|Beatrice de Gelder and Marco Tamietto (2007), Scholarpedia, 2(10):3555.||doi:10.4249/scholarpedia.3555||revision #20914 [link to/cite this article]|
Affective blindsight refers to the residual visual ability of patients with damage to the primary visual cortex (V1, striate cortex) to react reliably to the emotional valence of stimuli presented to their blind visual fields and whose presence and properties they are unable to report. To the extent that non-conscious vision can be created by experimental techniques in normally seeing individuals, their residual abilities are also referred to as affective blindsight when it concerns to emotional stimuli.
Setting the stage and defining boundaries
The notion that emotional information is processed notwithstanding limited visual awareness or attention represents a radical departure from previous psychodynamic conceptualizations of non-conscious perception of emotions toward evolutionary and neuroscientific accounts. After some initial reports mainly concerning subliminal perception, non-conscious emotion perception is systematically investigated by cognitive neuroscientists only since the early eighties (de Gelder, de Haan, & Heywood, 2001; Eastwood & Smilek, 2005; Kunst-Wilson & Zajonc, 1980).
There are now many studies showing that appropriate reactions to emotional stimuli can be retained in normal observers even when stimulus awareness is prevented by experimental manipulation like visual masking or application of transcranial magnetic stimulation (TMS) over primary visual areas, which temporarily render a stimulus invisible (Dimberg, Thunberg, & Elmehed, 2000; Esteves & Ohman, 1993; Jolij & Lamme, 2005; Killgore & Yurgelun-Todd, 2004; Liddell et al., 2005; Morris, Ohman, & Dolan, 1998; Murphy & Zajonc, 1993; Niedenthal, 1990; Pessoa, Japee, Sturman, & Ungerleider, 2006; Pessoa, Japee, & Ungerleider, 2005; Tamietto & de Gelder, in press; Whalen et al., 2004; Whalen et al., 1998; Williams et al., 2006; Williams et al., 2004). This suggests that there might be a non-conscious system for visually based affect evaluation that is functional not only in patients with cortical blindness, but also in neurologically intact observers in which conditions of cortical blindness have been created for the purpose of an experimental session. This non-conscious system seems to operate in parallel with the normal, predominantly cortical, processing routes and with characteristics that are possibly different from that of conscious emotion recognition. Nonetheless, it is currently a matter of debate whether masking or other techniques create a situation which is a functionally equivalent of affective blindsight (Marzi, Minelli, & Savazzi, 2004). Also unclear is the role played by the various visual areas in the cortex and which backward projections to V1 are still possible in neurologically intact cases presented with backward masking (Macknik & Livingstone, 1998). Studies of non-conscious vision in healthy observers can thus provide only partial support for non-conscious emotional processing and its neural underpinnings.
The same holds true for studies that investigate perceptual recognition of emotions in the absence of stimulus awareness in neurological populations with lesions to brain areas that are not primarily visual. For instance, a striking dissociation between the loss of conscious perception and the preservation of residual, non-conscious processing of some stimulus attributes (included emotional valence) is present also in conditions such as hemispatial neglect or visual extinction that follow injury to the parietal lobe in the right cerebral hemisphere (Tamietto, Geminiani, Genero, & de Gelder, 2007; Tamietto et al., 2005; Vuilleumier, 2005; Vuilleumier & Schwartz, 2001). In these cases, however, the deficit of visual awareness is remarkably different from that shown by blindsight patients and arises at later stages of stimulus processing that are most likely related to the ability to orient attention toward stimuli presented in a portion of the space, rather than to a defect directly due to visual perception (Driver & Mattingley, 1998). Consistent with this, conscious detection of a stimulus by neglect patients depends heavily on whether other stimuli are presented simultaneously, and may be transiently improved by several clinical interventions.
Affective blindsight in cortically blind persons thus presents the clearest case of non-conscious emotion perception because, when visual parameters like luminance are carefully controlled, the patients literally cannot see nor visually acknowledge the presence of a stimulus. Investigation of this condition offers a unique opportunity to understand the neuro-functional bases of emotion perception without awareness. Its importance is directly related to the fact that emotional processing in the absence of stimulus awareness is an important component of the emotional capabilities of neurologically intact individuals.
