Out of Sight but Not Out of Mind: The Role of Vision in Pain Perception

FIGURE 13-1 Modality-specific and multimodal brain responses in response to nociceptive, non-nociceptive somatosensory, auditory, and visual stimulation. Brain areas displaying a significant activation to all four types of sensory stimuli are shown in yellow. Voxels displaying selective activation to nociceptive and non-nociceptive somatosensory stimulation are shown in cyan. Voxels displaying significant activation only to nociceptive somatosensory stimuli are shown in red. Non-nociceptive somatosensory-specific, auditory-specific, and visual-specific voxels are shown in purple, blue, and green, respectively. (Adapted from Mouraux et al. [30] with permission.)

THE ROLE OF VISION IN SOMATOSENSORY PERCEPTION IN SIGHTED INDIVIDUALS

When information is transmitted to the brain through multiple senses, there are significant performance benefits in terms of speed and accuracy [42]. The effects are enhanced when different sensory stimuli are presented in a spatially and temporally congruent manner.

Visual–Tactile Integration

The multisensory benefits of visual–tactile integration have been repeatedly demonstrated. People are more likely to detect a near-threshold tactile stimulus when it is presented concurrently with a visual stimulus, compared to when it is presented alone [19]. Even if the visual stimulus is entirely task-irrelevant, it can enhance the detection of a tactile stimulus. Serino et al. [38] showed enhanced detection of subthreshold tactile stimuli on observers’ faces when they saw a face being touched by hands rather than a face being merely approached by hands. Even when the visual stimulus, the forearm, was both uninformative and irrelevant to the task, performance benefited, suggesting that vision is able to focus tactile attention and to modulate somatosensory cortical activity [43].

Visual Information of One’s Own Body Modulates Pain

Similar to the effects of vision on touch, pain perception is also modulated by specific visual information. As many of us have probably already experienced, after banging our knee the perception of pain can be minimized or exacerbated, depending on whether the knee looks normal or injured. This anecdotal observation has been confirmed in controlled studies reporting that viewing an undamaged body part can have an analgesic effect [23]. This “visually induced analgesia” suggests an interplay between the brain’s pain network and a posterior network for body perception, which modulates the experience of pain. Mancini and colleagues [25] further explored the relationship between pain and body perception by artificially distorting the image of the hand so that it appeared excessively larger or smaller than usual. When participants gazed toward the distorted reflection of their hands while receiving a noxious thermal stimulus, their experience of pain was inversely correlated with the relative size of their hands: enlarging the hand had an analgesic effect, whereas reducing its size resulted in increased pain ratings [25]. Interestingly, the analgesic effect of vision is strongly linked to embodiment; the magnitude of the effect is positively correlated with the belief that the hand is one’s own [25].

Neural networks involved in body image overlap considerably with the pain matrix, making these regions (especially the posterior parietal and somatosensory cortices) possible candidates for involvement in visual analgesia [24]. Neuroimaging studies have shown that extensive areas of the posterior parietal and inferotemporal cortices are involved in body representation, including the extrastriate body area, the fusiform body area, and have revealed a topographic map of viewed body parts throughout the visual cortex [6, 31, 33]. Interestingly, Longo et al. [24] reported that activity within parts of the pain matrix, like the primary somatosensory and the operculoinsular cortices, show reduced activity to a noxious heat stimulus when viewing the body compared to viewing an object. These authors further reported an increased functional coupling between the posterior parietal nodes of the visual body network and areas of the cortical pain network during viewing of the stimulated hand, compared to viewing an object. This finding suggests that multisensory interactions involving the perception of one’s own body underlie visual analgesia [12]. However, the perception of one’s own body is not directly linked to visual analgesic effects because opposite effects are reported when we view other people’s bodies in pain. Reports of vicarious pain demonstrate a relationship between observed and experienced pain [2, 48].

