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Neurons in primary visual cortex respond to binocular disparity, the raw material of stereoscopic depth perception. Although these neurons are probably essential to depth perception, a recent study has shown that they are unable to compute depth itself.
Learning leads to neural changes often considered to be driven by 'smart' areas of the brain. A recent study of the cellular changes that underlie perceptual learning has found that plasticity in the primary visual cortex V1 is necessary for learning and the changes that correlate with learning are more complex than one might expect.
Recent experiments clarify the role of the primary visual cortex and other cortical areas in visual awareness and blindsight.
The finding that mental imagery is associated with activity in primary visual cortex has important implications for sensory processing in the brain.
Neuronal activity is important for both the initial formation and the subsequent refinement of anatomical and physiological features of the mammalian visual system. Here we examine recent evidence concerning the role that spontaneous activity plays in axonal segregation, both of retinogeniculate afferents into eye-specific layers and of geniculocortical afferents into ocular dominance bands. We also assess the role of activity in the generation and plasticity of orientation selectivity in the primary visual cortex. Finally, we review recent challenges to textbook views on how inputs representing the two eyes interact during the critical period of visual cortical plasticity.
Human observers are exquisitely sensitive to tilt in the orientation of a line. We can detect rotations away from the vertical of 0.5 degrees. It has been suggested [1,2] that this accuracy is a result of the orientation-selectivity of simple cells in the primary visual cortex (V1), many of which have receptive fields with an elliptical shape [3]. However, it is possible to sense the tilt of many stimuli that are unlikely to have their tilt directly encoded by such cells. For example, a garment such as a tie with diagonal stripes would predominantly stimulate cells in V1 tuned to an orientation of the stripes; yet we could tell whether or not the garment as a whole was tilted from the vertical. The perception of oriented textures is subject to systematic errors, however. A striking example is the Fraser 'twisted cord' illusion (Fig. 1) in which we see the global orientation of the horizontal texture-defined lines as being tilted in the direction of its locally tilted segments. If the component segments are at a larger angle (30 degrees) to the global orientation, on the other hand, the perceived shift is in the opposite direction. We have measured these effects psychophysically, and we propose a model in which second-order orientation units receive excitation from V1 units of similar orientation, but inhibition from V1 units of dissimilar orientation. Our model correctly predicts that making the textures different in average brightness from the background will reduce the illusions.
Previous methods for analyzing optical imaging data have relied heavily on temporal averaging. However, response dynamics are rich sources of information. Here, we develop and present a method that combines principal component analysis and multitaper harmonic analysis to extract the statistically significant spatial and temporal response from optical imaging data. We apply the method to both simulated data and experimental optical imaging data from the cat primary visual cortex.
Cortical activity evoked by repeated identical sensory stimulation is extremely variable. The source of this variability is often assigned to "random ongoing background activity" which is considered to be irrelevant to the processing of the stimuli and can therefore be eliminated by ensemble averaging. In this work, we studied the single-trial variability in neuromagnetic responses elicited by circular checkerboard pattern stimuli with radii of 1.8 degrees, 3.7 degrees, and 4.5 degrees. For most of the MEG sensors over the occipital areas, the averaged signal showed a clear early (N70m) response following the stimulus onset and this response was modulated by the checkerboard size. A data-driven spatial filter was used to extract one of the many possible composite time courses of single-trial activity corresponding to the complex of N70m generators. Pattern analysis principles were then employed to analyze, classify, and handle the extracted temporal patterns. We explored whether these patterns correspond to distinct response modes, which could characterize the evoked response better than the averaged signal and over an extended range of latencies around N70m. A novel scheme for detecting and organizing the structure in single-trial recordings was utilized. This served as a basis for comparisons between runs with different checkerboard sizes and provided a causal interpretation of variability in terms of regional dynamics, including the relatively weak activation in primary visual cortex. At the level of single trial activity, the polymorphic response to a simple stimulus is generated by a coupling of polymodal areas and cooperative activity in striate and extrastriate areas. Our results suggest a state-dependent response with a wide range of characteristic time scales and indicate the ongoing activity as a marker of the responsiveness state.
