Sonia Poltoratski

Functionally Defined White Matter Reveals Segregated Pathways in Human Ventral Temporal Cortex Associated with Category-Specific Processing

It is unknown if the white-matter properties associated with specific visual networks selectively affect category-specific processing. In a novel protocol we combined measurements of white-matter structure, functional selectivity, and behavior in the same subjects. We find two parallel white-matter pathways along the ventral temporal lobe connecting to either face-selective or place-selective regions. Diffusion properties of portions of these tracts adjacent to face- and place-selective regions of ventral temporal cortex correlate with behavioral performance for face or place processing, respectively. Strikingly, adults with developmental prosopagnosia (face blindness) express an atypical structure-behavior relationship near face-selective cortex, suggesting that white-matter atypicalities in this region may have behavioral consequences. These data suggest that examining the interplay between cortical function, anatomical connectivity, and visual behavior is integral to understanding functional networks and their role in producing visual abilities and deficits.

The association of color memory and the enumeration of multiple spatially overlapping sets

Using dot displays, Halberda, Sires, and Feigenson (2006) showed that observers could simultaneously encode the numerosity of two spatially overlapping sets and the superset of all items at a glance. With the brief display and the masking used in Halberda et al., the task required observers to encode the colors of each set in order to select and enumerate all the dots in that set. As such, the observed capacity limit for set enumeration could reflect a limit in visual short-term memory (VSTM) capacity for the set color rather than a limit in set enumeration per se. Here, we largely replicated Halberda et al. and found successful enumeration of approximately two sets (the superset was not probed). We also found that only about two and a half colors could be remembered from the colored dot displays whether or not the enumeration task was performed concurrently with the color VSTM task. Because observers must remember the color of a set prior to enumerating it, the under three-item VSTM capacity for color necessarily dictates that set enumeration capacity in this paradigm could not exceed two sets. Thus, the ability to enumerate multiple spatially overlapping sets is likely limited by VSTM capacity to retain the discriminating feature of these sets. This relationship suggests that the capacity for set enumeration cannot be considered independently from the capacity for the sets defining features.

Reduced spatial integration in the ventral visual cortex underlies face recognition deficits in developmental prosopagnosia

Developmental prosopagnosia (DP) is characterized by deficits in face recognition without gross brain abnormalities. However, the neural basis of DP is not well understood. We measured population receptive fields (pRFs) in ventral visual cortex of DPs and typical adults to assess the contribution of spatial integration to face processing. While DPs showed typical retinotopic organization of ventral visual cortex and normal pRF sizes in early visual areas, we found significantly reduced pRF sizes in face-selective regions and in intermediate areas hV4 and VO1. Across both typicals and DPs, face recognition ability correlated positively with pRF size in both face-selective regions and VO1, whereby participants with larger pRFs perform better. However, face recognition ability is correlated with both pRF size and ROI volume only in face-selective regions. These findings suggest that smaller pRF sizes in DP may reflect a deficit in spatial integration affecting holistic processing required for face recognition.

Characterizing the effects of feature salience and top-down attention in the early visual system

The visual system employs a sophisticated balance of attentional mechanisms: salient stimuli are prioritized for visual processing, yet observers can also ignore such stimuli when their goals require directing attention elsewhere. A powerful determinant of visual salience is local feature contrast: if a local region differs from its immediate surround along one or more feature dimensions, it will appear more salient. We used high-resolution functional MRI (fMRI) at 7T to characterize the modulatory effects of bottom-up salience and top-down voluntary attention within multiple sites along the early visual pathway, including visual areas V1-V4 and the lateral geniculate nucleus (LGN). Observers viewed arrays of spatially distributed gratings, where one of the gratings immediately to the left or right of fixation differed from all other items in orientation or motion direction, making it salient. To investigate the effects of directed attention, observers were cued to attend to the grating to the left or right of fixation, which was either salient or nonsalient. Results revealed reliable additive effects of top-down attention and stimulus-driven salience throughout visual areas V1-hV4. In comparison, the LGN exhibited significant attentional enhancement but was not reliably modulated by orientation- or motion-defined salience. Our findings indicate that top-down effects of spatial attention can influence visual processing at the earliest possible site along the visual pathway, including the LGN, whereas the processing of orientation- and motion-driven salience primarily involves feature-selective interactions that take place in early cortical visual areas.NEW & NOTEWORTHY While spatial attention allows for specific, goal-driven enhancement of stimuli, salient items outside of the current focus of attention must also be prioritized. We used 7T fMRI to compare salience and spatial attentional enhancement along the early visual hierarchy. We report additive effects of attention and bottom-up salience in early visual areas, suggesting that salience enhancement is not contingent on the observers attentional state.

Contrasting Bottom-up Saliency and Top-Down Attention in the Early Visual Pathway.

The visual system employs a sophisticated balance of attentional mechanisms: individuals can willfully guide attention to serve their goals, but still notice salient information in the environment outside of their current locus of attention. At its simplest, this saliency can be defined as a measure of feature contrast across the visual field, such that a local region that is unlike its surrounding context along one or more feature dimensions is deemed salient. Here, we used high-resolution fMRI at 7T to investigate whether the effects of bottom-up saliency and top-down voluntary attention operate independently or interactively in the human visual system, and to determine the stages of visual processing at which these mechanisms first emerge. We measured BOLD responses to a near-peripheral Gabor grating, presented among a field of other gratings. Depending on its orientation relative to the contextual patches, this stimulus patch could be either salient - for example, a vertically oriented Gabor patch amongst horizontal patches - or not salient, sharing all of its features with the surrounding context. To concurrently manipulate top-down attention, observers performed a demanding task that directed covert spatial attention to either the salient or non-salient patch. This design allowed us to directly compare the magnitude of BOLD responses to salient and non-salient items, and to attended and non-attended items. We found evidence of independent, additive effects of top-down attention and bottom-up saliency in retinotopic visual areas V1, V2, V3, and hV4. In contrast, fMRI response amplitudes in the lateral geniculate nucleus (LGN) showed significant enhancement due to top-down attention, but showed no evidence of reliable modulation by orientation saliency. We conclude that saliency representation emerges in feature-selective populations of neurons at early cortical stages of visual processing, and can be distinguished from independent effects of top-down spatial attention. Meeting abstract presented at VSS 2015.