Elliot Freeman | City, University of London (original) (raw)

Papers by Elliot Freeman

ResearchOnline - JCU (James Cook University), Jun 1, 2008

F1000Research, Aug 1, 2015

Perception, 2006

Recent brain-imaging studies of bistable phenomena (eg binocular rivalry and bistable apparent mo... more Recent brain-imaging studies of bistable phenomena (eg binocular rivalry and bistable apparent motion) have shown neural correlates of subjective switches in extrastriate visual cortical areas. Little is known about the exact role of such switch-related activations in the dynamic resolution of perceptual conflicts. Here, we used functional magnetic resonance imaging to investigate the fine-tuning of switch-related responses to the visual features that are perceived to change in bistable motion stimuli. We examined the phenomenon of ...

Journal of Vision, Apr 13, 2012

Oscillatory synchronization of neuronal populations has been proposed to play a role in perceptua... more Oscillatory synchronization of neuronal populations has been proposed to play a role in perceptual integration and attentional processing. However, some conflicting evidence has been found with respect to its causal relevance for sensory processing, particularly when using flickering visual stimuli with the aim of driving oscillations. We tested psychophysically whether the relative phase of gamma frequency flicker (60 Hz) between stimuli modulates well-known facilitatory lateral interactions between collinear Gabor patches (Experiment 1) or crowding of a peripheral target by irrelevant distractors (Experiment 2). Experiment 1 assessed the impact of suprathreshold Gabor flankers on detection of a near-threshold central Gabor target ("Lateral interactions paradigm"). The flanking stimuli could flicker either in phase or in anti-phase with each other. The typical facilitation of target detection was found with collinear flankers, but this was unaffected by flicker phase. Experiment 2 employed a "crowding" paradigm, where orientation discrimination of a peripheral target Gabor patch is disrupted when surrounded by irrelevant distractors. We found the usual crowding effect, which declined with spatial separation, but this was unaffected by relative flicker phase between target and distractors at all separations. These results imply that externally driven manipulations of gamma frequency phase cannot modulate perceptual integration in vision.

Journal of Experimental Psychology: Human Perception and Performance, Aug 1, 2018

Sight and sound are out of synch in different people by different amounts for different tasks. Bu... more Sight and sound are out of synch in different people by different amounts for different tasks. But surprisingly, different concurrent measures of perceptual asynchrony correlate negatively (Freeman, Ipser et al, 2013. Cortex 49, 2875-2887): thus if vision subjectively leads audition in one individual, the same individual might show a visual lag in other measures of audiovisual integration (e.g. McGurk illusion, Stream-Bounce illusion). This curious negative correlation was first observed between explicit temporal order judgements and implicit phoneme identification tasks, performed concurrently as a dual task, using incongruent McGurk stimuli. Here we used a new set of different of explicit and implicit tasks and congruent stimuli, to test whether this negative correlation persists across testing sessions, and whether it might be an artefact of using specific incongruent stimuli. None of these manipulations eliminated the negative correlation between explicit and implicit measures. This supports the generalisability and validity of the phenomenon, and offers new theoretical insights into its explanation. Our previously proposed 'temporal renormalization' theory assumes that the timings of sensory events registered within the brain's different multimodal sub-networks are each perceived relative to a representation of the typical average timing of such events across the wider network. Our new data suggest that this representation is stable and generic, rather than dependent on specific stimuli or task contexts, and that it may be acquired through experience with a variety of simultaneous stimuli. Our results also add further evidence that speech comprehension may be improved in some individuals by artificially delaying voices relative to lip-movements.

Journal of Vision, Mar 24, 2010

Abstract We examined whether segmentation can alleviate the effect of a superimposed mask. We mea... more Abstract We examined whether segmentation can alleviate the effect of a superimposed mask. We measured contrast increment thresholds for a horizontal static test grating (2 cpd, 30% contrast) under three conditions: 1) the test grating presented alone; 2) with a similar-sized superimposed mask grating of orthogonal orientation, creating a plaid; 3) with a superimposed mask having twice the diameter of the test. We hypothesized that this large mask condition might facilitate segmentation of the test. Masks had fixed contrast of 30%. ...