OBJECTIVE: The cortical magnification factor characterizes the area of human primary visual cortex activated by a stimulus as a function of angular distance from an observer's line of sight. This study estimates human cortical magnification using an electrophysiological method with excellent temporal resolution: visual evoked potential (VEP) dipole source localization. METHODS: For each of 60 independently modulated checkerboard patches within the central 18 deg of the visual field, location, orientation, magnitude, and time-course of the dipole current source that best described the VEP distribution across a multi-electrode array was obtained. At numerous eccentricities, cortical magnification was determined using two different techniques: (1) the distance between each pair of adjacent stimulus patches was matched to the corresponding distance between adjacent cortical sources; and (2) the area of each stimulus patch was matched to the magnitude of the corresponding cortical source (which was assumed to be proportional to cortical area). RESULTS: The estimates of human cortical magnification using our electrophysiological method were similar to previous estimates from psychophysics, cortical stimulation, and functional magnetic resonance imaging. CONCLUSIONS: The concordance of results provided by these disparate technologies, with differing spatial and temporal limitations, supports their combination in studying the spatio-temporal dynamics of human brain function.
In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into area V1 in order to demonstrate the detailed morphology of individual axons terminating in prestriate area MT. On the basis of 24 axon reconstructions, several representative (but not necessarily comprehensive) characteristics have been identified: (1) Most axons arborize in a patchy manner over a widespread territory, frequently greater than 1.0 mm and often up to 1.5 x 1.8 mm (dimensions uncorrected for shrinkage). (2) Terminal arbors are distributed to layers 3, 4, and 6. Those in layer 6 need not be in register with those in the upper layers. (3) Number and size of terminal arbors are variable. One axon may have 1-4 arbors in the middle layers; typically at least one of these will have a diameter of 200-250 microns, while the others may be less developed. There are from 1-3 arbors in layer 6, usually 50 microns (but sometimes up to 100 microns) in diameter. (4) Terminal boutons are of mixed morphology, but usually beaded and large (up to 3.0 microns). (5) In the white matter, many axons travel in the external sagittal stratum but some are part of the U-fiber system. Axons commonly branch, sometimes at depths up to 0.75-1.0 mm, below the gray matter of MT. In summary, these axons are not stereotyped, but rather vary in the number and size of their terminal arbors, as well as in their branching and overall geometry. Connections from area V1 to MT have been associated with the magnocellular-dominated "processing channel." As widespread arborizations and bistratified terminations are common to both striate axons in MT and to geniculocortical axons in layer 4C alpha of primary visual cortex, these features might be correlated with magnocellular-specific processing requirements.
The distribution of ganglion cells and displaced amacrine cells was determined in whole-mounted Aotus retinae. In contrast to diurnal simians, Aotus has only a rudimentary fovea. Ganglion cell density decreases towards the periphery at approximately the same rate along all meridians, but is 1.2-1.8 times higher in the nasal periphery when compared to temporal region at the same eccentricities. The total number of ganglion cells varied from 421,500 to 508,700. Ganglion cell density peaked at 15,000/mm2 at 0.25 mm dorsal to the fovea. The displaced amacrine cells have a shallow density gradient, their peak density in the central region is about 1500-2000/mm2 and their total number varied from 315,900 to 482,800. Comparison between ganglion cell density and areal cortical magnification factor for the primary visual cortex, area 17, shows that there is not a simple proportional representation of the ganglion cell distribution. There is an overrepresentation of the central 10 deg of the visual field in the visual cortex. The present results for Aotus and the results of a similar analysis of data from other primates indicate that the overrepresentation of the central visual field is a general feature of the visual system of primates.
The organization of the primary visual cortex (VI) of the common marmoset (Callithrix jacchus) was studied both physiologically and by means of transneuronal labelling of geniculocortical afferents. We addressed the question whether monocular deprivation (MD) could stabilize segregation into ocular dominance (OD) columns, which are not seen in normal adult marmosets but are present in juvenile animals (Spatz, 1979, 1989). Properties of neurons in normal marmosets closely resembled those of other New-World and Old-World monkeys and orderly tangential progressions of preferred orientation were observed. However, in contrast to species that display well-defined OD columns, neurons of layer 4 in V1 of normal adult marmosets received balanced inputs from the two eyes. Early MD (even though followed by prolonged binocular experience into adulthood) resulted in a reduction of cell size in laminae of the lateral geniculate nucleus with input from the deprived eye and a dramatic overall shift in ocular dominance towards the nondeprived eye in the cortex. However, isolated clusters of cells dominated by the deprived eye were found in both layers 4 and 6. Injection of lectin-conjugated horseradish peroxidase (WGA-HRP) into the deprived eye revealed elongated patches of terminal label, about 350 microns wide, in flat-mounted sections through layer 4. Afferent segregation was sharper and more regular in the region of V1 representing parafoveal visual space than in that representing the fovea. Our findings support the notion that all Old-World and New-World monkeys possess the capacity for segregation of geniculocortical afferents into OD columns.