First Findings and First Questions
The first report that a patient with blindsight could discriminate with a reliability exceeding chance level the emotion of stimuli he could not consciously perceive was published by de Gelder and coworkers in 1999 (de Gelder, Vroomen, Pourtois, & Weiskrantz, 1999). The study was conducted on the well-know patient GY with blindness in his right visual field following damage to the left occipital lobe, and consisted of four different experiments in which short video fragments and still images showing different facial expressions were used as stimuli. Common to all experiments was the use of direct methodologies requiring the patient to “guess” in various forced-choice conditions the emotion conveyed by stimuli he remained unaware of. At that time, there was initial evidence from animal and human studies that subcortical structures (like amygdale in the medial temporal lobe) were able to survey for emotionally-laden stimuli in the environment and to initiate appropriate responses toward them even before a detailed perceptual analysis was provided by primary sensory cortices (LeDoux, 1996; Morris et al., 1998; Morris, Ohman, & Dolan, 1999; Whalen et al., 1998). However, until recently investigation of non-conscious perception in blindsight had focused predominantly on basic psychophysical properties such as discrimination of simple shapes, gratings, movement, or color (Weiskrantz, 1986, 2004; Weiskrantz, Warrington, Sanders, & Marshall, 1974). The finding that blindsight subjects can discriminate something as subtle as facial expressions without the contribution of primary visual cortex is however less puzzling when viewed against a broader biological context. Indeed, behavioral manifestations of emotion in the face or by whatever other means, including vocalizations and emotional body language, have a high communicative function in many species (Darwin, 1872; de Gelder, 2006; Hatfield, Cacioppo, & Rapson, 1994).
As befits a new phenomenon, a number of studies have addressed alternative explanations that do not need to invoke a non-cortical route (Cowey, 2004; de Gelder, Vroomen, Pourtois, & Weiskrantz, 2000; Heywood & Kentridge, 2000; Pessoa, 2005). One such possibility may be that simple and easily discriminated visual features (e.g., eyes wide open) are systematically associated with a specific facial expression. When this simple feature is first shown in the intact field and then in the blind one, correct responses may be based on the association of the single and easiest feature to the correct response. This possibility of interhemispheric transfer has been conclusively discarded when evidence of affective blindsight emerged in patients with total bilateral cortical blindness that are by definition unable to use visual information perceived in the intact field (Hamm et al., 2003; Pegna, Khateb, Lazeyras, & Seghier, 2005). Nonetheless, a number of questions still remained and were challenge in following studies; the most interesting and extensively studied being a) which stimulus categories and/or attributes can give rise to affective blindsight; b) if there is an influence of non-consciously perceived emotions upon conscious recognition of other stimuli; c) the anatomical details of the neural pathway sustaining affective blindsight.
Specificity of affective blindsight for particular emotional stimuli or attributes
Until very recently, most investigations of human emotions predominately concentrated on processes associated to conscious or non-conscious perception of facial expressions (Adolphs, 2002). So, it is not surprising that affective blindsight was initially tested with facial expressions. More surprising was the fact that in the first behavioral study only moving stimuli but not still images of facial expressions appeared to support affective blindsight. If movement was the critical aspect to support non-conscious discrimination of different emotional expressions, one would expect blindsight also for other attributes that critically rely on movement, such as facial speech. This prediction was tested in later studies that, however, found blindsight only for emotional facial expressions, but not for facial speech (de Gelder, Vroomen, Pourtois, & Weiskrantz, 2000). Other facial attributes such as personal identity or gender were also tested with negative results, thereby suggesting that neither movement nor non-emotional facial attributes are per se the determinant of the phenomenon. More directly, in later research affective blindsight emerged very clearly also when still images of facial expressions were used, especially if the patients were tested with indirect methodologies (see below) that typically do not require the subjects to make guesses about visual events they do not perceive consciously (Anders et al., 2004; de Gelder, Morris, & Dolan, 2005; de Gelder, Pourtois, van Raamsdonk, Vroomen, & Weiskrantz, 2001; de Gelder, Pourtois, & Weiskrantz, 2002; Pegna et al., 2005). Still unanswered is the issue of whether affective blindsight is induced by non-conscious processing of overall face configuration or by individual key features in the face. There is evidence that the eye region is particularly salient in conveying emotion information (namely of fear), and that the most ancient parts of our visual and emotion systems in the brain seem tuned to detect this simple signal rather than the whole face configuration (Morris, deBonis, & Dolan, 2002; Whalen et al., 2004). Nonetheless, a direct test of this issue in blindsight patients is still missing.
