Katharina Weiß

Simultaneity and temporal order perception: Different sides of the same coin? Evidence from a visual prior-entry study.

Attended stimuli are perceived as occurring earlier than unattended stimuli. This phenomenon of prior entry is usually identified by a shift in the point of subjective simultaneity (PSS) in temporal order judgements (TOJs). According to its traditional psychophysical interpretation, the PSS coincides with the perception of simultaneity. This assumption is, however, questionable. Technically, the PSS represents the temporal interval between two stimuli at which the two alternative TOJs are equally likely. Thus it also seems possible that observers perceive not simultaneity, but uncertainty of temporal order. This possibility is supported by prior-entry studies, which find that perception of simultaneity is not very likely at the PSS. The present study tested the percept at the PSS in prior entry, using peripheral cues to orient attention. We found that manipulating attention caused varying temporal perceptions around the PSS. On some occasions observers perceived the two stimuli as simultaneous, but on others they were simply uncertain about the order in which they had been presented. This finding contradicts the implicit assumption of most models of temporal order perception, that perception of simultaneity inevitably results if temporal order cannot be discriminated.

When circles become triangular: how transsaccadic predictions shape the perception of shape

Human vision is characterized by a consistent pattern of saccadic eye movements. With each saccade, internal object representations change their retinal position and spatial resolution. This raises the question as to how peripheral perception is affected by imminent saccadic eye movements. Here, we suggest that saccades are accompanied by a prediction of their perceptual consequences (i.e., the foveation of the target object). Accordingly, peripheral perception should be biased toward previously associated foveal input. In this study, we first exposed participants to an altered visual stimulation where one object systematically changed its shape during saccades. Subsequently, participants had to judge the shape of briefly presented peripheral saccade targets. The results showed that targets were perceived as less curved for objects that previously changed from more circular in the periphery to more triangular in the fovea. Similarly, shapes were perceived as more curved for objects that previously changed from triangular to circular. Thus, peripheral perception seems to depend not solely on the current input but also on memorized experiences, enabling predictions about the perceptual consequences of saccadic eye movements.

Associating peripheral and foveal visual input across saccades: A default mode of the human visual system?

Spatial processing resolution of a particular object in the visual field can differ considerably due to eye movements. The same object will be represented with high acuity in the fovea but only coarsely in periphery. Herwig and Schneider (in press) proposed that the visual system counteracts such resolution differences by predicting, based on previous experience, how foveal objects will look in the periphery and vice versa. They demonstrated that previously learned transsaccadic associations between peripheral and foveal object information facilitate performance in visual search, irrespective of the correctness of these associations. False associations were learned by replacing the presaccadic object with a slightly different object during the saccade. Importantly, participants usually did not notice this object change. This raises the question of whether perception of object continuity is a critical factor in building transsaccadic associations. We disturbed object continuity during learning with a postsaccadic blank or a task-irrelevant shape change. Interestingly, visual search performance revealed that neither disruption of temporal object continuity (blank) nor disruption of spatial object continuity (shape change) impaired transsaccadic learning. Thus, transsaccadic learning seems to be a very robust default mechanism of the visual system that is probably related to the more general concept of action-effect learning.

At the mercy of prior entry: Prior entry induced by invisible primes is not susceptible to current intentions

If one of two events is attended to, it will be perceived earlier than a simultaneously occurring unattended event. Since 150 years, this effect has been ascribed to the facilitating influence of attention, also known as prior entry. Yet, the attentional origin of prior-entry effects(1) has been repeatedly doubted. One criticism is that prior-entry effects might be due to biased decision processes that would mimic a temporal advantage for attended stimuli. Although most obvious biases have already been excluded experimentally (e.g. judgment criteria, response compatibility) and prior-entry effects have shown to persist (Shore, Spence, & Klein, 2001), many other biases are conceivable, which makes it difficult to put the debate to an end. Thus, we approach this problem the other way around by asking whether prior-entry effects can be biased voluntarily. Observers were informed about prior entry and instructed to reduce it as far as possible. For this aim they received continuous feedback about the correctness of their temporal judgments. If elicited by invisible primes the effect could not be reduced at all and only by 12 ms if elicited by visible cues. This challenges decision biases as primary source of prior-entry effects - at least if attention is oriented exogenously.

Attention and the speed of information processing: posterior entry for unattended stimuli instead of prior entry for attended stimuli.

