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Near, yet so far: The effect of pictorial cues on spatial attention
Brain and Cognition 76 (2011) 349–352
Contents lists available at ScienceDirect
Brain and Cognition journal homepage: www.elsevier.com/locate/b&c
Near, yet so far: The effect of pictorial cues on spatial attention Michael E.R. Nicholls a,⇑, Jason D. Forte b, Tobias Loetscher a, Catherine A. Orr b, Mark J. Yates b, John L. Bradshaw c a
School of Psychology, Flinders University, Australia Department of Psychology, University of Melbourne, Australia c Department of Psychology, Monash University, Australia b
a r t i c l e
i n f o
Article history: Accepted 25 April 2011
Keywords: Extrapersonal Peripersonal Dorsal Ventral Pseudoneglect
a b s t r a c t Distinct cognitive and neural mechanisms underlie perception and action in near (within-reach) and far (outside-reach) space. Objects in far space can be brought into the brain’s near-space through tool-use. We determined whether a near object can be pushed into far space by changing the pictorial context in which it occurs. Participants (n = 372) made relative length judgements for lines presented in near space, but superimposed over photographs of near and far objects. The left segment of the line was overestimated in the baseline and near-context conditions whereas the right was overestimated in the farcontext. The change from leftward to rightward overestimation is the same when lines are physically shifted from near to far space. Because participants did not have to do anything in relation to the photograph, the results suggest that simply viewing images with a near/far context can cause a shift of attention along the distal/proximal axis, which may reflect differential activation of the ventral/dorsal visual streams. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction Although it may appear seamless, the brain divides threedimensional space into a number of categories. One category runs along the proximal/distal axis. Near (peri-personal) space is generally considered to comprise the part of space that is within reach, whereas far (extra-personal) space is outside of reach. Whether there is a sharp boundary in the way the brain codes near and far space is a matter of debate. While Berti et al. (2002) concluded that there was a clear distinction between near and far space in neglect patients, studies in the general population suggest a gradual change as an object moves along the proximal/distal axis (see, Longo & Lourenco, 2006, 2007). Irrespective of whether the boundary is sharp or gradual, different neurological mechanisms are associated with processing near and far space (for a review, see Konen and Kastner (2008)). Lesion studies in primates reveal that the post-arcuate (area 6) and pre-arcuate (area 8) areas are associated with processing near and far space, respectively (Rizzolatti, Matelli, & Pavesi, 1983). For humans, regional cerebral blood flow research reveals activation of brain regions associated with the dorsal (intraparietal sulcus) and ventral (medial temporal cortex) streams for the bisection of lines placed in near and far space, respectively (Weiss et al., 2000). ⇑ Corresponding author. Address: School of Psychology, Flinders University, Bedford Park, SA 5052, Australia. Fax: +61 3 9347 6618. E-mail address: mike.nicholls@flinders.edu.au (M.E.R. Nicholls).
