Global shape aftereffects in composite radial frequency patterns

YesIndividual radial frequency (RF) patterns are generated by modulating a circle's radius as a sinusoidal function of polar angle and have been shown to tap into global shape processing mechanisms. Composite RF patterns can reproduce the complex outlines of natural shapes and examining these stimuli may allow us to interrogate global shape mechanisms that are recruited in biologically relevant tasks. We present evidence for a global shape aftereffect in a composite RF pattern stimulus comprising two RF components. Manipulations of the shape, location, size and spatial frequency of the stimuli revealed that this aftereffect could only be explained by the attenuation of intermediate-level global shape mechanisms. The tuning of the aftereffect to test stimulus size also revealed two mechanisms underlying the aftereffect; one that was tuned to size and one that was invariant. Finally, we show that these shape mechanisms may encode some RF information. However, the RF encoding we found was not capable of explaining the full extent of the aftereffect, indicating that encoding of other shape features such as curvature are also important in global shape processing.This research was supported by a Biotechnology and Biological Sciences Research Council (BBSRC) grant #BB/L007770/1

Retinotopic remapping of the visual system in deaf adults

Sound is a vital cue in helping hearing people orient their gaze and attention towards events outside their central line of sight, especially in the far periphery, where vision is poor. Without sound cues, deaf individuals must rely on vision as an ‘early warning system’ for peripheral events, and in fact numerous behavioural studies demonstrate that deaf adults have superior visual sensitivity, particularly to far peripheral stimuli. We asked whether an increased demand on peripheral vision throughout development might be reflected in early visual brain structures. Using functional magnetic resonance imaging (fMRI), we mapped visual field representations in 16 early, profoundly deaf adults and 16 hearing age-matched controls. To target the far periphery, we used wide-field retinotopic mapping stimuli to map visual field eccentricity out to 72°, well beyond conventional mapping studies. Deaf individuals exhibited a larger representation of the far peripheral visual field in both the primary visual cortex and the lateral geniculate nucleus of the thalamus. Importantly, this was not due to a total expansion of the visual map, as there was no difference between groups in overall size of either structure, but a smaller representation of the central visual field in the deaf group, suggesting a redistribution of neural resources. Here, we demonstrate for the first time that the demands placed on vision due to lifelong hearing loss can sculpt visual maps at the first level of inputs from the retina, increasing neural resources for processing stimuli in the far peripheral visual field

Assessing functional reorganization in visual cortex with simulated retinal lesions

Macular degeneration (MD) causes central vision loss, removing input to corresponding representations in the primary visual cortex. There is disagreement concerning whether the cortical regions deprived of input can remain responsive, and the source of reported cortical responses is still debated. To simulate MD in controls, normally sighted participants viewed a bright central disk to adapt the retina, creating a transient ‘retinal lesion’ during a functional MRI experiment. Participants viewed blocks of faces, scrambled faces and uniform grey stimuli, either passively or whilst performing a one-back task. To assess the impact of the simulated lesion, participants repeated the paradigm using a more conventional mean luminance simulated scotoma without adaptation. Our results suggest our attempt to create a more realistic simulation of a lesion did not impact on responses in the representation of the simulated lesion. While most participants showed no evidence of stimulus-driven activation within the lesion representation, a few individuals (22%) exhibited responses similar to a participant with juvenile MD who completed the same paradigm (without adaptation). Reliability analysis showed that responses in the representation of the lesion were generally consistent irrespective of whether positive or negative. We provide some evidence that peripheral visual stimulation can also produce responses in central representations in controls while performing a task. This suggests that the ‘signature of reorganization of visual processing’, is not found solely in patients with retinal lesions, consistent with the idea that activity may be driven by unmasked top–down feedback

Pigmentation predicts the shift in the line of decussation in humans with albinism

