Spacing of cytochrome oxidase blobs in visual cortex of normal and strabismic monkeys

Some models of visual cortical development are based on the assumption that the tangential organization of V1 is not determined prior to visual experience. In these models, correlated binocular activity is a key element in the formation of visual cortical columns, and when the degree of interocular correlation is reduced the models predict an increase in column spacing. To examine this prediction we measured the spacing of columns, as defined by cytochrome oxidase (CO) blobs, in the visual cortex of monkeys whose binocular vision was either normal or disrupted by a strabismus. The spatial distribution of blobs was examined in seven normal and five strabismic macaques. Tangential sections through the upper layers of the visual cortex were stained to reveal the two-dimensional (2D) pattern of CO blobs. Each blob was localized and their center-to-center spacing, packing arrangement and density were calculated using 2D nearest-neighbor spatial analyses. The mean center-to-center spacing of blobs (590 microm for normally reared and 598 microm for strabismic macaques) and the mean density of blobs (3.67 blobs/mm2 for normally reared and 3.45 blobs/mm2 for strabismic macaques) were not significantly different. In addition, the 2D packing arrangement of the blobs was not affected by strabismus. While it is clear that neural activity plays a key role in the elaboration and refinement of ocular dominance cortical modules, we conclude that it does not determine the spatial period of the pattern of CO blobs. This suggests that aspects of the neural circuitry underlying the columnar architecture of the visual cortex are established prenatally and its fundamental periodicity is not modifiable by experience.      

Neural activity in primate parietal area 7a related to spatial analysis of visual mazes

Cognitive psychological studies of humans and monkeys solving visual mazes have provided evidence that a covert analysis of the maze takes place during periods of eye fixation interspersed between saccades, or when mazes are solved without eye movements. We investigated the neural basis of this process in posterior parietal cortex by recording the activity of single neurons in area 7a during maze solution. Monkeys were required to determine from a single point of fixation whether a critical path through the maze reached an exit or a blind ending. We found that during this process the activity of approximately one in four neurons in area 7a was spatially tuned to maze path direction. We obtained evidence that path tuning did not reflect a covert saccade plan insofar as the majority of neurons active during maze solution were not active on a delayed-saccade control task, and the minority that were active on both tasks did not exhibit congruent spatial tuning in the two conditions. We also obtained evidence that path tuning during maze solution was not due to the locations of visual receptive fields mapped outside the behavioral context of maze solution, in that receptive field centers and preferred path directions were not spatially aligned. Finally, neurons tuned to path direction were not present in area 7a when a na?ve animal viewed the same visual maze stimuli but did not solve them. These data support the hypothesis that path tuning in parietal cortex is not due to the lower level visual features of the maze stimulus, but rather is associated with maze solution, and as such, reflects a cognitive process applied to a complex visual stimulus.     

Sensory regulation of immediate-early genes c-fos and zif268 in monkey visual cortex at birth and throughout the critical period

The postnatal development of ocular dominance columns (ODCs) in monkey visual cortex provides an exquisite model for studying mechanisms of experience-guided neuronal plasticity. While the presence of columns at birth in Old World monkeys is now well established, it remains unclear whether cortical neurons at this early stage are capable of modulating gene expression in response to changing sensory conditions. Using a set of monocular deprivation and stimulation protocols, we examined activity-driven expression of the immediate-early genes (IEGs) c-fos and zif268 during the critical period of development. We observed well-delineated patterns of ODCs produced by sensory regulation of both IEGs throughout the critical period, starting as early as the first postnatal day. The expression levels are similar in layers II/II, IVC and VI throughout development, with no selective decline in the thalamorecepient layer (layer IVC) of adult monkeys. A narrow strip of non-columnar c-Fos expression was observed at the border of layers IVC and V. Our results show that neurons in monkey visual cortex are equipped at birth with the molecular machinery for coupling sensory inputs to active genomic responses and that this responsivity extends throughout the critical period. The findings are discussed within the context of a possible role for IEGs in sensory-driven cortical plasticity during development.     