The dorsal/ventral distinction between near and far space may be aligned with functional implications related to ‘actionable’ objects. Milner and Goodale (1995) suggested that the dorsal stream is responsible for the visual control of action whereas the ventral stream enables the visual representation of the environment. Dorsal stream activation for near space may therefore reflect the fact that the object is also actionable. The connection between near space, actionable object and the dorsal visual stream, may not be that strong, however. In a PET study, Weiss, Marshall, Zilles, and Fink (2003) found that the neural representations of near and far space were independent of the motor/perceptual nature of the task. Another spatial category runs along the lateral axis and is defined by the body’s midline. Patients with damage to the posterior parietal cortex in the right hemisphere often suffer from visuospatial neglect (Nichelli, Rinaldi, & Cubelli, 1989). For objects placed in near space, the neglect manifests as an inability to perceive and/or attend to stimuli placed in the contralesional (left) hemispace (Nichelli et al., 1989). For some patients a dissociation has been observed whereby leftward neglect is observed for objects placed in near space, but not far space (Halligan & Marshall, 1991). For other patients, the reverse is true and they show leftward inattention for objects in far, but not near space (Vuilleumier, Valenza, Mayer, Reverdin, & Landis, 1998). Near and far space also interacts with attentional asymmetries in the intact brain. When an object is placed in near space, the features on the right receive less attention. As a consequence, when
0278-2626/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2011.04.006
M.E.R. Nicholls et al. / Brain and Cognition 76 (2011) 349–352
asked to bisect a line placed in near space, participants reliably place the bisector slightly to the left of the true centre (McCourt, 2001). This bias is thought to originate from the same cognitive/ neural mechanisms that give rise to clinical neglect (Bjoertomt, Cowey, & Walsh, 2002), and for this reason, it is often referred to as pseudoneglect. Pseudoneglect is also affected by the manipulation of near and far space. When lines are placed outside of reach, the leftward bisection bias can be annulled (Bjoertomt et al., 2002; McCourt & Garlinghouse, 2000) or revered towards a rightward bisection bias (Longo & Lourenco, 2006). The boundary between near and far space is not fixed and can be manipulated. In the primate brain, objects out of reach can be brought into ‘reach’ and processed by dorsal stream mechanisms through the use of a rake (Iriki, Tanaka, & Iwamura, 1996). For humans, Berti and Frassinetti (2000) found that, when using a laser pointed to make bisections, a neglect patient bisected to the right for lines placed in near space – but not far space. If the same patients used a stick to make the bisection, rightward bisections were observed in both conditions. Berti and Frassinetti (2000) suggested that tool use caused a remapping of space whereby an object in far space was brought into ‘reach’ by use of the tool. A similar effect has been observed for pseudoneglect in the general population. Longo and Lourenco (2006) presented lines at distances ranging from 0.3 m to 1.2 m. When the bisection was made using a laser pointer, leftward bisections were observed for near stimuli, which reversed to a rightward bisection bias for far stimuli. When the bisection was performed using a stick, however, leftward bisections were observed irrespective of viewing distance. Research that brings distant objects into near space raises the interesting possibility that the reverse may also be possible. That is, can an object in near space be pushed into far space? Lourenco and Longo (2009) investigated this issue by attaching weights to participants’ wrists as they bisected lines in near and far space. While the normal left/right shift in bisection was observed for near/far space, the point at which the change occurred was closer. They suggested that the extra effort associated with the weights reduced the size of near space and pushed some near objects into far space. The current study will also investigate whether objects can be pushed from near to far space, but will focus on perceptual manipulations. Research with stimuli such as numbers (e.g. ‘2’ or ‘9’) or objects that have a positional context (e.g. ‘hat or ‘boots’) has shown that the mere presentation of such stimuli causes an automatic shift of attention in lateral (Nicholls, Loftus, & Gevers, 2008) or vertical (Estes, Verges, & Barsalou, 2008) space. For the proximal/distal axis, Robertson and Kim (1999) demonstrated that perceptual depth induced through an ‘Ames-like’ room illusion affected shifts of spatial attention within near or far space. Research also indicates that the focus of attention can be ‘depth aware’ (Atchley, Kramer, Theeuwes & Anderson, 1997). With this research in mind, the present study investigated whether simply changing the pictorial context in which a stimulus occurs can produce attentional effects that are consistent with a shift between near and far space. Participants completed three conditions. In the baseline condition, pre-bisected lines were shown on a plain background and participants judged the relative lengths of the left and right sides of a line. In line with a large body of research (see Jewell and McCourt (2000) for a review) it was expected that the length on the left side would be overestimated due to the effects of pseudoneglect. In the near and far conditions, the lines were superimposed over photographs of near and far objects, respectively. The near conditions showed photographs of objects within reach (a light switch and a filing cabinet drawer). Because these objects are both located in near space, they are also both potentially actionable. Given that the dorsal stream is specialised for processing objects in near space and for programming action in space, it
