_The Impact of Glass Surfaces on Vision
The Impact of Glass Barriers on the Perception of Action and Other Dynamic Events
To date, a number of developmental studies have investigated how young infants behave when they are presented with transparent barrier in their peripersonal space (Diamond, Prevor, Callender, & Druin, 1997; Noland, 2008). A typical paradigm of these studies presents infants with a small pane of glass or plexiglas which separates them from a desirable toy. The researchers measure whether infants attempt to reach through the barrier (failed attempt) or pull the barrier down or reach around it (successful attempt; e.g., (Shinskey & Munakata, 2001). Findings from these developmental studies generally indicate that until nearly 1 year of age, infants have difficulty accounting for the impenetrability of such transparent barriers and consistently try to reach through them rather than around (e.g., Shinskey & Munakata, 2001; Diamond, 1990). The fact that infants can negotiate the transparent barrier after one year of age has been attributed to increased working memory capacity to represent the barrier (Lockman & Adams, 2001), or the need to move around it to reach the toy (Diamond, 1991). Taken together, this developmental work hints that with experience and maturation, the human brain can take into account the physical properties of a transparent barrier that permits vision but precludes action.
A limited complimentary literature has investigated the behavioural consequences of a transparent barrier within neurologically healthy adults (Kitagawa & Spence, 2005) and braindamaged patients (Farne, Dematte, & Ladavas, 2003). Both of these studies made use of a paradigm that investigated visual-tactile interactions. Farne et al (2003) employed a crossmodal extinction paradigm with right-hemisphere lesioned patients, and Kitagawa and Spence (2005) used a crossmodal congruency paradigm. The basic idea behind both paradigms is that an expectation of the hand being touched is established, and participants report which hand or where one of their hands was touched. A transparent barrier between the stimulating device and the participant’s hand is either present or absent. Both studies report that even when participants can clearly see the transparent barrier covering the hand, behavioural performance is indistinguishable whether the barrier is present or absent. Both teams of researchers interpret this as meaning that higher-level cognitive knowledge that a transparent barrier would prevent any contact from occurring is superseded by the fact that the barrier is transparent, and thus participants can see that the stimulating device is close to their hand (Farne, et al., 2003; Kitagawa & Spence, 2005). The overarching message, then, is that even when we have full awareness of the presence of a solid, transparent surface that prevents objects or people on the other side from contacting us, we still behave as if there is no barrier there. Thus, at least in certain circumstances, it would appear that top-down cognitive influences of knowing that a barrier exists does not reliably alter our perception of events occurring on the other side of the barrier if we can see through it– it is as if the barrier does not exist at all.
These findings present an intriguing conundrum about how we represent transparent surfaces in our environment. To date, it remains unknown how our brains encode the presence of a clear surface separating us from a moving object or another person. It could be the case that even though no behavioural differences emerged from the two behavioural studies performed with adults (Farne et al., 2003; Kitagawa & Spence, 2005), the brain actually encodes the transparent surface in a meaningful way that has not yet been satisfactorily addressed by behavioural experiments. The experiments in this section aim to advance the limited state of knowledge about how we incorporate the presence of a transparent barrier into perception of dynamic events in the environment by measuring neural and psychophysiological responses when observing events that incorporate a transparent barrier. We accomplish this via three related functional neuroimaging experiments, detailed below.