Neural and hemodynamic responses elicited by forelimb- and photo-stimulation in channelrhodopsin-2 mice: insights into the hemodynamic point spread function.

Hemodynamic responses are commonly used to map brain activity; however, their spatial limits have remained unclear because of the lack of a well-defined and malleable spatial stimulus. To examine the properties of neural activity and hemodynamic responses, multiunit activity, local field potential, cerebral blood volume (CBV)-sensitive optical imaging, and laser Doppler flowmetry were measured from the somatosensory cortex of transgenic mice expressing Channelrhodopsin-2 in cortex Layer 5 pyramidal neurons. The magnitude and extent of neural and hemodynamic responses were modulated using different photo-stimulation parameters and compared with those induced by somatosensory stimulation. Photo-stimulation-evoked spiking activity across cortical layers was similar to forelimb stimulation, although their activity originated in different layers. Hemodynamic responses induced by forelimb- and photo-stimulation were similar in magnitude and shape, although the former were slightly larger in amplitude and wider in extent. Altogether, the neurovascular relationship differed between these 2 stimulation pathways, but photo-stimulation-evoked changes in neural and hemodynamic activities were linearly correlated. Hemodynamic point spread functions were estimated from the photo-stimulation data and its full-width at half-maximum ranged between 103 and 175 µm. Therefore, submillimeter functional structures separated by a few hundred micrometers may be resolved using hemodynamic methods, such as optical imaging and functional magnetic resonance imaging.

Coordinated optimization of visual cortical maps (I) symmetry-based analysis.

In the primary visual cortex of primates and carnivores, functional architecture can be characterized by maps of various stimulus features such as orientation preference (OP), ocular dominance (OD), and spatial frequency. It is a long-standing question in theoretical neuroscience whether the observed maps should be interpreted as optima of a specific energy functional that summarizes the design principles of cortical functional architecture. A rigorous evaluation of this optimization hypothesis is particularly demanded by recent evidence that the functional architecture of orientation columns precisely follows species invariant quantitative laws. Because it would be desirable to infer the form of such an optimization principle from the biological data, the optimization approach to explain cortical functional architecture raises the following questions: i) What are the genuine ground states of candidate energy functionals and how can they be calculated with precision and rigor? ii) How do differences in candidate optimization principles impact on the predicted map structure and conversely what can be learned about a hypothetical underlying optimization principle from observations on map structure? iii) Is there a way to analyze the coordinated organization of cortical maps predicted by optimization principles in general? To answer these questions we developed a general dynamical systems approach to the combined optimization of visual cortical maps of OP and another scalar feature such as OD or spatial frequency preference. From basic symmetry assumptions we obtain a comprehensive phenomenological classification of possible inter-map coupling energies and examine representative examples. We show that each individual coupling energy leads to a different class of OP solutions with different correlations among the maps such that inferences about the optimization principle from map layout appear viable. We systematically assess whether quantitative laws resembling experimental observations can result from the coordinated optimization of orientation columns with other feature maps.

Coordinated optimization of visual cortical maps (II) numerical studies.

In the juvenile brain, the synaptic architecture of the visual cortex remains in a state of flux for months after the natural onset of vision and the initial emergence of feature selectivity in visual cortical neurons. It is an attractive hypothesis that visual cortical architecture is shaped during this extended period of juvenile plasticity by the coordinated optimization of multiple visual cortical maps such as orientation preference (OP), ocular dominance (OD), spatial frequency, or direction preference. In part (I) of this study we introduced a class of analytically tractable coordinated optimization models and solved representative examples, in which a spatially complex organization of the OP map is induced by interactions between the maps. We found that these solutions near symmetry breaking threshold predict a highly ordered map layout. Here we examine the time course of the convergence towards attractor states and optima of these models. In particular, we determine the timescales on which map optimization takes place and how these timescales can be compared to those of visual cortical development and plasticity. We also assess whether our models exhibit biologically more realistic, spatially irregular solutions at a finite distance from threshold, when the spatial periodicities of the two maps are detuned and when considering more than 2 feature dimensions. We show that, although maps typically undergo substantial rearrangement, no other solutions than pinwheel crystals and stripes dominate in the emerging layouts. Pinwheel crystallization takes place on a rather short timescale and can also occur for detuned wavelengths of different maps. Our numerical results thus support the view that neither minimal energy states nor intermediate transient states of our coordinated optimization models successfully explain the architecture of the visual cortex. We discuss several alternative scenarios that may improve the agreement between model solutions and biological observations.