Relation between Patterns of Intrinsic Lateral Connectivity, Ocular Dominance, and Cytochrome Oxidase-Reactive Regions in Macaque Monkey Striate Cortex

To help understand the role of long-range, clustered lateralconnections in the superficial layers of macaque striate cortex(area V1), we have examined the relationship of the patternsof intrinsic connections to cytochrome oxidase (CO) blobs, interblobs,and ocular dominance (OD) bands, using biocytin based neuroanatomicaltracing, CO histochemistry, and optical imaging. Microinjectionsof biocytin in layer 3 resulted in an asymmetric field (averageanisotropy of 1.8; maximum spread—3.7 mm) of labeled axonterminal clusters in layers 1–3, with the longer axisof the label spread oriented orthogonal to the rows of blobsand imaged OD stripes, parallel to the V1/V2 border. These labeledterminal patches (n = 186) from either blob or interblob injections(n = 20) revealed a 71% (132 out of 186) commitment of patchesto the same compartment as the injection site; 11% (20 out of186) to the opposite compartment, and 18% (34 out of 186) toborders of blob-interblob compartments, indicating that theconnectivity pattern is not strictly blob to blob, or interblobto interblob (p < 0.005; $$$²) In injections placed withinsingle OD domains (n = 11), 54% of the resulting labeled terminalpatches (43 out of 79) fell into the same OD territories asthe injection sites, 28% (22 out of 79) into the opposite ODregions, and 18% (14 out of 79) on borders, showing some connectionalbias toward same-eye compartments (p < 0.02; ANOVA). Individualinjection cases, however, varied in the degree (50–100%for CO patterns, 22–100% for 0D patterns) to which theyshowed same-compartment connectivity. These results reveal thatwhile connectivity between similar compartments predominates(e.g., blob to blob, right eye column to right eye column),interactions do occur between functionally different regions.     

Laminar and Columnar Organization of Immunoreactivity for Calcineurin, a Calcium- and Calmodulin-regulated Protein Phosphatase, in Monkey Striate Cortex

Protein phosphorylation and dephosphorylation play an importantrole in neuronal signal transduction. In this study the distributionof calcineurin, a calcium/calmodulin-dependent protein phosphatase,was investigated in the striate cortex of two Old World monkeys,Macaca fascicularis and Papio anubis, using a well-characterized,affinity-purified polyclonal antibody to calcineurin. In orderto relate the calcineurin distributions to established cytochemicalmarkers, adjacent sections were processed for the visualizationof cytochrome oxidase. The staining patterns obtained from thetwo species were remarkably similar. The results indicate that(1) monkey striate cortex exhibits strong calcineurinlike immunoreactivitythat is present both in the neuropil and in neurons, most ofwhich have characteristics of pyramidal cells; (2) the distributionof calcineurin is laminar specific; and (3) it is complementaryto that of cytochrome oxidase activity with respect to bothits laminar and its tangential pattern. In sections perpendicularto the cortical lamination calcineurin immunoreactivity is highin layers II and III, reduced in layer IVA, nearly as denseas in supragranular layers in layer IVB, minimal in layer IVC,and again enhanced, but not as much as in supragranular layers,in layers V and VI. In addition to these lamina-specific variations,the density of calcineurin-like immunoreactivity exhibits aperiodic modulation along trajectories parallel to the pialsurface that is most marked in layer III but also discernablein infragranular layers. Accordingly, in tangential sectionsthrough supragranular layers the calcineurin distribution ismosaic-like with patches of high density corresponding to cytochromepoorregions (interblob regions) and zones of low density correspondingto areas of high cytochrome oxidase activity (blobs).     