Monocular deprivation in adult macaques produces a rapid down-regulation in GABA and GABAA receptor subunit immunoreactivity in deprived-eye columns of primary visual cortex (VI) but a significantly delayed GABA reduction in deprived layers of the dorsal lateral geniculate nucleus (LGN). These findings, suggesting that normal inhibitory neurotransmission persists in LGN at a time when VI inhibitory mechanisms are greatly altered, are consistent with physiological studies that have demonstrated a greater degree of functional plasticity in VI than in LGN. Nonetheless, functional adaptation to partial loss of visual input has been detected in the LGN, indicating that synaptic plasticity takes place in this nucleus. In the present study, evidence for early changes in inhibitory neurotransmission were examined with immunocytochemical methods to determine if, in the absence of early GABA regulation, GABAA receptor subunits in macaque LGN are affected by adult deprivation. Immunoreactivity for alpha 1 and beta 2/3 subunits of the GABAA receptor was intense within the magnocellular layers and more modest in the parvocellular layers and intercalated layers. In all layers, immunoreactivity was present in the cytoplasm and along the surfaces of relatively large somata and in dense tangles of processes in the neuropil. Double-labeling experiments demonstrated that somata and processes immunoreactive for alpha 1 and beta 2/3 were surrounded by GABA terminals but no cell intensely immunoreactive for either subunit expressed immunoreactivity for GABA, itself. Following periods of monocular deprivation by tetrodotoxin (TTX) injection for 4 days or longer, layers deprived of visual activity displayed levels of alpha 1 and beta 2/3 immunoreactivity markedly lower than those displayed by the adjacent, normally active layers. Such changes were greater as the period of deprivation increased. The changes included a loss of immunostaining in and around somata and in many neuropil elements of deprived layers. These data indicate that GABA and GABAA receptor subunits alpha 1 and beta 2/3 are expressed by separate populations of neurons in macaque LGN that are differentially regulated by visual activity. The findings suggest that rapid, activity-dependent regulation of postsynaptic receptors represents one mechanism for altering synaptic strength in the adult macaque visual system.
We studied the anatomical substrates of spatial vision in a New World monkey, the marmoset Callithrix jacchus. This species has good visual acuity and a foveal specialization which is qualitatively similar to that of humans and other Old World primates. We measured the spatial density of retinal ganglion cells and photoreceptors, and calculated the relative numbers of these cell populations. We find that ganglion cells outnumber photoreceptors by between 2.4:1 and 4.2:1 in the fovea. The peak sampling density of ganglion cells is close to 550,000 cells/mm2. This value falls by almost 1000-fold between the fovea and peripheral retina; a value which approaches recent estimates of the centroperipheral ganglion cell gradient for human and macaque monkey retina and primary visual cortex. The marmoset shows a sex-linked polymorphism of color vision: all male and some female marmosets are dichromats. Six of the retinas used in the present study came from animals whose chromatic phenotype was identified in electrophysiological experiments and confirmed by polymerase chain reaction (PCR) amplification of cone opsin encoding genes. One animal was a trichromat and the others were dichromats. Antibodies against short wavelength-sensitive (SWS) cones labeled close to 8% of all cones near the fovea of one dichromat animal, consistent with electrophysiological evidence that the SWS system is present in all marmosets. The topography and spatial density of cone photoreceptors and ganglion cells was similar to that reported for macaque retina, and we found no obvious difference between dichromatic and trichromatic marmoset retinas. These results reinforce the view that the main determinate of primate foveal topography is the requirement for maximal spatial resolution.
The receptive fields of cells in the primary visual cortex (area 17 or V1) show clear orientation selectivity, unlike those of the lateral geniculate nucleus (LGN) cells which provide their visual input. The intrinsic circuitry of V1 cells is believed to be partly responsible for this selectivity. We investigated the influence of ascending projections from neurons in the lower layers (5 and 6) of V1 on the orientation selectivity of single neurons in the upper layers (2,3, and 4) by reversibly inactivating ("blocking") lower layer neural activity with iontophoretic application of gamma-aminobutyric acid (GABA) while recording from upper layer cells in the prosimian primate, Galago crassicaudatus. During lower layer blocking, the majority (20/28 = 71.4%) of upper layer neurons exhibited a change in the orientation of their preferred stimulus, a reduction in their orientation tuning, and/or an increase in their response amplitude. Twelve (42.9%) neurons exhibited shifts in their preferred orientation averaging 11 (+/- 4) deg. These neurons were located on average, 272 (+/- 120) microns tangential from the vertical axis of the pipette center. Eleven neurons (39.2%) exhibited an average reduced orientation tuning of 52.5%. Their average location was 230 +/- (115) microns away from the vertical axis of the pipette. Five (17.9%) neurons with average location 145 (+/- 75) microns from the vertical axis exhibited both effects. Two (7.1%) neurons that exhibited significant increases in response amplitude to stimulus angles within 10 deg of the peak excitatory stimulus without changes in orientation selectivity or tuning were located less than 100 microns from the vertical axis. The effects on the orientation tuning of cells were restricted in all cases to within +/- 30 deg of the preferred stimulus orientation. This suggests that layer blocking affects cells with preferred stimulus orientations similar to those of the recorded neurons. Only cells located within 500 microns tangential to the vertical axis of the injection site exhibited these effects. These results suggest that cells within layers 5 and 6 provide organized, orientation-tuned inhibition that sharpens the orientation tuning of cells in the upper cortical layers within the same, or closely neighboring, cell columns.
