[Hospital care for acute stroke at the Palestrina hospital: comparison of cost and quality of care with an "hospital at home" care model]. / Analisi dei ricoveri ospedalieri per patologia cerebrovascolare nell'Ospedale di Palestrina (RM): confronto dei costi e della qualità assistenziale con un modello di Ospedalizzazione Domiciliare.

INTRODUCTION: In the Lazio Region (Central Italy) the Emergency Care network has been recently re-arranged in order to improve the effectiveness of care. The aim of this paper is to assess the impact of stroke patient care on both cost and organization of the hospital of Palestrina (Lazio Region). METHODOLOGY: An analysis of cost and path of care of all stroke patients (239) with stroke attending the hospital in 2010 has been carried out. RESULTS: The care was more expensive than the one offered to the stroke patients attending the "Hospital at Home" service managed by the San Giovanni Battista Hospital in Turin; moreover the care was incomplete because of the lack of rehabilitation services in the hospital catchment area. CONCLUSION: The care model offered to stroke patient hit by stroke in Palestrina needs to be deeply revised.

Lightness constancy in primary visual cortex.

When the illumination of a visual scene changes, the quantity of light reflected from objects is altered. Despite this, the perceived lightness of the objects generally remains constant. This perceptual lightness constancy is thought to be important behaviorally for object recognition. Here we show that interactions from outside the classical receptive fields of neurons in primary visual cortex modulate neural responses in a way that makes them immune to changes in illumination, as is perception. This finding is consistent with the hypothesis that the responses of neurons in primary visual cortex carry information about surface lightness in addition to information about form. It also suggests that lightness constancy, which is sometimes thought to involve "higher-level" processes, is manifest at the first stage of visual cortical processing.

Visual neuroscience: illuminating the dark corners.

Recent experiments suggest that our perception of lightness involves a sophisticated interpretation of illumination and shadow. This finding challenges common notions about hierarchical processing and the neural basis of perception.

Neural correlates of perceived brightness in the retina, lateral geniculate nucleus, and striate cortex.

Brightness changes can be induced in a static gray field by modulating the luminance of surrounding areas. We used this induction phenomenon to investigate the neural representation of perceived brightness. Extracellular recordings were made in striate cortex, the lateral geniculate nucleus (LGN), and the optic tract of anesthetized cats using stimuli that produced brightness induction. While a cell's receptive field (RF) was covered by uniform gray illumination, the luminance of rectangular flanking regions was modulated sinusoidally in time, inducing brightness changes in the RF. We looked for a correspondence between the modulation of a cell's response and stimulus conditions that did or did not produce perceptual changes in brightness. We found that the responses of retinal ganglion cell axons in the optic tract were never correlated with brightness. On the other hand, many neurons in striate cortex and a small fraction in the LGN responded in a phase-locked manner at the temporal frequency of the flank modulation, even though the flanks were 3-7 degrees beyond the edges of the RF. Only in striate cortex were cells found that had responses correlated with brightness under all stimulus conditions. These findings suggest that brightness information is explicitly represented in the responses of neurons in striate cortex as part of a neural representation of object surfaces.

Monocular deprivation induces homosynaptic long-term depression in visual cortex.

Brief monocular deprivation during early postnatal development can lead to a depression of synaptic transmission that renders visual cortical neurons unresponsive to subsequent visual stimulation through the deprived eye. The Bienenstock-Cooper-Munro (BCM) theory proposes that homosynaptic mechanisms of long-term depression (LTD) account for the deprivation effects. Homosynaptic depression, by definition, occurs only at active synapses. Thus, in contrast to the commonly held view that the synaptic depression caused by monocular deprivation is simply a result of retinal inactivity, this theoretical framework indicates that the synaptic depression may actually be driven by the residual activity in the visually deprived retina. Here we examine the validity of this idea by comparing the consequences of brief monocular deprivation by lid suture with those of monocular inactivation by intra-ocular treatment with tetrodotoxin. Lid suture leaves the retina spontaneously active, whereas tetrodotoxin eliminates all activity. In agreement with the BCM theory, our results show that monocular lid suture causes a significantly greater depression of deprived-eye responses in kitten visual cortex than does treatment with tetrodotoxin. These findings have important implications for mechanisms of experience-dependent plasticity in the neocortex.