Saccade reward signals in posterior cingulate cortex.

Movement selection depends on the outcome of prior behavior. Posterior cingulate cortex (CGp) is strongly connected with both limbic and oculomotor circuitry, and CGp neurons respond following saccades, suggesting a role in signaling the motivational outcome of gaze shifts. To test this hypothesis, single CGp neurons were studied in monkeys while they shifted gaze to visual targets for liquid rewards that varied in size or were delivered probabilistically. CGp neurons responded following saccades as well as following reward delivery, and these responses were correlated with reward size. CGp neurons also responded following the omission of predicted rewards. The timing of CGp activation and its modulation by reward could provide signals useful for updating representations of expected saccade value.

Number and topography of cones, rods and optic nerve axons in New and Old World primates.

To better understand the evolution of spatial and color vision, the number and spatial distributions of cones, rods, and optic nerve axon numbers were assessed in seven New World primates (Cebus apella, Saimiri ustius, Saguinus midas niger, Alouatta caraya, Aotus azarae, Calllithrix jacchus, and Callicebus moloch). The spatial distribution and number of rods and cones was determined from counts of retinal whole mounts. Optic axon number was determined from optic nerve sections by electron microscopy. These data were amassed with existing data on retinal cell number and distribution in Old World primates, and the scaling of relative densities and numbers with respect to retinal area, eye and brain sizes, and foveal specializations were evaluated. Regular scaling of all cell types was observed, with the exceptionally large, rod-enriched retina of the nocturnal owl monkey Aotus azarae, and the unusually high cone density of the fovea of the trichromatic howler monkey Alouatta caraya presenting interesting variations on this basic plan. Over all species, the lawful scaling of rods, cones, and retinal ganglion cell number is hypothesized to result from a conserved sequence of cell generation that defends retinal acuity and sensitivity over a large range of eye sizes.

Spatial coding of position and orientation in primary visual cortex.

We examined the spatial distribution of population activity in primary visual cortex (V1) of tree shrews with optical imaging and electrophysiology. A line stimulus, thinner than the average V1 receptive field, evoked a broad strip of neural activity of nearly constant size for all stimulus locations tested within the central 10 degrees of visual space. Stimuli in adjacent positions activated highly overlapping populations of neurons; nevertheless, small changes in stimulus position produced orderly changes in the location of the peak of the population response. Statistically significant shifts in the population response were found for stimulus displacements an order of magnitude smaller than receptive field width, down to the limit of optical imaging resolution. Based on the pattern of population activity, we conclude that the map of visual space in V1 is orderly at a fine scale and has uniform coverage of position and orientation without local relationships in the mapping of these features.

Ocular dominance development revisited.

New approaches to the study of ocular dominance development, a model system for the development of neural architecture, indicate that eye-specific columns in primary visual cortex emerge substantially before the onset of the critical period, during which neural connections can be altered by visual experience. The timing, speed and specificity of column emergence implicate molecular patterning mechanisms, along with patterns of neural activity, in the generation of this columnar architecture.

High-resolution in vivo imaging of hippocampal dendrites and spines.

Structural changes in hippocampal dendrites and dendritic spines are thought to be a consequence of a wide range of experience- and activity-dependent manipulations. We explored the dynamics of hippocampal dendritic spines in vivo by developing a surgical preparation of the adult mouse brain that enabled two-photon imaging of fluorescently labeled CA1 pyramidal neurons. Dendritic trees and spines were repeatedly visualized over many hours in exquisite detail. We tested spine stability under both control conditions and during prolonged epileptic seizures. Remarkably, spines remained structurally stable after 30 min of experimental induction of epileptic seizures. Spines began to disappear only several hours after induction of epileptic activity. We thus demonstrate that this technique provides a methodology for direct in vivo optical studies of the intact mammalian hippocampus.

Molecular organization of the ferret visual thalamus.

The visual system encodes and deciphers information using parallel, anatomically segregated, processing streams. To reveal patterns of gene expression in the visual thalamus correlated with physiological processing streams, we designed a custom ferret cDNA microarray. By isolating specific subregions and layers of the thalamus, we identified a set of transcription factors, including Zic2, Islet1, and Six3, the unique distribution profiles of which differentiated the lateral geniculate nucleus (LGN) from the associated perigeniculate nucleus. Within the LGN, odd homeobox1 differentiated the A layers, which contain X cells and Y cells, from the C layers. One neuron-specific protein, Purkinje cell protein 4 (PCP4), was strongly expressed in Y cells in the ferret LGN and in the magnocellular layers of the primate LGN. In the ferret LGN, PCP4 expression began as early as postnatal day 7 (P7), suggesting that Y cells are already specified by P7. These results reveal a rich molecular repertoire that correlates with functional divisions of the LGN.

