Dagmar Müller

Sit-and-Wait Strategies in Dynamic Visual Search

The role of memory in visual search has lately become a controversial issue. Horowitz and Wolfe (1998) observed that performance in a visual search task was little affected by whether the stimuli were static or randomly relocated every 111 ms. Because a memory-based mechanism, such as inhibition of return, would be of no use in the dynamic condition, Horowitz and Wolfe concluded that memory is likewise not involved in the static condition. However, Horowitz and Wolfe could not effectively rule out the possibility that observers adopted a different strategy in the dynamic condition than in the static condition. That is, in the dynamic condition observers may have attended to a subregion of the display and waited for the target to appear there (sit-and-wait strategy). This hypothesis is supported by experimental data showing that performance in their dynamic condition does not differ from performance in another dynamic condition in which observers are forced to adopt a sit-and-wait strategy by being presented with a limited region of the display only.

NMDA receptor characterization and subunit expression in rat cultured mesencephalic neurones.

NMDA‐induced changes in free intracellular Ca2+ concentration ([Ca2+]i) were determined in individual cultured rat mesencephalic neurones by the fura‐2 method. mRNA expression encoding NMDA receptor subunits (NR1, NR2A‐D) was examined by RT–PCR. NMDA (1–100 μM, plus 10 μM glycine) induced a concentration‐dependent increase in [Ca2+]i (EC50=5.7 μM). The effect of NMDA was virtually insensitive to tetrodotoxin (0.3 μM) and nitrendipine (1 μM), but dependent on extracellular Ca2+. 5,7‐Dichlorokynurenic acid (10 μM), a specific antagonist at the glycine binding site on the NMDA receptor, abolished the NMDA response. Memantine, an open‐channel blocker, and ifenprodil, a preferential non‐competitive NR1/NR2B receptor antagonist diminished the NMDA effect with an IC50 value of 0.17 and 1 μM, respectively. Ethanol at 50 and 100 mM caused about 25 and 45%‐inhibition, respectively. Agarose gel analysis of the PCR products followed by ethidium bromide fluorescence or CSPD chemiluminescence detection revealed an almost exclusive expression of the NR1 splice variants lacking exon (E) 5 and E22. The 3′ splice form without both E21 and E22 exceeded that containing E21 by approximately 4 fold. The relative amounts of NR2A, NR2B, NR2C corresponded to approximately 1:2:1. NR2D mRNA was also detectable. In conclusion, mesencephalic neurones bear ethanol‐sensitive NMDA receptors which might be involved in the development of ethanol dependence and withdrawal. The high affinity of NMDA to this receptor, its sensitivity to ifenprodil and memantine may suggest that the mesencephalic NMDA receptor comprises the NR1 splice variant lacking E5, NR2B, and NR2C, respectively.

Amyloid precursor protein in guinea pigs — complete cDNA sequence and alternative splicing

We present the cDNA sequence of the guinea pig amyloid precursor protein comprising the complete coding sequence of 770 amino-acid residues. By alternative splicing of three exons transcripts encoding for 695, 714 and 751 amino acids and all forms previously denoted as L-APP are also generated. Guinea pig amyloid precursor protein was shown to exhibit extensive sequence similarity to its human and murine homologues of approx. 97% at the protein level which implies an evolutionary conserved but yet not fully understood physiological function.

Visual object representations can be formed outside the focus of voluntary attention: Evidence from event-related brain potentials

There is an ongoing debate whether visual object representations can be formed outside the focus of voluntary attention. Recently, implicit behavioral measures suggested that grouping processes can occur for task-irrelevant visual stimuli, thus supporting theories of preattentive object formation (e.g., Lamy, D., Segal, H., & Ruderman, L. Grouping does not require attention. Perception and Psychophysics, 68, 17–31, 2006; Russell, C., & Driver, J. New indirect measures of “inattentive” visual grouping in a change-detection task. Perception and Psychophysics, 67, 606–623, 2005). We developed an ERP paradigm that allows testing for visual grouping when neither the objects nor its constituents are related to the participants task. Our paradigm is based on the visual mismatch negativity ERP component, which is elicited by stimuli deviating from a regular stimulus sequence even when the stimuli are ignored. Our stimuli consisted of four pairs of colored discs that served as objects. These objects were presented isochronously while participants were engaged in a task related to the continuously presented fixation cross. Occasionally, two color deviances occurred simultaneously either within the same object or across two different objects. We found significant ERP differences for same- versus different-object deviances, supporting the notion that forming visual object representations by grouping can occur outside the focus of voluntary attention. Also our behavioral experiment, in which participants responded to color deviances—thus, this time the discs but, again, not the objects were task relevant—showed that the object status matters. Our results stress the importance of early grouping processes for structuring the perceptual world.

Temporal grouping affects the automatic processing of deviant sounds.

The present study investigates the influence of auditory temporal grouping on automatic sound processing. We compared the mismatch negativity (MMN) event-related potential elicited by successive deviant tones in a condition in which sounds were organized in tone pairs with respective MMN obtained in a non-paired sequence. When successive deviants belonged to different tone pairs the second deviant elicited a higher MMN amplitude than the corresponding deviant in the non-paired condition. Moreover, this temporal grouping had immediate effects on the extraction of rules as revealed by the finding that first and single deviants at the second position of a tone pair elicited larger MMNs than corresponding deviants in the non-paired condition or first and single deviants at the first position of a tone pair. Results demonstrate that auditory objects generated by temporal proximity are treated as relatively encapsulated units and that automatic deviance-detection may rely on multiple hierarchically organized representations of sound sequences.

Deviance-repetition effects as a function of stimulus feature, feature value variation, and timing: a mismatch negativity study.