Integration of surface information in primary visual cortex.

Ample evidence suggests that primary visual cortex is involved in the perception of form, and there is increasing evidence that it may also be important in the perception of surfaces. Perceptual qualities of surfaces, such as brightness, are based on extensive integration of information throughout the visual field. In primary visual cortex, we found that the responses of neurons to surfaces were also influenced by the intensity and organization of light in large portions of the visual field. Interactions with surrounding stimuli typically extended 10 to 20 degrees beyond a cell's receptive field the same spatial scale as perceptual interactions. Moreover, there were both facilitatory and inhibitory influences, just as there are additive and substractive perceptual interactions. Surprisingly, influences from outside the receptive field obtained with surface stimuli did not reliably correlate with influences recorded with gratings. These properties suggest that the underlying neuronal interactions may serve as the fundamental building blocks of surface perception.

Filling-in percepts produced by luminance modulation.

We report that when the luminance of a homogeneous spot of light is gradually increased or decreased, there are conditions in which the brightness of the spot is spatially nonuniform. When the spot luminance is increased, brightness changes in the spot's center lag behind changes at the edge and brightness appears to sweep inward. Conversely, if the luminance of the spot is decreased, there is a relative lag in the darkening toward the center of the spot and darkness seems to spread inward. In Experiment 1 we found that with both increasing and decreasing luminance sweeps, the strength of the brightness filling effects was strongest with luminance sweep durations of 0.25-0.5 sec. In Experiment 2, the sweep duration was held constant at 0.5 sec; the filling effect was seen when the dwell time spent at each luminance step was less than about 100 msec, but nonuniformities were not observed at longer dwell times. In Experiment 3, a spot of light was positioned to surround the optic disk in one eye. Surprisingly, when the spot was luminance modulated from bright to dark, darkness appeared to sweep from the edge to the center of the modulated disk, even though most of the disk's interior was imaged on a portion of the retina devoid of photoreceptors. These findings are consistent with the hypothesis that a neural filling-in mechanism in visual cortex plays a key role in brightness perception.

The representation of brightness in primary visual cortex.

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

Temporal limits of brightness induction and mechanisms of brightness perception.

The luminance of a squarewave grating was modulated in a manner such that every other stripe temporally varied between bright and dark and the intervening stripes had constant luminance. This produces brightness induction in the constant stripes, roughly in antiphase to the luminance modulation. We used this stimulus as a probe to explore the temporal properties of brightness induction and the mechanisms determining perceived brightness. Over a range of spatial frequencies we measured: (1) the highest temporal frequency at which brightness induction occurs; (2) the magnitude of induced brightness; and (3) the temporal phase of the induced brightness modulation. We find that brightness induction ceases with luminance modulation above a cutoff temporal frequency that depends on spatial frequency. The magnitude of induced brightness modulation is greatest at low spatial frequencies and low temporal frequencies. Induced brightness lags behind the luminance modulation and this phase lag increases as spatial frequency decreases. All of these findings can be understood as consequences of an induction process that takes longer to complete as the induction region increases in size.

Feature-specific effects of selective visual attention.

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observer's primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties.

Brightness perception and filling-in.

Three experiments were performed in which a stimulus with homogeneous color and luminance was masked by a second stimulus containing contours. In the first experiment the target was a large white disk and the mask was a white circle concentric with the disk but of smaller radius. We found that the mask had a large (up to 2 log unit) suppressive effect on the brightness of the target, but only inside the radius of the mask. With monoptic presentation of target and mask, the greatest suppression was observed with an SOA of 50-100 msec. With dichoptic presentation the strongest suppression was obtained with simultaneous stimuli. The second experiment demonstrated that the latest time at which masking was effective was correlated with the distance between the edge of the target stimulus and the contour in the mask. One possible explanation of the results from these two experiments is that the masking contour is interfering with the propagation of a brightness signal from the target's border. In the third experiment gaps were introduced into the masking circle. Surprisingly, even with rather large gaps there was significant suppression of brightness in the center of the target. We have encountered difficulties attempting to account for these findings with known physiological mechanisms such as lateral inhibition. A qualitative explanation of the results that looks promising is a two-component process involving brightness filling-in and smoothing to satisfy fixed boundary conditions at contours.

