Alessio Fracasso

Topographic representations of object size and relationships with numerosity reveal generalized quantity processing in human parietal cortex

Significance Processing of quantities such as object sizes and numbers relies on analyses of sensory information and informs cognitive tasks such as decision making and mathematics. Whereas sensory processing is organized into topographic maps reflecting sensory organ structure, organization of cognitive processing is poorly understood. We demonstrate topographic representation of object size-tuned responses. This arises separately from object number tuning, but these two quantities are associated in overlapping maps. This generalized quantity representation may allow us to consider object size and number together when making decisions. Optimization of cognitive processing using topographic maps may be a common organizing principle in association cortex, as it is in sensory processing. Linking cognitive representations in maps of related features may support increasingly abstract cognition. Humans and many animals analyze sensory information to estimate quantities that guide behavior and decisions. These quantities include numerosity (object number) and object size. Having recently demonstrated topographic maps of numerosity, we ask whether the brain also contains maps of object size. Using ultra-high-field (7T) functional MRI and population receptive field modeling, we describe tuned responses to visual object size in bilateral human posterior parietal cortex. Tuning follows linear Gaussian functions and shows surround suppression, and tuning width narrows with increasing preferred object size. Object size-tuned responses are organized in bilateral topographic maps, with similar cortical extents responding to large and small objects. These properties of object size tuning and map organization all differ from the numerosity representation, suggesting that object size and numerosity tuning result from distinct mechanisms. However, their maps largely overlap and object size preferences correlate with numerosity preferences, suggesting associated representations of these two quantities. Object size preferences here show no discernable relation to visual position preferences found in visuospatial receptive fields. As such, object size maps (much like numerosity maps) do not reflect sensory organ structure but instead emerge within the brain. We speculate that, as in sensory processing, optimization of cognitive processing using topographic maps may be a common organizing principle in association cortex. Interactions between object size and numerosity maps may associate cognitive representations of these related features, potentially allowing consideration of both quantities together when making decisions.

Unseen complex motion is modulated by attention and generates a visible aftereffect.

The relationship between attention and awareness and the processing of visual information outside of attention and awareness remain controversial issues. We employed the motion aftereffect (MAE) illusion and continuous flash suppression (CFS) to study the behavioral effects of unseen and unattended visual motion. The main finding was that either withdrawal of attention or the lack of visual awareness on the adaptors did not eliminate the formation of translational MAEs, spiral MAEs, or the interocular transfer of the MAE. However, no spiral MAE was generated when attention was diverted from the unseen spiral adaptors. Interestingly, all MAEs that arose in the absence of awareness or in the absence of attention were reduced in size. The pattern of results is consistent with suggestions that the magnitude of visual motion adaptation depends on both attention and awareness.

Systematic variation of population receptive field properties across cortical depth in human visual cortex

Receptive fields (RFs) in visual cortex are organized in antagonistic, center-surround, configurations. RF properties change systematically across eccentricity and between visual field maps. However, it is unknown how center-surround configurations are organized in human visual cortex across lamina. We use sub-millimeter resolution functional MRI at 7Tesla and population receptive field (pRF) modeling to investigate the pRF properties in primary visual cortex (V1) across cortical depth. pRF size varies according to a U-shaped function, indicating smaller pRF center size in the middle compared to superficial and deeper intra-cortical portions of V1, consistent with non-human primate neurophysiological measurements. Moreover, a similar U-shaped function is also observed for pRF surround size. However, pRF center-surround ratio remains constant across cortical depth. Simulations suggest that this pattern of results can be directly linked to the flow of signals across cortical depth, with the visual input reaching the middle of cortical depth and then spreading towards superficial and deeper layers of V1. Conversely, blood-oxygenation-level-dependent (BOLD) signal amplitude increases monotonically towards the pial surface, in line with the known vascular organization across cortical depth. Independent estimates of the haemodynamic response function (HRF) across cortical depth show that the center-surround pRF size estimates across cortical depth cannot be explained by variations in the full-width half maximum (FWHM) of the HRF.