Aside from facial expressions, other stimulus categories have been used to test whether affective blindsight could be extended to stimuli other than faces. Thus far, the most studied categories are affective scenes and bodily expressions of emotions. Generally, negative results have been reported for scenes with both direct and indirect methods, suggesting that the appraisal of the emotional content of complex pictures requires cognitive and semantic processing that depends critically on conscious visual perception, which is prevented by V1 damage in blindsight patients (de Gelder et al., 2005; de Gelder et al., 2002). On the other hand, behavioral and neuroimaging results have shown that affective blindsight for bodily expressions may be at least as clearly established as that previously reported for facial expressions, and sustained by a partly overlapping neural pathway (de Gelder & Hadjikhani, 2006; Tamietto, Weiskrantz, Geminiani, & de Gelder, 2007). This indicates that implicit processing of emotions in blindsight does not seem to be specific for faces but rather, and more generically, for biologically primitive emotional expressions that are clearly associated with action tendencies.
The influence of non-conscious emotions on conscious experience
Initially, the main emphasis of research on affective blindsight has been in documenting the dissociation between conscious and non-conscious emotional processing and the presence of the latter in blindsight patients. Findings about subliminal perception in the 50s raised the question of whether unseen information influences our conscious perception of the seen world. New experiments on affective blindsight investigated possible on-line interaction between the aware and the unaware modes of emotional processing, and the influence exerted by unseen emotions over ongoing recognition of other consciously perceived stimuli (de Gelder et al., 2005; de Gelder et al., 2001; de Gelder et al., 2002; Tamietto et al., 2007). This series of studies took advantage of indirect methods of testing which, as compared to direct “guessing” methods, cannot be influenced by deliberate response strategies used by the patient. In these methods two stimuli are simultaneously presented, one projected to the blind field of which the patient is unaware while the other is consciously perceived. The patient is asked to respond to the normally perceived stimulus and conclusions about non-conscious processing are based on evidence that conscious evaluation of the former stimulus is biased by the presence of the latter unseen stimulus (Marzi, Tassinari, Aglioti, & Lutzemberger, 1986). This approach has been successful in demonstrating both visual/visual as well as cross-modal interactions (visual/auditory) between consciously and non-consciously perceived emotional stimuli. For instance, conscious recognition of facial expressions is speeded up if another face showing the same expression is presented in the blind field (de Gelder et al., 2005; de Gelder et al., 2001). Interestingly, the influence of non-conscious processing over conscious perception does not seem related to the physical/perceptual similarities between stimuli, but rather appears to be driven by the emotional information conveyed by the stimuli. Indeed, a bias from unseen to seen stimulus is also present when two stimuli have very different physical properties, such as a facial and a bodily expression, but represent congruent or incongruent information like, for example, a happy face paired with a fearfull body expression (Tamietto et al., 2007). The same type of interaction has been also studied in multisensory conditions with unseen visual stimuli modulating the auditory processing of an audiovisual stimulus pair. Thus, presenting to the blind field an incongruent visual affective information (a facial expression) bias the judgment of the emotional prosody of a sentence (de Gelder et al., 2005; de Gelder et al., 2002).
Different methods for common purposes
Various methodologies have been used to investigate affective blindsight and uncover the specific details of this phenomenon. Direct and indirect behavioural methods gave rise to the original discovery of affective blindsight and are still an indispensable tool to ascertain non-conscious processing and determine which stimulus categories and attributes can be processed in the absence of awareness (de Gelder et al., 2001; de Gelder et al., 1999). The use of neuroimaging techniques like functional magnetic resonance imaging (fMRI) provided direct evidence regarding the spatial properties of the actual pathway sustaining affective blindsight and the neural structures involved in it (Anders et al., 2004; de Gelder & Hadjikhani, 2006; de Gelder et al., 2005; Morris, DeGelder, Weiskrantz, & Dolan, 2001; Pegna et al., 2005). The low temporal resolution of fMRI, however, still does not allow a fine-grained analysis of the temporal dynamics among the various areas implicated. This problem has been partially circumvented by the use of electroencephalography, which has a temporal resolution of the order of milliseconds, but a low spatial resolution (de Gelder et al., 2002; Rossion et al., 2000). The joint and simultaneous use of both techniques seems very promising to gain a better insight of the spatial and temporal properties of the neurophysiology of affective blindsight, but has not been used yet. Likewise, the peripheral physiological changes that may be induced in blindsight patients by the presentation of unseen emotions are still poorly understood, but there is initial evidence that non-consciously perceived emotions may elicit arousal and expressive reactions in the patients that are consistent with the affective valence of the unseen stimuli, as measured by electromyography and pupillometry (Tamietto, Weiskrantz, Geminiani, & de Gelder, in preparation).