Cortex, Oct 1, 2020

Visual motion or flashing lights can evoke auditory sensations in some people. This largescale in... more Visual motion or flashing lights can evoke auditory sensations in some people. This largescale internet study aimed to validate a combined subjective/objective test of the genuineness of this putative form of synaesthesia (visually-evoked auditory response, vEAR). Correlations were measured between each individual's ratings of the vividness of auditory sensations evoked by a series of looping videos, and measurement of the videos' physical low-level motion energy, calculated using Adelson and Bergen's (1985) computational model of low-level visual motion processing. The strength of this association for each individual provided a test of how strongly subjective vEAR was driven by objective motion energy ('ME-sensitivity'). A second aim was to infer whether vEAR depends on cortical excitation and/or disinhibition of early visual and/or auditory brain areas. To achieve this, correlations were measured between the above vEAR measures and visual contrast surround-suppression, which is thought to index lateral inhibition in the early visual system. As predicted by a disinhibition account of vEAR, video ratings were overall higher in individuals showing weaker surround-suppression. Interestingly, surround-suppression and ME-sensitivity did not correlate. Additionally, both surround-suppression and ME-sensitivity each independently predicted different clusters of trait measures selected for their possible association with cortical excitability and/or disinhibition: Surround-suppression was associated with vEAR self-ratings and auditory-evoked visual phosphenes, while MEsensitivity was independently associated with ratings of other traits including susceptibility to migraine and pattern glare. Altogether, these results suggest there are two independent mechanisms underlying vEAR and its associated traits, based putatively on cortical disinhibition versus excitability.

Journal of Vision, Jun 29, 2012

We used fMRI to examine the neural correlates of subjective reversals for bistable structure-from... more We used fMRI to examine the neural correlates of subjective reversals for bistable structure-from-motion. We compared transparent random-dot kinematograms depicting either a cylinder rotating in depth, or two flat surfaces translating in opposite directions at apparently different depths. For both such stimuli the motion of dots on the different apparent depth planes typically appears to reverse direction periodically on prolonged viewing. Yet for cylindrical but not flat stimuli, such subjective reversals also coincide with apparent reversal of 3D rotation direction. We hypothesised that the lateral occipital complex (region LOC), sensitive to 3D form, might show greater event-related activity for subjective reversals of cylindrical than flat stimuli; conversely, motion-sensitive hMT+/V5 should respond in common to subjective reversals for either type of stimuli, since both are perceived as changes in planar motion. We obtained an event-related measure of neural activity associated with subjective reversals, after first factoring out block-related differences between cylindrical versus flat stimuli (and thereby the associated low-level blocked stimulus differences). In support of our hypothesis, only the cylindrical stimuli produced reversal-related activity in contralateral human LOC. In contrast, the hMT+/V5 complex was activated alike by subjective reversals for both cylindrical and flat stimuli. Intriguingly V1 also showed (contralateral) specificity for rotational reversals, suggesting a possible feedback influence from LOC. These results reveal specific neural correlates for subjective switches of 3D rotation versus translation, as distinct from subjective reversals in general.

