Modeling Surround Suppression in V1 Neurons with a Statistically-Derived Normalization Model

We examine the statistics of natural monochromatic images decomposed using a multi-scale wavelet basis. Although the coefficients of this representation are nearly decorrelated, they exhibit important higher-order statistical dependencies that cannot be eliminated with purely linear processing. In particular, rectified coefficients corresponding to basis functions at neighboring spatial positions, orientations and scales are highly correlated. A method of removing these dependencies is to divide each coefficient by a weighted combination of its rectified neighbors. Several successful models of the steady-state behavior of neurons in primary visual cortex are based on such "divisive normalization" computations, and thus our analysis provides a theoretical justification for these models. Perhaps more importantly, the statistical measurements explicitly specify the weights that should be used in computing the normalization signal. We demonstrate that this weighting is qualita..

On the functions, mechanisms, and malfunctions of intracortical contextual modulation

A broad neuron-centric conception of contextual modulation is reviewed and re-assessed in the light of recent neurobiological studies of amplification, suppression, and synchronization. Behavioural and computational studies of perceptual and higher cognitive functions that depend on these processes are outlined, and evidence that those functions and their neuronal mechanisms are impaired in schizophrenia is summarized. Finally, we compare and assess the long-term biological functions of contextual modulation at the level of computational theory as formalized by the theories of coherent infomax and free energy reduction. We conclude that those theories, together with the many empirical findings reviewed, show how contextual modulation at the neuronal level enables the cortex to flexibly adapt the use of its knowledge to current circumstances by amplifying and grouping relevant activities and by suppressing irrelevant activities

Perceptual Learning of Lateral Interactions in the near-periphery of the visual field: New Perspectives for patients with Macular Degeneration

