Synesthetes show normal sound-induced flash fission and fusion illusions.

Idiopathic synesthesia, a neurological condition in which a stimulus in one sense generates a concurrent experience in a different sense, is often considered an example of multisensory integration. Consequently it has been suggested that synesthetes should experience multisensory illusions more consistently and compellingly than typical participants. To test this we measured the sound induced flash fission and fusion illusions in 22 coloured hearing synesthetes and 31 control participants. Analysis of the data using signal detection analysis, however, indicated no difference between the groups, either in perception or response bias, but a secondary analysis of the data did show evidence of a decline in the illusions for synesthetes with increasing age.

Integration and segregation of multiple motion signals by neurons in area MT of primate.

We used multielectrode arrays to measure the response of populations of neurons in primate middle temporal area to the transparent motion of two superimposed dot fields moving in different directions. The shape of the population response was well predicted by the sum of the responses to the constituent fields. However, the population response profile for transparent dot fields was similar to that for coherent plaid motion and hence an unreliable cue to transparency. We then used single-unit recording to characterize component and pattern cells from their response to drifting plaids. Unlike for plaids, component cells responded to the average direction of superimposed dot fields, whereas pattern cells could signal the constituent motions. This observation provides support for a strong prediction of the Simoncelli and Heeger (1998) model of motion analysis in area middle temporal, and suggests that pattern cells have a special status in the processing of superimposed dot fields.

Gain control in the response of human visual cortex to plaids.

A recent intrinsic signal optical imaging study in tree shrew showed, surprisingly, that the population response of V1 to plaid patterns comprising grating components of equal contrast is predicted by the average of the responses to the individual components (MacEvoy SP, Tucker TR, Fitzpatrick D. Nat Neurosci 12: 637-645, 2009). This prompted us to compare responses to plaids and gratings in human visual cortex as a function of contrast and orientation. We found that the functional MRI (fMRI) blood oxygenation level-dependent (BOLD) responses of areas V1-V3 to a plaid comprising superposed grating components of equal contrast are significantly higher than the responses to a single component. Furthermore, the orientation response profile of a plaid is poorly predicted from a linear combination of the responses to its components. Together, these results indicate that the model of MacEvoy et al. (2009) cannot, without modification, account for the fMRI BOLD response to plaids in human visual cortex.

Color responsiveness argues against a dorsal component of human V4.

The retinotopic organization, position, and functional responsiveness of some early visual cortical areas in human and non-human primates are consistent with their being homologous structures. The organization of other areas remains controversial. A critical debate concerns the potential human homologue of macaque area V4, an area very responsive to colored images: specifically, whether human V4 is divided between ventral and dorsal components, as in the macaque, or whether human V4 is confined to one ventral area. We used fMRI to define these areas retinotopically in human and to test the impact of image color on their responsivity. We found a robust preference for full-color movie segments over a luminance-matched achromatic version in ventral V4 but little or no preference in the vicinity of the putative dorsal counterpart. Contrary to previous reports that visual field coverage in the ventral part of V4 is deficient without the dorsal part, we found that coverage in ventral V4 extended to the lower vertical meridian, including the entire contralateral hemifield. Together these results provide evidence against a dorsal component of human V4. Instead, they are consistent with human V4 being a single, ventral region that is sensitive to the chromatic components of images.

Rapid serial visual presentation of motion: short-term facilitation and long-term suppression.

The visual system can detect coherent motion in the midst of motion noise. This is accomplished with motion-sensitive channels, each of which is tuned to a limited range of motion directions. Our aim was to show how a single channel is affected by motions both within and outside its tuning range. We used a psychophysical reverse-correlation procedure. An array of dots moved coherently with a new, randomly chosen, direction every 14 or 28 ms. Human subjects pressed a key whenever they saw upwards movement. The results were analyzed by finding two motion directions before each key-press: the first preceded the key-press by the reaction time, and the second preceded the first by a variable interval. There were two main findings. First, the subject was significantly more likely to press the key when the vector average of the two motions was in the target direction. This effect was short-lived: it was only seen for inter-stimulus intervals of several tens of milliseconds. Second, motion detection was reduced when the target direction was preceded by a motion of similar direction 100-200 ms earlier. The results support the idea that a motion-sensitive channel sums sub-optimal inputs, and is suppressed by similar motion in the long term.

Combination of subcortical color channels in human visual cortex.

Mechanisms of color vision in cortex have not been as well characterized as those in sub-cortical areas, particularly in humans. We used fMRI in conjunction with univariate and multivariate (pattern) analysis to test for the initial transformation of sub-cortical inputs by human visual cortex. Subjects viewed each of two patterns modulating in color between orange-cyan or lime-magenta. We tested for higher order cortical representations of color capable of discriminating these stimuli, which were designed so that they could not be distinguished by the postulated L-M and S-(L + M) sub-cortical opponent channels. We found differences both in the average response and in the pattern of activity evoked by these two types of stimuli, across a range of early visual areas. This result implies that sub-cortical chromatic channels are recombined early in cortical processing to form novel representations of color. Our results also suggest a cortical bias for lime-magenta over orange-cyan stimuli, when they are matched for cone contrast and the response they would elicit in the L-M and S-(L + M) opponent channels.