Non-Conscious Processing of Motion Coherence Can Boost Conscious Access

Research on the scope and limits of non-conscious vision can advance our understanding of the functional and neural underpinnings of visual awareness. Here we investigated whether distributed local features can be bound, outside of awareness, into coherent patterns. We used continuous flash suppression (CFS) to create interocular suppression, and thus lack of awareness, for a moving dot stimulus that varied in terms of coherence with an overall pattern (radial flow). Our results demonstrate that for radial motion, coherence favors the detection of patterns of moving dots even under interocular suppression. Coherence caused dots to break through the masks more often: this indicates that the visual system was able to integrate low-level motion signals into a coherent pattern outside of visual awareness. In contrast, in an experiment using meaningful or scrambled biological motion we did not observe any increase in the sensitivity of detection for meaningful patterns. Overall, our results are in agreement with previous studies on face processing and with the hypothesis that certain features are spatiotemporally bound into coherent patterns even outside of attention or awareness.

Lines of Baillarger in vivo and ex vivo: Myelin contrast across lamina at 7 T MRI and histology

The human cerebral cortex is characterized by a number of features that are not uniformly distributed, such as the presence of multiple cytoarchitectonic elements and of myelinated layers running tangentially to the cortex surface. The presence and absence of these features are the basis of the parcellation of the cerebral cortex in several areas. A number of areas show myelin increases localized within the cortex, e.g., the stria of Gennari located in layer IV of the primary visual cortex. Sub-millimeter MRI can resolve myelin variations across the human cortex and may allow in vivo parcellation of these brain areas. Here, we image within-area myelination. We modified a T1-weighted (T1-w) MPRAGE sequence to enhance myelin visualization within the cortex. First, we acquired images from an ex vivo sample, and compared MRI laminar profiles from calcarine (corresponding to primary visual cortex) and extra-calcarine areas with histology sections from the same locations. Laminar profiles between myelin stained sections and the T1-w images were similar both in calcarine as well as extra-calcarine cortex. In calcarine cortex, the profile reveals the stria of Gennari. In extra-calcarine cortex, a similar profile exists which we suggest corresponds to the lines of Baillarger. Next, we adapted the same sequence to image within-area myelination in vivo. Also in in vivo data, we discriminated similar laminar profiles in calcarine and extra-calcarine cortex, extending into parietal and frontal lobes. We argue that this myelin pattern outside the calcarine cortex represents the lines of Baillarger.

Ultra-high field MRI: Advancing systems neuroscience towards mesoscopic human brain function

ABSTRACT Human MRI scanners at ultra‐high magnetic field strengths of 7 T and higher are increasingly available to the neuroscience community. A key advantage brought by ultra‐high field MRI is the possibility to increase the spatial resolution at which data is acquired, with little reduction in image quality. This opens a new set of opportunities for neuroscience, allowing investigators to map the human cortex at an unprecedented level of detail. In this review, we present recent work that capitalizes on the increased signal‐to‐noise ratio available at ultra‐high field and discuss the theoretical advances with a focus on sensory and motor systems neuroscience. Further, we review research performed at sub‐millimeter spatial resolution and discuss the limits and the potential of ultra‐high field imaging for structural and functional imaging in human cortex. The increased spatial resolution achievable at ultra‐high field has the potential to unveil the fundamental computations performed within a given cortical area, ultimately allowing the visualization of the mesoscopic organization of human cortex at the functional and structural level. HIGHLIGHTSUltra‐high field MRI (UHF) provides improved sensitivity and specificity.UHF provides access to human mesoscopic organization, including laminae and columns.The mesoscopic scale may contain the fundamental computational unit of the brain.This may be the most fundamental unit required to understand human brain function.

Waves of visibility: probing the depth of inter-ocular suppression with transient and sustained targets

In order to study non-conscious visual processing, researchers render otherwise consciously perceived images into invisible stimuli. Through the years, several psychophysical techniques have been developed for this purpose. Yet the comparison of experimental results across techniques remains a difficult task as the depth of suppression depends on the interactions between the type of stimuli and the suppression methods employed. This poses a limit to the inferences that researchers make about the extent of non-conscious processes. We investigated the mechanisms underlying inter-ocular suppression during continuous flash suppression (CFS) and dichoptic visual masking using a transient onset target stimulus and a variety of stimulus/mask temporal manipulations. We show that target duration, timing of target onset, and mask frequency are key aspects of inter-ocular suppression during CFS with transient targets. The differences between our results and sustained target CFS studies suggest that two distinct mechanisms are involved in the detection of transient and prolonged target stimuli during CFS. Our results provide insight into the dynamics of CFS together with evidence for similarities between transient target CFS and dichoptic visual masking.