One way in which peripheral vision may acquire the functional role of the fovea, a part of the retina preferentially used for complex visual tasks (such as reading and face recognition) is by modulating the strength of intracortical connections in the humans’ visual areas with perceptual learning. Perceptual learning is a practice-dependent improvement in a visual task performance that can persist for several months, and is specific for stimulus, task, eye presentation and retinal locus of stimulation. These specificity effects have been explained on the basis of neural plasticity, consisting in long-term modifications of a number of mechanisms in the early visual cortices, that are selective for basic stimulus attributes (Karni & Sagi, 1991, 1993; Ahissar & Hochstein, 1993, 1996; Casco & Campana, 2001). Perceptual learning experiments with stimuli involving lateral masking (Polat & Sagi, 1994b, 1995; Polat, Ma-Naim, Belkin & Sagi, 2004) have suggested that practice is able to modulate short- and long-range lateral interactions between neurons responding to collinear elements. These studies showed that contrast thresholds for a target are modulated by the presence collinear flankers, and the type of modulation depends on the distance between the central target and the flankers: inhibitory for short target-to-flankers distance, and facilitatory for longer distances. With the training the suppression from the short target-flanker separation can be reduced and facilitation at relatively long target-flanker separation increases. These studies suggest that practice on lateral interactions increases the efficacy of the collinear interactions between neighbouring neurons, an effect that enhances connectivity with remote neurons via a cascade of local interactions. Most importantly, perceptual learning on lateral interactions has been showed to be useful for improving contrast sensitivity in people with normal low vision (Tan & Fong, 2008; Polat, 2009) or with impaired lateral interactions such as amblyopia (Polat et al., 2004). Notably, these studies showed that, differently from previous perceptual learning experiments where non transfer of learning to different stimulus attributes was observed, the effect of training on lateral interactions transferred to higher level visual tasks, like visual acuity (VA) (Tan & Fong, 2008) producing a long-lasting perceptual benefit in everyday visual tasks. However, in all these studies stimuli were presented in the fovea. This thesis aimed at investigating the possibility that the effect of training also improves lateral interaction in retinal regions eccentric with respect to the fovea. Lateral interactions strongly depend on eccentricity: in the periphery they are mostly inhibitory (Petrov, Carandini & McKee, 2005; Cavanaugh et al., 2002). These evidence leads to the hypothesis that peripheral vision may acquire some of the functional role of the fovea only if inhibition is reduced. Shani & Sagi (2005) showed that collinear facilitation in the near-periphery is weak and that perceptual learning seems not to be effective in modulating lateral interactions. However, in their study the training was very short, and only one target-to-flanker distance was tested. If lateral interactions could be modulated in the near periphery, and transfer to visual tasks such as VA and crowding was possible, this could be extremely important for rehabilitation of individuals with loss of central vision, such as in macular degeneration. These type of patients, after the loss of central vision, are forced to use the periphery of the visual field for the most demanding visual tasks like face recognition and reading. In Experiment 1, we were interested in verifying whether inhibitory lateral interactions in the near-periphery (4 degrees of eccentricity) may be reduced by training and if training transfers to other visual functions. Eight subjects were trained with different spatial frequencies (1, 2, 4, and 8 cycles per degree - cpd) and different target-to-flankers separations (2, 3, 4, and 8). Before the practice sessions subjects performed a series of pre-tests aimed at measuring their peripheral contrast sensitivity function (CSF), peripheral visual acuity (VA) and crowding effect. Consistently with previous studies (Petrov et al., 2005; Cavanagh et al., 2002), results of the Experiment 1 showed that, in the near periphery, lateral interactions are inhibitory even at target-to-flanker distances (4) where facilitatory interactions are found in central vision. Facilitation was reported for a target-to-flankers distance of 8, consistently with the most recent investigation on peripheral lateral interactions (Lev & Polat, 2011). Most importantly, Experiment 1 showed that lateral interactions in parafoveal vision can be modulated by training, reducing the inhibition, and that perceptual learning transfers to other visual abilities, leading to a reduction of crowding. Since learning specificity is viewed as the main indicator of the level of processing at which learning takes place, and since learning of lateral interaction has been shown to transfer to different visual functions, in Experiment 2 we tested the specificity of learning to basic stimulus features such as target-flankers local and global orientation and retinal position. We trained 4 new subjects in contrast detection of a collinearly flanked vertical target, and found a significant learning effect for the trained configuration but no transfer of learning to either the same stimulus presented in a symmetrical retinal location, nor to a 45 deg oriented collinear target-flankers configuration, presented in the same retinal position as the learning stimulus. The finding that these transfer stimuli are immune to perceptual learning of vertical orientations strongly suggests that the modulation of lateral interactions through perceptual learning is functionally specific, and that transfer to different visual functions can only occur when these are based on the specific early mechanism that is learned. In Experiment 3, we aimed at exploiting the effects of the perceptual learning of lateral interactions for improving peripheral vision in patients affected by macular degeneration. Training consisted in a contrast detection of a Gabor target with collinear high contrast Gabor flankers, at different target-to-flankers separations, located in their preferential retinal locus (PRL, the new fixation point that spontaneously emeges in this type of patients) and in a symmetrical location. The rationale behind the measuring or lateral interactions and the training in the PRL and in another retinal location was to point out possible differences in intracortical connectivity for this new fixation point respect to other retinal spots. Consistently with other studies (Dilks, Baker, Peli and Kanwisher, 2009), we did not find major differences in terms of lateral interactions and perceptual learning effects between PRL and the symmetrical locus. Training increased contrast sensitivity, and, despite not having any effect on crowding, improved visual acuity in the maculopathy subjects. The absence of crowding reduction could be due to a “roof effect”, since this type of patients naturally train their peripheral view, probably reaching their maximal performance even before the training.. Nevertheless, the improvement of visual acuity opens new perspectives for the rehabilitation of patients with macular degeneration, but also for improving peripheral vision in normal-sighted subjects, since recent studies showed the important role of the periphery of the visual field in tasks such as postural stability, locomotion and driving. In Experiment 4, we investigated the architecture of peripheral lateral interactions in a maculopathy patients, finding collinear facilitation at shorter target-to-flankers separation respect to normal-sighted subjects. Interestingly, collinear facilitation was reported for target presentation in the PRL but not in the No-PRL, where collinear interactions were only inhibitory. Moreover, perceptual learning training appeared to be effective in modulating lateral interactions only in the PRL, questioning the hypothesis of a “use-dependent” cortical reorganization, supported, among the others, by Dilks et al. (2009)