Storing memories of ongoing, everyday experiences requires a high degree of plasticity, but retaining these memories demands protection against changes induced by further activity and experience. Models in which memories are stored through switch-like transitions in synaptic efficacy are good at storing but bad at retaining memories if these transitions are likely, and they are poor at storage but good at retention if they are unlikely. We construct and study a model in which each synapse has a cascade of states with different levels of plasticity, connected by metaplastic transitions. This cascade model combines high levels of memory storage with long retention times and significantly outperforms alternative models. As a result, we suggest that memory storage requires synapses with multiple states exhibiting dynamics over a wide range of timescales, and we suggest experimental tests of this hypothesis.

It is well known that the density of neurons varies within the adult brain. In neocortex, this includes variations in neuronal density between different lamina as well as between different regions. Yet the concomitant variation of the microvessels is largely uncharted. Here we present automated histological, imaging, and analysis tools to simultaneously map the locations of all neuronal and non-neuronal nuclei and the centerlines and diameters of all blood vessels within thick slabs of neocortex from mice. Based on total inventory measurements of different cortical regions (~ 107 cells vectorized across brains), these methods revealed: (1) In three dimensions, the mean distance of the center of neuronal somata to the closest microvessel was 14 μm. (2) Volume samples within lamina of a given region show that the density of microvessels does not match the strong laminar variation in neuronal density. This holds for both agranular and granular cortex. (3) Volume samples in successive radii from the midline to the ventral-lateral edge, where each volume summed the number of cells and microvessels from the pia to the white matter, show a significant correlation between neuronal and microvessel densities. These data show that while neuronal and vascular densities do not track each other on the 100 μm scale of cortical lamina, they do track each other on the 1 – 10 mm scale of the cortical mantle. The absence of a disproportionate density of blood vessels in granular lamina is argued to be consistent with the initial locus of functional brain imaging signals.

Neural activity in the brain is followed by localized changes in blood flow and volume. We address the relative change in volume for arteriole vs. venous blood within primary vibrissa cortex of awake, head-fixed mice. Two-photon laser-scanning microscopy was used to measure spontaneous and sensory evoked changes in flow and volume at the level of single vessels. We find that arterioles exhibit slow (<1 Hz) spontaneous increases in their diameter, as well as pronounced dilation in response to both punctate and prolonged stimulation of the contralateral vibrissae. In contrast, venules dilate only in response to prolonged stimulation. We conclude that stimulation that occurs on the time scale of natural stimuli leads to a net increase in the reservoir of arteriole blood. Thus, a "bagpipe" model that highlights arteriole dilation should augment the current "balloon" model of venous distension in the interpretation of fMRI images.ocalized changes in the flow and volume of oxygenated blood in the brain are commonly used as a correlate of heightened neural activity. For two important imaging modalities, blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) (1) and intrinsic optical signal imaging (IOS) (2), the signals are generated by a complex interplay of the rate of oxidative metabolism, the flux of blood in the underlying angioarchitecture, and changes in vascular volume (3). The locus for the increase in vascular volume that follows sensory stimulation (i.e., arterioles or venules) is an enduring controversy that bears directly on interpreting and quantifying fMRI signals (4-6). To resolve this question, we used in vivo two-photon laserscanning microscopy to image spontaneous and sensory evoked vascular dynamics in the vibrissa area of parietal cortex of awake, head-fixed mice. All data were collected through a reinforced thin-skull window (7) (Fig. 1A); this method obviates potential complications from inflammation or changes in cranial pressure that may occur with a craniotomy (8). ResultsImages of the pial surface and measurements of the diameter of the lumen of surface arterioles and venules were performed while the mouse sat passively (Fig. 1B). Dilations greater than 5% of the baseline diameter occurred with frequencies of 0.07 ± 0.05 Hz (mean ± SD, n = 118 arteries in six mice). The diameters of short segments of arterioles (red in Fig. 1B) exhibited relatively large spontaneous increases in the spectral range between 0.1 Hz and 1 Hz ( Fig. 1 C and D). The peak amplitude of these fluctuations was 23% ± 10% of the initial vessel diameter across all arterioles, with instances of a 50% increases in diameter. Further, these lowfrequency oscillations were strongly coherent and synchronous over a distance of several hundred micrometers across cortex (198 pairs of vessels, in six mice, that were separated by 20-315 μm; slope of coherence = 0.001 μm −1 [nonsignificant (NS)] and slope of phase shift = 0.0009 rad/μm

We present a method to form an optical window in the mouse skull that spans millimeters and is stable for months without inflammation of the brain. This enabled us to repeatedly image blood flow in cortical capillaries of awake animals and determine long-range correlations in speed. We further demonstrate repeated cortical imaging of dendritic spines, microglia, and angioarchitecture, as well as illumination to drive motor output via optogenetics and induce microstrokes via photosensitizers.

Spontaneous fluctuations in hemodynamic signals in the absence of a task or overt stimulation are used to infer neural activity. We tested this coupling by simultaneously measuring neural activity and changes in cerebral blood volume (CBV) in the somatosensory cortex of awake, head-fixed mice during periods of true rest, and during whisker stimulation and volitional whisking. Here we show that neurovascular coupling was similar across states, and large spontaneous CBV changes in the absence of sensory input were driven by volitional whisker and body movements. Hemodynamic signals during periods of rest were weakly correlated with neural activity. Spontaneous fluctuations in CBV and vessel diameter persisted when local neural spiking and glutamatergic input was blocked, and during blockade of noradrenergic receptors, suggesting a non-neuronal origin for spontaneous CBV fluctuations. Spontaneous hemodynamic signals reflect a combination of behavior, local neural activity, and putatively non-neural processes.