Attention has been shown to modulate the visual evoked potential (VEP) recorded to reversing achromatic patterns. However, the chromatic onset VEP appears to be robust to attentional shifts. Functional magnetic resonance imaging (fMRI) responses to both chromatic and achromatic reversing patterns are also affected by attention. Resolution and comparison of these results is problematic due to differences in presentation mode, stimulus parameters, and the source of the response. Here, we report the results of experiments using comparable perceptual contrasts, pattern reversals, and a co-extensive and highly demanding multiple object tracking (MOT) task while exploring the effects of attentional modulation across both the chromatic (L − M) and (S − (L + M)) and the achromatic visual pathways. Our findings indicate that although achromatic VEPs are modulated by attention, chromatic VEPs are more robust to attentional modulation, even when using comparable stimulus presentation modes and in the presence of a highly demanding distractor task. In addition, we found that the majority of the modulation appears to be from a relative decrease in response due to the distractor task rather than a relative increase in response during heightened attention to the stimulus.

Recent studies on neural pathways in a broad spectrum of vertebrates suggest that, in addition to migration and an increase in the number of certain select neurons, a significant aspect of neural evolution is a “parcellation” (segregation-isolation) process that involves the loss of selected connections by the new aggregates. A similar process occurs during ontogenetic development. These findings suggest that in many neuronal systems axons do not invade unknown territories during evolutionary or ontogenetic development but follow in their ancestors' paths to their ancestral targets; if the connection is later lost, it reflects the specialization of the circuitry.

The pattern of interspecific variability suggests (1) that overlap of circuits is a more common feature in primitive (generalized) than in specialized brain organizations and (2) that most projections, such as the retinal, thalamotelencephalic, corticotectal, and tectal efferent ones, were bilateral in the primitive condition. Specialization of these systems in some vertebrate groups has involved the selective loss of connections, resulting in greater isolation of functions. The parcellation process may also play an important role in cell diversification.

The parcellation process as described here is thought to be one of several underlying mechanisms of evolutionary and ontogenetic differentiation.

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 μm2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.

Modulation by two temporal frequencies differentiates visual processing at the fundamentals (1Fs), second harmonics (2Fs), and second-order intermodulation components (IMCs), the latter created neurally as the sum or difference of the two modulation frequencies. Steady-state VEPs were recorded while stereo-normal adults viewed luminance or grating stimuli modulated by up to three temporal frequencies under dichoptic, monocular, or ordinary (binocular) viewing conditions arranged using liquid crystal light shutters. In Experiment 1, modulation of luminance by a single temporal frequency produced strong 1F and 2F VEP components, but modulation of gratings produced only 2Fs. Modulation by two temporal frequencies resulted in IMCs, often in the absence of evoked activity in the EEC at the 1Fs. IMCs were generally larger during pattern as compared to luminance modulation. Amplitudes of 1Fs and IMCs were smaller, but 2Fs were larger, during dichoptic as compared to ordinary viewing. Although the 2F to a single modulation presented to one eye was not reduced when a second frequency was added to the opposite eye, monocular IMCs were diminished when a frequency was added to the opposite eye. We conclude that 2Fs and IMCs are associated with different neural substrates. Results are consistent with a two pathway model with one pathway having a nonlinear filter prior to binocular combination, the other pathway having a nonlinearity following binocular linear summation. Implications of these data for binocular function are discussed.

The distribution of somatostatin (SS)-containing neurons was investigated by immunocytochemical methods in the central visual system of adult, developing, and retina-ablated pigeons. In normal adult brains, SS-positive cells and processes were present in the optic tectum, the nucleus of the basal optic root, the visual Wulst, and the ectostriatum. During development, progressive increase or decrease in the numerical density and the total number of SS-containing neurons occurred as determined by quantitative analysis. Changes in SS immunoreactivity also occurred as a consequence of unilateral and bilateral retina removal immediately after hatching, i.e. before retinofugal connections have been established. In spite of the segregation of visual inputs due to the almost completely crossed retinal projections, unilateral and bilateral deafferentation differentially affected SS-containing visual regions. In addition, different effects were observed on the relative packing density of labeled cells as compared to their total number. A possible role of retinal axons in regulating the distribution of SS immunoreactivity was suggested by its altered expression induced by retinal deafferentation. In addition, parallels with the distribution of SS immunoreactivity in the pigeon’s visual system were used to suggest possible equivalence between cell populations in the avian and the mammalian brains.