Visual analgesia may seem somewhat paradoxical as one would rather expect that directed attention would lead to enhanced pain responses [34]. Indeed, not all studies have replicated the visual analgesic effect. In fact, Valentini and colleagues [47] found no analgesic effect when seeing the hand in a normal position. Instead, these authors reported that viewing the hand in a crossed position had an analgesic effect compared to viewing the hand in its hemispace, or viewing an object placed in the contralateral space, suggesting that proprioceptive information can modulate pain perception. Another recent study also failed to replicate the visual analgesic effect [44]. These authors compared the effects of direct versus mirror vision of the stimulated hand or an object on nociceptive and non-nociceptive stimuli, while measuring event-related potentials (ERPs). Results showed that looking at the hand compared to an object did not modulate the subjective pain perception; however, it reduced the magnitude of the nociceptive N240 wave, and enhanced the magnitude of the non-nociceptive P200. The results of these two recent studies question both the robustness and the ubiquity of visual analgesia. Skin color can also affect pain responses. Inflamed or injured skin tends to be red, whereas a more bluish skin color is typically associated with cold skin. When painful thermal stimuli are applied to a red patch of skin on an embodied virtual arm, participants have lower thresholds for pain compared to when the embodied arm is blue [26]. An increased pain perception is also observed when a painful stimulus is felt at the same time as viewing a hand being pricked with a needle, compared to viewing a hand poked with a Q-tip [13]. Unlike the visual analgesic effect, this increased pain response is associated with reduced α-band activity in the posterior cingulate cortex and fusiform gyrus [14].

Visual Information of Someone Else’ Body Modulates Visual-Nociceptive Integration

Seeing another person in pain makes people more sensitive to pain [17], whereas observing a stoic person’s lack of response to pain can inhibit the experience of pain in the observer [46]. Several factors, in particular empathy, facilitate or interfere with somatosensation as a result of observing someone in pain. Empathy is generally defined as the capacity to understand and respond to the unique affective experiences of another person [11]. Although empathy for a co-fellow’s pain and pain in one’s own body are perceived differently, they are represented very similarly in the brain [17, 39].

Associative learning plays an important role in explaining whether a stimulus is perceived as painful or not. During our lives, we learn to associate the visual images of our wounds with pain. Thus, through classical conditioning, seeing injuries (such as others bleeding) may induce the conditioned autonomic response to pain. The conditioned response to painful images, such as images of people in pain, can then modulate one’s own pain perception [36]. The association has been demonstrated through a conditioning paradigm in the laboratory study conducted by Wunsch and colleagues [50]. They found that painful and nonpainful thermal stimuli were perceived as more intense and more unpleasant when they were immediately preceded by one of the conditioned aversive stimuli (e.g., an image of a mutilated body), compared to the same tactile stimulation preceded by one of the images that were used in the neutral or positive conditioning sequences.

Vicarious Pain or “I Feel Your Pain”

Vicarious somatosensory experiences occur when one has a sensation of pain, or more often a “tingling,” without being touched at all, after observing another in pain. In contrast to empathic pain, where a tactile stimulus is perceived as more unpleasant when empathy is activated, vicarious pain is a tactile or nociceptive sensation without any direct input to the skin. Vicarious pain sensations have been reported both in patients and in healthy control populations.

Vicarious pain intense enough to be problematic has only been reported in patients with a history of intense, traumatic pain [2]. Since phantom pain is often triggered by thinking about, observing, or inferring that another person is in pain, it is often categorized as a vicarious pain [10]. Thus, the population with the highest number of vicarious pain reports are amputees with phantom pain [10]. Indeed, memories from painful experiences appear to play a role in determining where on the body the vicarious pain is felt [18].

Normal individuals without a history of trauma also sometimes report sensations simply from observing others in pain, without any stimulation to the skin [48]. Vicarious “pain” is fairly weak and low in intensity, and does not resemble the pain that is actually felt. Roughly 30% of a normal population will report low-intensity vicarious sensations, labeled as “tingling,” from looking at images and video clips depicting painful events [32]. When looking at painful scenes, vicarious pain responders have stronger activation of emotional (insular cortex) and sensory (i.e., secondary somatosensory cortex) brain regions associated with pain, compared to people who do not report such sensations [32].

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