Clara Casco

I further develop the Vannuscorps et al. [(2021). Shape-centered representations of bounded regions of space mediate the perception of objects. Cognitive neuropsychology, 1–50.] interpretation of Davida's deficit, as based on a failure of mapping information carried by the parvocellular pathway onto non-retinotopic coordinates. I assumed that magno-parvo cooperation is required to bind local features into 2-D shapes with sharp edges, although the relative role of either subsystem may depend on the task. My suggestion is that perception is un impaired when the imbalance is in favour of the magno system; that is, with images blurred or made up of either disconnected elements or isolated line segments not requiring binding. Conversely, misperception occurs when the task involves an imbalance in favour of parvo analysis; i.e., when orientation judgment is based mainly on the highest levels of parvo analysis devoted to binding in the ventral stream. In these last conditions, Davida's misperception of orientation of these 2-D shapes may result from a switch of magno-parvo cooperation to conflict.

Background: In recent years, the introduction of visual rehabilitation for patients with homonymous visual field defects has been met with both enthusiasm and caution. Despite the evidence that restitutive training results in expansion of the visual field, several concerns have been raised.Objective: We tested the effectiveness of a new rehabilitative protocol called “Neuro Restoration Training” (NRT) in reducing visual field defects and in restituting visual functions in the restored hemianopic area.Methods: Ten patients with homonymous visual field defects (lesion age >6 months) where trained in detecting low contrast Gabor patches randomly presented in the blind field, which refers to regions of 0 dB sensitivity, and along the hemianopic boundary between absolute (0 dB) and partial blindness (>0 dB). Training included static, drifting, and flickering Gabors in different blocks. Positions along the hemianopic boundary were systematically shifted toward the blind field according to the threshold reduction during the training. Before and after the training, we assessed visual field expansion and improvement in different high-level transfer tasks (i.e., letter identification and shape recognition) performed in the hemianopic boundary and in the blind field.Results: NRT led to significant visual field enlargement (≈5 deg), as indicated by the conventional Humphrey perimetry, and two custom made evaluations of visual field expansion with eye movement control (one static and one dynamic). The restored area acquired new visual functions such as small letter recognition and perception of moving shapes. Finally, for some patients, NRT also improved detection, either aware or not, of high contrast flickering grating and recognition of geometrical shapes entirely presented within the blind field.Conclusion: These results suggest that NRT may lead to visual field enlargement and translate into untrained visual functions.

Visual perception relies on low-level encoding of local orientation. Recent studies show an age-dependent impairment in orientation discrimination of stimuli embedded in external noise, suggesting that encoding of orientation is inefficient in older adults. In the present study we ask whether aging also reduces decoding, i.e. selecting the neural representations of target orientation while discarding those conflicting with it. We compared younger and older participants capability (mean age 24 and 68 years respectively) in discriminating whether the orientation of a Gabor target was left or right from the vertical. We measured (d’), an index of discrimination sensitivity, for orientation offset ranging from 1 to 12 deg. In the isolated target condition, d’ was reduced by aging and, in the older group, did not increase with orientation offset, thus resulting in a larger group difference at large than small orientation offsets from the vertical. Moreover, oriented elements in the background impaired more discrimination in the older group. However, distractors reduced more d’ when target-background orientation offset was large than when target and flanker had similar orientation, indicating that the effect of the background was not local, i.e. due to target inhibition by similarly oriented flankers. Altogether, these results indicate that ageing reduces the efficiency in discarding the response to orientations differing from the target. Our results suggest that neural decision-making mechanisms, involving not only signal enhancement but also non-signal inhibition, become inefficient with age. This suggestion is consistent with the neurophysiological evidence of inefficient visual cortical inhibition in aging.

Purpose: This study aims at comparing participants with juvenile macular degeneration (MD) and normally sighted observers in their sensitivity to mirror and translational symmetry. Methods: We measured in 25 normal sighted and 9 MD participants sensitivity ( d’) to detect the symmetry of two dot patterns presented at the opposite sides of their central scotoma. Results: At a large dot patterns separation (13.3 deg), at which detection failed in normally sighted observers, MD patients had high sensitivity to mirror symmetry, whereas translational symmetry was undetected. Conclusions: The mirror-translational dissociation is not predicted by the well-known phenomenon of shrinking the location of images surrounding the scotoma. Our results indicate higher capacity of MD with respect to normally sighted observers to organize mirror symmetric dot patterns far apart into a unique percept. Our results suggest that MD have acquired the capability to use information only present in mirror symmetry, i.e., the co-aligned position of the centre of low-frequency filters connecting symmetric dot pairs on opposite sides of the scotoma. This relevant functional change in vision of MD patients may find its explanation in a functionally acquired high-level cortical representation of visual input.