was expected that this condition would activate the dorsal visual stream. As a result, there should be a leftward attentional bias, which causes the length of left side of the line should be overestimated. The far condition showed photographs of far objects (a classical building facade and a verandah) that were not immediately actionable. It was anticipated that these images would activate the ventral stream, which is specialised for distance perception and perception independent of action. Consistent with research in this area (Longo & Lourenco, 2006), a rightward attentional bias was predicted in this condition. 2. Method 2.1. Participants The sample was drawn from a large undergraduate class (m = 106, f = 358). To exclude individuals who were perhaps not attending to the task, participants with an accuracy score below chance (50%) were excluded. Participants who were not righthanded were also excluded. This left a sample of 372 participants (m = 80, f = 292) with a median age of 19 years. Participants gave informed consent and the study was approved by the Human Research Ethics Committee at the University of Melbourne. 2.2. Apparatus and stimuli Horizontal lines were defined by two short bars (10 mm long and 2 mm thick), which were separated by a horizontal distance of 140 mm. A third bar was placed between the flankers either 2 mm to the left or right of the true horizontal centre. For the baseline condition, three black bars were superimposed over a black horizontal line and placed in the centre of a sheet of white A4 paper. There were four baseline trials, with half of the trials bisected to the left or right (see Fig. 1). For the near and far conditions, three white bars were superimposed over black and white photographic images, which were 250 mm wide and 160 mm high. The three bars were aligned with
** **
*
Rightward bias
10
Lateral bias (%)
350
5 0 ##
##
#
-5 -10
Leftward bias
-15 -20
Baseline
Near
Far
Distance Context Fig. 1. Mean lateral bias (with ±SE bars) for the baseline, near and far conditions. Negative and positive lateral bias scores indicate leftward and rightward biases, respectively. Results for one sample t-tests, comparing each of the conditions with zero are shown along the x axis (# = p < .05; ## = p < .001). Results of post hoc comparison t-tests are shown with the bars above ( = p < .05; = p < .001). Examples of the stimuli used in the baseline, near and far conditions are shown below.
M.E.R. Nicholls et al. / Brain and Cognition 76 (2011) 349–352
a natural horizontal line that ran through the image. There were four images: Two were taken of distant objects (a classical building facade and a verandah) and two were of close objects (a filing cabinet and a light switch). There were four repeats of each image. Two images contained a line bisected to the left and two were bisected to the right. The actual horizontal position of the ‘line’ within the photograph was jittered by 5 mm between trials to discourage the use of landmark cues. Each trial was printed on a separate piece of A4 paper (20 pages in all). The order in which the trials occurred was randomised and the pages were stapled together. Half of the participants received a booklet in which the images were in their normal orientation. The other participants received a booklet where the entire image had been left/right reversed. Mirror imaging was introduced to control for asymmetries that occur in natural images, such as shadows and trees. 2.3. Procedure The experiment was carried out in a class-room setting. Participants sat at a desk with the response booklet placed on the table in front of them, aligned with their midline. Participants were asked to look at each image for 10 s. This relatively long exposure duration was chosen to increase the chance that the near/far pictorial context would influence the participant’s cognitive state. The possibility that such long presentations could induce adaptation after-effects was avoided by introducing a relatively long inter stimulus interval of approximately 20 s. Following the inspection period of 10 s, participants judged whether the left or right side of the line was longer. The response was recorded by the participant below the image. Participants did not move onto the next image until asked to do so by the experimenter after a delay of approximately 20 s. 3. Results Lateral bias was calculated by subtracting the number of ‘left longer’ responses from the number of ‘right longer’ responses and then converting to a percentage of the number of trials in that condition. Scores therefore ranged from 100 (the left side always seems longer) to +100 (the right side always seems longer). Scores approaching zero indicate no bias to either the left or right. The lateral biases for the baseline, near and far conditions are shown in Fig. 1. One sample t-tests comparing the samples with zero, revealed significant leftward biases for the baseline (t(371) = 4.56, p < .001) and near (t(371) = 3.05, p < .001) conditions and a rightward bias for the far condition (t(371) = 2.11, p < .05). A mixedmodel ANOVA with distance (baseline, near & far) and sex (male, female) as within- and between-participants factors was used to analyse the data. There was a significant effect of distance (F (2, 740) = 15.13, p < .001, g2p = .04). Post-hoc comparisons revealed significant differences for all points of comparison between the three conditions (all ps < .05). There was no effect of sex and no interaction. 