A novel algorithm for optimal image thresholding of biological data.

With the proliferation of both in vivo and in vitro microscopy techniques in the neurosciences, increased attention has been placed on the development of image analysis techniques. As experiments can produce large numbers of high bit depth images, automated processing methods have become necessary for handling these data sets. Thresholding, whereby a high bit depth image is converted into a binary image in order to identify a feature of interest, is one such standard automated technique; but the method of selecting an appropriate threshold value is far from standard. We present a novel algorithm, maximum correlation thresholding (MCT), that thresholds images accurately and efficiently without relying on any assumptions of the statistics of the image. As MCT produces thresholded images that preserve the most salient elements in the image, the algorithm performs as well as a trained user on a range of neurobiological data and in a variety of noisy conditions or when preprocessing steps preceded the thresholding operation. Our method will thus allow neuroscientists to automate image thresholding using a robust, computationally efficient algorithm, ultimately aiding in accurate image quantification and analysis.

End-on imaging: a new perspective on dorsoventral development in Drosophila embryos.

Drosophila ventral furrow formation has frequently been used as a model to study developmentally-regulated cell-shape changes. However, a technique to follow all cellular changes during this process within a single living embryo has been lacking. We describe a novel technique, called "end-on imaging", to collect time-lapse images of transversely mounted living embryos. End-on imaging revealed several new features of dorsoventral development. First, we observed a wave of syncytial nuclear divisions predicting the location of the ventral furrow. Second, we determined that there is a 5-min gap between the end of cellularization and the start of ventral furrow formation, suggesting that the two processes may share the same pool of cytoskeletal components. Lastly, we show that apical-membrane flattening, the first step in ventral furrow formation, is due to the ventral cells pushing against the vitelline membrane, rather than flattening the dome-shaped, apical surfaces of these cells by a pulling or constriction motion.

Development of cortical circuits: lessons from ocular dominance columns.

The development of ocular dominance columns has served as a Rosetta stone for understanding the mechanisms that guide the construction of cortical circuits. Traditionally, the emergence of ocular dominance columns was thought to be closely tied to the critical period, during which columnar architecture is highly susceptible to alterations in visual input. However, recent findings in cats, monkeys and ferrets indicate that columns develop far earlier, more rapidly and with considerably greater precision than was previously suspected. These observations indicate that the initial establishment of cortical functional architecture, and its subsequent plasticity during the critical period, are distinct developmental phases that might reflect distinct mechanisms.

Visual and saccade-related activity in macaque posterior cingulate cortex.

Previous neurophysiological studies have reported that neurons in posterior cingulate cortex (PCC) respond after eye movements, and that these responses may vary with ambient illumination. In monkeys, PCC neurons also respond after the illumination of large visual patterns but not after the illumination of small visual targets on either reflexive saccade tasks or peripheral attention tasks. These observations suggest that neuronal activity in PCC is modulated by behavioral context, which varies with the timing and spatial distribution of visual and oculomotor events. To test this hypothesis, we measured the spatial and temporal response properties of single PCC neurons in monkeys performing saccades in which target location and movement timing varied unpredictably. Specifically, an unsignaled delay between target onset and movement onset permitted us to temporally dissociate changes in PCC activity associated with either event. Response fields constructed from these data demonstrated that many PCC neurons were activated after the illumination of small contralateral visual targets, as well as after the onset of contraversive saccades guided by those targets. In addition, the PCC population maintained selectivity for small contralateral targets during delays of up to 600 ms. Overall, PCC activation was highly variable trial to trial and selective for a broad range of directions and amplitudes. Planar functions described response fields nearly as well as broadly tuned 2-dimensional Gaussian functions. Additionally, the overall responsiveness of PCC neurons decreased during delays when both a fixation stimulus and a saccade target were visible, suggesting a modulation by divided attention. Finally, the strength of the neuronal response after target onset was correlated with saccade accuracy on delayed-saccade trials. Thus PCC neurons may signal salient visual and oculomotor events, consistent with a role in visual orienting and attention.