Unconscious Vision Research Articles

Role of corpus callosum in unconscious vision

The existence of unconscious visually triggered behavior in patients with cortical blindness (e.g., homonymous hemianopia) has been amply demonstrated and the neural bases of this phenomenon have been thoroughly studied. However, a crosstalk between the two hemispheres as a possible mechanism of unconscious or partially conscious vision has not been so far considered. Thus, the aim of this study was to assess the relationship between structural and functional properties of the corpus callosum (CC), as shown by probabilistic tractography (PT), behavioral detection/discrimination performance and level of perceptual awareness in the blind field of patients with hemianopia. Twelve patients were tested in two tasks with black-and-white visual square-wave gratings, one task of movement and the other of orientation. The stimuli were lateralized to one hemifield either intact or blind. A PT analysis was carried out on MRI data to extract fiber properties along the CC (genu, body, and splenium). Compared with a control group of participants without brain damage, patients showed lower FA values in all three CC sections studied. For the intact hemifield we found a significant correlation between PT values and visual detection/discrimination accuracy. For the blind hemifield the level of perceptual awareness correlated with PT values for all three CC sections in the movement task. Importantly, significant differences in all three CC sections were found also between patients with above-vs. chance detection/discrimination performance while differences in the genu were found between patients with and without perceptual awareness. Overall, our study provides evidence that the properties of CC fibers are related to the presence of unconscious stimulus detection/discrimination and to hints of perceptual awareness for stimulus presentation to the blind hemifield. These results underline the importance of information exchange between the damaged and the healthy hemisphere for possible partial or full recovery from hemianopia.

Conscious and Unconscious Vision Transmission to Brain over Behavior

Our goal is to apply historical evidence with longitudinal records of human underlining brain led behavior, to compensate for the findings based on conventional brain signal and image measurements taken at a resting position. This compensation, due to unconsciousness, is expected to play a major role in one’s decision making process. Using masks to distract vision, also measuring facial muscle movements, we were able to separate cognition through conscious and unconscious vision. Two types of masks are used over face image arrays under different cultural backgrounds to exercise 3 sequences of tests: (1) randomly generated masks anterior and posterior to target images; (2) Tai Chi Tu as masks, to take cognizance of longitudinal changes of the environment; (3) for each mask, contrasting European and Asian perception of facial emotional images. Our proposed approach is an attempt to investigate spatio-temporal effect about brain decision-making when historical evidence is considered to provoke the unconsciousness. Evidence and environmental influences over brain are commonly transmitted from vision. By masking vision at varying epoch, rates of longitudinal changes are examined for impact on perception, even when these rates have not been normalized for testing. Some justification of mask usage, its exposure duration and frequency are designed into our testing procedures for sensitivity studies. Significantly more testing is expected to understand the ambiguity between unconscious and conscious vision and to justify the proposed type of longitudinal changes on brain triggered behavior.

Contents of Unconscious Color Perception

In the contemporary discussions concerning unconscious perception it is not uncommon to postulate that content and phenomenal character are ‘orthogonal’, i.e., there is no type of content which is essentially conscious, but instead, every representational content can be either conscious or not. Furthermore, this is not merely treated as a thesis justified by theoretical investigations, but as supported by empirical considerations concerning the actual functioning of the human cognition. In this paper, I address unconscious color perception and argue for a negative thesis—that the main experimental paradigms used in studying unconscious color perception do not provide support for the position that conscious and unconscious color representations have the same type of content. More specifically, I claim that there is no significant support for the claim that unconscious vision categorically represents surface colors.

The neural correlates of the visual consciousness in schizophrenia: an fMRI study