4. Discussion The pictorial context in which the lines were presented had a significant effect on line bisection. Although there is no direct evidence that the near/far images actually affected the cognitive processing of distance, the results are in accord with such a shift. Consistent with the effect of pseudoneglect, participants overestimated the length of the left side of the line in the baseline condition. Unexpectedly, the leftward bias was stronger in the baseline condition compared to the near condition. This effect may be
351
related to the nature of the background. Because the baseline condition presented the stimuli against a plain background, it is possible that it provided a measure of pseudoneglect that was free of bilateral distracters, which can reduce the leftward bias commonly found in line bisection tasks (Berti, Maravita, Frassinetti, & Umilta’, 1995). It is also possible the strong leftward bias in the baseline condition reflects the operation of a near/far gradient discussed by Longo and Lourenco (2006, 2007). In this case, the line in the near condition is perceived to be set at the same viewing distance as the baseline condition plus the ‘distance’ from the camera’s lens to the object. As a result, the lines in the near condition are perceived to be further away than the baseline condition, which results in a shift away from a strong leftward bias. The left side of the line was judged to be longer in the near condition whereas the right side was judged to be longer in the far condition and this resulted in a significant difference between the near and far conditions. The difference between the near and far conditions is unlikely to be related to an illusory lengthening effect in the far condition (e.g. the Ames illusion, Gehringer & Engel, 1986). If perceived length increased in the far condition, the leftward bias should have increased in this condition – due to a well known effect of stimulus length on pseudoneglect (Jewell & McCourt, 2000; McCourt & Jewell, 1999). Instead, the results of changing the pictorial context from near to far seem remarkably similar to studies that have physically moved the stimuli from near to far space (Longo & Lourenco, 2006). Previous research has brought distant objects into near space through tool use. It is possible that tool use induces an effect by switching activation from the ventral stream to the dorsal stream, which is specialised for the visual control of action (Milner & Goodale, 1995). The present study is quite different. The task was deliberately designed so that the motor/perceptual demands were identical in the near and far conditions. In addition, no overt response was directed towards the stimuli. The present results therefore cannot be the result of dorsal stream activation through overt motor activity. The results suggest that simply changing the representational context in which the stimuli occurred was sufficient to bring about a shift between ventral and dorsal stream processing. In this regard, the results are similar to perceptual studies of number/space (Nicholls et al., 2008) and spatial embodiment (Estes et al., 2008), which also found that changing the context of a stimulus brings about an automatic shift of attention. The fact that the distribution of attention is affected by pictorial context is also relevant to the discussion of the level at which attention in near/far space is controlled. Although low level cues such as ocular convergence are undoubtedly important (Atchley, Kramer, Theeuwes, & Andersen, 1997), high level cues can also induce a shift of attention along the proximal/distal axis. Robertson and Kim (1999) demonstrated that space, as it is perceived, affected shifts of attention within a depth plane. Our study demonstrates that the perceptual context in which a stimulus occurs shifts attention between depth planes. Besides being close, the near images also showed objects that were actionable. Chao and Martin (2000) demonstrated that images of tools, but not other objects selectively activated dorsal stream mechanisms. The fact that the images of the light switch and filing cabinet were potentially actionable may have also played an important role in the effect observed in the current study. Using classic visual illusions of depth (such as the Ponzo illusion) would be an interesting way of investigating the effect of perceived depth independent of the semantic content of the images. In summary, our results show that pictorial context can influence whether a stimulus is processed as near or far. The shift in attentional asymmetries suggests that near objects were pushed into far space. Unlike previous studies that have brought far space into reach using tools, the current effect is unlikely to reflect overt
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M.E.R. Nicholls et al. / Brain and Cognition 76 (2011) 349–352
motor activity. Instead, it is possible that context has an effect through covert activation of the dorsal and ventral visual streams.