Parvalbumin expression in visual cortical interneurons depends on neuronal activity and TrkB ligands during an Early period of postnatal development

The differentiation of cortical interneurons is controlled by environmental factors. Here, we describe the role of activity and neurotrophins in regulating parvalbumin (PARV) expression using organotypic cultures (OTC) of rat visual cortex as model system. In OTC, PARV expression was dramatically delayed. The organotypic proportion of approximately 6% PARV neurons was not established before 50-70 DIV, whereas in vivo all neurons are present until P20. Thalamic afferents increased cortical PARV mRNA in OTC, but not to the age-matched in vivo level. During the first 10 DIV, BDNF and NT-4 accelerated PARV mRNA expression in a Trk receptor and MEK2 dependent manner. The BDNF action required PI3 kinase signalling. PARV expression required activity. The proportion of neurons which managed to up-regulate PARV was inversely related to the duration of early transient periods of activity deprivation. Long-term activity-deprived OTC completely failed to up-regulate PARV mRNA. Both TrkB ligands failed to promote PARV expression in activity-deprived OTC. However, a few basket and chandelier neurons were observed, suggesting that the development of class-specific morphological features is activity-independent. Once established, PARV expression became resistant to late-onset activity deprivation. In conclusion, PARV expression depended on activity and TrkB ligands which appear to prime the PARV expression already before its developmental onset.     

The connections of layer 4 subdivisions in the primary visual cortex (V1) of the owl monkey

The primary visual cortex (V1) of primates receives signals from parallel lateral geniculate nucleus (LGN) channels. These signals are utilized by the laminar and compartmental [i.e. cytochrome oxidase (CO) blob and interblob] circuitry of V1 to synthesize new output pathways appropriate for the next steps of analysis. Within this framework, this study had two objectives: (i) to analyze the con- nections between primary input and output layers and compartments of V1; and (ii) to determine differences in connection patterns that might be related to species differences in physiological properties in an effort to link specific pathways to visual functions. In this study we examined the intrinsic interlaminar connections of V1 in the owl monkey, a nocturnal New World monkey, with a special emphasis on the projections from layer 4 to layer 3. Interlaminar connections were labeled via small iontophoretic or pressure injections of tracers [horseradish peroxidase, biocytin, biotinylated dextrine amine (BDA) or cholera toxin subunit B conjugated to colloidal gold particles]. Our most significant finding was that layer 4 (4C of Brodmann) can be divided into three tiers based upon projections to the superficial layers. Specifically, we find that 4alpha (4Calpha), 4beta (4Cbeta) and 4ctr send primary projections to layers 3C (4B), 3Bbeta (4A) and 3Balpha (3B), respectively. Examination of laminar structure with Nissl staining supports a tripartite organization of layer 4. The cortical output layer above layer 3Balpha (3B) (e.g. layer 3A) does not appear to receive any direct connections from layer 4 but receives heavy input from layers 3Balpha (3B) and 3C (4B). Some connectional differences also were observed between the subdivisions of layer 3 and the infragranular layers. No consistent differences in connections were observed that distinguished CO blobs from interblobs or that could be correlated with differences in visual lifestyle (nocturnal versus diurnal) when compared with connectional data in other primates. Re-examination of data from previous studies in squirrel and macaque monkeys suggests that the tripartite organization of layer 4 and the unique projection pattern of layer 4ctr are not restricted to owl monkeys, but are common to a number of primate species.     

Development of cytochrome oxidase blobs in visual cortex of normal and visually deprived cats

Cytochrome oxidase (CO) blobs are central to our understanding of the columnar organization and parallel pathways in primate and cat visual cortex. In primates, development of blobs and their relationship with other columnar features of the visual cortex begins pre-natally, before visual experience. In kittens, the supragranular layers differentiate post-natally, after eye opening, raising the possibility that visual experience may influence the development of blobs in cat V1. We have examined the development of blobs in unfolded and flattened sections through the visual cortex of normally reared, dark-reared, monocularly deprived and binocularly deprived kittens. Blobs were found in superficial layers of V1 of normally reared kittens as early as 2 weeks of age, although at this age the overall CO staining in V1 was lighter than in V2. By 6 weeks of age the blobs were adult-like. A patchy pattern of CO staining was also found in V2 of young kittens but not in adults. Visual experience was not necessary for expression of the blobs and monocularly deprived kittens had well developed blobs, indicating that strong Y cell drive is not necessary for the development of blobs in cat V1. CO blobs appear in kitten V1 very early in post-natal development and their expression is independent of visual experience, suggesting that they may be an intrinsic feature of V1 organization.     

