Optimal Encoding of Interval Timing in Expert Percussionists, J Neurosci, 3 (32), 1056-1060.

We measured temporal reproduction in human subjects with various levels of musical expertise: expert drummers, string musicians, and non-musicians. While duration reproduction of the non-percussionists showed a characteristic central tendency or regression to the mean, drummers responded veridically. Furthermore, when the stimuli were auditory tones rather than flashes, all subjects responded veridically. The behavior of all three groups in both modalities is well explained by a Bayesian model that seeks to minimize reproduction errors by incorporating a central tendency prior, a probability density function centered at the mean duration of the sample. We measured separately temporal precision thresholds with a bisection task; thresholds were twice as low in drummers as in the other two groups. These estimates of temporal precision, together with an adaptable Bayesian prior, predict well the reproduction results and the central tendency strategy under all conditions and for all subject groups. These results highlight the efficiency and flexibility of sensorimotor mechanisms estimating temporal duration.

“Non-retinotopic processing” in Ternus motion displays modeled by spatiotemporal filters,J Vis, 1 (12),

Recently, M. Boi, H. Ogmen, J. Krummenacher, T. U. Otto, & M. H. Herzog (2009) reported a fascinating visual effect, where the direction of apparent motion was disambiguated by cues along the path of apparent motion, the Ternus-Pikler group motion, even though no actual movement occurs in this stimulus. They referred to their study as a “litmus test” to distinguish “non-retinotopic” (motion-based) from “retinotopic” (retina-based) image processing. We adapted the test to one with simple grating stimuli that could be more readily modeled and replicated their psychophysical results quantitatively with this stimulus. We then modeled our experiments in 3D (x, y, t) Fourier space and demonstrated that the observed perceptual effects are readily accounted for by integration of information within a detector that is oriented in space and time, in a similar way to previous explanations of other motion illusions. This demonstration brings the study of Boi et al. into the more general context of perception of moving objects.

The effects of cross-sensory attentional demand on subitizing and on mapping number onto space,Vision Res,

Various aspects of numerosity judgments, especially subitizing and the mapping of number onto space, depend strongly on attentional resources. Here we use a dual-task paradigm to investigate the effects of cross-sensory attentional demands on visual subitizing and spatial mapping. The results show that subitizing is strongly dependent on attentional resources, far more so than is estimation of higher numerosities. But unlike many other sensory tasks, visual subitizing is equally affected by concurrent attentionally demanding auditory and tactile tasks as it is by visual tasks, suggesting that subitizing may be amodal. Mapping number onto space was also strongly affected by attention, but only when the dual-task was in the visual modality. The non-linearities in numberline mapping under attentional load are well explained by a Bayesian model of central tendency.

Spatiotemporal dynamics of perisaccadic remapping in humans revealed by classification images,J Vis, 4 (12), 11.

We actively scan our environment with fast ballistic movements called saccades, which create large and rapid displacements of the image on the retina. At the time of saccades, vision becomes transiently distorted in many ways: Briefly flashed stimuli are displaced in space and in time, and spatial and temporal intervals appear compressed. Here we apply the psychophysical technique of classification images to study the spatiotemporal dynamics of visual mechanisms during saccades. We show that saccades cause gross distortions of the classification images. Before the onset of saccadic eye movements, the positive lobes of the images become enlarged in both space and in time and also shifted in a systematic manner toward the pre-saccadic fixation (in space) and anticipated in time by about 50 ms. The transient reorganization creates a spatiotemporal organization oriented in the direction of saccadic-induced motion at the time of saccades, providing a potential mechanism for integrating stimuli across saccades, facilitating stable and continuous vision in the face of constant eye movements.

Visual motion distorts visual and motor space, J Vis, 2 (12),

Mapping of number onto space is fundamental to mathematics and measurement. Previous research suggests that while typical adults with mathematical schooling map numbers veridically onto a linear scale, pre-school children and adults without formal mathematics training, as well as individuals with dyscalculia, show strong compressive, logarithmic-like non-linearities when mapping both symbolic and non-symbolic numbers onto the numberline. Here we show that the use of the linear scale is dependent on attentional resources. We asked typical adults to position clouds of dots on a numberline of various lengths. In agreement with previous research, they did so veridically under normal conditions, but when asked to perform a concurrent attentionally-demanding conjunction task, the mapping followed a compressive, non-linear function. We model the non-linearity both by the commonly assumed logarithmic transform, and also with a Bayesian model of central tendency. These results suggest that veridical representation numerosity requires attentional mechanisms.

Spatiotopic perceptual maps in humans: evidence from motion adaptation,Proc Biol Sci, 1740 (279), 3091-3097.