In the current literature, two distinct and opposite models are suggested to explain the consciousness disorders in schizophrenia. The first one suggests that consciousness disorders rely on a low-level processing deficit, when the second model suggests that consciousness disorders rely on disruption in the ability to consciously access information, with preserved unconscious processing. The current study aims to understand the mechanisms associated with visual consciousness disorder in order to pave the road that will settle the debate regarding these hypotheses. During a functional magnetic resonance imaging session, 19 healthy participants (HC) and 15 patients with schizophrenia (SCZ) performed a visual detection task to compare the neural substrates associated with the conscious access to the visual inputs. The visual detection threshold was significantly higher in SCZ than in HC [t(32) = 3.37, p = 0.002]. Whole-brain ANOVA demonstrated that around the visual detection threshold patients with SCZ failed to activate a large network of brain areas compared to HC. (1) During conscious vision, HC engaged more the left cuneus and the right occipital cortex than patients with SCZ, (2) during unconscious vision, HC engaged a large network that patients with SCZ failed to activate, and finally, (3) during the access to consciousness process, patients with SCZ failed to activate the anterior cingulate cortex. These results suggest that the consciousness disorders in schizophrenia rely on specific dysfunctions depending on the consciousness stage. The disorders of the conscious vision are associated with dysfunction of occipital areas while the ones associated with unconscious vision rely on a large widespread network. Finally, the conscious access to the visual inputs is impaired by a dysfunction of the anterior cingulate cortex. The current study suggests that none of the two suggested models can explain consciousness disorders in schizophrenia. We suggest that there is an alternative model supporting that the conscious access to visual inputs is due to a disengagement of the supragenual anterior cingulate during the unconscious processing of the visual inputs associated with a sensory deficit.

Visual Functions Generating Conscious Seeing.

Visual functions are reviewed that coincide with conscious as opposed to unconscious vision. Four stages of vision are identified, going from the fully invisible, to subjectively invisible, unattended, and clearly visible. It is proposed that feature extraction, categorization, and some aspects of visual inference occur during full and subjective invisibility. Functions related to perceptual organization, such as grouping and figure-ground segregation, occur during inattention as well as full visibility. It is argued that perceptual organization is the function that is central to understanding the transition from unconscious to conscious seeing. It is discussed what this implies for theories of consciousness such as Recurrent Processing Theory, Higher Order Thought Theory, Integrated Information Theory, and Global Neuronal Workspace Theory.

Dissociations of conscious and unconscious perception in TMS-induced blindsight

As with some patients with primary visual cortex (V1) damage, transcranial magnetic stimulation (TMS) over V1 reliably induces blindsight, whereby observers can correctly discriminate the attributes of visual stimuli despite being unable to detect them. This TMS-induced blindsight has been demonstrated to reflect a form of unconscious vision that relies upon different neural pathways than with conscious vision. However, the timing of the neural processes mediating TMS-induced blindsight has been unclear, especially when considering suggestions that TMS interferes with feedback processes to V1 that mediate conscious visual perception. To better elucidate the neural mechanisms that give rise to blindsight, we tested TMS-induced blindsight for the orientation of visual stimuli across a range of stimulus onset asynchronies (SOAs) to assess how different latencies of visual cortex disruption, relative to a visual stimulus, affect detection rates and forced-choice discrimination accuracy. At all TMS latencies, including at SOAs with substantial visual suppression from TMS, discrimination performance was significantly above-chance, demonstrating the consistency of TMS-induced blindsight. Crucially, we observed two windows of maximum visual suppression from TMS at SOAs between 65 and 105 ms, but consistent above-chance discrimination performance accuracy across these windows. However, at longer SOAs, detection and discrimination covaried, suggesting a dependency of discrimination performance on detection only when detection rates exceed threshold levels of normal vision. Taken together, these results indicate that unconscious discrimination occurs independently of detection, including at TMS intervals that optimally interfere with conscious visual perception. They further suggest that forced-choice discrimination is less dependent on feedback processes to V1 than visual awareness and that TMS-induced blindsight is not the same as near-threshold vision.

More than blindsight: Case report of a child with extraordinary visual capacity following perinatal bilateral occipital lobe injury