Hierarchical development of the primate visual cortex, as revealed by neurofilament immunoreactivity: early maturation of the middle temporal area (MT)

It has been suggested that the development of the cerebral cortex reflects its hierarchical organization, with the primary sensory areas being the first to reach structural and functional maturity, and higher-order association areas being the last. In the present study, we labelled the cortex of New World marmoset monkeys of late fetal and early postnatal ages with an antibody to non-phosphorylated neurofilament, a marker of structural maturation of a subset of pyramidal cells. Supporting the concept of hierarchical maturation, we found that at birth labelled cells were found in the primary visual, auditory and somatosensory areas, but not in most other cortical fields. The exception was visual area MT, which revealed an infragranular pattern of labelling comparable to the one observed in the primary areas, as well as some supragranular staining. In MT, an adult-like pattern of labelled cells, including both supragranular and infragranular layer neurons, emerged within the first postnatal month. In comparison, the development of other extrastriate areas was delayed, with the first signs of neurofilament staining not present until the third week. The present results support the concept of MT as another primary visual area, an idea previously advanced on the basis of functional and anatomical evidence.     

Estrogen replacement increases spinophilin-immunoreactive spine number in the prefrontal cortex of female rhesus monkeys

While studies have shown that estrogen affects hippocampal spine density and function, behavioral studies in humans and nonhuman primates have also implicated the prefrontal cortex in the effects of estrogen on cognition. However, the potential for similar estrogen-induced increases in spines and synapses in the prefrontal cortex has not been investigated in primates. Moreover, it is not known if such an estrogen effect would be manifested throughout the neocortex or primarily in the regions involved in cognition. Therefore, we investigated the effects of estrogen on dendritic spines in the prefrontal and primary visual cortices of young rhesus monkeys. Young female monkeys were ovariectomized and administered either estradiol cypionate or vehicle by intramuscular injection. Using an antibody against the spine-associated protein, spinophilin, spine numbers were estimated in layer I of area 46 and in layer I of the opercular portion of area V1 (V1o). Spine numbers in layer I of area 46 were significantly increased (55%) in the ovariectomy + estrogen group compared to the ovariectomy + vehicle group, yet spine numbers in layer I of area V1o were equivalent across the two groups. The present results suggest that estrogen's effects on synaptic organization influence select neocortical layers and regions in a primate model, and provide a morphological basis for enhanced prefrontal cortical functions following estrogen replacement.     

The morphology of the koniocellular axon pathway in the macaque monkey

The key objective of this study was to determine the distribution and morphology of koniocellular (K) lateral geniculate nucleus (LGN) axons in primary visual cortex (V1) of the macaque monkey. In particular, we were interested in understanding whether subpopulations of K axons exist in this species and, if so, if these subpopulations arise from different K layers of the LGN. Restricted injections of the tracers, biotinilated dextran amine, or Phaseolus vulgaris leucoagglutinin were targeted to specific LGN K layers under electrophysiological guidance and immunocytochemistry was used to visualize labeled axons in cortex that were subsequently reconstructed through serial sections. A total of 36 complete axons and 166 axon segments were reconstructed. Our results identified at least 2 main subpopulations of K axons in macaque V1 based on branching patterns and bouton distribution. Axons that arise primarily from LGN layers K1 and K2 are morphologically simple and tend to branch in cortical layers 1 and 3A. These axons give rise to fewer boutons than seen in axons arising from the dorsal K LGN layers K3-K6. Axons that arise from LGN layers K3-K6 terminate as complex, focused arbors in the cytochrome oxidase (CO) blobs in layer 3Balpha, with only occasional simple projections to the more superficial layers of cortex. Combined with previous observations, our data suggest that there are at least 3 subclasses of K LGN axons in macaque monkey that are similar to K axons identified earlier in both nocturnal simian owl monkeys (Ding and Casagrande 1997) and in prosimian, bush babies (Lachica and Casagrande 1992) suggesting that the LGN K channels that terminate in the CO blobs and in layer 1 are not unique to macaque monkeys but are a common primate feature.     

