Blindsight In Normal Observers Research Articles

Masked blindsight in normal observers: Measuring subjective and objective responses to two features of each stimulus

Recent visual masking studies that have measured visual awareness with graded subjective scales have often failed the show any evidence for unconscious visual processing in normal observers in a paradigm similar to that used in studies on blindsight patients. Without any reported awareness of the target, normal observers typically cannot discriminate target's features better than chance. The present study examined processing of color and orientation by measuring graded awareness and forced-choice discriminations for both features in each trial. When no awareness for either feature was reported, discrimination of each feature succeed better than expected by chance, even when the other feature was incorrectly discriminated in the same trial. However, the characteristics of the mask determined whether or not masked blindsight was observed. We conclude that when the processing channels are free from intra-channel interference, unbound or weakly bound features can guide behaviour without any reported awareness in normal observers.

Normal observers show no evidence for blindsight in facial emotion perception

Some researchers have argued that normal human observers can exhibit “blindsight-like” behavior: the ability to discriminate or identify a stimulus without being aware of it. However, we recently used a bias-free task to show that what looks like blindsight may in fact be an artifact of typical experimental paradigms’ susceptibility to response bias. While those findings challenge previous reports of blindsight in normal observers, they do not rule out the possibility that different stimuli or techniques could still reveal perception without awareness. One intriguing candidate is emotion processing, since processing of emotional stimuli (e.g. fearful/happy faces) has been reported to potentially bypass conscious visual circuits. Here we used the bias-free blindsight paradigm to investigate whether emotion processing might reveal “featural blindsight,” i.e. ability to identify a face’s emotion without introspective access to the task-relevant features that led to the discrimination decision. However, we saw no evidence for emotion processing “featural blindsight”: as before, whenever participants could identify a face’s emotion they displayed introspective access to the task-relevant features, matching predictions of a Bayesian ideal observer. These results add to the growing body of evidence that perceptual discrimination ability without introspective access may not be possible for neurologically intact observers.

Bayesian ideal observer predicts weak forms of blindsight in normal observers

It has been reported that damage to V1 can lead to blindsight: such patients can discriminate simple visual stimuli above chance and yet allegedly report no conscious visual percept. It has also been suggested that similar forms of unconscious vision can be demonstrated in normal observers, i.e. under certain experimental conditions (e.g. low-contrast masked stimuli) subjects can perform above-chance discrimination even when rating confidence/stimulus visibility at the lowest level. However, from a signal detection theoretic (SDT) perspective, these findings may be trivial: subjects may report low confidence/invisibility only because the signal falls below an arbitrary confidence/visibility criterion, rather than being truly invisible. We therefore investigated whether perceptual discrimination without subjective awareness can be demonstrated unequivocally by the rigorous standards of ideal observer analyses. To bypass the criterion problem mentioned above, we probed confidence with a 2-interval forced-choice procedure (Barthelme & Mamassian, 2009, PLoS CB): normal observers viewed paired intervals of masked stimuli, discriminated the orientation of each (left/right tilted), and then chose which decision they were more confident in (Expt 1) or which stimulus was more visible (Expt 2). Importantly, in one interval the stimulus had zero contrast, meaning above-chance orientation discrimination was impossible. Strong evidence for blindsight would consist of subjects' discriminating the stimulus above-chance yet failing to consistently select it as more confident/visible relative to the blank interval. Such suboptimal behavior would violate common models of perception. In both experiments, intriguingly, we found that observers judged confidence/visibility with lower sensitivity than they could discriminate orientation, according to SDT measures, suggesting there may be evidence for compromised subjective awareness. Crucially, however, a Bayesian ideal observer model predicts similar behavior. We argue that traditional SDT analyses make unrealistic assumptions about what observers can do in these tasks, and show with simulations how misleading results could be obtained via conventional procedures. Meeting abstract presented at VSS 2015. Language: en

Absolute and relative blindsight.