A physiological correlate of the Pulfrich effect in cortical neurons of the cat.

When a swinging pendulum is viewed with a light-attenuating filter before one eye, the pendulum bob is perceived to move in an elliptical path in depth. It is believed that the filter causes this illusion, the Pulfrich effect, by delaying processing of the image in the filtered eye relative to that of the unfiltered eye. We sought a physiological correlate of this effect by studying binocular integration in cortical neurons of cats while they viewed moving stimuli. Special attention was focused on single unit disparity tuning because it is widely believed that depth perception is related to the responses of disparity selective neurons in visual cortex. We found that placing a filter before one of the cat's eyes produced a temporal delay in the cortical response. The temporal delay was always associated with a shift in the neuron's spatial disparity tuning. The observed temporal delays and disparity shifts are comparable with the magnitude of the Pulfrich effect in humans.

Cortical processing of hyperacuity tasks.

Spatial discrimination thresholds were determined by having subjects make comparisons between stimuli presented successively at the same or at different locations in the visual field. Two tasks were employed, discrimination of line orientation and discrimination of the distance between two parallel lines (spatial interval discrimination). We find that discrimination thresholds based on the comparison of features in spatially-separated stimuli are comparable to those based on the comparison of features in two stimuli presented successively at the same location. This holds even when the stimuli are presented in nonhomologous positions in the visual field or are presented in a manner such that they activate cells in opposite cortical hemispheres in the early visual areas. This shows that discriminability is not determined solely by differences in the firing rates of striate or pre-striate neurons. Rather, it appears that the attributes of visual stimuli are precisely coded and available for comparison at higher levels of visual processing. Implications of this finding for models of hyperacuity are discussed.

Subjective contours, tilt aftereffects, and visual cortical organization.

The tilt aftereffect (TAE) was used to study interactions between real and subjective contours. Subjects adapted to either real or illusory lines and were then shown test stimuli containing real or illusory lines. In our first experiment, we found that there is a marked asymmetry in the interactions between real and subjective stimuli. Adaptation to real lines produces comparable TAEs with real and subjective test lines. With either type of test stimulus the maximum effect occurs with a 10-20 deg difference between the orientations of the adaptation and test stimuli. Also, there is a strong TAE when the adaptation and test stimuli contain only subjective lines. However, there is a significantly weaker TAE when the adaptation stimulus is subjective and the test stimulus is real. In a second experiment we find that interocular transfer of tilt aftereffects is greater when the test stimulus is subjective than when it is real. These results are consistent with physiological reports that a subset of orientation selective cells in visual cortex is responsive to subjective contours and that these cells are more binocular, on average, than those responsive only to real contours. Our findings also suggest that the perception of subjective contours is based on the activation of neurons with properties, such as orientation selectivity, which are characteristic of early visual cortical areas.

Blockade of intracortical inhibition in kitten striate cortex: effects on receptive field properties and associated loss of ocular dominance plasticity.

We have investigated the importance of GABAergic inhibition for the receptive field properties and plasticity of cells in the visual cortex of kittens. Osmotic minipumps were used to continuously infuse the GABA-antagonist, bicuculline methiodide (BIC), into striate cortex. Extracellular recordings were made during BIC infusion to assess neuronal response properties during the blockade of inhibition. Recordings were also made from other kittens after concurrent monocular deprivation and BIC infusion to investigate the importance of response selectivity for ocular dominance plasticity. The minipump delivery technique was used to produce a large volume of cortex presumably free of GABA-ergic inhibition. Compared to recordings in saline-infused control hemispheres, about half of the cells in bicuculline-infused hemispheres had abnormally low orientation selectivity. The low selectivity was generally accompanied by marked anomalies in several other receptive field properties. Particularly striking was the large size of the receptive fields. At eccentricities less than 10 deg many receptive fields subtended from 10 to over 30 deg of arc. The less selective neurons also had abnormal responses to flashed stimuli, giving strong transient responses to the onset and offset of large stationary stimuli which filled their receptive fields. These results imply that intracortical inhibition normally suppresses responses to stimuli within a large excitatory zone beyond the classical receptive field. Inhibition is necessary for the normal orientation selectivity of many cells, although the selectivity may be partially established by the cell's excitatory input. Additionally, intracortical inhibition appears to be necessary for the antagonism and segregation of ON and OFF receptive field subregions. In our study of plasticity, we exploited the fact that BIC treatment greatly increases the range of stimuli that activate cortical neurons. Kittens were monocularly deprived for 7 days concurrently with cortical infusion of BIC. After cessation of the drug treatment, physiological recordings were made. Response properties had returned to normal but neurons in BIC-infused hemispheres had a significantly reduced ocular dominance shift compared to neurons in control hemispheres. This is probably related to the reduced selectivity of cells during BIC infusion. The suggestion here is that there is diminished ocular dominance plasticity in BIC-infused hemispheres because of an increased probability of correlated activity between spontaneous discharge from the closed eye and the cortical activity evoked by the open eye afferents.