We investigated spatial suppression of a drifting Gabor target of 0.5 c/° induced by adjacent and iso-oriented stationary Gabors (flankers) whose spatial frequency differed by ±1 and ±2 octaves to that of the drifting target. Stimuli (target and flankers) were presented for 33 ms. Results showed greater spatial suppression when the spatial frequency of the stationary but transient flanking Gabors was either equal or 1–2 octaves lower than when it was 1–2 octaves higher than the target's spatial frequency. This asymmetry was evident only for the drifting target, but not for the stationary target. In addition, we investigated whether perceptual learning (PL) reduced the spatial suppression induced by the flankers. We found that PL increased contrast sensitivity for the target, but only when it was reduced by the lateral masking flankers, and its effect did not transfer to an isolated drifting target of equal or higher spatial frequency. These results suggest that PL selectively affects suppressive interactions rather than contrast gain. We suggest that the suppressive effect of low spatial frequency flankers and the lack of suppression with high spatial frequency flankers may reflect two complementary phenomena: camouflage by the transient flankers (i.e., context) and breaking of camouflage by form-motion segmentation. Camouflage may result because both target and flankers activate the motion (magnocellular) system. Breaking of camouflage instead may occur when target and flankers' spatial frequency are more suitable for quasi-independent activation of the form system (by the flankers) and the motion system (by the target).

Purpose: In this study we investigated in observers with low myopia: (i) the pattern of lateral interactions between stimuli activating early cortical analyzers and its modulation by perceptual learning (PL), and (ii) whether PL transferred to untrained stimuli and tasks and whether it exhibits interocular transfer. Method: Participants (seven adults with low myopia) performed 12 training sessions. Participants were trained on a contrast detection task of a central Gabor target flanked by two co-oriented and co-aligned high contrast Gabor patches. Target-to-flankers separation along the vertical axis was varied from 2 wavelengths (λ) to 8λ. Results: The results showed that before PL facilitatory lateral interactions in the myopic eye were reduced in strength, but PL increased contrast sensitivity and improved facilitatory lateral interactions. However, PL did not transfer to different local/global orientations and lower spatial frequencies. On the other hand, PL resulted in an enhancement of the contrast sensitivity function (CSF) and of the uncorrected visual acuity (UCVA) both in the trained and untrained eye. Conclusions: Such improvements seem to be associated to a modulation of lateral interactions between target and flankers and it is likely to take place at a level in which the inputs from the two eyes converge.

The brain can retain speed information in early visual short-term memory in an astonishingly precise manner. We investigated whether this (early) visual memory system is active during the extrapolation of occluded motion and whether it reflects speed misperception due to contrast and size. Experiments 1A and 2A showed that reducing target contrast or increasing its size led to an illusory speed underestimation. Experiments 1B, 2B, and 3 showed that this illusory phenomenon is reflected in the memory of speed during occluded motion, independent of the range of visible speeds, of the length of the visible trajectory or the invisible trajectory, and of the type of task. These results suggest that illusory speed is retained in memory during invisible motion.

The predominant view of motion and form processing in the human visual system assumes that these two attributes are handled by separate and independent modules. Motion processing involves filtering by direction-selective sensors, followed by integration to solve the aperture problem. Form processing involves filtering by orientation-selective and size-selective receptive fields, followed by integration to encode object shape. It has long been known that motion signals can influence form processing in the well-known Gestalt principle of common fate; texture elements which share a common motion property are grouped into a single contour or texture region. However, recent research in psychophysics and neuroscience indicates that the influence of form signals on motion processing is more extensive than previously thought. First, the salience and apparent direction of moving lines depends on how the local orientation and direction of motion combine to match the receptive field properties of motion-selective neurons. Second, orientation signals generated by “motion-streaks” influence motion processing; motion sensitivity, apparent direction and adaptation are affected by simultaneously present orientation signals. Third, form signals generated by human body shape influence biological motion processing, as revealed by studies using point-light motion stimuli. Thus, form-motion integration seems to occur at several different levels of cortical processing, from V1 to STS.