Research

Brief Research Summary

Method: Tadin lab uses psychophysics, transcranial magnetic stimulation (TMS), fMRI, and eye-tracking to investigate neural mechanisms of visual perception in normal and special populations. We are also starting a pharmacological GABA/glutamate manipulation study to determine roles of cortical inhibition in a range of visual and cognitive functions. In that study, we will use magnetic resonance spectroscopy (MRS) and paired-pulse TMS to assess pharmacological changes and correlate those measures with behavioral changes.
Topics: Current topics include motion perception, binocular rivalry, visual awareness, contextual interactions, perceptual learning, visual adaptation, attention and temporal dynamics of vision.
Research examples: Our psychophysical work has revealed several counterintuitive characteristics of human motion perception and linked these findings with cortical center-surround mechanisms. Follow-up work investigated temporal and spatial properties of center-surround interactions across visual sub-modalities in normal, schizophrenic and MDMA-user populations. Another line of research uses binocular rivalry and visual crowding as experimental methods for studying the characteristics of visual awareness. Combined use of rigorous psychophysical methods and TMS allows us to make causal inferences about the neural mechanisms of visual perception. These lines of research are supplemented with related investigations of visual processing in special populations, including schizophrenic patients, low-vision children, chronic drug users and by measuring behavioral changes caused by pharmacological manipulations. Funding: Currently, our research is funded by generous start-up funds from the University of Rochester and the Center for Visual Science.
Research Resources:
Simens 3T Trio fMRI scanner, located at the The Rochester Center for Brain Imaging BrainSight frameless stereotaxic system
Transcranial Magnetic Stimulation (TMS) facilities Eye-tracking systems
Virtual Reality facility
Equipment for visual psychophysics

