Top-down Biases in Visual Search
This is the abstract of a recent study (Töllner et al., 2012, Cereb Cor), which explored whether and how top-down factors, such as expectancies or experiences, can bias feedforward visual processing:
Visual search for feature singletons is slowed when a task-irrelevant, but more salient distracter singleton is concurrently presented. While there is a consensus that this distracter interference effect can be influenced by internal system settings, it remains controversial at what stage of processing this influence starts to affect visual coding. Advocates of the ‘‘stimulus-driven’’ view maintain that the initial sweep of visual processing is entirely driven by physical stimulus attributes and that top-down settings can bias visual processing only after selection of the most salient item. By contrast, opponents argue that top-down expectancies can alter the initial selection priority, so that focal attention is ‘‘not automatically’’ shifted to the location exhibiting the highest feature contrast. To precisely trace the allocation of focal attention, we analyzed the Posterior-Contralateral-Negativity (PCN) in a task in which the likelihood (expectancy) with which a distracter occurred was systematically varied. Our results show that both high (vs. low) distracter expectancy and experiencing a distracter on the previous trial speed up the timing of the target-elicited PCN. Importantly, there was no distracter-elicited PCN, indicating that participants did not shift attention to the distracter before selecting the target. This pattern unambiguously demonstrates that preattentive vision is top-down modifiable.
Töllner, T., Rangelov, D., & Müller, H.J. (2012). How the speed of motor-response decisions, but not focal-attentional selection, differs as a function of task set and target prevalence. Proceedings of the National Academy of Sciences, 109, E1990-E1999.
Töllner, T., Müller, H.J., & Zehetleitner, M. (2012). Top-down dimensional weight set determines the capture of visual attention: Evidence from the PCN component. Cerebral Cortex, 22(7), 1554-1563.
Wykowska, A., Schubö, A. (2011). Irrelevant singletons in visual search do not capture attention but canproduce non-spatial filtering costs. Journal of Cognitive Neuroscience, 23, 645–660.
Wykowska, A., Schubö, A. (2010). On the temporal relation of top-down and bottom-up mechanisms during guidance of attention. Journal of Cognitive Neuroscience, 22, 640-654.
Töllner, T., Zehetleitner, M., Gramann, K., & Müller, H.J. (2010). Top-down weighting of visual dimensions: Behavioral and electrophysiological evidence. Vision Research, 50 (14), 1372-1381.
Bottom-up Biases in Visual Search
This is the summary of a recent study (Töllner et al., 2011, PLoS) in which we asked whether the Posterior-Contralateral-Negativity (PCN), and thus the timing of focal-attentional selection, can be used as an electro-cortical correlate of visual saliency:
The notion of a saliency-based processing architecture underlying human vision is central to a number of current theories of visual selective attention. On this view, focal attention is guided by an overall-saliency map of the scene, which integrates (sums) signals from pre-attentive sensory feature-contrast computations (e.g., for color, motion, etc.). By linking the Posterior Contralateral Negativity (PCN) component to reaction time (RT) performance, we tested one specific prediction of such salience summation models: expedited shifts of focal-attention to targets with low, as compared to high, target-distracter similarity. For two feature-dimensions (color and orientation), we observed decreasing RTs with increasing target saliency. Importantly, this pattern was systematically mirrored by the timing, as well as amplitude, of the PCN. This pattern demonstrates that visual saliency is a key determinant of the time it takes for focal-attention to be engaged onto the target item, even when it is just a feature singleton.
Töllner, T., Zehetleitner, M., Gramann, K., & Müller, H.J. (2011). Stimulus saliency modulates pre-attentive processing speed in human visual cortex. PLoS ONE, 6(1), e16276.
Töllner, T., Zehetleitner, M., Krummenacher, J., Müller, H.J. (2011). Perceptual basis of redundancy gains in visual pop-out search. Journal of Cognitive Neuroscience, 23, 137-150.
Intertrial Biases in Visual Search
Here is the abstract of a study (Töllner et al., 2008, JEP:HPP) in which we coupled mental chronometry to both PCN and LRP component in order to illuminate the (pre-attentive versus post-selective) origin of the dimension-specific intertrial facilitation effect in visual search:
In cross-dimensional visual search tasks, target discrimination is faster when the previous trial contained a target defined in the same visual dimension as the current trial. The dimension-weighting account (DWA; A. Found & H. J. Müller, 1996) explains this intertrial facilitation by assuming that visual dimensions are weighted at an early perceptual stage of processing. Recently, this view has been challenged by models claiming that intertrial facilitation effects are generated at later stages that follow attentional target selection (K. Mortier, J. Theeuwes, & P. A. Starreveld, 2005). To determine whether intertrial facilitation is generated at a perceptual stage, at the response selection stage, or both, the authors focused on specific event-related brain potential components (directly linkable to perceptual and response-related processing) during a compound search task. Visual dimension repetitions were mirrored by shorter latencies and enhanced amplitudes of the PCN, suggesting a facilitated allocation of attentional resources to the target. Response repetitions and changes systematically modulated the lateralized readiness potential amplitude, suggesting a benefit from residual activations of the previous trial biasing the correct response. Overall, the present findings strengthen the DWA by indicating a perceptual origin of dimension change costs in visual search.
Wiegand, I., Finke, K., Müller, H.J., & Töllner, T. (2013). Event-related potentials dissociate perceptual from response-related age effects in visual search. Neurobiology of Aging, 34, 973-985.
Töllner, T., Gramann, K., Kiss, M., Müller, H.J., & Eimer, M. (2008). Electrophysiological markers of visual dimension changes and response changes. Journal of Experimental Psychology: Human Perception and Performance, 34 (3), 531-542.