Carolina Murd

Target-mask shape congruence impacts the type of metacontrast masking

Visual metacontrast masking may depend on the time intervals between target and mask in two qualitatively different ways: in type-A masking the smaller the mask delay from target the stronger the masking while in type-B masking maximal masking effect is obtained with a larger temporal delay of the mask. Variability in the qualitative apperance of masking functions has been explained by variability in stimuli parameters and tasks. Recent research on metacontrast masking has surprisingly shown that both of these types of functions can be found with an identical range of stimulation parameters depending on individual differences between observers. Here we show that obtaining clear-cut type-A masking depends on whether target and mask shapes are congruent or incongruent and whether observers use the cues available due to the congruence factor. Conspicuously expressed type-A masking is selectively associated with incongruent target-mask pairings. In the latter conditions target identification level significantly drops with the shortest target-to-mask delays.

Caffeine enhances frontal relative negativity of slow brain potentials in a task-free experimental setup.

State dependent effects on brain processes are difficult to study due to the task-related confounds. Even in simple task environments external stimuli inevitably interact with dynamically changing states of the brain. Psychopharmacological manipulation and transcranial magnetic stimulation can be used independently of variations in subjects experimental task and environmental stimulation. Our aim was to show the investigative potential of combining these two methods for studying the effects of the state of the brain on the dynamics of task-free evoked brain activity. Caffeine was used for inducing higher arousal state and transcranial magnetic stimulation was used to evoke widespread bioelectrical responses of the brain. Occipitally delivered magnetic pulses caused increased global negativity of the brain potentials, but no speed-up of brain potentials when caffeine was administered. The relative negativization effect was most clearly expressed in slow potentials and as measured from frontal and parietal electrodes. This study shows how the causal effects of brain states on neural processes can be studied without the confounding influence of experimental task and stimuli.

Spatially localized motion aftereffect disappears faster from awareness when selectively attended to according to its direction

In searching for the target-afterimage patch among spatially separate alternatives of color-afterimages the target fades from awareness before its competitors (Bachmann, T., & Murd, C. (2010). Covert spatial attention in search for the location of a color-afterimage patch speeds up its decay from awareness: Introducing a method useful for the study of neural correlates of visual awareness. Vision Research 50, 1048-1053). In an analogous study presented here we show that a similar effect is obtained when a target spatial location specified according to the direction of motion aftereffect within it is searched by covert top-down attention. The adverse effect of selective attention on the duration of awareness of sensory qualiae known earlier to be present for color and periodic spatial contrast is extended also to sensory channels carrying motion information.

Covert spatial attention in search for the location of a color-afterimage patch speeds up its decay from awareness: Introducing a method useful for the study of neural correlates of visual awareness

Previous research has reported that attention to color afterimages speeds up their decay. However, the inducing stimuli in these studies have been overlapping, thereby implying that they involved overlapping receptive fields of the responsible neurons. As a result it is difficult to interpret the effect of focusing attention on a phenomenally projected target-afterimage. Here, we present a method free from these shortcomings. In searching for a target-afterimage patch among spatially separate alternatives the target fades from awareness before its competitors. This offers a good means to study neural correlates of visual awareness unconfounded with attention and enabling a temporally extended pure phenomenal experience free from simultaneous inflow of sensory transients.

Detection of colour changes in a moving object

The colour-changing stimulus paradigm is based on a tacit assumption that kinematic attributes (velocity, movement direction) do not affect the detection of colour change (). In this study three experiments are reported that clearly demonstrate that the time needed to detect changes in colouration of a moving stimulus becomes shorter with its velocity. The reduction of reaction time with increase of velocity is a purely kinematic effect independent on the reduction of reaction time caused by the stimulus uncertainty effects. It is concluded that colour coding mechanisms are not totally ignorant about movement parameters.

Repetitive TMS over V5/MT shortens the duration of spatially localized motion aftereffect: The effects of pulse intensity and stimulation hemisphere

Causal relevance of the cortical area V5/MT for motion (aftereffect) perception has been shown when rTMS pulses have been applied onto this area, leading to disruption of the percept. Typically, the inducing and test stimuli have consisted in a spatially contiguous area from where stimulation is presented. Observers have had no need to divide attention between spatially remote areas including motion-related signals with different vectors. Here we present experimental results showing that an adverse effect of rTMS on motion aftereffect can be obtained when contralateral V5/MT is stimulated and subjects have to report which one of the two simultaneous aftereffect percepts separated into two hemifields decays before the other. The effect appears stronger following right hemisphere V5/MT stimulation and is clearly evident even with weak rTMS pulses.

