Iiris Luiga

Illusory reversal of temporal order: the bias to report a dimmer stimulus as the first

When two objects are presented in rapid succession, observers find it difficult to discriminate their temporal order. Below certain limit (e.g., 20-70 ms), the rate of correct temporal order judgement is reported to be about 50% (i.e., close to chance level). However we have found stimulus conditions where order discrimination drops significantly below chance level: the stimulus that is presented as the second is reported as the first. It is necessary that the stimuli are very brief, spatially overlapping, clear-cut backward and forward masking is absent, stimulus onset asynchronies are very short, and luminance contrast of the following stimulus is considerably lower than luminance contrast of the first stimulus. The higher the contrast ratio, the stronger the order reversal effect. However, because also in the conditions where the two stimuli are presented synchronously, the dimmer target is perceived as the first, the effect should be attributed to some implicit bias which enforces subjects to regard a more contrasted stimulus as the one that appears subsequent to the less contrasted stimulus.

Variations in backward masking with different masking stimuli: II. The effects of spatially quantised masks in the light of local contour interaction, interchannel inhibition, perceptual retouch, and substitution theories.

In part I we showed that with spatially non-overlapping targets and masks both local metacontrast-like interactions and attentional processes are involved in backward masking. In this second part we extend the strategy of varying the contents of masks to pattern masking where targets and masks overlap in space, in order to compare different masking theories. Images of human faces were backward-masked by three types of spatially quantised masks (the same faces as targets, faces different from targets, and Gaussian noise with power spectra typical for faces). Configural characteristics, rather than the spectral content of the mask, predicted the extent of masking at relatively long stimulus onset asynchronies (SOAs). This poses difficulties for the theory of transient-on-sustained inhibition as the principal mechanism of masking and also for local contour interaction being a decisive factor in pattern masking. The scale of quantisation had no effect on the masking capacity of noise masks and a strong effect on the capacity of different-face masks. Also, the decrease of configural masking with an increase in the coarseness of the quantisation of the mask highlights ambiguities inherent in the re-entrance-based substitution theory of masking. Different masking theories cannot solve the problems of masking separately. They should be combined in order to create a complex, yet comprehensible mode of interaction for the different mechanisms involved in visual backward masking.

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.

Luminance processing in object substitution masking

We probed how processing of luminance increments and decrements interacts with attention dependent substitution masking. Results showed that a target was identified better when surrounded by an opposite polarity mask as compared to the same polarity mask. Opposite polarity mask decreased an effect of distracters, indicating influence on the time of directing attention to a target. The opposite polarity mask decreased masking when delayed for longer than 100 ms. Stimuli with the same polarity but different contrast showed increased masking with high contrast mask. Luminance processing, particularly polarity processing, probably enables faster formation of distinct object representation, interacting with attentional selection processes in object substitution masking.

Variations in backward masking with different masking stimuli: I. Local interaction versus attentional switch.

The types of stimuli used as targets and masks considerably change the masking functions in a way that requires us to abandon any single mechanism of masking as the sole explanation of backward masking. In the first of two reports in which the problem of the mask-dependence of masking is addressed, we explore the role of the relative spatial positioning of targets and masks in order to differentiate between local interaction and attentional models. If single letters were masked by double-letter masks then the relative spatial arrangement of the letters, which was changed in order to vary the involvement of metacontrast-like processes, had an effect at shorter SOAs, but not at longer SOAs where strong masking still persisted. This poses difficulties for proposing local contour interaction as the main mechanism of masking. Similarly, crowding effects alone cannot explain the results. Backward masking also involves attention being directed to working-memory processing of the succeeding object while abandoning the preceding object.

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.

Delayed offset of distracters masks a local target

Object substitution masking (OSM) is observed when a brief target surrounded with a mask is presented among distracter stimuli and cannot be identified when it and the distracters disappear but the mask remains in view. We probed whether OSM also occurs without a local mask object when the distracters remain after target offset. We also varied the congruence between the local target and the global search display and the grouping properties of the delayed offset distracters. A target was briefly presented in a global object configuration of distracters that had delayed offset. Results showed that OSM could be observed with delayed offset of distracters grouped into a global mask shape. Congruence of the shapes of the global and local objects did not affect OSM, suggesting that a generalized abstract visual pattern representation of the global object may not be involved in OSM nor did the grouping properties of the delayed offset distracters influence OSM.