Remapping of the line motion illusion across eye movements

Although motion processing in the brain has been classically studied in terms of retinotopically defined receptive fields, recent evidence suggests that motion perception can occur in a spatiotopic reference frame. We investigated the underlying mechanisms of spatiotopic motion perception by examining the role of saccade metrics as well as the capacity of trans-saccadic motion. To this end, we used the line motion illusion (LMI), in which a straight line briefly shown after a high contrast stimulus (inducer) is perceived as expanding away from the inducer position. This illusion provides an interesting test of spatiotopic motion because the neural correlates of this phenomenon have been found early in the visual cortex and the effect does not require focused attention. We measured the strength of LMI both with stable fixation and when participants were asked to perform a 10° saccade during the blank ISI between the inducer and the line. A strong motion illusion was found across saccades in spatiotopic coordinates. When the inducer was presented near in time to the saccade cue, saccadic latencies were longer, saccade amplitudes were shorter, and the strength of reported LMI was consistently reduced. We also measured the capacity of the trans-saccadic LMI by varying the number of inducers. In contrast to a visual-spatial memory task, we found that the LMI was largely eliminated by saccades when two or more inducers were displayed. Together, these results suggest that motion perceived in non-retinotopic coordinates depends on an active, saccade-dependent remapping process with a strictly limited capacity.

Laminar imaging of positive and negative BOLD in human visual cortex at 7 T

&NA; Deciphering the direction of information flow is critical to understand the brain. Data from non‐human primate histology shows that connections between lower to higher areas (e.g. retina→V1), and between higher to lower areas (e.g. V1←V2) can be dissociated based upon the distribution of afferent synapses at the laminar level. Ultra‐high field scanners opened up the possibility to image brain structure and function at an unprecedented level of detail. Taking advantage of the increased spatial resolution available, it could theoretically be possible to disentangle activity from different cortical depths from human cerebral cortex, separately studying different compartments across depth. Here we use half‐millimeter human functional and structural magnetic resonance imaging (fMRI, MRI) to derive laminar profiles in early visual cortex using a paradigm known to elicit two separate responses originating from an excitatory and a suppressive source, avoiding any contamination due to blood‐stealing. We report the shape of laminar blood level oxygenation level dependent (BOLD) profiles from the excitatory and suppressive conditions. We analyse positive and negative %BOLD laminar profiles with respect to the dominating linear trend towards the pial surface, a confounding feature of gradient echo BOLD fMRI, and examine the correspondence with the anatomical landmark of input‐related signals in primary visual cortex, the stria of Gennari. Highlights7 T fMRI has enabled the visualization of fine scales of organization in human cortex.0.55 mm isotropic human gradient‐echo fMRI and 0.55 mm myelin‐sensitive structural MRI.Laminar BOLD profiles from the excitatory and suppressive conditions.

Spatiotopic updating facilitates perception immediately after saccades

As the neural representation of visual information is initially coded in retinotopic coordinates, eye movements (saccades) pose a major problem for visual stability. If no visual information were maintained across saccades, retinotopic representations would have to be rebuilt after each saccade. It is currently strongly debated what kind of information (if any at all) is accumulated across saccades and when this information becomes available after a saccade. Here, we use a motion illusion to examine the accumulation of visual information across saccades. In this illusion, an annulus with a random texture slowly rotates and is then replaced with a second texture (motion transient). With increasing rotation durations, observers consistently perceive the transient as large rotational jumps in the direction opposite to rotation direction (backward jumps). We first show that accumulated motion information is updated spatiotopically across saccades. Then, we show that this accumulated information is readily available after a saccade, immediately biasing postsaccadic perception. The current findings suggest that presaccadic information is used to facilitate postsaccadic perception and are in support of a forward model of transsaccadic perception, aiming at anticipating the consequences of eye movements and operating within the narrow perisaccadic time window.