiluminant colour contrast. All these stimuli had identical power but different phase spectra, either highly congruent (even or odd symmetry stimuli) or random (noise). At equiluminance, V1 BOLD activity showed no preference between congruent‐ and random‐phase stimuli, as well as no preference between even and odd symmetric stimuli. Areas higher in the visual hierarchy, both along the dorsal pathway (caudal part of the intraparietal sulcus, dorsal LO and V3A) and the ventral pathway (V4), responded preferentially to odd symmetry over even symmetry stimuli, and to congruent over random phase stimuli. Interestingly, V1 showed an equal increase in BOLD activity at each alternation between stimuli of different symmetry, suggesting the existence of specialised mechanisms for the detection of edges and lines such as even‐ and odd‐chromatic receptive fields. Overall the results indicate a high selectivity of colour‐selective neurons to spatial phase along both the dorsal and the ventral pathways in humans.