Seeing and Perceiving, 2012

Due to physical and neural delays, the sight and sound of a person speaking causes a cachophony o... more Due to physical and neural delays, the sight and sound of a person speaking causes a cachophony of asynchronous events in the brain. How can we still perceive them as simultaneous? Our converging evidence suggests that actually, we do not. Patient PH, with midbrain and auditory brainstem lesions, experiences voices leading lipmovements by approximately 200 ms. In temporal order judgements (TOJ) he experiences simultaneity only when voices physically lag lips. In contrast, he requires the opposite visual lag (again of about 200 ms) to experience the classic McGurk illusion (e.g., hearing 'da' when listening to /ba/ and watching lips say [ga]), consistent with pathological auditory slowing. These delays seem to be specific to speech stimuli. Is PH just an anomaly? Surprisingly, neuro-typical individual differences between temporal tuning of McGurk integration and TOJ are actually negatively correlated. Thus some people require a small auditory lead for optimal McG but an auditory lag for subjective simultaneity (like PH but not as extreme), while others show the opposite pattern. Evidently, any individual can concurrently experience the same external events as happening at different times. These dissociative patterns confirm that distinct mechanisms for audiovisual synchronization versus integration are each subject to different neural delays. To explain the apparent repulsion of their respective timings, we propose that multimodal synchronization is achieved by discounting the average neural event time within each modality. Lesions or individual differences which slow the propagation of neural signals will then attract the average, so that relatively undelayed neural signals will be experienced as occurring relatively early.

Perception, Aug 1, 1996

The flexibility of perceptual grouping was examined from a functional perspective. If particular ... more The flexibility of perceptual grouping was examined from a functional perspective. If particular wholes are good for particular tasks, we can bend the rules of Gestalt organisation in order to see them? Evidence was gained by observing the effects of Gestalt factors on task performance, in the context of subjects' experience and expectation. Ellipsoids were split into two semi-ellipses by a central gap of variable width, which controlled the relative dominance of a one-whole percept (Narrow-Gap) or a two-parts percept (Wide-Gap). In a sequential comparison task, a reference ellipsoid was paired either with another ellipsoid (Whole/Whole), or with a single semi-ellipse (Whole/Part). Subjects responded “different” to a change in the shape of one semi-ellipse. Stimuli appeared for 50 ms and the reference stimulus was masked with stimulus onset asynchronies (SOAs) of 50 to 250 ms. Subjects were first trained either on Whole/Whole or Whole/Part only; both comparisons types were then mixed randomly. A loss of discriminability was predicted for Whole/Whole with Wide-Gap, and for Whole/Part with Narrow-Gap, but only for untrained and unexpected comparisons. The predicted interaction was significant. The effects were particularly strong with 50 ms SOA, suggesting an early involvement of task-contextual factors in perceptual grouping, and showing no evidence of global priority. Subjects appeared to readily learn flexible grouping strategies, abstracting only the behaviourally relevant relationships, and thus offsetting the effects of stimulus structure on perceptual grouping.

Nature Neuroscience, Sep 17, 2001

Detection of an oriented visual target can be facilitated by collinear visual flankers. Such late... more Detection of an oriented visual target can be facilitated by collinear visual flankers. Such lateral interactions are thought to reflect integrative processes in low-level vision. In past studies, the flankers were task-irrelevant, and were typically assumed to be unattended. Here we manipulated attention to the flankers directly, by requiring observers to judge the relative alignment of two flankers while ignoring a second flanker-pair. Under identical stimulus conditions, attended flankers produced typical lateral interactions, but ignored flankers did not. These data show that lateral interactions can depend on attention to the flanking context, revealing the functional consequences of attentional modulation in low-level vision.

Neuropsychologia, Sep 1, 2011

The magnitude dimensions of number, time and space have been suggested to share some common magni... more The magnitude dimensions of number, time and space have been suggested to share some common magnitude processing, which may imply symmetric interaction among dimensions. Here we challenge these suggestions by presenting a double dissociation between two neuropsychological patients with left (JT) and right (CB) parietal lesions and selective impairment of number and time processing respectively. Both patients showed an influence of task-irrelevant number stimuli on time but not space processing. In JT otherwise preserved time processing was severely impaired in the mere presence of task-irrelevant numbers, which themselves could not be processed accurately. In CB, impaired temporal estimation was influenced by preserved number processing: small numbers made (already grossly underestimated) time intervals appear even shorter relative to large numbers. However, numerical estimation was not influenced by time in healthy controls and in both patients. This new double dissociation between number and time processing and the asymmetric interaction of number on time: (1) provides further support to the hypothesis of a partly shared magnitude system among dimensions, instead of the proposal of a single, fully shared system or of independent magnitude systems which would not explain dissociations or interactions among dimensions; (2) may be explained in terms of a stable hierarchy of dimensions, with numbers being the strongest.