In albinism a large proportion of nerve fibres originating in temporal retina cross the midline at the chiasm and project to the contralateral hemisphere. Studies in rodents with albinism have suggested that the extent of this misrouting at the chiasm is inversely related to pigmentation levels. Here, we examine whether there is evidence for a similar relationship in humans with albinism. Functional MRI was performed on 18 subjects with albinism, 17 control subjects and six controls with nystagmus as they underwent hemifield visual stimulation of nasal or temporal retina. Functional activation in 16 coronal slices beginning at the posterior occipital lobes were analysed and the extent of hemispheric response lateralization at each slice position was determined. During temporal retina stimulation, the control response was lateralized to the hemisphere ipsilateral to the stimulated eye for all slices. In albinos, the response in posterior slices was predominantly in the contralateral hemisphere, consistent with misrouting of temporal retina fibres. However, as slice location became progressively anterior, response lateralization reverted to the ipsilateral hemisphere. The slice location at which the transition from contra- to ipsilateralization occurred provided an estimate of the extent of fibre misrouting in the individual. The slice transition location correlated negatively with pigmentation level, providing the first evidence for a relationship between pigmentation and the extent of misrouting in humans with albinism

Organization of the visual cortex in human albinism

In albinism there is an abnormal projection of part of the temporal retina to the visual cortex contralateral to the eye. This projection, together with the normally routed fibers from nasal retina, provides a cortical hemisphere with visual input from more than the normal hemifield of visual space. In many mammalian models of albinism, a possible sensory mismatch in the visual cortex is avoided either by reorganization of the thalamocortical connections to give the abnormal input an exclusive cortical representation, or by the abnormal input being substantially suppressed. In this study we examine, with fMRI, how the human visual cortex topographically maps its input in albinism. We find that the input from temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas. The abnormal routing of temporal fibers is not total, with the line of decussation shifting to between 6 and 14degrees into temporal retina. Our results indicate that the abnormal input to visual cortex in human albinism does not undergo topographic reorganization between the thalamus and cortex. Furthermore, the abnormal input is not significantly suppressed in either striate or extrastriate areas. The topographic mapping that we report in human does not conform, therefore, to the commonly observed patterns in other mammals but takes the form of the "true albino" pattern that has been reported rarely in cat and in the only other individual primate studied

Induced deficits in speed perception by transcranial magnetic stimulation of human cortical areas V5/MT+ and V3A

noIn this report, we evaluate the role of visual areas responsive to motion in the human brain in the perception of stimulus speed. We first identified and localized V1, V3A, and V5/MT+ in individual participants on the basis of blood oxygenation level-dependent responses obtained in retinotopic mapping experiments and responses to moving gratings. Repetitive transcranial magnetic stimulation (rTMS) was then used to disrupt the normal functioning of the previously localized visual areas in each participant. During the rTMS application, participants were required to perform delayed discrimination of the speed of drifting or spatial frequency of static gratings. The application of rTMS to areas V5/MT and V3A induced a subjective slowing of visual stimuli and ( often) caused increases in speed discrimination thresholds. Deficits in spatial frequency discrimination were not observed for applications of rTMS to V3A or V5/MT+. The induced deficits in speed perception were also specific to the cortical site of TMS delivery. The application of TMS to regions of the cortex adjacent to V5/MT and V3A, as well as to area V1, produced no deficits in speed perception. These results suggest that, in addition to area V5/MT+, V3A plays an important role in a cortical network that underpins the perception of stimulus speed in the human brain.BBSR

Specialized and independent processing of orientation and shape in visual field maps LO1 and LO2

We identified human visual field maps, LO1 and LO2, in object-selective lateral occipital cortex. Using transcranial magnetic stimulation (TMS), we assessed the functions of these maps in the perception of orientation and shape. TMS of LO1 disrupted orientation, but not shape, discrimination, whereas TMS of LO2 disrupted shape, but not orientation, discrimination. This double dissociation suggests that specialized and independent processing of different visual attributes occurs in LO1 and LO2