Biology subjects

Harrison, W. J.

Publications and source records attributed to Harrison, W. J..

4 recordsLinked to original sources

bioRxiv2017

Selective attention modulates surface filling-in

The visual system is required to compute objects from partial image structure so that figures can be segmented from their backgrounds. Although early clinical, behavioral, and modeling data suggested that such computations are performed pre-attentively, recent neurophysiological evidence suggests that surface filling-in is influenced by attention. In the present study we developed a variant of the classical Kanizsa illusory triangle to investigate whether voluntary attention modulates perceptual filling-in. Our figure consists of \"pacmen\" positioned at the tips of an illusory 6-point star and alternating in polarity such that two illusory triangles are implied to compete with one another within the figure. On each trial, observers were cued to attend to only one triangle, and then compared its lightness with a matching texture-defined triangle. We found that perceived lightness of the illusory shape depended on the polarity of pacmen framing the attended triangle, although the magnitude of this effect was weaker than when all inducers were of the same polarity. Our findings thus reveal that voluntary attention can influence lightness filling-in, and provide important data linking neurophysiological effects to phenomenology.

neuroscience↗

bioRxiv2017

Visual working memory is independent of the cortical spacing between memoranda

The sensory recruitment hypothesis states that visual short term memory is maintained in the same visual cortical areas that initially encode a stimulus features. Although it is well established that the distance between features in visual cortex determines their visibility, a limitation known as crowding, it is unknown whether short term memory is similarly constrained by the cortical spacing of memory items. Here we investigated whether the cortical spacing between sequentially presented memoranda affects the fidelity of memory in humans (of both sexes). In a first experiment, we varied cortical spacing by taking advantage of the log-scaling of visual cortex with eccentricity, sequentially presenting memoranda in peripheral vision along either the radial or tangential visual axis with respect to the fovea. In a second experiment, we sequentially presented memoranda either within or beyond the critical spacing of visual crowding, a distance within which visual features cannot be perceptually distinguished due to their nearby cortical representations. In both experiments and across multiple measures, we found strong evidence that the ability to maintain visual features in memory is unaffected by cortical spacing. These results indicate that the neural architecture underpinning working memory has properties inconsistent with the known behaviour of sensory neurons in visual cortex. Instead, the dissociation between perceptual and memory representations supports a role of higher cortical areas, such as posterior parietal or prefrontal regions, or may involve an as yet unspecified mechanism in visual cortex in which stimulus features are bound to their temporal order.\n\nSignificance StatementAlthough much is known about the resolution with which we can remember visual objects, the cortical representation of items held in short term memory remains contentious. A popular hypothesis suggests that memory of visual features is maintained via the recruitment of the same neural architecture in sensory cortex that encodes stimuli. We investigated this claim by manipulating the spacing in visual cortex between sequentially presented memoranda such that some items shared cortical representations more than others, while preventing perceptual interference between stimuli. We found clear evidence that short term memory is independent of the intra-cortical spacing of memoranda, revealing a dissociation between perceptual and memory representations. Our data indicate that working memory relies on different neural mechanisms from sensory perception.

neuroscience↗

bioRxiv2017

Wakefulness state modulates conscious access: Suppression of auditory detection in the transition to sleep

Mapping the reports of awareness and its neural underpinnings is instrumental to understand the limits of human perception. The capacity to become aware of objects in the world may be studied by suppressing faint target stimuli with strong masking stimuli, or - alternatively - by manipulating the level of wakefulness from full alertness to mild drowsiness. By combining these two approaches, we studied how perceptual awareness is modulated by decreasing wakefulness. We found dynamic changes in behavioural and neural signatures of conscious access in humans between awake and drowsy states. Behaviourally, we show a decrease in the steepness of the psychophysical function for conscious access in drowsy trials. Neural mapping showed delayed processing of target-mask interaction as the consciousness transition progressed, suggesting that the brain resolution of conscious access shifts from early sensory/perceptual to decision-making stages of processing. Once the goal to report the awareness of a target is set, the system behaviourally adapts to rapid changes in wakefulness, revealing the flexibility of the neural signatures of conscious access, and its suppression, to maintain performance. Significance statementMaintaining full alertness for long periods of time in attentionally demanding situations is challenging and may lead to a decrease in performance. We show the effect of wakefulness fluctuations on behaviour and brain dynamics that humans use to maintain performance. We reveal the neural strategies we have to cope with drowsiness by shifting the weights to more flexible brain processes and relaxing the precision of the decisions we take.

neuroscience↗

bioRxiv2016

Visual crowding is a combination of an increase of positional uncertainty, source confusion, and featural averaging

Although we perceive a richly detailed visual world, our ability to identify 1 individual objects is severely limited in clutter, particularly in peripheral vision. Models of such crowding have generally been driven by the phenomenological misidentifications of crowded targets: using stimuli that do not easily combine to form a unique symbol (e.g. letters or objects), observers typically confuse the source of objects and report either the target or a distractor, but when continuous features are used (e.g. orientated gratings or line positions) observers report a feature somewhere between the target and distractor. To reconcile these accounts, we develop a hybrid method of adjustment that allows detailed analysis of these multiple error categories. Observers reported the orientation of a target, under several distractor conditions, by adjusting an identical foveal target. We apply new modelling to quantify whether perceptual reports show evidence of positional uncertainty, source confusion, and featural averaging on a trial-by-trial basis. Our results show that observers make a large proportion of source-confusion errors. However, our study also reveals the distribution of perceptual reports that underlie performance in this crowding task more generally: aggregate errors cannot be neatly labelled because they are heterogeneous and their structure depends on target-distractor distance.

neuroscience↗