Injury to the primary visual cortex (V1, striate cortex) and the geniculostriate pathway in adults results in cortical blindness, abolishing conscious visual perception. Early studies by Larry Weiskrantz and colleagues demonstrated that some patients with an occipital-lobe injury exhibited a degree of unconscious vision and visually-guided behaviour within the blind field. A more recent focus has been the observed phenomenon whereby early-life injury to V1 often results in the preservation of visual perception in both monkeys and humans. These findings initiated a concerted effort on multiple fronts, including nonhuman primate studies, to uncover the neural substrate/s of the spared conscious vision. In both adult and early-life cases of V1 injury, evidence suggests the involvement of the Middle Temporal area (MT) of the extrastriate visual cortex, which is an integral component area of the dorsal stream and is also associated with visually-guided behaviors. Because of the limited number of early-life V1 injury cases for humans, the outstanding question in the field is what secondary visual pathways are responsible for this extraordinary capacity? Here we report for the first time a case of a child (B.I.) who suffered a bilateral occipital-lobe injury in the first two weeks postnatally due to medium-chain acyl-Co-A dehydrogenase deficiency. At 6 years of age, B.I. underwent a battery of neurophysiological tests, as well as structural and diffusion MRI and ophthalmic examination at 7 years. Despite the extensive bilateral occipital cortical damage, B.I. has extensive conscious visual abilities, is not blind, and can use vision to navigate his environment. Furthermore, unlike blindsight patients, he can readily and consciously identify happy and neutral faces and colors, tasks associated with ventral stream processing. These findings suggest significant re-routing of visual information. To identify the putative visual pathway/s responsible for this ability, MRI tractography of secondary visual pathways connecting MT with the lateral geniculate nucleus (LGN) and the inferior pulvinar (PI) were analysed. Results revealed an increased PI-MT pathway in the left hemisphere, suggesting that this pulvinar relay could be the neural pathway affording the preserved visual capacity following an early-life lesion of V1. These findings corroborate anatomical evidence from monkeys showing an enhanced PI-MT pathway following an early-life lesion of V1, compared to adults.

Early processing in primary visual cortex is necessary for conscious and unconscious vision while late processing is necessary only for conscious vision in neurologically healthy humans

The neural mechanisms underlying conscious and unconscious visual processes remain controversial. Blindsight patients may process visual stimuli unconsciously despite their V1 lesion, promoting anatomical models, which suggest that pathways bypassing the V1 support unconscious vision. On the other hand, physiological models argue that the major geniculostriate pathway via V1 is involved in both unconscious and conscious vision, but in different time windows and in different types of neural activity. According to physiological models, feedforward activity via V1 to higher areas mediates unconscious processes whereas feedback loops of recurrent activity from higher areas back to V1 support conscious vision. With transcranial magnetic stimulation (TMS) it is possible to study the causal role of a brain region during specific time points in neurologically healthy participants. In the present study, we measured unconscious processing with redundant target effect, a phenomenon where participants respond faster to two stimuli than one even when one of the stimuli is not consciously perceived. We tested the physiological feedforward-feedback model of vision by suppressing conscious vision by interfering selectively either with early or later V1 activity with TMS. Our results show that early V1 activity (60ms) is necessary for both unconscious and conscious vision. During later processing stages (90ms), V1 contributes selectively to conscious vision. These findings support the feedforward-feedback-model of consciousness.

Unconscious vision spots the animal but not the dog: Masked priming of natural scenes.

The functional role of consciousness has been traditionally assumed to be related to high-level executive functions, but recent theories of visual consciousness suggest qualitative differences between conscious and unconscious processes also in lower level visual processes. We tested how specific is the information that can be extracted by unconscious processes from natural scenes. Prime images which were suppressed from consciousness by continuous flash suppression facilitated categorization of visible targets at superordinate level (animal vs. non-animal) when the prime shared a category membership with the target. Suppressed prime images did not have any effect on categorization at the basic level (e.g., horse vs. other animal). Priming occurred at basic level categorization only when the prime images were available to consciousness. This pattern supports a "coarse-to-fine" model in which the visual system can unconsciously access coarse representations, but consciousness is needed for finer analysis of visual scenes.

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

Pupillary light reflex to light inside the natural blind spot.

When a light stimulus covers the human natural blind spot (BS), perceptual filling-in corrects for the missing information inside the BS. Here, we examined whether a filled-in surface of light perceived inside the BS affects the size of the short-latency pupillary light reflex (PLR), a pupil response mediated by a subcortical pathway for unconscious vision. The PLR was not induced by a red surface that was physically absent but perceptually filled-in inside the BS in the presence of a red ring surrounding it. However, a white large disk covering the BS unexpectedly induced a larger PLR than a white ring surrounding the BS border did, even though these two stimuli must be equivalent for the visual system, and trial-by-trial percepts did not predict PLR size. These results suggest that some physiological mechanism, presumably the retinal cells containing the photopigment melanopsin, receives the light projected inside the BS and enhances PLR.

