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

Embedding of Cortical Representations by the Superficial Patch System

Pyramidal cells in layers 2 and 3 of the neocortex of many species collectively form a clustered system of lateral axonal projections (the superficial patch system—Lund JS, Angelucci A, Bressloff PC. 2003. Anatomical substrates for functional columns in macaque monkey primary visual cortex. Cereb Cortex. 13:15-24. or daisy architecture—Douglas RJ, Martin KAC. 2004. Neuronal circuits of the neocortex. Annu Rev Neurosci. 27:419-451.), but the function performed by this general feature of the cortical architecture remains obscure. By comparing the spatial configuration of labeled patches with the configuration of responses to drifting grating stimuli, we found the spatial organizations both of the patch system and of the cortical response to be highly conserved between cat and monkey primary visual cortex. More importantly, the configuration of the superficial patch system is directly reflected in the arrangement of function across monkey primary visual cortex. Our results indicate a close relationship between the structure of the superficial patch system and cortical responses encoding a single value across the surface of visual cortex (self-consistent states). This relationship is consistent with the spontaneous emergence of orientation response-like activity patterns during ongoing cortical activity (Kenet T, Bibitchkov D, Tsodyks M, Grinvald A, Arieli A. 2003. Spontaneously emerging cortical representations of visual attributes. Nature. 425:954-956.). We conclude that the superficial patch system is the physical encoding of self-consistent cortical states, and that a set of concurrently labeled patches participate in a network of mutually consistent representations of cortical inpu

A Dynamic Neural Field Model of Mesoscopic Cortical Activity Captured with Voltage-Sensitive Dye Imaging

A neural field model is presented that captures the essential non-linear characteristics of activity dynamics across several millimeters of visual cortex in response to local flashed and moving stimuli. We account for physiological data obtained by voltage-sensitive dye (VSD) imaging which reports mesoscopic population activity at high spatio-temporal resolution. Stimulation included a single flashed square, a single flashed bar, the line-motion paradigm – for which psychophysical studies showed that flashing a square briefly before a bar produces sensation of illusory motion within the bar – and moving squares controls. We consider a two-layer neural field (NF) model describing an excitatory and an inhibitory layer of neurons as a coupled system of non-linear integro-differential equations. Under the assumption that the aggregated activity of both layers is reflected by VSD imaging, our phenomenological model quantitatively accounts for the observed spatio-temporal activity patterns. Moreover, the model generalizes to novel similar stimuli as it matches activity evoked by moving squares of different speeds. Our results indicate that feedback from higher brain areas is not required to produce motion patterns in the case of the illusory line-motion paradigm. Physiological interpretation of the model suggests that a considerable fraction of the VSD signal may be due to inhibitory activity, supporting the notion that balanced intra-layer cortical interactions between inhibitory and excitatory populations play a major role in shaping dynamic stimulus representations in the early visual cortex

Spatio- temporal dynamics of odor representations in the mammalian olfactory bulb

We explored the spatio-temporal dynamics of odor-evoked activity in the rat and mouse main olfactory bulb (MOB) using voltage-sensitive dye imaging (VSDI) with a new probe. The high temporal resolution of VSDI revealed odor-specific sequences of glomerular activation. Increasing odor concentrations reduced response latencies, increased response amplitudes, and recruited new glomerular units. However, the sequence of glomerular activation was maintained. Furthermore, we found distributed MOB activity locked to the nasal respiration cycle. The spatial distribution of its amplitude and phase was heterogeneous and changed by sensory input in an odor-specific manner. Our data show that in the mammalian olfactory bulb, odor identity and concentration are represented by spatio-temporal patterns, rather than spatial patterns alone