Orientation and color columns in monkey visual cortex

The literature on orientation and color columns in monkey visual cortex is reviewed. The orientation column model most consistent with existing data is one containing 'stripes' of alternating positive and negative orientation 'singularities' (cytochrome oxidase blobs) which run along the centers of ocular dominance (OD) columns, with horizontal and vertical orientations alternating at interblob centers. Evidence is summarized suggesting that color is mapped continuously across the monkey's primary visual cortex, with the ends of the spectrum located at 'red' and 'blue' cytochrome oxidase blobs and extra-spectral purple located between adjacent red and blue blobs in the same OD column. In the orientation column model, the 'linear zones' of Obermayer and Blasdel have the appearance of the lines on a pumpkin. A pinwheel model of color columns, consistent with existing data, includes spectral and extra-spectral colors as spokes. Spectral iso-color lines run across iso-orientation lines in linear zones, while extra-spectral iso-color lines occupy the 'saddle points' of Obermayer and Blasdel. The color column model accounts for closure of the perceptual color circle, as proposed by Isaac Newton in 1704, but does not account for color opponency.     

Loss of neurofilament labeling in the primary visual cortex of monocularly deprived monkeys

Visual experience during early life is important for the development of neural organizations that support visual function. Closing one eye (monocular deprivation) during this sensitive period can cause a reorganization of neural connections within the visual system that leaves the deprived eye functionally disconnected. We have assessed the pattern of neurofilament labeling in monocularly deprived macaque monkeys to examine the possibility that a cytoskeleton change contributes to deprivation-induced reorganization of neural connections within the primary visual cortex (V-1). Monocular deprivation for three months starting around the time of birth caused a significant loss of neurofilament labeling within deprived-eye ocular dominance columns. Three months of monocular deprivation initiated in adulthood did not produce a loss of neurofilament labeling. The evidence that neurofilament loss was found only when deprivation occurred during the sensitive period supports the notion that the loss permits restructuring of deprived-eye neural connections within the visual system. These results provide evidence that, in addition to reorganization of LGN inputs, the intrinsic circuitry of V-1 neurons is altered when monocular deprivation occurs early in development.     

Regulation of Calcium-Binding Protein Immunoreactivity in GABA Neurons of Macaque Primary Visual Cortex

Monocular deprivation produces an imbalance in visual drivefrom the two eyes, which in adult macaque V1 leads to markedchanges in the neurochemistry of GABA interneurons. Such changeswere further examined by studying immunoreactivity for calbindin,calretinin, and parvalbumin, three calcium-binding proteinsthat mark distinct subpopulations of GABA neurons, in macaquesthat had been monocularly deprived by intravitreal injectionof tetrodotoxin. Deprivation for 5 d or longer produced a reversalin the normal pattern of calbindin immunostaining in layer III,from one in which intense neuronal immunostaining surroundedthe cytochrome oxidase-rich puffs to one in which it occupiedthe puffs. Over the same period, calbindin immunostaining inother layers was reduced across the entire width of deprived-eyecolumns or extended into flanking regions of normal-eye columns.In contrast, reduction in parvalbumin immunostaining occurredonly in deprived-eye columns and included only terminals withshort periods of deprivation (up to 17 d) but both terminalsand somata with longer periods. No change in calretinin immunoreactivitywas observed. These findings demonstrate that GABA neurons ofmacaque V1 vary in their response to monocular deprivation accordingto their neurochemistry and position, suggesting that the weightof inputs from the two eyes and the intrinsic characteristicsof each GABA population determine how a neuron responds to achange in visual input.     