Psychophysical "blinding" methods reveal a functional hierarchy of unconscious visual processing.

Numerous non-invasive experimental "blinding" methods exist for suppressing the phenomenal awareness of visual stimuli. Not all of these suppressive methods occur at, and thus index, the same level of unconscious visual processing. This suggests that a functional hierarchy of unconscious visual processing can in principle be established. The empirical results of extant studies that have used a number of different methods and additional reasonable theoretical considerations suggest the following tentative hierarchy. At the highest levels in this hierarchy is unconscious processing indexed by object-substitution masking. The functional levels indexed by crowding, the attentional blink (and other attentional blinding methods), backward pattern masking, metacontrast masking, continuous flash suppression, sandwich masking, and single-flash interocular suppression, fall at progressively lower levels, while unconscious processing at the lowest levels is indexed by eye-based binocular-rivalry suppression. Although unconscious processing levels indexed by additional blinding methods is yet to be determined, a tentative placement at lower levels in the hierarchy is also given for unconscious processing indexed by Troxler fading and adaptation-induced blindness, and at higher levels in the hierarchy indexed by attentional blinding effects in addition to the level indexed by the attentional blink. The full mapping of levels in the functional hierarchy onto cortical activation sites and levels is yet to be determined. The existence of such a hierarchy bears importantly on the search for, and the distinctions between, neural correlates of conscious and unconscious vision.

The magnocellular system versus the dorsal stream

In his opinion paper, Breitmeyer (2014) sought to identify the contributions from the magno- and parvocellular systems to conscious and unconscious vision. One approach employed in this attempt was to study the effect of contrast. This was based on the well established finding that magno- and parvocellular cells differ in their contrast-response functions (Kaplan and Shapley, 1986): Magnocellular responses increase rapidly with contrast at low contrast levels but saturate at medium contrasts. Parvocellular responses, on the other hand, increase with contrast at a relatively constant rate. In his Figure 5, Breitmeyer makes the suggestion based on the effect of contrast, that visible priming reflects magnocellular activity whereas invisible priming reflects parvocellular responses. In Figure ​Figure11 is shown (solid line) the function: Figure 1 The contrast-response curve from Breitmeyer (2014) (solid line) generated form Equation 1 which was by Breitmeyer attributed to magnocellular responses. Also shown are contrast-response curves for magnocellular cells (dotted line) and for cells in cortical ... r=(66.5 c2.9)/(0.0762.9+c2.9) (1) where c denotes contrast, which was assumed by Breitmeyer to reflect the contrast-response relationship of magnocellular neurons. Sclar et al. (1990) found that contrast-response data from visual neurons can be described by the equation: r=rmax cn/(cn+c50n) (2) where c denotes contrast and the constants rmax, c50, and n differ for the different types of neurons. For magnocellular cells these values were 52.7, 0.11, and 1.2, respectively (Sclar et al., 1990). By using these values in Equation 2 the dotted curve in Figure ​Figure11 was generated. The functions in the figure have been normalized so as to give a response of 1.0 for a contrast of 1.0. This is the only adjustment made. As can be seen, the function for the magnocellular cells from Sclar et al. does not provide a particularly close fit to the function of Breitmeyer. This prompts the question: Would functions for other types of cells provide a better fit? Sclar et al. (1990) found that c50 (in Equation 2) for MT cells increases with decreasing stimulus size and that for very small stimuli it is comparable to that for magnocellular neurons. However, the exponent, n, was was found to be unchanged and the maximum response, rmax, underwent only minor changes. The latter, however, is irrelevant in the present context since the responses are normalized. Using Equation 2 MT cell responses have been calculated for large and small stimuli. For large stimuli the constants rmax, c50, and n were set to 36, 0.07, and 3.0 (from Sclar et al., 1990). In the case of small stimuli the value 0.11 (i.e., the value for magnocellular cells from Sclar et al.) was used for c50. As can be seen, the curves for MT cells (marked MT-L and MT-S for large and small stimuli, respectively) provide closer fits to the curve of Breitmeyer than does the function for magnocellular responses and the function of Breitmeyer falls between the two curves for MT cells. Area MT is part of the dorsal cortical stream. The ventral and dorsal cortical streams represent two sets of cortical areas (Merigan and Maunsell, 1993). Many authors (e.g., Breitmeyer, 2014) have conflated the magno- and parvocellular systems with, respectively, the dorsal and ventral streams. This, however, faces some difficulties. In the case of the dorsal stream, although it clearly receives a substantial portion of its input from the magnocellular system it also receives sizable inputs from the koniocellular (Sincich et al., 2004) and parvocellular systems (Nassi et al., 2006). In the case of the ventral stream, as exemplified by Area V4, lesion studies have indicated that it receives about equally strong inputs from the magno- and parvocellular systems (Ferrera et al., 1994). Also, lesions placed in the dorsal and ventral streams have quite different effects from those placed in the magno- and parvocellular systems (Merigan and Maunsell, 1993). Figure ​Figure11 emphasizes that the magnocellular system and the dorsal stream (exemplified here by Area MT) also differ in regard to contrast-response functions.