The concept of relative blindsight, referring to a difference in conscious awareness between conditions otherwise matched for performance, was introduced by Lau and Passingham (2006) as a way of identifying the neural correlates of consciousness (NCC) in fMRI experiments. By analogy, absolute blindsight refers to a difference between performance and awareness regardless of whether it is possible to match performance across conditions. Here, we address the question of whether relative and absolute blindsight in normal observers can be accounted for by response bias. In our replication of Lau and Passingham's experiment, the relative blindsight effect was abolished when performance was assessed by means of a bias-free 2AFC task or when the criterion for awareness was varied. Furthermore, there was no evidence of either relative or absolute blindsight when both performance and awareness were assessed with bias-free measures derived from confidence ratings using signal detection theory. This suggests that both relative and absolute blindsight in normal observers amount to no more than variations in response bias in the assessment of performance and awareness. Consideration of the properties of psychometric functions reveals a number of ways in which relative and absolute blindsight could arise trivially and elucidates a basis for the distinction between Type 1 and Type 2 blindsight.

Relative blindsight arises from a criterion confound in metacontrast masking: Implications for theories of consciousness

Relative blindsight is said to occur when different levels of subjective awareness are obtained at equality of objective performance. Using metacontrast masking, Lau and Passingham (2006) reported relative blindsight in normal observers at the shorter of two stimulus-onset asynchronies (SOAs) between target and mask. Experiment 1 replicated the critical asymmetry in subjective awareness at equality of objective performance. We argue that this asymmetry cannot be regarded as evidence for relative blindsight because the observers' responses were based on different attributes of the stimuli (criterion contents) at the two SOAs. With an invariant criterion content (Experiment 2), there was no asymmetry in subjective awareness across the two SOAs even though objective performance was the same. Experiment 3 examined the effect of criterion level on estimates of relative blindsight. Collectively, the present results question whether metacontrast masking is a suitable paradigm for establishing relative blindsight. Implications for theories of consciousness are discussed.

Relative blindsight in normal observers and the neural correlate of visual consciousness

By using a paradigm based on metacontrast masking, we created experimental conditions in which the subjective report of consciousness differs but the objectively measured ability to discriminate visual targets does not. This approach allowed us to study the neural correlate of consciousness while having performance levels carefully matched in healthy human subjects. A comparison of the neural activity associated with these conditions as measured by functional MRI showed that conscious perception is associated with spatially specific activity in the mid-dorsolateral prefrontal cortex (area 46). Further analysis confirms that this activation is not only free from any performance confound, but is also not driven by differences in the timing of the physical stimuli. Our results suggest that the prefrontal cortex is important for the essentially subjective aspects of conscious perception.

Inducing blindsight in normal observers.

We attempted to induce blindsight in normal observers, in an effort to replicate and extend the findings of Kolb and Braun (1995). In that demonstration, observers were able to localize a target in the absence of visual awareness, indicated by the lack of a correlation between localization accuracy and confidence ratings. Replication of this work seemed essential, given the failed attempt by Morgan, Mason, and Solomon (1997). A key aspect of the present work was the use of a pointing response, which is believed to have access to the unconscious representations subserving blindsight. In the critical rivalrous condition, the display consisted of Gabor patterns presented dichoptically with orthogonal orientation in each eye. Binocular summation of left and right images combined to give the appearance of a uniform plaid, camouflaging the texture-defined target. Our attempt to demonstrate blindsight in normal observers was unsuccessful, in that the localization accuracy of the texture-defined target and the observers' confidence ratings were positively correlated. Although the replication was unsuccessful, the results are valuable in that they provide closure to this widely publicized but fruitless line of inquiry.

Blindsight in normal observers.

Some patients with lesions in visual cortex lack conscious visual experience but, when tested, exhibit a significant ability, termed 'blindsight', to discriminate visual stimuli. Here we report two different visual displays that induce blindsight in normal observers. Using an objective measure, we show that conscious experience remains defective at presentation times much longer (1 s) than the onset of visual sensitivity (approximately 60 ms). To obtain this effect, we generate a contrast between visual textures and then conceal the contrast by superimposing 'complementary' textures. Complementarity can involve either opposite motion or binocular rivalry and orthogonal orientation. In both cases, observers locate the texture contrast reliably but do not, by either subjective or objective measures, consciously experience it. Taken together with present knowledge of the visual cortical site(s) at which opposite motion and rivalrous orientation interact, this observation bears upon the functional anatomy of conscious visual experience.