Applying the LATER model to reaction time data: an open-source toolkit.

Analysing reaction time distributions can provide insights into decision-making processes in the brain. The Linear Approach to Threshold with Ergodic Rate (LATER) model is arguably the simplest model for predicting reaction time distributions and can summarise distributions with as few as two free parameters. However, the coordinates for visualising and fitting distributions using LATER ("reciprobit" space) are irregular, making the application of this simple model inaccessible to those without a programming background.

Estimating lighting direction in scenes with multiple objects.

To recover the reflectance and shape of an object in a scene, the human visual system must account for the properties of the light illuminating the object. Here, we examine the extent to which multiple objects within a scene are utilised to estimate the direction of lighting in a scene. In Experiment 1, we presented participants with rendered scenes that contained 1, 9, or 25 unfamiliar blob-like objects and measured their capacity to discriminate whether a directional light source was left or right of the participants' vantage point.

Relating Sound and Sight in Simulated Environments.

The auditory signals at the ear can be affected by components arriving both directly from a sound source and indirectly via environmental reverberation. Previous studies have suggested that the perceptual separation of these contributions can be aided by expectations of likely reverberant qualities. Here, we investigated whether vision can provide information about the auditory properties of physical locations that could also be used to develop such expectations.

Sensory attenuation is modulated by the contrasting effects of predictability and control.

Self-generated stimuli have been found to elicit a reduced sensory response compared with externally-generated stimuli. However, much of the literature has not adequately controlled for differences in the temporal predictability and temporal control of stimuli. In two experiments, we compared the N1 (and P2) components of the auditory-evoked potential to self- and externally-generated tones that differed with respect to these two factors.

Seeing the Intensity of a Sound-producing Event Modulates the Amplitude of the Initial Auditory Evoked Response.

An auditory event is often accompanied by characteristic visual information. For example, the sound level produced by a vigorous handclap may be related to the speed of hands as they move toward collision. Here, we tested the hypothesis that visual information about the intensity of auditory signals are capable of altering the subsequent neurophysiological response to auditory stimulation.

Surface curvature from kinetic depth can affect lightness.

The light reaching the eye confounds the proportion of light reflected from surfaces in the environment with their illumination. To achieve constancy in perceived surface reflectance (lightness) across variations in illumination, the visual system must infer the relative contribution of reflectance to the incoming luminance signals. Previous studies have shown that contour and stereo cues to surface shape can affect the lightness of sawtooth luminance profiles.

The ability to tickle oneself is associated with level of psychometric schizotypy in non-clinical individuals.

A recent study (Lemaitre et al., 2016, Consciousness and Cognition, 41, 64-71) found that non-clinical individuals who scored highly on a psychometric scale of schizotypy were able to tickle themselves. The present study aimed to extend this finding by investigating whether the ability to tickle oneself was associated with level of psychometric schizotypy considered as a continuous variable.

Correlates of Perceptual Orientation Biases in Human Primary Visual Cortex.

Vision can be considered as a process of probabilistic inference. In a Bayesian framework, perceptual estimates from sensory information are combined with prior knowledge, with a stronger influence of the prior when the sensory evidence is less certain. Here, we explored the behavioral and neural consequences of manipulating stimulus certainty in the context of orientation processing.

No apparent influence of psychometrically-defined schizotypy on orientation-dependent contextual modulation of visual contrast detection.

We investigated the relationship between psychometrically-defined schizotypy and the ability to detect a visual target pattern. Target detection is typically impaired by a surrounding pattern (context) with an orientation that is parallel to the target, relative to a surrounding pattern with an orientation that is orthogonal to the target (orientation-dependent contextual modulation). Based on reports that this effect is reduced in those with schizophrenia, we hypothesised that there would be a negative relationship between the relative score on psychometrically-defined schizotypy and the relative effect of orientation-dependent contextual modulation.

Human visual cortical responses to specular and matte motion flows.

Determining the compositional properties of surfaces in the environment is an important visual capacity. One such property is specular reflectance, which encompasses the range from matte to shiny surfaces. Visual estimation of specular reflectance can be informed by characteristic motion profiles; a surface with a specular reflectance that is difficult to determine while static can be confidently disambiguated when set in motion.