A direct comparison of unconscious face processing under masking and interocular suppression.

Different combinations of forward and backward masking as well as interocular suppression have been used extensively to render stimuli invisible and to study those aspects of visual stimuli that are processed in the absence of conscious experience. Although the two techniques—masking vs. interocular suppression—obviously differ both in their applications and mechanisms, only little effort has been made to compare them systematically. Yet, such a comparison is crucial: existing discrepancies in the extent of unconscious processing inferred from these two techniques must be reconciled, as our understanding of unconscious vision should be independent of the technique used to prevent visibility. Here, we studied similarities and differences between faces rendered invisible by masking vs. interocular suppression using a priming paradigm. By carefully equating stimulus strength across the two techniques, we analyzed the effects of face primes with the same viewpoint (repetition priming, Experiment 1) and of face primes with a different viewpoint (identity priming, Experiment 2) on the reaction times for a fame categorization task. Overall, we found that the magnitude of both repetition and identity priming largely depended on stimulus visibility. Moreover, when the primes were subjectively invisible, both repetition and identity priming were found to be qualitatively stronger under masking than under interocular suppression. Taken together, these results help refine our understanding of which level of visual processing each technique disrupts, and illustrate the importance of systematic methodological comparisons in the field of unconscious vision.

Unconscious vision and executive control: how unconscious processing and conscious action control interact.

Research on unconscious or unaware vision has demonstrated that unconscious processing can be flexibly adapted to the current goals of human agents. The present review focuses on one area of research, masked visual priming. This method uses visual stimuli presented in a temporal sequence to lower the visibility of one of these stimuli. In this way, a stimulus can be masked and even rendered invisible. Despite its invisibility, a masked stimulus if used as a prime can influence a variety of executive functions, such as response activation, semantic processing, or attention shifting. There are also limitations on the processing of masked primes. While masked priming research demonstrates the top-down dependent usage of unconscious vision during task-set execution it also highlights that the set-up of a new task-set depends on conscious vision as its input. This basic distinction captures a major qualitative difference between conscious and unconscious vision.

Unlike in Clinical Blindsight Patients, Unconscious Processing of Chromatic Information Depends on Early Visual Cortex in Healthy Humans

Large amount of data concerning the neural mechanisms that mediate unconscious vision come from cortically blind patients who can process stimuli presented in their blind visual field. How well the findings generalize to neurologically healthy humans remains open. Using transcranial magnetic stimulation (TMS), we studied whether chromatic processing depends on the early visual cortex in healthy participants. We employed a phenomenon called the redundant target effect (RTE): simple reaction times to two stimuli are faster than to one stimulus, even when one of the stimuli is presented outside consciousness. The RTE produced by chromatically defined stimuli disappeared when the contralateral stimulus of two bilateral stimuli was suppressed from consciousness. In contrast to studies on blindsight patients, the results imply that the early visual cortex is necessary for the processing of chromatic information in neurologically healthy humans.