Selected Research Topics & Approaches

Perceptual correlates of neural center-surround interactions in motion perception
If a biological mechanism is effective, and thus adaptive, it often recurs in related applications. In the brain, one such mechanism is interaction between center and surround regions of a neuron’s receptive field – a ubiquitous property found in most sensory systems, presumably tailored for extracting features that are differentially distributed over space. In motion processing, neurophysiologists have investigated center-surround interactions for over twenty years (Tadin & Lappin, 2005a). Proposed functional roles of this conceptually simple neural mechanism include figure-ground segregation, guidance of slow pursuit eye movements and the analysis of object shape. Corresponding behavioral work, however, had been limited in part because a well-defined perceptual correlate of center-surround interactions has not been described. We were the first to provide a systematic behavioral description of center-surround interactions in motion perception. Based on the converging evidence from a series of experiments, we linked these results with suppressive center-surround receptive fields, such as those found in cortical area MT (Tadin et al., 2003). The main result is surprising and counterintuitive: observers’ ability to perceive motion of a high contrast object substantially deteriorates as the object size increases. At low contrast, however, performance improves as the size increases, indicating an adaptive process that depends on stimulus strength. This contrast-dependency of surround interactions has subsequently been confirmed with single cell recordings in cortical area MT (Pack et al., 2005). In the initial study (Tadin et al., 2003), we ensured that this result is replicable using a range of psychophysical methods and that it generalizes to several stimulus types. In follow-up studies we have: developed a model of the effects of size and contrast on motion perception (Tadin & Lappin, 2005b), revealed fine temporal properties of center-surround interactions using a reverse correlation method (Tadin, Lappin & Blake, 2006), demonstrated adaptive center surround-interactions using binocular rivalry as an experimental tool (Paffen et al., 2006), and extended the original findings using reaction times and diffusion modeling (ongoing research). In addition, the Paffen et al. (2006) study demonstrated that center-surround interactions analogous to those in motion perception occur in orientation and color processing – a result nicely dovetailing with the ubiquity of center-surround interactions in the brain.
Functional role of center-surround interactions A description of a visual mechanism should only be considered the first stage in a research program. Establishing functional roles of the described mechanism is arguably a more important second step. For example, both neurophysiologists and psychologists have hypothesized that center-surround receptive field interactions play an important role in our ability to visually separate objects from their backgrounds. This hypothesis, however, has received almost no experimental support. We and other groups have found that patients diagnosed with schizophrenia and elderly observers exhibit reduced center-surround suppression (Tadin et al., 2006; Betts et al., 2005). Interestingly, both elderly and schizophrenic patients have deficits in figure-ground segregation – an observation consistent with a functional link between two deficits (Tadin & Blake, 2005). Indeed, in a series of experiments (Tadin et al., in press), we showed that center-surround suppression is affected by contextual manipulations changing figure ground relationships.
Vision in special populations
The previous section illustrates how work with special populations can go beyond a mere documentation of abnormalities and actually provide insights about the nature and functional roles of an unimpaired visual system. In addition to the work with schizophrenic patients (Tadin et al., 2006), we developed a training procedure that could be used to train patients with impairments such as macular degeneration to use a bioptic telescope (Tadin et al., in preparation), started a study to document visual deficits of heavy MDMA users (collaboration with Ron Cowan at Vanderbilt Psychiatry) and are investigating perceptual benefits of video-game playing (Green & Bavelier, 2003) to low-vision children (collaboration with Joe Lappin and Anne Corn). We are currently exploring the integrity of contextual processing in schizophrenia in both vision and working memory (collaboration with Sohee Park). This line of research has the promise of two benefits: (1) it potentially yields insights into normal functioning of the visual system and (2) has a real potential to translate into tangible improvements in visual functioning of the affected populations.
Binocular rivalry and visual awareness
Our lab is interested in binocular rivalry for three reasons: (1) its inherent significance and fascination, (2) its potential as a valuable experiment tool, and (3) its usefulness as an experimental method for studying visual awareness. In work exploring the nature of rivalry per se, we have investigated the attentional control of binocular rivalry (Chong, Tadin & Blake, 2005) and showed that TMS of early visual areas affects binocular rivalry dynamics in a retinotopic fashion, while having no effect on a related phenomenon of stimulus rivalry (Pearson, Tadin & Blake, 2007). These TMS findings indicate that binocular rivalry is mediated, at least in part, by early visual processes that are localized in the cortical representation of visual space. The dynamics of binocular rivalry are sensitive to the strength of the rival stimuli. We exploited this rivalry characteristic to devise a modality-independent experimental measure and demonstrated the key shared properties of center-surround mechanisms in motion, orientation and color perception (Paffen et al., 2006). Finally, using rivalry and visual crowding to manipulate phenomenal visibility of visual stimuli, we showed that fluctuations in visual awareness affect the build-up of early visual aftereffects (Blake et al., 2006). This finding stresses the importance of low-level mechanisms in both rivalry and visual crowding. Moreover, our finding undermines conclusions drawn from the results of two seminal studies (Blake & Fox, 1975; He et al., 1996), whose results were interpreted as psychophysical evidence against the direct role of primary visual cortex neurons in visual awareness.
Transcranial magnetic stimulation (TMS) TMS is the only non-invasive technique that can reversibly interfere with brain activity with reasonable spatial and high temporal resolution. TMS methodology is still relatively young, and, not surprisingly, most studies test a small number of experimental conditions. Our aim is to better integrate TMS with rigorous psychophysical design and, where beneficial, conduct in-depth explorations. To illustrate, in our first TMS study (Pearson, Tadin & Blake, 2007), in addition to several control experiments, we investigated: the effects of TMS over foveal and peripheral regions of V1/V2 on binocular rivalry, the dependency of the effect of TMS on individual differences in binocular rivalry, the interaction between TMS and modulations of rivalry dynamics within observers, and the effects of TMS on a related process of stimulus rivalry. Our current TMS project is exploring fine temporal properties of center-surround interactions in cortical area MT. This work was motivated by a reverse correlation study (Tadin, Lappin & Blake, 2006) where we showed that excitatory center and inhibitory surround signals can be observed as separate in time – an observation naturally lending itself to TMS exploration. Preliminary results indicate that TMS can improve motion discriminations of large moving objects, presumably by selectively interfering with inhibitory surround signals. Finally, in collaboration with Joel Pearson, we started to investigate the role of cortical feedback on the binding of visual features in the primary visual cortex.