Antibody labeling of the calcium-binding protein calbindin 28kD (CaBP) and gamma-aminobutyric acid (GABA) is altered by short-term monocular deprivation in the lateral geniculate nucleus and visual cortex of adult primates. It is not known whether these alterations occur in other subcortical visual structures. We therefore have examined antibody labeling to CaBP and GABA in the superior colliculus (SC) of visually deprived Rhesus monkeys. One group was monocularly enucleated as adults. The other monkeys experienced different types of monocular and binocular deprivation from birth, including occlusion of one eye, and/or surgically induced aphakia, optically corrected with extended-wear contact lenses, or an intraocular lens implant. Some of these monkeys also had one eye enucleated prior to perfusion.

In the SC of normal monkeys, CaBP-immunoreactive neurons formed three laminar tiers within SC, one within the zonal layer (ZL) and upper superficial gray layer (SGL), another bridging the optic and intermediate gray layers, and a third within the deep gray layer. CaBP neurons within the upper tier had small pyriform or stellate morphologies while those in the deeper tiers were slightly larger neurons, most with a stellate morphology. GABA-immunoreactive neurons were densely distributed within the SGL and more sparsely distributed within the deeper layers. These cells were mostly small neurons with horizontal, pyriform, or stellate morphologies.

Neither monocular enucleation nor occlusion nor aphakia combined with continuous occlusion of the fellow eye produced any visible reduction in antibody labeling in cells or neuropil within the SC. Full-field measures of labeling intensity (optical density) within the ZL and upper SGL revealed no consistent differences between the SC contralateral or ipsilateral to the affected eye in either CaBP- or GABA-labeled sections. Measures of the optical density, number, and size of labeled neurons also showed no consistent effects of enucleation and/or occlusion. We therefore conclude that the retino-geniculostriate and retino-collicular systems differ in their response to deprivation which is likely due to the significant overlap of retinal axons from the two eyes that occurs in the SC of the Rhesus monkey.

Visual cortical areas are richly but selectively connected by “patchy” projections. We characterized these connections physiologically with cross-correlograms (CCHs), calculated for neuron pairs or small groups located one each in visual areas 17 and 18 of the cat. The CCHs were then compared to the visuotopic and orientation match of the neurons' receptive fields (RFs).

For both spontaneous and visually driven activity, most non-flat correlograms were centered; i.e. the most likely temporal relationship between spikes in the two areas is a synchronous one. Although spikes are most likely to occur simultaneously, area 17 spikes may occur before area 18 or vice versa, giving the cross-correlogram peak a finite width (temporal dispersion). Cross-correlograms fell into one of three groups according to their full-width at half peak height: 1–8 ms (modal width, 3 ms), 15–65 ms (modal width 30 ms), or 100–1000 ms (modal width 400 ms). These classificatory groups are nonoverlapping; the three types of coupling appeared singly and in combination.

Neurons whose receptive fields (RFs) are nonoverlapping or cross-oriented may yet be coupled, but the coupling is more likely to be the broadest type of coupling than the medium-dispersed type. The sharpest type of coupling is found exclusively between neurons with at least partially overlapping RFs and mostly between neurons whose stimulus orientation preferences matched to within 22.5 deg. The maximum spatial dispersion observed in the RFs of coupled neurons compares well with the maximum divergence seen anatomically in the A18/A17 projection system.

We suggest three different mechanisms to produce each of the three different degrees of observed spatial and temporal coherence. All mechanisms use common input of cortical origin. For medium and broad coupling, this common input arises from cell assemblies split between both sides of the 17/18 projection system, but acting synchronously. Such distributed common-input cell assemblies are a means of overcoming sparse connectivity and achieving synaptic transmission in the pyramidal network.

