Distinguishing perceptual shifts from response biases
Friday, May 11, 3:30 – 5:30 pm
Organizer: Joshua Solomon, City University London
Presenters: Sam Ling, Vanderbilt; Keith Schneider, York University; Steven Hillyard, UCSD; Donald MacLeod, UCSD; Michael Morgan, City University London, Max Planck Institute for Neurological Research, Cologne; Mark Georgeson, Aston University
Sensory adaptation was originally considered a low-level phenomenon involving measurable changes in sensitivity, but has been extended to include many cases where a change in sensitivity has yet to be demonstrated. Examples include adaptation to blur, temporal duration and face identity. It has also been claimed that adaptation can be affected by attention to the adapting stimulus, and even that adaptation can be caused by imaging the adapting stimulus. The typical method of measurement in such studies involves a shift in the mean (p50) point of a psychometric function, obtained by the Method of Single Stimuli. In Signal Detection Theory, the mean is determined by a decision rule, as opposed to the slope which is set by internal noise. The question that arises is how we can distinguish shifts in mean due to a genuine adaptation process from shifts due to a change in the observer�s decision rule. This was a hot topic in the 60�s, for example, in discussion between Restle and Helson over Adaptation Level Theory, but it has become neglected, with the result that any shift in the mean of a psychometric function is now accepted as evidence for a perceptual shift. We think that it is time to revive this issue, given the theoretical importance of claims about adaptation being affected by imagination and attention, and the links that are claimed with functional brain imaging.
Attention alters appearance
Sam Ling, Vanderbilt University
Maintaining veridicality seems to be of relatively low priority for the human brain; starting at the retina, our neural representations of the physical world undergo dramatic transformations, often forgoing an accurate depiction of the world in favor of augmented signals that are more optimal for the task at hand. Indeed, visual attention has been suggested to play a key role in this process, boosting the neural representations of attended stimuli, and attenuating responses to ignored stimuli. What, however, are the phenomenological consequences of attentional modulation? I will discuss a series of studies that we and others have conducted, all converging on the notion that attention can actually change the visual appearance of attended stimuli across a variety of perceptual domains, such as contrast, spatial frequency, and color. These studies reveal that visual attention not only changes our neural representations, but that it can actually affect what we think we see.
Attention increases salience and biases decisions but does not alter appearance.
Keith Schneider, York University
Attention enhances our perceptual abilities and increases neural activity. Still debated is whether an attended object, given its higher salience and more robust representation, actually looks any different than an otherwise identical but unattended object. One might expect that this question could be easily answered by an experiment in which an observer is presented two stimuli differing along one dimension, contrast for example, to one of which attention has been directed, and must report which stimulus has the higher apparent contrast. The problem with this sort of comparative judgment is that in the most informative case, that in which the two stimuli are equal, the observer is also maximally uncertain and therefore most susceptible to extraneous influence. An intelligent observer might report, all other things being equal, that the stimulus about which he or she has more information is the one with higher contrast. (And it doesn’t help to ask which stimulus has the lower contrast, because then the observer might just report the less informed stimulus!) In this way, attention can bias the decision mechanism and confound the experiment such that it is not possible for the experimenter to differentiate this bias from an actual change in appearance. It has been over ten years since I proposed a solution to this dilemma�an equality judgment task in which observers report whether the two stimuli are equal in appearance or not. This paradigm has been supported in the literature and has withstood criticisms. Here I will review these findings.
Electrophysiological Studies of the Locus of Perceptual Bias
Steven Hillyard, UCSD
The question of whether attention makes sensory impressions appear more intense has been a matter of debate for over a century. Recent psychophysical studies have reported that attention increases the apparent contrast of visual stimuli, but there is still a controversy as to whether this effect is due to the biasing of decisions as opposed to the altering of perceptual representations and changes in subjective appearance. We obtained converging neurophysiological evidence while observers judged the relative contrast of Gabor patch targets presented simultaneously to the left and right visual fields following a lateralized cue (auditory or visual). This non-predictive cueing boosted the apparent contrast of the Gabor target on the cued side in association with an enlarged neural response in the contralateral visual cortex that began within 100 ms after target onset. The magnitude of the enhanced neural response in ventral extrastriate visual cortex was positively correlated with perceptual reports of the cued-side target being higher in contrast. These results suggest that attention increases the perceived contrast of visual stimuli by boosting early sensory processing in the visual cortex.
Adaptive sensitivity regulation in detection and appearance
Donald MacLeod, UCSD
The visual system adapts to changing levels of stimulation with alterations of sensitivity that are expressed both in in changes in detectability, and in changes of appearance. The connection between these two aspects of sensitivity regulation is often taken for granted but need not be simple. Even the proportionality between ‘thresholds’ obtained by self-setting and threshold based on reliability of detection (e.g. forced-choice) is not generally expected except under quite restricted conditions and unrealistically simple models of the visual system. I review some of the theoretical possibilities in relation to available experimental evidence. Relatively simple mechanistic models provide opportunity for deviations from proportionality, especially if noise can enter into the neural representation at multiple stages. The extension to suprathreshold appearance is still more precarious; yet remarkably, under some experimental conditions, proportionality with threshold sensitivities holds, in the sense that equal multiples of threshold match.
Observers can voluntarily shift their psychometric functions without losing sensitivity
Michael Morgan, City University London, Max Planck Institute for Neurological Research, Cologne, Barbara Dillenburger, Sabine Raphael, Max Planck; Joshua A. Solomon, City University
Psychometric sensory discrimination functions are usually modeled by cumulative Gaussian functions with just two parameters, their central tendency and their slope. These correspond to Fechner�s �constant� and �variable� errors, respectively. Fechner pointed out that even the constant error could vary over space and time and could masquerade as variable error. We wondered whether observers could deliberately introduce a constant error into their performance without loss of precision. In three-dot vernier and bisection tasks with the method of single stimuli, observers were instructed to favour one of the two responses when unsure of their answer. The slope of the resulting psychometric function was not significantly changed, despite a significant change in central tendency. Similar results were obtained when altered feedback was used to induce bias. We inferred that observers can adopt artificial response criteria without any significant increase in criterion fluctuation. These findings have implications for some studies that have measured perceptual �illusions� by shifts in the psychometric functions of sophisticated observers.
Sensory, perceptual and response biases: the criterion concept in perception
Mark Georgeson, Aston University
Signal detection theory (SDT) established in psychophysics a crucial distinction between sensitivity (or discriminability, d�) and bias (or criterion) in the analysis of performance in sensory judgement tasks. SDT itself is agnostic about the origins of the criterion, but there seems to be a broad consensus favouring �response bias� or �decision bias�. And yet, perceptual biases exist and are readily induced. The motion aftereffect is undoubtedly perceptual – compelling motion is seen on a stationary pattern � but its signature in psychophysical data is a shift in the psychometric function, indistinguishable from �response bias�. How might we tell the difference? I shall discuss these issues in relation to some recent experiments and modelling of adaptation to blur (Elliott, Georgeson & Webster, 2011). A solution might lie in dropping any hard distinction between perceptual shifts and decision biases. Perceptual mechanisms make low-level decisions. Sensory, perceptual and response criteria might be represented neurally in similar ways at different levels of the visual hierarchy, by biasing signals that are set by the task and by the history of stimuli and responses (Treisman & Williams, 1984). The degree of spatial localization over which the bias occurs might reflect its level in the visual hierarchy. Thus, given enough data, the dilemma (are aftereffects perceptual or due to response bias?) might be resolved in favour of such a multi-level model.