Scene coherence can affect the local response to natural images in human V1.

Neurons in primary visual cortex (V1) can be indirectly affected by visual stimulation positioned outside their receptive fields. Although this contextual modulation has been intensely studied, we have little notion of how it manifests with naturalistic stimulation. Here, we investigated how the V1 response to a natural image fragment is affected by spatial context that is consistent or inconsistent with the scene from which it was extracted.

Sensitivity to the visual field origin of natural image patches in human low-level visual cortex.

Asymmetries in the response to visual patterns in the upper and lower visual fields (above and below the centre of gaze) have been associated with ecological factors relating to the structure of typical visual environments. Here, we investigated whether the content of the upper and lower visual field representations in low-level regions of human visual cortex are specialised for visual patterns that arise from the upper and lower visual fields in natural images. We presented image patches, drawn from above or below the centre of gaze of an observer navigating a natural environment, to either the upper or lower visual fields of human participants (n = 7) while we used functional magnetic resonance imaging (fMRI) to measure the magnitude of evoked activity in the visual areas V1, V2, and V3.

Orientation decoding: Sense in spirals?

The orientation of a visual stimulus can be successfully decoded from the multivariate pattern of fMRI activity in human visual cortex. Whether this capacity requires coarse-scale orientation biases is controversial. We and others have advocated the use of spiral stimuli to eliminate a potential coarse-scale bias-the radial bias toward local orientations that are collinear with the centre of gaze-and hence narrow down the potential coarse-scale biases that could contribute to orientation decoding.

Regions of mid-level human visual cortex sensitive to the global coherence of local image patches.

The global structural arrangement and spatial layout of the visual environment must be derived from the integration of local signals represented in the lower tiers of the visual system. This interaction between the spatially local and global properties of visual stimulation underlies many of our visual capacities, and how this is achieved in the brain is a central question for visual and cognitive neuroscience. Here, we examine the sensitivity of regions of the posterior human brain to the global coordination of spatially displaced naturalistic image patches.

Consequences of polar form coherence for fMRI responses in human visual cortex.

Relevant features in the visual image are often spatially extensive and have complex orientation structure. Our perceptual sensitivity to such spatial form is demonstrated by polar Glass patterns, in which an array of randomly-positioned dot pairs that are each aligned with a particular polar displacement (rotation, for example) yield a salient impression of spatial structure. Such patterns are typically considered to be processed in two main stages: local spatial filtering in low-level visual cortex followed by spatial pooling and complex form selectivity in mid-level visual cortex.

Impaired processing of binaural temporal cues to auditory scene analysis in schizophrenia.

It is well established that individuals with schizophrenia demonstrate alterations in auditory perception beginning at the very earliest stages of information processing. However, it is not clear how these impairments in basic information processing translate into high-order cognitive deficits. Auditory scene analysis allows listeners to group auditory information into meaningful objects, and as such provides an important link between low-level auditory processing and higher cognitive abilities.

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.

Cortical and behavioral sensitivity to eccentric polar form.

Patterns composed of local features aligned relative to polar angle, yielding starbursts, concentric circles, and spirals, can inform the understanding of spatial form perception. Previous studies have shown that starburst and concentric form instantiated in Glass patterns are, relative to spirals, both more readily detected in noise and evoke higher levels of blood-oxygen level-dependent (BOLD) signal, as measured with functional magnetic resonance imaging (fMRI), in the retinotopic cortex. However, such studies have typically presented the polar form at the center of gaze, which confounds the distribution of local orientations relative to fixation with variations in polar form.

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.

Orientation-selective chromatic mechanisms in human visual cortex.

We used functional magnetic resonance imaging (fMRI) at 3T in human participants to trace the chromatic selectivity of orientation processing through functionally defined regions of visual cortex. Our aim was to identify mechanisms that respond to chromatically defined orientation and to establish whether they are tuned specifically to color or operate in an essentially cue-invariant manner. Using an annular test region surrounded inside and out by an inducing stimulus, we found evidence of sensitivity to orientation defined by red-green (L-M) or blue-yellow (S-cone isolating) chromatic modulations across retinotopic visual cortex and of joint selectivity for color and orientation.

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.

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.

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.

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.