Marcello Maniglia

Reducing crowding by weakening inhibitory lateral interactions in the periphery with perceptual learning.

We investigated whether lateral masking in the near-periphery, due to inhibitory lateral interactions at an early level of central visual processing, could be weakened by perceptual learning and whether learning transferred to an untrained, higher-level lateral masking known as crowding. The trained task was contrast detection of a Gabor target presented in the near periphery (4°) in the presence of co-oriented and co-aligned high contrast Gabor flankers, which featured different target-to-flankers separations along the vertical axis that varied from 2λ to 8λ. We found both suppressive and facilitatory lateral interactions at target-to-flankers distances (2λ - 4λ and 8λ, respectively) that were larger than those found in the fovea. Training reduces suppression but does not increase facilitation. Most importantly, we found that learning reduces crowding and improves contrast sensitivity, but has no effect on visual acuity (VA). These results suggest a different pattern of connectivity in the periphery with respect to the fovea as well as a different modulation of this connectivity via perceptual learning that not only reduces low-level lateral masking but also reduces crowding. These results have important implications for the rehabilitation of low-vision patients who must use peripheral vision to perform tasks, such as reading and refined figure-ground segmentation, which normal sighted subjects perform in the fovea.

Implied motion from static photographs influences the perceived position of stationary objects.

A growing amount of evidence suggests that viewing a photograph depicting motion activates the same direction-selective neurons involved in the perception of real motion. It has been shown that prolonged exposure (adaptation) to photographs depicting directional motion can induce motion adaptation and consequently motion aftereffect. The present study investigated whether adapting to photographs depicting humans, animals, and vehicles that move leftward or rightward also generates a positional aftereffect (the motion-induced position shift--MIPS), in which the perceived spatial position of a target pattern is shifted in the opposite direction to that of adaptation. Results showed that adapting to still photographs depicting objects that move in a particular direction shifts the perceived position of subsequently presented stationary objects opposite to the depicted adaptation direction and that this effect depends on the retinotopic location of the adapting stimulus. These results suggest that the implied motion could activate the same direction-selective and speed-tuned mechanisms that produce positional aftereffect when viewing real motion.

The fastest (and simplest), the earliest: The locus of processing of rapid forms of motion aftereffect

Adaptation to directional motion has been shown to bias the perceived direction of a subsequently presented stationary or flickering test stimulus toward the opposite direction with respect to that of adaptation. This phenomenon, called motion aftereffect, is usually generated with adaptation periods of tens of seconds or minutes and has been shown to depend upon the functional integrity of visual area V5/MT. Rapid forms of MAE, arising and decaying within half a second (rMAE), can also be generated with sub-second adaptation durations. In order to investigate the neural substrate underlying the rMAE, repetitive transcranial magnetic stimulation (rTMS) has been used just after the adaptation stimulus over areas V1/V2, V5/MT, or over the vertex. Results showed that, besides some reduction in strength of the rMAE when rTMS was delivered over V5/MT, it was maximally disrupted when stimulation was delivered over early visual areas V1/V2. This is the first study where a causal role of early visual cortices in MAE is demonstrated. Moreover, this finding supports the existence of multiple loci along the visual stream in which gain control takes place and generates the MAE as a byproduct. The specific locus is likely to depend on the specific stimulus used.

Common (and multiple) neural substrates for static and dynamic motion after-effects: A rTMS investigation

INTRODUCTION Prolonged exposure to directional motion (adaptation) biases the perceived direction of subsequently presented test stimuli towards the opposite direction with respect to that of adaptation (i.e., motion after-effect; MAE). Different neural populations seem to be involved in the generation of the MAE, depending on the spatiotemporal characteristics of both adapting and test stimuli. Although the tuning mechanisms of the neural populations involved in the MAE have been psychophysically identified, the specific loci along the motion processing hierarchy where the different types of MAE take place is still debated. METHOD In this study, by using repetitive transcranial magnetic stimulation (rTMS) delivered during the inter-stimulus interval (ISI) between adapting and test patterns, we investigated the cortical locus of processing of static MAE (sMAE) and dynamic MAE (dMAE). RESULTS Results showed that rTMS over V2/V3 or V5/MT decreased the perceived duration of both sMAE and dMAE, although rTMS over V2/V3 decreased mainly the perceived duration of sMAE. CONCLUSIONS sMAE and dMAE rely on the same cortical structures present at intermediate and low-levels of motion processing, although low-level visual areas (e.g., V2/V3) show a prevalence of neurons responsible for sMAE.

The origin of the audiovisual bounce inducing effect: A TMS study

The audiovisual bounce inducing effect (ABE) is a bouncing percept induced by the presence of a sound at the moment of two moving objects intercepting in a motion display otherwise perceived as streaming. The origin of the ABE is still debated: the effect could arise from the subtraction of attentional resources caused by the sound (needed to favor the perception of streaming), and/or from the cross-modal integration (binding) of visual and auditory information: indeed bouncing-like sounds are best in inducing the ABE. The neural mechanism responsible for the ABE is still unknown. Here, by using offline TMS, we investigated the role of the posterior parietal cortex (PPC), thought to be involved in both attentional and binding processes, in the generation of the ABE. Results show that disrupting the functional integrity of the right (but not the left) PPC has the effect of weakening the binding of cross-modal information, which reduces the magnitude of the ABE. Indeed, if the effect of parietal stimulation was merely to disrupt attention, we would expect an increase (not a decrease) of bouncing percepts. The present study not only shows the involvement of the right PPC in the ABE, but also support the notion that cross-modal binding (and not attention) is at the origin of the ABE.