The retinal projections of the thirteen-lined ground squirrel were determined by tracing anterograde transport of intravitreally injected horseradish peroxidase (HRP) or wheat-germ conjugated horseradish peroxidase (WGA-HRP). Label was seen in the suprachiasmatic nucleus and adjacent anterior hypothalamic area, the accessory optic system (the medial, dorsal, and lateral terminal nuclei), the dorsal and ventral lateral geniculate nuclei, the intergeniculate leaflet, the pretectal nuclei (the anterior, posterior, and olivary pretectal nuclei and the nucleus of optic tract), and the superior colliculus. Most of these structures were labeled bilaterally, with dense contralateral label and sparse ipsilateral label, a pattern typical for animals with laterally placed eyes. However, the suprachiasmatic nucleus and the nucleus of the optic tract received input only from the contralateral eye. In contrast to previous degeneration studies, the sensitive HRP tracers (in conjunction with cytochrome-oxidase reactivity) revealed an elaborate organization within the lateral geniculate nucleus (dorsal LGN, ventral LGN, and intergeniculate leaflet) that is consistent with existing organizational schemes for other mammalian species.

The calcium-binding proteins calbindin (CaBP) and parvalbumin (PV) are important in regulating intracellular calcium in brain cells. PV immunoreactivity is reduced by enucleation in the lateral geniculate nucleus (LGN) and by enucleation and visual deprivation in the striate cortex of adult monkeys. The effects of enucleation and visual deprivation on CaBP immunoreactivity in the LGN are not known. We therefore have studied cells and neuropil in the LGN that are labeled by antibodies to CaBP in normal and visually deprived Rhesus monkeys to determine if there is an effect on this calcium-binding protein. One group of monkeys had one eye removed 2 weeks to 4.3 years before sacrifice. A second group had one eye occluded with opaque lenses from infancy without enucleation. A final group had one eye occluded long-term followed by short-term enucleation 2 weeks before sacrifice.

In normal monkeys, CaBP-immunoreactive neurons were found throughout the LGN. They were sparsely distributed within the six main laminae, and more densely distributed within layer S and the interlaminar zones (ILZ). The labeled ILZ neurons had a distinct morphology, with fusiform somata and elaborate dendritic trees that were confined primarily to the ILZ. Most CaBP-labeled neurons in the main layers had dendrites that radiated in all directions from the soma. ILZ and main layer cells labeled by CaBP thus probably represent two different cell types.

Monocular enucleation with or without occlusion produced a significant reduction in antibody labeling in the deafferented laminae. Field measures revealed an average 11.5% reduction in optical density in each deafferented lamina compared to its adjacent, nondeprived layer. The differences in field optical density between deprived and nondeprived layers were statistically significant. CaBP neurons were still visible, but the optical density of antibody labeling in these cells also was reduced. Occlusion without enucleation had no effect. Thus, deafferentation, but not light deprivation, reduces concentrations of CaBP in monkey LGN. This effect is different than that seen in striate cortex of adult monkeys, where visual deprivation as well as enucleation alters CaBP immunoreactivity.

Disturbances of visual perception frequently accompany neurodegenerative disorders but have been little studied in Huntington's disease (HD) gene carriers. We used psychophysical tests to assess visual perception among individuals in the prediagnostic and early stages of HD. The sample comprised four groups, which included 201 nongene carriers (NG), 32 prediagnostic gene carriers with minimal neurological abnormalities (PD1); 20 prediagnostic gene carriers with moderate neurological abnormalities (PD2), and 36 gene carriers with diagnosed HD. Contrast sensitivity for stationary and moving sinusoidal gratings, and tests of form and motion discrimination, were used to probe different visual pathways. Patients with HD showed impaired contrast sensitivity for moving gratings. For one of the three contrast sensitivity tests, the prediagnostic gene carriers with greater neurological abnormality (PD2) also had impaired performance as compared with NG. These findings suggest that early stage HD disrupts visual functions associated with the magnocellular pathway. However, these changes are only observed in individuals diagnosed with HD or who are in the more symptomatic stages of prediagnostic HD. (JINS, 2008, 14, 446–453.)

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5° diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.