Sight unseen: an exploration of conscious and unconscious vision

Prologue 1. A tragic accident 2. Doing without seeing 3. When vision for action fails 4. The origins of vision: from modules to models 5. Streams within streams 6. Why do we need two systems? 7. Getting it all together 8. Postscript: Dee's life 15 years on Epilogue

Brain-Stimulation Induced Blindsight: Unconscious Vision or Response Bias?

A dissociation between visual awareness and visual discrimination is referred to as “blindsight”. Blindsight results from loss of function of the primary visual cortex (V1) which can occur due to cerebrovascular accidents (i.e. stroke-related lesions). There are also numerous reports of similar, though reversible, effects on vision induced by transcranial Magnetic Stimulation (TMS) to early visual cortex. These effects point to V1 as the “gate” of visual awareness and have strong implications for understanding the neurological underpinnings of consciousness. It has been argued that evidence for the dissociation between awareness of, and responses to, visual stimuli can be a measurement artifact of the use of a high response criterion under yes-no measures of visual awareness when compared with the criterion free forced-choice responses. This difference between yes-no and forced-choice measures suggests that evidence for a dissociation may actually be normal near-threshold conscious vision. Here we describe three experiments that tested visual performance in normal subjects when their visual awareness was suppressed by applying TMS to the occipital pole. The nature of subjects’ performance whilst undergoing occipital TMS was then verified by use of a psychophysical measure (d') that is independent of response criteria. This showed that there was no genuine dissociation in visual sensitivity measured by yes-no and forced-choice responses. These results highlight that evidence for visual sensitivity in the absence of awareness must be analysed using a bias-free psychophysical measure, such as d', In order to confirm whether or not visual performance is truly unconscious.

Dissociation of visual localization and visual detection in rhesus monkeys (Macaca mulatta)

Conscious and unconscious cognitive processes contribute independently to human behavior and can be dissociated. For example, humans report failing to see objects clearly in the periphery while simultaneously being able to grasp those objects accurately (Milner in Proc R Soc B Biol Sci 279:2289-2298, 2012). Knowing whether similar dissociations are present in nonverbal species is critical to our understanding of comparative psychology and the evolution of brains. However, such dissociations are difficult to detect in nonhumans because verbal reports of experience are the main way we discriminate putative conscious from unconscious processing. We trained monkeys in a localization task in which they responded to the location where a target appeared, and a matched detection task in which they reported the presence or absence of the same target. We used masking to manipulate the visibility of targets. Accuracy was high in both tasks when stimuli were unmasked and was attenuated by visual masking. At the strongest level of masking, performance in the detection task was at chance, while localization remained significantly above chance. Critically, errors in the detection task were predominantly misses, indicating that the monkeys' behavior remained under stimulus control, but that the monkeys did not detect the target despite above-chance localization. While these results cannot establish the existence of phenomenal vision in monkeys, the dissociation of visually guided action from detection parallels the dissociation of conscious and unconscious vision seen in humans.

Perception-action dissociations depend on the luminance contrast of the stimuli

The observation that near-threshold low-contrast visual distractors can equally influence perceptual state and goal-directed motor responses was recently taken as an argument against a sharp separation between a conscious vision for perception and an unconscious vision for action. However, data supporting the dual visual system theory have principally involved high-contrast stimuli. In the present study, we assessed the effect of varying the contrast of a near-threshold visual distractor while keeping its visibility constant with backward noise masks. Eight participants performed fast manual reaching movements toward a highly visible target while subsequently reporting the presence/absence of a near-threshold distractor appearing at the opposite location with respect to the body midline. For all distractor contrasts, hand trajectory deviations toward the distractor were observed when the distractor was present and detected. When the distractor remained undetected deviations also occurred, but for higher contrasts. The subliminal motor effect traditionally observed in visual masking studies may therefore primarily depend on the luminance contrast of the interfering stimuli. These results suggest that dissociations between perceptual and motor responses can be explained by a single-signal model involving differential thresholds for perception and action that are specifically modulated as a function of both the requirements of the task and the contrast level of the stimuli. Such modulation is compatible with neurophysiological accounts of visual masking in which feedforward activation to--and feedback activation from--higher visual areas are correlated with the actual presence of the stimulation and its conscious perception, respectively.