Although no visual inputs arise from the blind spot, the same visual attribute there as in the visual field surrounding the blind spot is perceived. Because of this remarkable "perceptual filling-in," a hole corresponding to the blind spot is not perceived, even when one eye is closed. Does the same phenomenon occur in the case of a scotoma in which visual inputs are lost postnatally due to a retinal lesion? We report that it did: in the macaque monkey, behavioral evidence for filling-in at a scotoma produced by a laser-induced monocular retinal lesion was obtained. The visual receptive fields of neurons in the primary visual cortex (V1) in and around the representation of the visual field corresponding to the scotoma were also mapped, and no clear difference between the retinotopic organization of this part in V1 and that found in the normal visual field was found. Also, perceptual filling-in was found to occur only two days after the lesion. These findings suggest that the normal visual system possesses a mechanism that yields filling-in when some part of the retina is damaged, and that such a mechanism requires no topographical reorganization in V1.
The middle temporal visual area, MT, is one of three major targets of the primary visual cortex, area 17, in primates. We assessed the contribution of area 17 connections to the responsiveness of area MT neurons to visual stimuli by first mapping the representation of the visual hemifield in MT of anesthetized owl monkeys with microelectrodes, ablating an electrophysiologically mapped part of area 17, and then immediately remapping MT. Before the lesions, neurons at recording sites throughout MT responded vigorously to moving slits of light and other visual stimuli. In addition, the relationship of receptive fields to recording sites revealed a systematic representation of the contralateral visual hemifield in MT, as reported previously for owl monkeys and other primates. The immediate effect of removing part of the retinotopic map in area 17 by gentle aspiration was to selectively deactivate the corresponding part of the visuotopic map in MT. Lesions of dorsomedial area 17 representing central and paracentral vision of the lower visual quadrant deactivated neurons in caudomedial MT formerly having receptive fields in the central and paracentral lower visual quadrant. Most neurons at recording sites throughout other parts of MT had normal levels of responsiveness to visual stimuli, and receptive-field locations that closely matched those before the lesion. However, neurons at a few sites along the margin of the deactivated zone of cortex had receptive fields that were slightly displaced from the region of vision affected by the lesion into other parts of the visual field, suggesting some degree of plasticity in the visual hemifield representation in MT.(ABSTRACT TRUNCATED AT 250 WORDS)
The lateral geniculate nucleus (LGN) of primates contains three classes of relay cells, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. At present, very little is known about either the structure or function of the K relay cells in New or Old World monkeys (simian primates). In monkeys, K cells are located between the main LGN layers and adjacent to the optic tract. For convenience, these intercalated cell layers are numbered K1-K4 starting closest to the optic tract with K1. The objective of this study was to examine the details of K axon morphology in the primary visual cortex (V1) of owl monkeys and to determine if different K layers give rise to distinct axon types. For this purpose, injections of WGA-HRP or PHA-L were made into specific K LGN layers and the distribution and morphology of the resulting labeled axons were analyzed. Injections of fluorescent tracers also were made within the superficial layers of V1 to further document connections via analysis of the patterns of retrogradely labeled cells in the LGN. Our main finding is that K axons in owl monkeys terminate as delicate focused arbors within single cytochrome oxidase (CO) blob columns in cortical layer III and within cortical layer I. Overall, the morphology of the K axons in these monkeys is quite similar to what we described previously for K geniculocortical axons in the distantly related bush baby (prosimian primate), suggesting that the basic features of this pathway are common to all primates. Our results also provide evidence that the axon arbors from different K layers are morphologically distinct; axons from LGN layer K1 project mainly to cortical layer I, while axons from LGN layer K3 chiefly terminate in cortical layer III. Taken together, these results imply that the basic features of axons within the K pathway are conserved across primates, and that the K axons from different K layers are likely to differ in function based upon their different morphologies.