Cross-modal circuitry between auditory and somatosensory areas of the cat anterior ectosylvian sulcal cortex: a 'new' inhibitory form of multisensory convergence

Examples of convergence of visual and auditory, or visual and somatosensory, inputs onto individual neurons abound throughout the brain, but substantially fewer incidences of auditory-somatosensory neurons have been reported. The present experiments sought to examine auditory-somatosensory convergence to assess whether there is a feature of this type of convergence that might obscure it from conventional methods of multisensory detection. Auditory-somatosensory convergence was explored in cat anterior ectosylvian sulcus (AES) cortex, where higher-order somatosensory area IV (SIV) and auditory field of the anterior ectosylvian sulcus (FAES) share a common border. While neuroanatomical tracers documented a projection from FAES to SIV, physiological studies failed to reveal the bimodal neurons expected from such cross-modal connectivity. Stimulation of FAES through indwelling electrodes also failed to excite any of the SIV neurons examined. However, when stimulation of auditory FAES was combined with somatosensory stimulation, a large majority (66%) of SIV neurons showed a significant response attenuation. FAES-induced response suppression was specific to SIV, could not be elicited by activating other auditory regions and was blocked by the microiontophoretic application of the GABAergic antagonist bicuculline methiodide. Based on these data, a novel, cross-modal circuit is proposed involving projections from auditory FAES to somatosensory SIV, where local inhibitory interneurons 'reverse the sign' of the cross-modal signals to produce auditory-somatosensory suppression. This form of excitatory-inhibitory multisensory convergence has not been reported before and suggests that the level of interaction between auditory and somatosensory modalities has been substantially underestimated.     

The projection from V1 to extrastriate area 21a: a second patchy efferent pathway colocalizes with the CO blob columns in cat visual cortex

The different patchy organizations of neurons projecting from primary visual cortex (area 17) to the various extrastriate areas may contribute to functional differences in the output to each of these areas. The pattern of neurons projecting to extrastriate area 21a was examined using large injections of retrograde tracers and compared to the pattern shown by neurons projecting to the lateral suprasylvian area (LS). Patches of neurons projecting to 21a showed a bimodal laminar distribution, with numerous labeled cells in the upper and lower third of layer 3 bracketing a sparsely labeled central third; LS-projecting neurons were confined to the lower and middle thirds of layer 3. The 21a projecting cells were relatively tighter in their clustering pattern than the LS projecting cells, i. e. the difference in labeling density between patch and interpatch zones was greater for 21a-projecting cells than for LS-projecting cells. As previously shown for the LS-projecting cells, patches of 21a-projecting cells colocalized with CO blob columns in area 17. Combined with our earlier results, this study shows that the CO blob compartments in area 17 give rise to at least two distinct efferent pathways, one projecting to LS and the other to 21a, and furthermore suggest that each pathway may carry unique information to its extrastriate target.     

Loss of visually driven synaptic responses in layer 4 regular-spiking neurons of rat visual cortex in absence of competing inputs

Monocular deprivation (MD) during development shifts the ocular preference of primary visual cortex (V1) neurons by depressing closed-eye responses and potentiating open-eye responses. As these 2 processes are temporally and mechanistically distinct, we tested whether loss of responsiveness occurs also in absence of competing inputs. We thus compared the effects of long-term MD in layer 4 regular-spiking pyramidal neurons (L4Ns) of binocular and monocular V1 (bV1 and mV1) with whole-cell recordings. In bV1, input depression was larger than potentiation, and the ocular dominance shift was larger for spike outputs. MD-but not retinal inactivation with tetrodotoxin-caused a comparable loss of synaptic and spike responsiveness in mV1, which is innervated only by the deprived eye. Conversely, brief MD depressed synaptic responses only in bV1. MD-driven depression in mV1 was accompanied by a proportional reduction of visual thalamic inputs, as assessed upon pharmacological silencing of intracortical transmission. Finally, sub- and suprathreshold responsiveness was similarly degraded in L4Ns of bV1 upon complete deprivation of patterned vision through a binocular deprivation period of comparable length. Thus, loss of synaptic inputs from the deprived eye occurs also in absence of competition in the main thalamorecipient lamina, albeit at a slower pace.     