References Atchley, P., Kramer, A. F., Theeuwes, J., & Andersen, G. J. (1997). Spatial cuing in a stereographic display: Evidence for a depth aware attentional focus. Psychonomic Bulletin & Review, 4, 524–529. Berti, A., & Frassinetti, F. (2000). When far becomes near: Remapping of space by tool use. Journal of Cognitive Neuroscience, 12, 415–420. Berti, A., Maravita, A., Frassinetti, F., & Umilta’, C. A. (1995). Unilateral neglect can be affected by stimuli in the neglected field. Cortex, 31, 331–343. Berti, A., Smania, M., Rabuffetti, M., Ferrarin, M., Spinazzola, L., D’Amico, A., et al. (2002). Coding of near and far space during walking in neglect patients. Neuropsychology, 16, 390–399. Bjoertomt, O., Cowey, A., & Walsh, V. (2002). Spatial neglect in near and far space investigated by repetitive transcranial magnetic stimulation. Brain, 125, 2012–2022. Chao, L. L., & Martin, A. (2000). Representation of manipulable man-made objects in the dorsal stream. Neuroimage, 12, 478–484. Estes, Z., Verges, M., & Barsalou, L. W. (2008). Head up, foot down: Object words orient attention to the objects’ typical location. Psychological Science, 19, 93–97. Gehringer, W. L., & Engel, E. (1986). Effect of ecological viewing conditions on the Ames’ distorted room illusion. Journal of Experimental Psychology: Human Perception and Performance, 12, 181–185. Halligan, P. W., & Marshall, J. C. (1991). Left neglect for near, but not far space in man. Nature, 350, 498–500. Iriki, A., Tanaka, M., & Iwamura, Y. (1996). Coding of modified body schema during tool use by macaque postcentral neurons. Neuroreport, 7, 2325–2330. Jewell, G., & McCourt, M. E. (2000). Pseudoneglect: A review and meta-analysis of performance factors in line bisection tasks. Neuropsychologia, 38, 93–110. Konen, C. S., & Kastner, S. (2008). Two hierarchically organised neural structures for object information in human visual cortex. Nature Neuroscience, 11, 224–231.
Longo, M. R., & Lourenco, S. F. (2006). On the nature of near space: Effects of tool use and the transition to far space. Neuropsychologia, 44, 977–981. Longo, M. R., & Lourenco, S. F. (2007). Space perception and body morphology: Extent of near space scales with arm length. Experimental Brain Research, 177, 285–290. Lourenco, S. F., & Longo, M. R. (2009). The plasticity of near space: Evidence for contraction. Cognition, 112, 451–456. McCourt, M. E. (2001). Performance consistency of normal observers in forcedchoice tachistoscopic visual line bisection. Neuropsychologia, 39, 1065–1076. McCourt, M. E., & Garlinghouse, M. (2000). Asymmetries of visuospatial attention are modulated by viewing distance and visual field elevation: Pseudoneglect in peripersonal and extrapersonal space. Cortex, 36, 715–732. McCourt, M. E., & Jewell, G. (1999). Visuospatial attention in line bisection: Stimulus modulation of pseudoneglect. Neuropsychologia, 37, 843–855. Milner, A. D., & Goodale, M. A. (1995). The visual brain in action. Oxford: Oxford University Press. Nichelli, P., Rinaldi, M., & Cubelli, R. (1989). Selective spatial attention and length representation in normal subjects and in patients with unilateral spatial neglect. Brain & Cognition, 9, 57–70. Nicholls, M. E. R., Loftus, A., & Gevers, W. (2008). Look, no hands: A perceptual task shows that number magnitude induces shifts of attention. Psychonomic Bulletin & Review, 15, 413–418. Rizzolatti, G., Matelli, M., & Pavesi, G. (1983). Deficits in attention and movement following the removal of postarcuate (area 6) and prearcuate (area 8) cortex in macaque monkeys. Brain, 106, 655–673. Robertson, L. C., & Kim, M. (1999). Effects of perceived space on spatial attention. Psychological Science, 10, 76–79. Vuilleumier, P., Valenza, N., Mayer, E., Reverdin, A., & Landis, T. (1998). Near and far visual space in unilateral neglect. Annals of Neurology, 43, 406–410. Weiss, P. H., Marshall, J. C., Wunderlich, G., Tellmann, L., Halligan, P. W., Freund, H., et al. (2000). Neural consequences of acting in near versus far space: A physiological basis for clinical dissociations. Brain: A Journal of Neurology, 123, 2531–2541. Weiss, P. H., Marshall, J. C., Zilles, K., & Fink, G. R. (2003). Are action and perception in near and far space additive of interactive factors. Neuroimage, 18, 837–846.