A theory for the use of visual orientation information which exploits the columnar structure of striate cortex.

A neural model is constructed based on the structure of a visual orientation hypercolumn in mammalian striate cortex. It is then assumed that the perceived orientation of visual contours is determined by the pattern of neuronal activity across orientation columns. Using statistical estimation theory, limits on the precision of orientation estimation and discrimination are calculated. These limits are functions of single unit response properties such as orientation tuning width, response amplitude and response variability, as well as the degree of organization in the neural network. It is shown that a network of modest size, consisting of broadly orientation selective units, can reliably discriminate orientation with a precision equivalent to human performance. Of the various network parameters, the discrimination threshold depends most critically on the number of cells in the hypercolumn. The form of the dependence on cell number correctly predicts the results of psychophysical studies of orientation discrimination. The model system's performance is also consistent with psychophysical data in two situations in which human performance is not optimal. First, interference with orientation discrimination occurs when multiple stimuli activate cells in the same hypercolumn. Second, systematic errors in the estimation of orientation can occur when a stimulus is composed of intersecting lines. The results demonstrate that it is possible to relate neural activity to visual performance by an examination of the pattern of activity across orientation columns. This provides support for the hypothesis that perceived orientation is determined by the distributed pattern of neural activity. The results also encourage the view that limits on visual discrimination are determined by the responses of many neurons rather than the sensitivity of individual cells.

Orientation discrimination as a function of stimulus eccentricity and size: nasal/temporal retinal asymmetry.

Orientation discrimination threshold is a monotonically increasing function of retinal eccentricity. Increasing stimulus length extends the range of eccentricities over which fine orientation discriminations can be made. Orientation discrimination thresholds at all eccentricities are determined by the size of the cortical image of the stimulus. Thresholds obtained using either nasal or temporal hemiretina are similar up to the blind spot, beyond which the temporal retina yields increasingly higher thresholds. The results are consistent with a recent theoretical study which predicts that orientation discrimination threshold is determined by the number of cortical cells activated by the discrimination target.

Two methods of catecholamine depletion in kitten visual cortex yield different effects on plasticity.

As first clearly demonstrated by the experiments of Wiesel and Hubel, the developing visual cortex is exquisitely sensitive to sensory deprivation. Temporary closure of one eye of a kitten during a critical period that extends from 3 weeks to 3 months of age results in a dramatic cortical reorganization such that most neurones, originally binocularly driven, are dominated exclusively by the open eye. Recently, attention has been directed to chemical factors which may influence the degree of plasticity during the critical period. The work of Kasamatsu and pettigrew suggests that cortical catecholamines, especially noradrenaline (NA), are essential for the normal plastic response to visual deprivation. In an effort to clarify the role of NA in visual cortical plasticity, we have monocularly deprived kittens whose cortex had been depleted of catecholamines by the neurotoxin 6-hydroxydopamine (6-OHDA). We used two strategies to deplete cortical NA: the first, pioneered by Kasamatsu el al., utilized osmotic minipumps to deliver 6-OHDA to visual cortex; the second involved systemic neonatal injections of 6-OHDA, a technique which has proved effective in rodents. We found, using high-pressure liquid chromatography (HPLC), that both techniques produced a substantial reduction in the level of cortical NA. However, single unit recording in area 17 revealed that the plastic response to monocular deprivation (MD) was only diminished in the kittens depleted by minipump.