Previous research in adults has demonstrated the utility of the visual evoked potential (VEP) to measure the integrity of the chromatic and achromatic visual pathways. The VEP has also been shown to be a valuable indicator of maturation of these pathways in infants up to 1 year of age. The present manuscript reports changes in the visual pathways from 2 years to adulthood as measured by the spatio-chromatic VEP. The responses to achromatic reversal stimuli designed to preferentially activate the low spatial-frequency achromatic (luminance) pathways appear adult-like by 1 year of age. The responses to low spatial-frequency isoluminant onset stimuli designed to preferentially activate the chromatic pathway do not appear as they do in the adult until after 12–13 years of age. The shapes of the chromatic VEP waveforms shift from a positive–negative complex to a negative–positive complex. These changes can be modeled by a decrease in the latency of a large negative component between the ages of 1 year and adulthood. The results suggest that for low spatial-frequency stimuli, there are long-term changes in the development of the chromatic pathways that are not observed in the low spatial-frequency achromatic pathways. The changes in the chromatic VEP waveforms with age may be a physiological correlate of reported behavioral changes.

The Cavalieri method and the optical fractionator were employed to estimate the volume and neuron numbers, respectively, of the dorsal lateral geniculate nucleus (dLGN) in seven adult male pigtail monkeys (Macaca nemestrina). Unbiased estimates were selectively obtained for the parvocellular (P), magnocellular (M), and interlaminar plus superficial (I + S) layers of the nucleus. The dLGN had a mean volume of 56.5 mm3, and contained on average 1.79 million neurons. The P layers contributed 64% of the volume and 83% of the neurons in the dLGN; the corresponding proportions for the other layers were 13% and 9% (M), and 23% and 8% (I + S). Interindividual variability was large for neuron counts, which varied within a two-fold range, and lower for volume estimates. Since no published data are available for the pigtail dLGN, the present results are compared with quantitative studies of the dLGN in other macaque species, placing special emphasis in the discussion of the methodologies used.

Monocular deprivation early in postnatal development leads to anatomical and physiological changes in the lateral geniculate nucleus (LGN) and visual cortex. Many of these changes are dependent upon activation of the NMDA receptor. We have examined the role of visual experience in modifying NMDAR1 subunit expression in the LGN of animals reared with various forms of visual deprivation. Following monocular deprivation initiated either at eye opening or at the peak of the critical period, there were approximately 20% fewer NMDAR1-immunopositive neurons in the deprived laminae of the LGN. The loss of NMDAR1-immunopositive neurons was found throughout both the binocular and monocular segments of the LGN and after monocular deprivation until just 3 weeks of age. These results indicate that the loss of NMDAR1 in the LGN following monocular deprivation does not simply reflect changes in the visual cortex. The loss of NMDAR1 expression was not necessarily permanent. Initiation of binocular vision at the peak of the critical period ameliorated the effect of monocular deprivation and the introduction of a period of reverse occlusion led to a complete reversal. Taken together, the results show that the expression of the NMDAR1 subunit in the LGN can be modified by the pattern of visual experience during postnatal development.

This article describes the anatomy of the visual pathways and how they should be assessed under anaesthesia. The differential diagnosis of asymmetrical pupils is illustrated with clinical examples and a strategy as to how they should be examined.

Antibodies directed against the calcium-binding proteins, parvalbumin and calbindin, can be used to label distinct neuronal subgroups in the primate visual pathway. We analyzed parvalbumin immunoreactivity (P-IR) and calbindin immunoreactivity (C-IR) in the lateral geniculate nucleus (LGN) and visual cortex of the marmoset, Callithrix jacchus. We compared marmosets which were identified as having dichromatic or trichromatic color vision. Within the LGN, the density of P-IR neurones is highest in the parvocellular and magnocellular laminae, but C-IR neurones are found mainly in the koniocellular division of the LGN, that is, the interlaminar zones and S laminae. Not all interlaminar zone cells are C-IR. In the visual cortex, P-IR neurones are present in all laminae except lamina 1, in areas V1 and V2. Neurones which are strongly C-IR are mainly located in laminae 2 and 3 in V1 and V2. Lightly C-IR neurones are concentrated in lamina 4, and are more numerous in V1 than in V2. Quantitative analysis showed no differences in the density or distribution of IR neurones in either LGN or visual cortex when dichromat and trichromat animals were compared. We conclude that this functional difference is not associated with differences in the neurochemistry of calcium-binding proteins in the primary visual pathways.