Visual perception: Spotlight on the primary visual cortex

AbstractVisual search tasks appear to involve spatially selective attention to the target, but evidence for attentional modulation in the visual area with the most precise retinotopic organization – V1 – has been elusive. Recent imaging studies show that spatial attention can indeed enhance visual responses in human V1

Visual Cortex: Two-Photon Excitement

Current in vivo methods for imaging the visual cortex lack the ability to map response properties at the level of single cells. A new technique using two-photon imaging of calcium signals has now overcome this limitation

Visual cortex maps are optimized for uniform coverage

articles Mammalian primary visual cortex contains a single continuous representation of retinotopic visual space, on which orderly, periodic maps of several different visual stimulus properties are superimposed. These properties include ocular dominance and preferred orientation [1][2] These maps are often structurally related. For example, in macaque monkey These findings are consistent with earlier suggestions 4 that visual cortex maps develop according to a combination of continuity and completeness constraints, which act in opposition. The continuity constraint specifies that neighboring cortical locations should have similar receptive field properties, whereas the completeness constraint ensures that all combinations of the parameters represented in individual maps are distributed uniformly over visual space. A quantitative measure of completeness, known as coverage uniformity, or c′, has been devised 16 . It is calculated in the following way: for a given combination of features (for example, some unique combination of orientation, spatial frequency, eye and retinal location), the total neural response, A, in the cortex is calculated, taking into account the spatial structure of the maps of these properties, and the receptive field tuning widths of individual cortical units. Uniform coverage means that A should be independent of the specific feature combination chosen. It is convenient to define c′ as the standard deviation of A divided by its mean, taken over some representative set of stimuli. This makes it a dimensionless measure of 'noise' in the cortical representation of a particular feature space. If c′ = 0, coverage is completely uniform; larger values correspond to an increasingly noisy representation: for example, if c′ = 1, the standard deviation of the signal across the feature space is equal to the mean. The hypothesis that cortical maps are organized so as to optimize (that is, minimize) c′ was tested here by systematically perturbing the spatial relationships between maps of orientation, ocular dominance and spatial frequency obtained simultaneously in area 17 of the cat 13 to see whether c′ is at a local minimum. Two different methods were used to do this, both of which left continuity in the individual maps unchanged. In the first, the spatial relationships were altered by various combinations of flips (mirror inversions) about either the horizontal or vertical axes and/or 180°rotation (equivalent to a mirror inversion about one axis followed by a mirror inversion about the other). For three rectangular maps, there are a total of 16 transformations that disturb the point-to-point relationships between the maps in a unique way. (Note that some combinations of flips are equivalent: for example, flipping two of the maps about the vertical axis is equivalent to flipping just the third one.) The second method examined the possibility that map structure is close to a local optimum for coverage uniformity. To test this, a single map was displaced sideways by a given number of pixels relative to the other two, which remained fixed relative to each other. Coverage was then calculated for the region common to all three maps. This was done separately for each of the ocular dominance, spatial frequency and orientation maps, for a range of Cat visual cortex contains a topographic map of visual space, plus superimposed, spatially periodic maps of ocular dominance, spatial frequency and orientation. It is hypothesized that the layout of these maps is determined by two constraints: continuity or smooth mapping of stimulus properties across the cortical surface, and coverage uniformity or uniform representation of combinations of map features over visual space. Here we use a quantitative measure of coverage uniformity (c') to test the hypothesis that cortical maps are optimized for coverage. When we perturbed the spatial relationships between ocular dominance, spatial frequency and orientation maps obtained in single regions of cortex, we found that cortical maps are at a local minimum for c'. This suggests that coverage optimization is an important organizing principle governing cortical map development

The variable link between PNA and NAO in observations and in multi-century CGCM simulations

The link between the Pacific/North American pattern (PNA) and the North Atlantic Oscillation (NAO) is investigated in reanalysis data (NCEP, ERA40) and multi-century CGCM runs for present day climate using three versions of the ECHAM model. PNA and NAO patterns and indices are determined via rotated principal component analysis on monthly mean 500 hPa geopotential height fields using the varimax criteria. On average, the multi-century CGCM simulations show a significant anti-correlation between PNA and NAO. Further, multi-decadal periods with significantly enhanced (high anti-correlation, active phase) or weakened (low correlations, inactive phase) coupling are found in all CGCMs. In the simulated active phases, the storm track activity near Newfoundland has a stronger link with the PNA variability than during the inactive phases. On average, the reanalysis datasets show no significant anti-correlation between PNA and NAO indices, but during the sub-period 1973–1994 a significant anti-correlation is detected, suggesting that the present climate could correspond to an inactive period as detected in the CGCMs. An analysis of possible physical mechanisms suggests that the link between the patterns is established by the baroclinic waves forming the North Atlantic storm track. The geopotential height anomalies associated with negative PNA phases induce an increased advection of warm and moist air from the Gulf of Mexico and cold air from Canada. Both types of advection contribute to increase baroclinicity over eastern North America and also to increase the low level latent heat content of the warm air masses. Thus, growth conditions for eddies at the entrance of the North Atlantic storm track are enhanced. Considering the average temporal development during winter for the CGCM, results show an enhanced Newfoundland storm track maximum in the early winter for negative PNA, followed by a downstream enhancement of the Atlantic storm track in the subsequent months. In active (passive) phases, this seasonal development is enhanced (suppressed). As the storm track over the central and eastern Atlantic is closely related to the NAO variability, this development can be explained by the shift of the NAO index to more positive values

Pairing-Induced Changes of Orientation Maps in Cat Visual Cortex

AbstractWe have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps