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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

“Blindsight” is a term first coined by Weiskrantz et al. in 1974 to describe residual visual performance in the cortically blind. It has been postulated that blindsight could be due to the retinotectal pathway projecting information past V1 to later cortical structures. Our interest was in whether the pathways responsible for blindsight could also support enumeration. We tested a 53-year-old male, C.H., who had suffered a medial right occipital lobe stroke seven months previous. The stroke resulted in an upper left homonymous quadrantanopia. In order to determine whether there were any residual abilities in the blind field, we first tested basic detection and discrimination skills. C.H. was able to determine whether or not a 7° X 7° visual angle object was presented in his blind field with near perfect accuracy though his accuracy was only 90% for smaller (4.6° X 2.2°) figures. C.H. also discriminated between large X′s and O′s and horizontal and vertical bars with good accuracy when the figures were large (78% for X vs. O and 73% for horizontal vs. vertical bars). Throughout these tests C.H. staunchly maintained that he could not see the stimuli and that he was only guessing. Because it was clear that C.H. had some residual ability in his blind field, we tested his enumeration. The enumeration task involved 1-3 items. These items were 1.5° Blindsight and enumeration: A case study http://www.journalofvision.org/content/10/7/898.short 1 of 2 2015-01-03, 8:42 PM X 1.5° black diamonds, 0-2 in his blind field and 1-3 in his non-blind field. Across all conditions, C.H. was able to use the information in his blind field and identify the total number of items presented at levels of performance that were well above chance. Because C.H. appears to use blind field information to enumerate, we postulate that enumeration ability may be mediated by the same structures that support blindsight. Received June 26, 2010. © 2010 ARVO Articles citing this article Blliindsiight and enumeratiion:: A case study J Vis August 12, 2010 10(7): 898 Abstract Blindsight and enumeration: A case study http://www.journalofvision.org/content/10/7/898.short 2 of 2 2015-01-03, 8:42 PM

Purpose: The ability in cortically blind patients to respond to visual stimuli without consciously experiencing has been termed ‘blindsight’. The goal of this study is to investigate the involvement of the superior colliculi (SC) in mediating blindsight in hemispherectomized (HS) subjects. Methods: We used the achromatic properties of collicular cells, which receive no input from S-cones, to design a reaction time test of a spatial summation effect (SSE), in which reaction times to two bilaterally presented stimuli are significantly faster compared to a single one. Achromatic (SC-visible) and blue/yellow (SC-invisible) gabor patches (1cpd, spatial σ=1cycle, temporal σ=250ms) were displayed on a CRT monitor and isolated either the achromatic or the S-cone opponent (blue/yellow) pathway. Stimuli were presented 10° in the right, left, or in both visual fields. Stimulus onset-time was randomized at 0/500/1000ms with an ITI of 2000ms. Fixation was monitored with an eye tracker. Three HS subjects with and two without blindsight were tested. Results: Subjects with blindsight showed a SSE to achromatic stimuli confirming the presence of blindsight. Presentation of blue/yellow stimuli, however, failed to alter reaction times demonstrating that their blindsight is colour-blind for blue/yellow stimuli. HS subjects without blindsight showed no SSE to either achromatic or blue/yellow stimuli. Conclusions: Blindsight, at least in HS subjects, is color blind, and hence we conclude that it is mediated by the SC. Furthermore, by testing HS subjects we could reject the possibility that spared islands of visual cortex subtend blindsight.

Humans can experience aftereffects from oriented stimuli that are not consciously perceived, suggesting that such stimuli receive cortical processing. Determining the physiological substrate of such effects has proven elusive owing to the low spatial resolution of conventional human neuroimaging techniques compared to the size of orientation columns in visual cortex. Here we show that even at conventional resolutions it is possible to use fMRI to obtain a direct measure of orientation-selective processing in V1. We found that many parts of V1 show subtle but reproducible biases to oriented stimuli, and that we could accumulate this information across the whole of V1 using multivariate pattern recognition. Using this information, we could then successfully predict which one of two oriented stimuli a participant was viewing, even when masking rendered that stimulus invisible. Our findings show that conventional fMRI can be used to reveal feature-selective processing in human cortex, even for invisible stimuli.

A 21-year-old male presented with occipital lobes that were extensively damaged by bilateral infarcts present at birth. The absence of the striate cortex was confirmed with anatomic and functional MRI and high-resolution EEG. His cortical visual impairment was severe, but he retained a remarkable ability to see fast-moving stimuli. Horizontal optokinetic nystagmus could be elicited from either eye. Resolution acuity was close to normal providing the patient was allowed to move his head and eyes. The direction of motion in random-dot patterns could be discriminated with perfect accuracy at speeds above 2 deg/s, and the patient reported that he could 'see' the motion at fast but not at slow speeds. This conscious residual vision for motion is known as Riddoch's phenomenon, but it has never been reported in the complete absence of the striate cortex. Functional neuroimaging revealed activation that was outside the motion-responsive regions of the extrastriate cortex. This case demonstrates remarkable plasticity in the human visual system and may have implications for understanding the functional organization of the motion pathways.

Can practice effects on unconscious stimuli lead to awareness? Can we "learn to see"? Recent evidence suggests that blindsight patients trained for an extensive period of time can learn to discriminate and consciously perceive stimuli that they were previously unaware of. So far, it is unknown whether these effects generalize to normal observers. Here we investigated practice effects in metacontrast masking. Subjects were trained for five consecutive days on the stimulus onset asynchrony (SOA) that resulted in chance performance. Our results show a linear increase in sensitivity (d') but no change in bias (c) for the trained SOA. This practice effect on sensitivity spreads to all tested SOAs. Additionally, we show that subjects rate their perceptual awareness of the target stimuli differently before and after training, exhibiting not only an increase in sensitivity, but also in the subjective awareness of the percept. Thus, subjects can indeed "learn to see."