The influence of global form on local orientation anisotropies in human visual cortex.

Perception of the spatial structure of the environment results from visual system processes which integrate local information to produce global percepts. Here, we investigated whether particular global spatial arrangements evoke greater responses in the human visual system, and how such anisotropies relate to those evident in the responses to the local elements that comprise the global form. We presented observers with Glass patterns; images composed of randomly positioned dot pairings (dipoles) spatially arranged to produce a percept of translational or polar global form. We used functional magnetic resonance imaging (fMRI) to infer the magnitude of neural activity within early retinotopic regions of visual cortex (V1, V2, V3, V3A/B, and hV4) while the angular arrangement of the dipoles was modulated over time to sample the range of orientations. For both translational and polar Glass patterns, V1 showed an increased response to vertical dipole orientations and all visual areas showed a bias towards dipole orientations that were radial to the point of fixation. However, areas V1, V2, V3, and hV4 also demonstrated a bias, only present for polar Glass patterns, towards dipole orientations that were tangential to the point of fixation. This enhanced response to tangential orientations within polar form indicates sensitivity to curvature or more global form characteristics as early as primary visual cortex.

Orientation anisotropies in human visual cortex.

Representing the orientation of features in the visual image is a fundamental operation of the early cortical visual system. The nature of such representations can be informed by considering anisotropic distributions of response across the range of orientations. Here we used functional MRI to study modulations in the cortical activity elicited by observation of a sinusoidal grating that varied in orientation. We report a significant anisotropy in the measured blood-oxygen level-dependent activity within visual areas V1, V2, V3, and V3A/B in which horizontal orientations evoked a reduced response. These visual areas and hV4 showed a further anisotropy in which increased responses were observed for orientations that were radial to the point of fixation. We speculate that the anisotropies in cortical activity may be related to anisotropies in the prevalence and behavioral relevance of orientations in typical natural environments.

Radial biases in the processing of motion and motion-defined contours by human visual cortex.

Luminance gratings reportedly produce a stronger functional magnetic resonance imaging (fMRI) blood oxygen level-dependent (BOLD) signal in those parts of the retinotopic cortical maps where they are oriented radially to the point of fixation. We sought to extend this finding by examining anisotropies in the response of cortical areas V1-V3 to motion-defined contour stimuli. fMRI at 3 Tesla was used to measure the BOLD signal in the visual cortex of six human subjects. Stimuli were composed of strips of spatial white noise texture presented in an annular window. The texture in alternate strips moved in opposite directions (left-right or up-down). The strips themselves were static and tilted 45 degrees left or right from vertical. Comparison with maps of the visual field obtained from phase-encoded retinotopic analysis revealed systematic patterns of radial bias. For motion, a stronger response to horizontal was evident within V1 and along the borders between V2 and V3. For orientation, the response to leftward tilted contours was greater in left dorsal and right ventral V1-V3. Radial bias for the orientation of motion-defined contours analogous to that reported previously for luminance gratings could reflect cue-invariant processing or the operation of distinct mechanisms subject to similar anisotropies in orientation tuning. Radial bias for motion might be related to the phenomenon of "motion streaks," whereby temporal integration by the visual system introduces oriented blur along the axis of motion. We speculate that the observed forms of radial bias reflect a common underlying anisotropy in the representation of spatiotemporal image structure across the visual field.

Orientation-specific contextual modulation of the fMRI BOLD response to luminance and chromatic gratings in human visual cortex.

The responses of orientation-selective neurons in primate visual cortex can be profoundly affected by the presence and orientation of stimuli falling outside the classical receptive field. Our perception of the orientation of a line or grating also depends upon the context in which it is presented. For example, the perceived orientation of a grating embedded in a surround tends to be repelled from the predominant orientation of the surround. Here, we used fMRI to investigate the basis of orientation-specific surround effects in five functionally-defined regions of visual cortex: V1, V2, V3, V3A/LO1 and hV4. Test stimuli were luminance-modulated and isoluminant gratings that produced responses similar in magnitude. Less BOLD activation was evident in response to gratings with parallel versus orthogonal surrounds across all the regions of visual cortex investigated. When an isoluminant test grating was surrounded by a luminance-modulated inducer, the degree of orientation-specific contextual modulation was no larger for extrastriate areas than for V1, suggesting that the observed effects might originate entirely in V1. However, more orientation-specific modulation was evident in extrastriate cortex when both test and inducer were luminance-modulated gratings than when the test was isoluminant; this difference was significant in area V3. We suggest that the pattern of results in extrastriate cortex may reflect a refinement of the orientation-selectivity of surround suppression specific to the colour of the surround or, alternatively, processes underlying the segmentation of test and inducer by spatial phase or orientation when no colour cue is available.