Why are nearly simultaneous stimuli frequently perceived in reversed order? The origin of errors in temporal judgments is a question older than experimental psychology itself. One of the earliest suspects is attention. According to the concept of prior entry, attention accelerates attended stimuli; thus they have “prior entry” to perceptive processing stages, including consciousness. Although latency advantages for attended stimuli have been revealed in psychophysical studies many times, these measures (e.g. temporal order judgments, simultaneity judgments) cannot test the prior-entry hypothesis completely. Since they assess latency differences between an attended and an unattended stimulus, they cannot distinguish between faster processing of attended stimuli and slower processing of unattended stimuli. Therefore, we present a novel paradigm providing separate estimates for processing advantages respectively disadvantages of attended and unattended stimuli. We found that deceleration of unattended stimuli contributes more strongly to the prior-entry illusion than acceleration of attended stimuli. Thus, in the temporal domain, attention fulfills its selective function primarily by deceleration of unattended stimuli. That means it is actually posterior entry, not prior entry which accounts for the largest part of the effect.

Feature prediction across eye movements is location specific and based on retinotopic coordinates

With each saccadic eye movement, internal object representations change their retinal position and spatial resolution. Recently, we suggested that the visual system deals with these saccade-induced changes by predicting visual features across saccades based on transsaccadic associations of peripheral and foveal input (Herwig & Schneider, 2014). Here we tested the specificity of feature prediction by asking (a) whether it is spatially restricted to the previous learning location or the saccade target location, and (b) whether it is based on retinotopic (eye-centered) or spatiotopic (world-centered) coordinates. In a preceding acquisition phase, objects systematically changed their spatial frequency during saccades. In the following test phases of two experiments, participants had to judge the frequency of briefly presented peripheral objects. These objects were presented either at the previous learning location or at new locations and were either the target of a saccadic eye movement or not (Experiment 1). Moreover, objects were presented either in the same or different retinotopic and spatiotopic coordinates (Experiment 2). Spatial frequency perception was biased toward previously associated foveal input indicating transsaccadic learning and feature prediction. Importantly, while this pattern was not bound to the saccade target location, it was seen only at the previous learning location in retinotopic coordinates, suggesting that feature prediction probably affects low- or mid-level perception.

How transsaccadic predictions shape the perception of shape

INTRODUCTION Human vision is characterized by consistent shifts between fixations and saccadic eye movements. With each saccade, internal object representations change their retinal position and spatial resolution which raises the question as to how extra-foveal perception is affected by upcoming saccadic eye movements. Recently, we suggested that saccades are accompanied by a prediction of their perceptual consequences-i.e., the foveation of the target object (Herwig & Schneider, 2014, Journal of Experimental Psychology: General). Accordingly, extra-foveal perception should be biased toward previously associated foveal input. Up to now, effects of transsaccadic feature prediction on extra-foveal perception have been exclusively reported for surface features (i.e., color and spatial frequency) which are known to play an important role in establishing object correspondence while moving the eyes. In the present study, we tested whether also the extra-foveal perception of visual shape is partly based on predicted postsaccadic foveal input. METHODS Sixteen participants in an eyetracking experiment first underwent a 30 min acquisition phase, where, unnoticed by most participants, one out of two objects systematically changed its shape during saccades. In the following test phase, participants had to judge the shape of briefly presented peripheral saccade target objects. RESULTS Peripheral saccade targets were perceived as less curved for objects which previously changed from more circular in the periphery to more triangular in the fovea compared to objects which did not change during acquisition. Likewise, shapes were perceived as more curved for objects which previously changed from triangular- to circular-like. CONCLUSION This result indicates that the extra-foveal perception of shape is specifically biased toward previously associated postsaccadic foveal input. Thus, extra-foveal perception seems to depend not solely on the current input but also on memorized experiences enabling predictions about the perceptual consequences of saccadic eye movements. Meeting abstract presented at VSS 2015.

Attention speeds up visual information processing: selection for perception or selection for action?

Attention speeds up information processing. Although this finding has a long history in experimental psychology, it has found less regard in computational models of visual attention. In psychological research, two frameworks explain the function of attention. Selection for perception emphasizes that perception- or consciousness-related processing presupposes selection of relevant information, whereas selection for action emphasizes that action constraints make selection necessary. In the present study, we ask whether or how far attention, as measured by the speed-up of information processing, is based on selection for perception or selection for action. The accelerating effect was primarily based on selection for perception, but there was also a substantial effect of selection for action.