Flash-lag effect: complicating motion extrapolation of the moving reference-stimulus paradoxically augments the effect

One fundamental property of the perceptual and cognitive systems is their capacity for prediction in the dynamic environment; the flash-lag effect has been considered as a particularly suggestive example of this capacity (Nijhawan in Nature 370:256–257, 1994, Behav Brain Sci 31:179–239, 2008). Thus, because of involvement of the mechanisms of extrapolation and visual prediction, the moving object is perceived ahead of the simultaneously flashed static object objectively aligned with the moving one. In the present study we introduce a new method and report experimental results inconsistent with at least some versions of the prediction/extrapolation theory. We show that a stimulus moving in the opposite direction to the reference stimulus by approaching it before the flash does not diminish the flash-lag effect, but rather augments it. In addition, alternative theories (in)capable of explaining this paradoxical result are discussed.

Scotomas induced by multiple, spatially invariant TMS pulses have stable size and subjective contrast

Transcranial magnetic stimulation (TMS) can be used for studying causal effects on visual phenomenology. Occipitally delivered TMS pulses when applied after a brief spatially extended visual reference stimulus induce a localized degrading effect on the visual quality of the reference, a subjective darkening called scotoma. The stability of the subjective characteristics of artificial scotomas has not been studied with advanced neuronavigation of TMS. In 3 experiments we studied the size and relative contrast of TMS-induced scotomas and looked for possible adaptation effects to TMS delivered to the same cortical location for many successive trials. MRI-based neuro-navigated biphasic single-pulse stimulation was used to show that (i) ISI values leading to scotomas in all individual subjects extend over a wide range of time intervals from 35 ms to 199 ms, (ii) the size of and relative decrease of contrast of scotoma area remained stable over multiple stimulations, and (iii) TMS effect on scotomas was location-specific so that carry-over effects from temporarily changed TMS location to another hemisphere were absent - returning back with stimulation to the original site from a temporarily changed site led to the previous value of scotoma expression.

Detection of colour change in moving objects: Temporal order judgment and reaction time analysis

The time needed to detect changes in the colouration of a single moving stimulus becomes shorter with its increasing velocity (Kreegipuu et al, 2006 Vision Research 46 1848–1855). We examined the ability to detect colour change in moving chromatic bars or sinusoidal gratings through temporal order judgment (TOJ) and reaction time (RT) tasks to test whether the effect of velocity found in a previous study is universal and holds for different tasks and stimuli. The results demonstrate that the TOJ and simple RT to the colour change of a moving grating are insensitive to stimulus velocity. Therefore, we conclude that the process of comparison of the two internal representations of external events does not have access to temporal information precise enough to estimate the exact time when something enters our subjective awareness. The motion effect on colour-change perception seems to be confined to a single stimulus that moves across the visual field, to events that contain some spatial predictability, and to tasks that reflect the time of the change relatively directly.

Variants of TPH2 interact with fast visual processing as assessed by metacontrast

Sensitivity to threatening or otherwise unpleasant visual stimuli has become a widely used measure of potential vulnerability/resilience. Basically, experiments using this strategy present brief stimuli, often followed by a mask, and individuals’ sensitivity is measured. However, it has not been asked whether the individual differences in threat detection or adaptive resilience associated with genetic variability-related endophenotypes might be just a function of some basic visual functions involved in processing and reporting brief visual stimuli without any emotional content. Effects attributed to emotional processing may be confounded by variability in simple basic visual skills. However, if simple visual skills are variable depending on common genetic variability, simple perceptual tests of screening for genetic risks can be developed. In a sample of normal human individuals, we studied the effects of a single nucleotide polymorphism (rs4570625) in the gene that encodes the rate-limiting enzyme in serotonin synthesis, TPH2, on metacontrast masking. Visual discrimination of target shapes that were incongruent with mask shapes was poorer in G homozygotes (typically considered more resilient individuals) compared with T-allele carriers and this effect was influenced by participants’ sex. Implications for the development of psychophysical testing-based methods of screening for vulnerability/resilience in relation to the pathology of the serotonergic system-related dysfunction are considered.