Contents lists available at ScienceDirect
Brain and Cognition journal homepage: www.elsevier.com/locate/b&c
Near, yet so far: The effect of pictorial cues on spatial attention Michael E.R. Nicholls a,⇑, Jason D. Forte b, Tobias Loetscher a, Catherine A. Orr b, Mark J. Yates b, John L. Bradshaw c a
School of Psychology, Flinders University, Australia Department of Psychology, University of Melbourne, Australia c Department of Psychology, Monash University, Australia b
a r t i c l e
i n f o
Article history: Accepted 25 April 2011
Keywords: Extrapersonal Peripersonal Dorsal Ventral Pseudoneglect
a b s t r a c t Distinct cognitive and neural mechanisms underlie perception and action in near (within-reach) and far (outside-reach) space. Objects in far space can be brought into the brain’s near-space through tool-use. We determined whether a near object can be pushed into far space by changing the pictorial context in which it occurs. Participants (n = 372) made relative length judgements for lines presented in near space, but superimposed over photographs of near and far objects. The left segment of the line was overestimated in the baseline and near-context conditions whereas the right was overestimated in the farcontext. The change from leftward to rightward overestimation is the same when lines are physically shifted from near to far space. Because participants did not have to do anything in relation to the photograph, the results suggest that simply viewing images with a near/far context can cause a shift of attention along the distal/proximal axis, which may reflect differential activation of the ventral/dorsal visual streams. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction Although it may appear seamless, the brain divides threedimensional space into a number of categories. One category runs along the proximal/distal axis. Near (peri-personal) space is generally considered to comprise the part of space that is within reach, whereas far (extra-personal) space is outside of reach. Whether there is a sharp boundary in the way the brain codes near and far space is a matter of debate. While Berti et al. (2002) concluded that there was a clear distinction between near and far space in neglect patients, studies in the general population suggest a gradual change as an object moves along the proximal/distal axis (see, Longo & Lourenco, 2006, 2007). Irrespective of whether the boundary is sharp or gradual, different neurological mechanisms are associated with processing near and far space (for a review, see Konen and Kastner (2008)). Lesion studies in primates reveal that the post-arcuate (area 6) and pre-arcuate (area 8) areas are associated with processing near and far space, respectively (Rizzolatti, Matelli, & Pavesi, 1983). For humans, regional cerebral blood flow research reveals activation of brain regions associated with the dorsal (intraparietal sulcus) and ventral (medial temporal cortex) streams for the bisection of lines placed in near and far space, respectively (Weiss et al., 2000). ⇑ Corresponding author. Address: School of Psychology, Flinders University, Bedford Park, SA 5052, Australia. Fax: +61 3 9347 6618. E-mail address: mike.nicholls@flinders.edu.au (M.E.R. Nicholls).
