Serge O. Dumoulin

The cortical deficit in humans with strabismic amblyopia

To further our understanding of the cortical deficit in strabismic amblyopia, we measured, compared and mapped functional magnetic resonance imaging (fMRI) activation between the fixing and fellow amblyopic eyes of ten strabismic amblyopes. Of specific concern was whether the function of any visual area was spared in strabismic amblyopia, as recently suggested by both positron emission tomography (PET) and fMRI studies, and whether there was a close relationship between the fMRI response and known psychophysical deficits. To answer these questions we measured the psychophysical deficit in each subject and used stimuli whose relationship to the psychophysical deficit was known. We observed that stimuli that were well within the amblyopic passband did produce reduced fMRI activation, even in visual area V1. This suggests that V1 is anomalous in amblyopia. A similar level of reduction was observed in V2. In two subjects, we found that stimuli outside the amblyopic passband produced activation in visual area V3A. We did not find a close relationship between the fMRI response reduction in amblyopia and either of the known psychophysical deficits even though the fMRI response reduction in amblyopia did covary with stimulus spatial frequency.

Selectivity of human retinotopic visual cortex to S-cone-opponent, L⁄M-cone-opponent and achromatic stimulation

Our aim was to make a quantitative comparison of the response of the different visual cortical areas to selective stimulation of the two different cone‐opponent pathways [long‐ and medium‐wavelength (L/M)‐ and short‐wavelength (S)‐cone‐opponent] and the achromatic pathway under equivalent conditions. The appropriate stimulus‐contrast metric for the comparison of colour and achromatic sensitivity is unknown, however, and so a secondary aim was to investigate whether equivalent fMRI responses of each cortical area are predicted by stimulus contrast matched in multiples of detection threshold that approximately equates for visibility, or direct (cone) contrast matches in which psychophysical sensitivity is uncorrected. We found that the fMRI response across the two colour and achromatic pathways is not well predicted by threshold‐scaled stimuli (perceptual visibility) but is better predicted by cone contrast, particularly for area V1. Our results show that the early visual areas (V1, V2, V3, VP and hV4) all have robust responses to colour. No area showed an overall colour preference, however, until anterior to V4 where we found a ventral occipital region that has a significant preference for chromatic stimuli, indicating a functional distinction from earlier areas. We found that all of these areas have a surprisingly strong response to S‐cone stimuli, at least as great as the L/M response, suggesting a relative enhancement of the S‐cone cortical signal. We also identified two areas (V3A and hMT+) with a significant preference for achromatic over chromatic stimuli, indicating a functional grouping into a dorsal pathway with a strong magnocellular input.

Cortical specialization for concentric shape processing

It is current dogma that neurons in primary visual cortex extract local edges from the scene, from which later visual areas reconstruct more meaningful shapes. In intermediate areas, such as area V4, responses are driven by features more complex than local oriented edges but more basic than meaningful shapes. The present study was motivated by the proposal that concentric (circular) shape processing is an important aspect of intermediate shape processing and is proposed to occur in area V4. However, previous studies are not able to discriminate between the number of orientations within the image nor how these orientations vary across space (orientation gradient, contrast or curvature) as opposed to concentric shape processing per se. We address the question whether V4 responses are driven by curvature or circularity. We use fMRI and tightly controlled narrowband stimuli with identical local and global properties. These patterns either form random or circular patterns with tightly matched orientation gradients and therefore similar curvature. We find stronger responses to circular patterns in areas V3/VP and V4. Our results suggest that extracting circular shape is an important step in intermediate shape processing.

Color responses of the human lateral geniculate nucleus: selective amplification of S‐cone signals between the lateral geniculate nucleno and primary visual cortex measured with high‐field fMRI

The lateral geniculate nucleus (LGN) is the primary thalamic nucleus that relays visual information from the retina to the primary visual cortex (V1) and has been extensively studied in non‐human primates. A key feature of the LGN is the segregation of retinal inputs into different cellular layers characterized by their differential responses to red‐green (RG) color (L/M opponent), blue‐yellow (BY) color (S‐cone opponent) and achromatic (Ach) contrast. In this study we use high‐field functional magnetic resonance imaging (4 tesla, 3.6u2003×u20033.6u2003×u20033u2003mm3) to record simultaneously the responses of the human LGN and V1 to chromatic and Ach contrast to investigate the LGN responses to color, and how these are modified as information transfers between LGN and cortex. We find that the LGN has a robust response to RG color contrast, equal to or greater than the Ach response, but a significantly poorer sensitivity to BY contrast. In V1 at low temporal rates (2u2003Hz), however, the sensitivity of the BY color pathway is selectively enhanced, rising in relation to the RG and Ach responses. We find that this effect generalizes across different stimulus contrasts and spatial stimuli (1‐d and 2‐d patterns), but is selective for temporal frequency, as it is not found for stimuli at 8u2003Hz. While the mechanism of this cortical enhancement of BY color vision and its dynamic component is unknown, its role may be to compensate for a weak BY signal originating from the sparse distribution of neurons in the retina and LGN.

Sparsely distributed contours dominate extra-striate responses to complex scenes

The human visual system exploits redundancy in natural scenes to derive useful information. Such redundancy is frequently associated with either contours or textures within images. In this study we use fMRI to evaluate how the total amount of contrast-energy contained in contours and textures within natural images affect responses in visual cortex. We used both the entire natural image and parts of it containing predominantly contour or texture information. We modified these natural images in order to match other image properties that are known to affect cortical responses as closely as possible. These modified natural images, i.e. pseudo-natural images, remain highly recognizable. We also used synthetic images without recognizable content but with closely matched image properties. We report that most of the primary visual cortex (V1) signal variations are explained by the total amount of contrast-energy in the images. Extra-striate visual cortex, on the other hand, is driven strongest by images containing sparsely distributed contours; independent of contrast-energy amount or recognizable image content. These results provide evidence for an initial representation of natural images in V1 based on local oriented filters. Later visual cortex (and to a modest degree V1) incorporates a facilitation of contour-based structure and suppressive interactions that effectively amplify sparse-contour information within natural images.

Centrifugal bias for second-order but not first-order motion.

Limited-lifetime Gabor stimuli were used to assess both first- and second-order motion in peripheral vision. Both first- and second-order motion mechanisms were present at a 20-deg eccentricity. Second-order motion, unlike first-order, exhibits a bias for centrifugal motion, suggesting a role for the second-order mechanism in optic flow processing.

How many positions can we perceptually encode, one or many?

Here we show that our sensitivity for discriminating relative position across the visual field is limited. In experiment 1 we show that we are much worse at detecting a texture defined by the relative position of elements within an array than would be expected if we had access to multiple estimates of relative position across the visual field. In experiment 2 we show that human performance is impaired for positional judgments when there is uncertainty as to which of a number of possible elements is misaligned. This impairment is greater than one would expect from an ideal observer model and greater than that found for a comparable task involving orientation. It is consistent with positional thresholds being determined by only one estimate of relative position. In experiment 3 we estimate the number of suprathreshold positional signals that can be pooled at the same time across the visual field using a standard summation variance paradigm. The results suggest that the human visual system is limited to one estimate of position, but additional estimates can be built up serially over time; however, this process is slow and probably cognitive in nature. These experiments taken as a whole suggest that only one estimate of relative position (i.e. relative to a predefined reference) at a time is accessible at the perceptual level.