Functional organization of color domains in V1 and V2 of macaque monkey revealed by optical imaging

Areas V1 and V2 of Macaque monkey visual cortex are characterized by unique cytochrome-oxidase (CO)-staining patterns. Initial electrophysiological studies associated CO blobs in V1 with processing of surface properties such as color and brightness and the interblobs with contour information processing. However, many subsequent studies showed controversial results, some supporting this proposal and others failing to find significant functional differences between blobs and interblobs. In this study, we have used optical imaging to map color-selective responses in V1 and V2. In V1, we find striking "blob-like" patterns of color response. Fine alignment of optical maps and CO-stained tissue revealed that color domains in V1 strongly associate with CO blobs. We also find color domains in V1 align along centers of ocular dominance columns. Furthermore, color blobs in V1 have low orientation selectivity and do not overlap with centers of orientation domains. In V2, color domains coincide with thin stripes; orientation-selective domains coincide with thick and pale stripes. We conclude that color and orientation-selective responses are preferentially located in distinct CO compartments in V1 and V2. We propose that the term "blob" encompasses both the concept of "CO blob" and "color domain" in V1.     

Search for color 'center(s)' in macaque visual cortex

It is often stated that color is selectively processed in cortical area V4, in both macaques and humans. However most recent data suggests that color is instead processed in region(s) antero-ventral to V4. Here we tested these two hypotheses in macaque visual cortex, where 'V4' was originally defined, and first described as color selective. Activity produced by equiluminant color-varying (versus luminance-varying) gratings was measured using double-label deoxyglucose in awake fixating macaques, in multiple areas of flattened visual cortex. Much of cortex was activated near-equally by both color- and luminance-varying stimuli. In remaining cortical regions, discrete color-biased columns were found in many cortical visual areas, whereas luminance-biased activity was found in only a few specific regions (V1 layer 4B and area MT). Consistent with a recent hypothesis, V4 was not uniquely specialized for color processing, but areas located antero-ventral to V4 (in/near TEO and anterior TE) showed more color-biased activity.     

Synaptic efficacy during repetitive activation of excitatory inputs in primate dorsolateral prefrontal cortex

Neurons in the monkey dorsolateral prefrontal cortex (DLPFC) fire persistently during the delay period of working memory tasks. To determine how repetitive firing affects the efficacy of synaptic inputs to DLPFC layer 3 neurons, we examined the effects of repetitive presynaptic stimulation on the amplitude and temporal summation of EPSPs. Recordings were obtained in monkey DLPFC brain slices from regular spiking (RS) pyramidal cells and two types of interneurons, fast spiking (FS) and adapting non-pyramidal (ANP) cells. Repetitive stimulation of presynaptic axons in layer 3 caused EPSP depression in RS and FS neurons, but EPSP facilitation in ANP cells. A shorter EPSP duration produced weaker temporal summation in FS neurons compared to the other cell classes. Thus, due to the combined effects of dynamic changes in EPSP amplitude and differences in temporal summation, the effect of a presynaptic spike train differed according to the postsynaptic cell class. Similar results were obtained when recording unitary EPSPs evoked in connected pairs of presynaptic RS pyramidal cells and postsynaptic RS, FS or ANP neurons. In addition, similar differences in the efficacy of sustained inputs among cell classes were observed when delay-related firing was reproduced in vitro by stimulating inputs with the timing of spike trains recorded from the DLPFC of monkeys performing a delayed-response task. We suggest that the transition from baseline firing rates to higher frequency delay-related firing may lead to the differential activation of distinct cell populations, with corresponding significant effects on the patterns of activity in local prefrontal circuits.