Finally, the use of TMS and of other methods to induce non-conscious perception in normal observers has revealed interesting parallels with the behavioural outcomes reported in affective blindsight and with its neurophysiological bases (Jolij & Lamme, 2005; Liddell et al., 2005; Morris et al., 1999; Tamietto & de Gelder, in press). In the future, a better understanding of the actual influences of these techniques on the intact brain will provide the bases for a proper comparison with blindsight patients and for a more comprehensive understanding of the systems involved in conscious and non-conscious emotional processing in the intact and damaged brain.
Neurophysiological bases of affective blindsight
The neuro-anatomical underpinnings of affective blindsight, like those of blindsight, are still not fully understood. Yet, in the case of affective blindsight, the debate is enriched by the numerous findings on affective processing derived from animal studies and the theoretical models they have produced. Earlier animal studies in rats underlined the role of midbrain structures in providing a rapid but coarse analysis of the affective value of auditory as well as visual stimuli even without the contribution of the primary sensory cortices (Campeau & Davis, 1995; Doron & Ledoux, 1999; Jones & Burton, 1976; LeDoux, 1996; Linke, De Lima, Schwegler, & Pape, 1999; Shi & Davis, 2001). A similar subcortical pathway was also envisaged in healthy human observer when facial expressions were subliminally presented (Morris et al., 1999). Thus, attention was suddenly focused on the functional integrity of this subcortical visual pathway in patients with affective blindsight and, indeed, the activation of subcortical structures composing this pathway has been repeatedly shown in different neuroimaging studies (de Gelder & Hadjikhani, 2006; de Gelder et al., 2005; Morris et al., 2001; Pegna et al., 2005). However, a conclusive demonstration of the anatomical connections between these various subcortical structures is not yet at a hand in primates (Cowey, 2004; Pessoa, 2005).
The involvement of the subcortical pathway in affective and “non-affective” blindsight has been mostly documented in patient GY, who suffered an occipital lesion very early in life (at age 7) (de Gelder et al., 2005; Morris et al., 2001; Sahraie et al., 1997). Thus, it is possible that post-lesion and experience-dependent plasticity have taken place in this patient. In this case the role of the subcortical pathway would not generalize to all patients showing affective blindsight. Even thought the presentation of affective stimuli to the blind fields of other patients also activated subcortical structures like the amygdala, the functional or anatomical connectivity of the different structures putatively implicated along this route have not been directly tested in patients different from GY. Interestingly, however, subliminal emotional expressions activate in healthy subjects the same subcortical pathway as that advocated to be the most likely alternative to emotional processing in cases of affective blindsight following striate cortex lesions (Liddell et al., 2005; Morris et al., 1999; Williams et al., 2006). Studies are now underway that will trace the pathways involved in non-cortical processing using tractography methods like diffusion tensor imaging (DTI).
Affective blindsight and emotional consciousness
Blindsight continues to fascinate philosophers because it rightly appears as a critical test case for theories of consciousness. Some of this fascination has rubbed on cognitive scientists and the neural basis of perception with and without awareness is now a lively topic of research (de Gelder et al., 2001; Weiskrantz, 1997). However, affective blindsight raises issues that only partly overlap with those of blindsight. With affective blindsight the question is not to see or not to see, and still to “see”. The reason is that the presence of an unseen affective stimulus may give raise to a chain of affective reactions in the organism that provide the blindsight subject with non-visual cues about the stimulus attributes. If emotions are characterised by the action tendencies they are associated with (Frijda, 2007), then the blindsight viewer may sense the properties of the stimulus he is presented and respond independently of any visual awareness of it. Unlike presentation of gratings or dot patterns to the blind field, presentation of affective images presumably resonates in the perceiving organism in other ways than by sending ripples through the visual system only. In future developments the scope of the debate on consciousness in perception will probably be broadened, as attention needs to be paid to the affective resonance of non-visual stimulus processing and the ways in which this provides an indirect but efficient basis for guessing the meaning of unseen stimuli. This is already suggested by the findings about the role parietal somatosensory cortex plays in affective blindsight (Anders et al., 2004) and by imitation-like reactions to unseen facial expressions (Tamietto et al., in preparation).
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