The human auditory pre-attentive change detection system, indicated by the mismatch negativity component (MMN) of the event-related brain potential is known to be sensitive for the successive presentation of tones deviating in frequency. The present study investigated deviance-repetition effects as a function of stimulus feature, feature value variation, and stimulus-onset asynchrony (SOA). For succession of frequency and location deviants carrying the same feature value, MMN elicited by the second deviant was reduced significantly. The reduction of MMN amplitude was markedly smaller, if the second of two successive deviants carries a different feature value, as compared with same feature value deviants. The variation of SOA across the temporal window of integration did not influence the deviance-repetition effects. We suggest that the pre-attentive change-detection system evaluates the informational content of the succession of deviants enabling an effective scanning of the auditory environment for potentially relevant events.

Impact of lower- vs. upper-hemifield presentation on automatic colour-deviance detection: a visual mismatch negativity study

The automatic processing of deviances from the temporal context of the visual environment has become an important topic in visual cognitive sciences, which is often investigated using the visual mismatch negativity (vMMN). This event-related potential (ERP) component is elicited by an irregular stimulus (e.g., a red disc) presented in a series of stimuli (e.g., green discs) comprising a temporal regularity (e.g., colour repetition). We determined the influence of lower- vs. upper-hemifield presentation of the irregular stimulus on the vMMN while using whole-field stimulus displays controlling for sustained shifts in spatial attention. Deviances presented in the lower hemifield elicited a larger vMMN than the ones presented in the upper hemifield at a latency of 200-280ms. However, this asymmetry was preceded by deviance-related hemifield effects already emerging at an earlier latency (110-150ms), where upper-hemifield deviances elicited a positive potential but lower-hemifield deviances did not. With variable resolution electromagnetic tomography (VARETA) early deviance-related activity was localised to retinotopically organised regions of the visual cortex (BA 17/18) and vMMN-sources were localised to the middle/superior occipital gyrus, to higher areas along the temporal visual stream, but also to BA 17/18. We argue that the upper/lower-hemifield vMMN asymmetry relies at least partially on the hemifield-dependent differential sensitivity of early deviance-related activity generated in retinotopically organised regions of the visual cortex. However, a superior automatic processing of deviances presented in the lower visual hemifield may also contribute to the effect.

Omission mismatch negativity builds up late.

The mismatch negativity (MMN) event-related potential (ERP) is elicited by irregular sounds in a regular sound sequence. MMN depends on the length of the preceding regular stimulation: in repetitive sequences, changes following only two or three repetitions may elicit MMN. Thus, MMN builds up rapidly. Infrequent omissions in a repetitive sound sequence may also elicit an MMN-like waveform. In this study, the build-up of this omission-related ERP was investigated. Trains of two to nine tones with omissions at the penultimate stimulus positions were presented. Results showed no consistent omission-related ERP. In contrast, when omissions occurred with 9% probability among continuously presented tones, the omission-related waveform was elicited. Thus, the omission-related ERP builds up later than MMN.

A temporal constraint for automatic deviance detection and object formation: A mismatch negativity study

The automatic detection of deviations within a constant sine wave tone is confined to the initial part of approximately 350 ms. When a deviation occurs beyond this critical limit, the mismatch negativity (MMN) - a deviance-related component of the event-related potential - is largely attenuated or even absent. However, for time-variant acoustic stimuli such as speech sounds or tonal patterns, MMN is also obtained for deviations beyond the initial 350 ms. We consider two hypotheses that can explain the MMN to time-variant sounds. One is that the terminal part of those sounds is represented as the spectral information varies over time (spectral-variation hypothesis). The other is that transients, occurring in time-variant signals, help to segment the long sounds into smaller units, each being not larger than the critical 350 ms (segmentation hypothesis). We measured MMN to duration shortenings (deviants) embedded in a sequence of 1000 ms long standard tones of increasing frequency (sweeps). The sweeps did or did not contain a noise burst. Results reveal a lack of MMN to the duration deviant in the sweep without a noise burst, which rules out the spectral-variation hypothesis. The presence of MMN to the duration deviant in the sweep with a noise burst supports the segmentation hypothesis. Thus, the results suggest a temporal constraint inherent to the processing of unstructured/unsegmented long tones. We argue that transients within a sound act as segmentation cues providing an automatic sound representation for which deviations can be detected.

Object-related regularities are processed automatically: evidence from the visual mismatch negativity

One of the most challenging tasks of our visual systems is to structure and integrate the enormous amount of incoming information into distinct coherent objects. It is an ongoing debate whether or not the formation of visual objects requires attention. Implicit behavioral measures suggest that object formation can occur for task-irrelevant and unattended visual stimuli. The present study investigated pre-attentive visual object formation by combining implicit behavioral measures and an electrophysiological indicator of pre-attentive visual irregularity detection, the visual mismatch negativity (vMMN) of the event-related potential. Our displays consisted of two symmetrically arranged, task-irrelevant ellipses, the objects. In addition, there were two discs of either high or low luminance presented on the objects, which served as targets. Participants had to indicate whether the targets were of the same or different luminance. In separate conditions, the targets either usually were enclosed in the same object or in two different objects (standards). Occasionally, the regular target-to-object assignment was changed (deviants). That is, standards and deviants were exclusively defined on the basis of the task-irrelevant target-to-object assignment but not on the basis of some feature regularity. Although participants did not notice the regularity nor the occurrence of the deviation in the sequences, task-irrelevant deviations resulted in increased reaction times. Moreover, compared with physically identical standard displays deviating target-to-object assignments elicited a negative potential in the 246–280 ms time window over posterio-temporal electrode positions which was identified as vMMN. With variable resolution electromagnetic tomography (VARETA) object-related vMMN was localized to the inferior temporal gyrus. Our results support the notion that the visual system automatically structures even task-irrelevant aspects of the incoming information into objects.