Pestilli, F., Ling, S. & Carrasco, M. (in press). A population-coding model of attention's influence on contrast response: estimating neural effects from psychophysical data. Vision Research.
Human psychophysics and monkey physiology studies have shown that attention
modulates early vision – contrast sensitivity and processing. But how can we bridge the
effects of attention on perceptual performance to their neural underpinnings? Here we
implement a population-coding model that estimates attentional effects on population
contrast response given psychophysical data. Model results show that whereas
endogenous (sustained, voluntary) attention changes population contrast-response via
contrast gain, exogenous (transient, involuntary) attention changes population contrast-
response via response gain.
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Carrasco, M., Fuller, S. & Ling, S. (in press). Transient attention does increase perceived contrast of suprathreshold stimuli: A reply to Prinzmetal, Long and Leonhardt (2008) . Perception & Psychophysics.
Carrasco, Ling and Read (2004) showed that transient attention increases perceived contrast. However, Prinzmetal, Long and Leonhardt (2008) suggest that for targets of low visibility, observers may bias their response towards the cued location, and propose a cue-bias explanation for our previous results. Our response is three-fold. First, we outline several key methodological differences between the studies that could account for the different results. We conclude that the cue-bias hypothesis is a plausible explanation for Prinzmetal et al.’s (2008) results, given the characteristics of their stimuli, but not for the studies by Carrasco and colleagues, in which the stimuli were suprathreshold (Carrasco et al., 2004; Fuller, Rodriguez & Carrasco, 2008; Ling & Carrasco, 2007). Second, we conduct a study to show that the stimuli used in our previous studies are not near-threshold, but suprathreshold (Experiment 1, Phase 1). Furthermore, we found an increase in apparent contrast for a high-contrast stimulus when it was precued, but not when it was postcued –providing more evidence against a cue-bias hypothesis (Experiment 1, Phase 2). We also show that the visibility of the stimuli in Prinzmetal et al. (2008) was much lower than that of Carrasco et al. (2004), rendering their stimuli susceptible to their cue-bias explanation (Experiment 2). Third, we present a comprehensive summary of all the control conditions used in different labs that have ruled out a cue bias explanation of the appearance studies. We conclude that a cue-bias explanation may operate with near- threshold and low-visibility stimuli, as was the case in Prinzmetal et al., but that such an explanation has no bearing on studies with suprathreshold stimuli. Consistent with our previous studies, the present data support the claim that attention does alter the contrast appearance of suprathreshold stimuli.
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Ling,
S., Liu, T. & Carrasco, M. (in press). How spatial and feature-based attention affect the gain and tuning of population responses. Vision Research.
How does attention optimize our visual system for the task at hand? Two mechanisms have been proposed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect performance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boosting the gain and sharpening the tuning of the population response.
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Ling,
S. & Carrasco, M. (2007). Transient covert
attention does alter appearance: A reply to Schneider (2006). Perception
& Psychophysics, 69(6), 1051-1058.
We
recently demonstrated that transient covert attention increases
the apparent contrast of a stimulus (Carrasco, Ling & Read,
2004). Schneider (2006) proposes that the observed increase
in apparent contrast is largely due to sensory interactions
occurring between the precue and stimulus, rather than attention.
Specifically, he asserts that cueing effects only occur at
contrasts near detection threshold, and that there are confounding
sensory interactions between the cue and stimulus at supra-threshold
detection contrasts. Our response is two-fold. First, we
outline the key methodological differences between our study
and Schneider’s that are likely to account for the
different results, and explain how we had ruled out the sensory
interaction explanation of the cue. Second, we directly test
the prediction put forth by Schneider: were the effects due
to sensory interactions, reversing the luminance polarity
of the precue in our paradigm should lead to differential
cueing effects. We replicate one of the experiments of our
original study, and add a condition in which the cue luminance
is either black or white. Our results replicated our previous
findings –they showed an increase in apparent contrast
to a stimulus when it was precued, even with high contrast
stimuli. Moreover, we found that the black cue and the white
cue had the same effect, thus ruling out the alternative
explanation proposed by Schneider. Transient attention does
alter contrast appearance.
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