The role of high-level visual areas in short- and longer-lasting forms of neural plasticity

Striate and extrastriate neurons present short-term synaptic depression and facilitation in response to brief stimulations. Recent psychophysical studies have shed light on some possible relationships between these short-term forms of neural plasticity and of psychophysical behavior. It has been shown that a brief adaptation to directional motion biases the perceived direction of a subsequently presented ambiguous test pattern towards the same direction to that of the adaptation (rapid visual motion priming--rVMP), but only after brief (40ms) adaptation-test blank intervals. Although when the adaptation duration is increased, the perceived motion direction of the ambiguous test pattern is biased towards the opposite direction to that of the adaptation pattern (rapid motion aftereffect--rMAE). In the present study we stimulated MT and MST neurons via the presentation of contracting and expanding circular gratings. Our aim was to assess whether rapid effects exist at these higher levels of processing where neurons respond to optic flow, and if such effects are present determine their timescale. Results revealed strong rMAEs and perceptual sensitization (PS), which is a long-lasting facilitation that increases gradually when using intermediate and long adaptation-test blank intervals. We did not observe any effect of rVMP. Our results are considered to reflect the competition between coexistent forms of short- and long-term synaptic depression and facilitation implemented at different visual cortical circuitries.

The spatial range of peripheral collinear facilitation

Contrast detection thresholds for a central Gabor patch (target) can be modulated by the presence of co-oriented and collinear high contrast Gabors flankers. In foveal vision collinear facilitation can be observed for target-to-flankers relative distances beyond two times the wavelength (λ) of the Gabor’s carrier, while for shorter relative distances (<2λ) there is suppression. These modulatory influences seem to disappear after 12λ. In this study, we measured contrast detection thresholds for different spatial frequencies (1, 4 and 6 cpd) and target-to-flankers relative distances ranging from 6 to 16λ, but with collinear configurations presented in near periphery at 4° of eccentricity. Results showed that in near periphery collinear facilitation extends beyond 12λ for the higher spatial frequencies tested (4 and 6 cpd), while it decays already at 10λ for the lowest spatial frequency used (i.e., 1 cpd). In addition, we found that increasing the spatial frequency the peak of collinear facilitation shifts towards larger target-to-flankers relative distances (expressed as multiples of the stimulus wavelength), an effect never reported neither for near peripheral nor for central vision. The results suggest that the peak and the spatial extent of collinear facilitation in near periphery depend on the spatial frequency of the stimuli used.

Fast random motion biases judgments of visible and occluded motion speed

ABSTRACT Human sensitivity to speed differences is very high, and relatively high when one has to compare the speed of an object that disappears behind an occluder with a standard. Nevertheless, different speed illusions (by contrast, adaptation, dynamic visual noise) affect proper speed judgment for both visible and occluded moving objects. In the present study, we asked whether an illusion due to non‐directional motion noise (random dynamic visual noise, rDVN) intervenes at the level of speed encoding, thus affecting speed discrimination, or at the level of speed decoding by non‐sensory decision‐making mechanisms, indexed by speed overestimation of visible and invisible motion. In Experiment 1, participants performing a temporal two‐Alternative Forced Choice task, judged the speed of a target moving in front of the rDVN or a static visual noise (SVN). In Experiment 2 and 3, the target disappeared behind the rDVN/SVN, and participants reported whether the target reappeared early or late (Experiment 2), or the time to contact (TTC) with the end of the occluded trajectory (Experiment 3). In Experiment 1 and 2, we found that rDVN affected the point of subjective equality (pse) of the individual’s psychometric function in a way indicating speed overestimation, while not affecting speed discrimination threshold (just noticeable differences, jnd). In Experiment 3 the rDVN reduced the TTC. Though not entirely consistent, our results suggest that a similar speed decoding mechanism, which read‐out motion information to form a perceptual decision, operates regarding of whether motion is visible or invisible.

Spontaneous and training-induced cortical plasticity in MD patients: Hints from lateral masking

Macular degeneration (MD) affects central vision and represents the leading cause of visual diseases in elderly population worldwide. As a consequence of central vision loss, MD patients develop a preferred retinal locus (PRL), an eccentric fixation point that replaces the fovea. Here, our aim was to determine whether and to what extent spontaneous plasticity takes place in the cortical regions formerly responding to central vision and whether a visual training based on perceptual learning (PL) can boost this plasticity within the PRL area. Spontaneous and PL-induced cortical plasticity were characterized by using lateral masking, a contrast sensitivity modulation induced by collinear flankers. This configuration is known to be sensitive to neural plasticity and underlies several rehabilitation trainings. Results in a group of 4 MD patients showed that collinear facilitation was similar to what observed in age- and eccentricity-matched controls. However, MD patients exhibited significantly reduced collinear inhibition, a sign of neural plasticity, consistent with the hypothesis of partial cortical reorganization. Three AMD patients from the same group showed a further reduction of inhibition after training, but not controls. This result suggests that PL might further boost neural plasticity, opening promising perspectives for the development of rehabilitation protocols for MD patients.