How our perceptual experience of the world remains stable and continuous despite the frequent repositioning eye movements remains very much a mystery. One possibility is that our brain actively constructs a spatiotopic representation of the world, which is anchored in external-or at least head-centred-coordinates. In this study, we show that the positional motion aftereffect (the change in apparent position after adaptation to motion) is spatially selective in external rather than retinal coordinates, whereas the classic motion aftereffect (the illusion of motion after prolonged inspection of a moving source) is selective in retinotopic coordinates. The results provide clear evidence for a spatiotopic map in humans: one which can be influenced by image motion.

Impaired visual size-discrimination in children with movement disorders,Neuropsychologia, 8 (50), 1838-1843.

Multisensory integration of spatial information occurs late in childhood, at around eight years (Gori, Del Viva, Sandini, & Burr, 2008). For younger children, the haptic system dominates size discrimination and vision dominates orientation discrimination: the dominance may reflect sensory calibration, and could have direct consequences on children born with specific sensory disabilities. Here we measure thresholds for visual discrimination of orientation and size in children with movement disorders of upper limbs. Visual orientation discrimination was very similar to the age-matched typical children, but visual size discrimination thresholds were far worse, in all eight individuals with early-onset movement disorder. This surprising and counterintuitive result is readily explained by the cross-sensory calibration hypothesis: when the haptic sense is unavailable for manipulation, it cannot be readily used to estimate size, and hence to calibrate the visual experience of size: visual discrimination is subsequently impaired. This complements a previous study showing that non-sighted children have reduced acuity for haptic orientation, but not haptic size, discriminations (Gori, Sandini, Martinoli, & Burr, 2010). Together these studies show that when either vision or haptic manipulation is impaired, the impairment also impacts on complementary sensory systems that are calibrated by that one.

When the world becomes ‘too real’: a Bayesian explanation of autistic perception,Trends Cogn Sci, 10 (16), 504-510.

Perceptual experience is influenced both by incoming sensory information and prior knowledge about the world, a concept recently formalised within Bayesian decision theory. We propose that Bayesian models can be applied to autism – a neurodevelopmental condition with atypicalities in sensation and perception – to pinpoint fundamental differences in perceptual mechanisms. We suggest specifically that attenuated Bayesian priors – ‘hypo-priors’ – may be responsible for the unique perceptual experience of autistic people, leading to a tendency to perceive the world more accurately rather than modulated by prior experience. In this account, we consider how hypo-priors might explain key features of autism – the broad range of sensory and other non-social atypicalities–in addition to the phenomenological differences in autistic perception.

Active movement restores veridical event-timing after tactile adaptation,J Neurophysiol, 8 (108), 2092-2100.

Growing evidence suggests that time in the subsecond range is tightly linked to sensory processing. Event-time can be distorted by sensory adaptation, and many temporal illusions can accompany action execution. In this study, we show that adaptation to tactile motion causes a strong contraction of the apparent duration of tactile stimuli. However, when subjects make a voluntary motor act before judging the duration, it annuls the adaptation-induced temporal distortion, reestablishing veridical event-time. The movement needs to be performed actively by the subject: passive movement of similar magnitude and dynamics has no effect on adaptation, showing that it is the motor commands themselves, rather than reafferent signals from body movement, which reset the adaptation for tactile duration. No other concomitant perceptual changes were reported (such as apparent speed or enhanced temporal discrimination), ruling out a generalized effect of body movement on somatosensory processing. We suggest that active movement resets timing mechanisms in preparation for the new scenario that the movement will cause, eliminating inappropriate biases in perceived time. Our brain seems to utilize the intention-to-move signals to retune its perceptual machinery appropriately, to prepare to extract new temporal information.

Constructing stable spatial maps of the world,Perception, 11 (41), 1355-1372.

To interact rapidly and effectively with our environment, our brain needs access to a neural representation—or map—of the spatial layout of the external world. However, the construction of such a map poses major challenges to the visual system, given that the images on our retinae depend on where the eyes are looking, and shift each time we move our eyes, head, and body to explore the world. Much research has been devoted to how the stability is achieved, with the debate often polarized between the utility of spatiotopic maps (that remain solid in external coordinates), as opposed to transiently updated retinotopic maps. Our research suggests that the visual system uses both strategies to maintain stability. f MRI, motion-adaptation, and saccade-adaptation studies demonstrate and characterize spatiotopic neural maps within the dorsal visual stream that remain solid in external rather than retinal coordinates. However, the construction of these maps takes time (up to 500 ms) and attentional resources. To solve the immediate problems created by individual saccades, we postulate the existence of a separate system to bridge each saccade with neural units that are ‘transiently craniotopic’. These units prepare for the effects of saccades with a shift of their receptive fields before the saccade starts, then relaxing back into their standard position during the saccade, compensating for its action. Psychophysical studies investigating localization of stimuli flashed briefly around the time of saccades provide strong support for these neural mechanisms, and show quantitatively how they integrate information across saccades. This transient system cooperates with the spatiotopic mechanism to provide a useful map to guide interactions with our environment: one rapid and transitory, bringing into play the high-resolution visual areas; the other slow, long-lasting, and low-resolution, useful for interacting with the world.