Experimental Brain Research, Jun 4, 2011

Several recent multisensory studies show that sounds can influence visual processing. Some visual... more Several recent multisensory studies show that sounds can influence visual processing. Some visual judgments can be enhanced for visual stimuli near a sound occurring around the same time. A recent TMS study (Romei et al. 2009) indicates looming sounds might influence visual cortex particularly strongly. But unlike most previous behavioral studies of possible audiovisual exogenous effects, TMS phosphene thresholds rather than judgments of external visual stimuli were measured. Moreover, the visual hemifield assessed relative to the hemifield of the sound was not varied. Here, we compared the impact of looming sounds to receding or ''static'' sounds, using auditory stimuli adapted from Romei et al. (2009), but now assessing any influence on visual orientation discrimination for Gabor patches (well-known to involve early visual cortex) when appearing in the same hemifield as the sound or on the opposite side. The looming sounds that were effective in Romei et al. (2009) enhanced visual orientation sensitivity (d 0) here on the side of the sound, but not for the opposite hemifield. This crossmodal, spatially specific effect was stronger for looming than receding or static sounds. Similarly to Romei et al. (2009), the differential effect for looming sounds was eliminated when using white noise rather than structured sounds. Our new results show that looming structured sounds can specifically benefit visual orientation sensitivity in the hemifield of the sound, even when the sound provides no information about visual orientation itself.

Cognition, Jun 1, 2001

Issues concerning selective attention provoke new questions about visual segmentation, and vice-v... more Issues concerning selective attention provoke new questions about visual segmentation, and vice-versa. We illustrate this by describing our recent work on grouping under conditions of inattention, on change blindness for background events and the residual processing of undetected background changes, on modal versus amodal completion in visual search, and the differential effects of these two forms of completion on attentional processes, and on attentional modulation of lateral interactions thought to arise in early visual cortex. Many of these results indicate that segmentation processes substantially constrain attentional processes, but the reverse in¯uence is also apparent, suggesting an interactive architecture. We discuss how the`proto-objects' revealed by studies of segmentation and attention (i.e. the segmented perceptual units which constrain selectivity) may relate to other object-based notions in cognitive science, and we wrestle with their relation to phenomenal visual awareness.

Frontiers in Psychology, 2011

To test whether atypical number development may affect other types of quantity processing, we inv... more To test whether atypical number development may affect other types of quantity processing, we investigated temporal discrimination in adults with developmental dyscalculia (DD). This also allowed us to test whether number and time may be sub-served by a common quantity system or decision mechanisms: if they do, both should be impaired in dyscalculia, but if number and time are distinct they should dissociate. Participants judged which of two successively presented horizontal lines was longer in duration, the first line being preceded by either a small or a large number prime ("1" or "9") or by a neutral symbol ("#"), or in a third task participants decided which of two Arabic numbers (either "1," "5," "9") lasted longer. Results showed that (i) DD's temporal discriminability was normal as long as numbers were not part of the experimental design, even as task-irrelevant stimuli; however (ii) task-irrelevant numbers dramatically disrupted DD's temporal discriminability the more their salience increased, though the actual magnitude of the numbers had no effect; in contrast (iii) controls' time perception was robust to the presence of numbers but modulated by numerical quantity: therefore small number primes or numerical stimuli seemed to make durations appear shorter than veridical, but longer for larger numerical prime or numerical stimuli. This study is the first to show spared temporal discriminationa dimension of continuous quantity-in a population with a congenital number impairment. Our data reinforce the idea of a partially shared quantity system across numerical and temporal dimensions, which supports both dissociations and interactions among dimensions; however, they suggest that impaired number in DD is unlikely to originate from systems initially dedicated to continuous quantity processing like time.