The dorsal/ventral distinction between near and far space may be aligned with functional implications related to ‘actionable’ objects. Milner and Goodale (1995) suggested that the dorsal stream is responsible for the visual control of action whereas the ventral stream enables the visual representation of the environment. Dorsal stream activation for near space may therefore reflect the fact that the object is also actionable. The connection between near space, actionable object and the dorsal visual stream, may not be that strong, however. In a PET study, Weiss, Marshall, Zilles, and Fink (2003) found that the neural representations of near and far space were independent of the motor/perceptual nature of the task. Another spatial category runs along the lateral axis and is defined by the body’s midline. Patients with damage to the posterior parietal cortex in the right hemisphere often suffer from visuospatial neglect (Nichelli, Rinaldi, & Cubelli, 1989). For objects placed in near space, the neglect manifests as an inability to perceive and/or attend to stimuli placed in the contralesional (left) hemispace (Nichelli et al., 1989). For some patients a dissociation has been observed whereby leftward neglect is observed for objects placed in near space, but not far space (Halligan & Marshall, 1991). For other patients, the reverse is true and they show leftward inattention for objects in far, but not near space (Vuilleumier, Valenza, Mayer, Reverdin, & Landis, 1998). Near and far space also interacts with attentional asymmetries in the intact brain. When an object is placed in near space, the features on the right receive less attention. As a consequence, when
0278-2626/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2011.04.006
M.E.R. Nicholls et al. / Brain and Cognition 76 (2011) 349–352
asked to bisect a line placed in near space, participants reliably place the bisector slightly to the left of the true centre (McCourt, 2001). This bias is thought to originate from the same cognitive/ neural mechanisms that give rise to clinical neglect (Bjoertomt, Cowey, & Walsh, 2002), and for this reason, it is often referred to as pseudoneglect. Pseudoneglect is also affected by the manipulation of near and far space. When lines are placed outside of reach, the leftward bisection bias can be annulled (Bjoertomt et al., 2002; McCourt & Garlinghouse, 2000) or revered towards a rightward bisection bias (Longo & Lourenco, 2006). The boundary between near and far space is not fixed and can be manipulated. In the primate brain, objects out of reach can be brought into ‘reach’ and processed by dorsal stream mechanisms through the use of a rake (Iriki, Tanaka, & Iwamura, 1996). For humans, Berti and Frassinetti (2000) found that, when using a laser pointed to make bisections, a neglect patient bisected to the right for lines placed in near space – but not far space. If the same patients used a stick to make the bisection, rightward bisections were observed in both conditions. Berti and Frassinetti (2000) suggested that tool use caused a remapping of space whereby an object in far space was brought into ‘reach’ by use of the tool. A similar effect has been observed for pseudoneglect in the general population. Longo and Lourenco (2006) presented lines at distances ranging from 0.3 m to 1.2 m. When the bisection was made using a laser pointer, leftward bisections were observed for near stimuli, which reversed to a rightward bisection bias for far stimuli. When the bisection was performed using a stick, however, leftward bisections were observed irrespective of viewing distance. Research that brings distant objects into near space raises the interesting possibility that the reverse may also be possible. That is, can an object in near space be pushed into far space? Lourenco and Longo (2009) investigated this issue by attaching weights to participants’ wrists as they bisected lines in near and far space. While the normal left/right shift in bisection was observed for near/far space, the point at which the change occurred was closer. They suggested that the extra effort associated with the weights reduced the size of near space and pushed some near objects into far space. The current study will also investigate whether objects can be pushed from near to far space, but will focus on perceptual manipulations. Research with stimuli such as numbers (e.g. ‘2’ or ‘9’) or objects that have a positional context (e.g. ‘hat or ‘boots’) has shown that the mere presentation of such stimuli causes an automatic shift of attention in lateral (Nicholls, Loftus, & Gevers, 2008) or vertical (Estes, Verges, & Barsalou, 2008) space. For the proximal/distal axis, Robertson and Kim (1999) demonstrated that perceptual depth induced through an ‘Ames-like’ room illusion affected shifts of spatial attention within near or far space. Research also indicates that the focus of attention can be ‘depth aware’ (Atchley, Kramer, Theeuwes & Anderson, 1997). With this research in mind, the present study investigated whether simply changing the pictorial context in which a stimulus occurs can produce attentional effects that are consistent with a shift between near and far space. Participants completed three conditions. In the baseline condition, pre-bisected lines were shown on a plain background and participants judged the relative lengths of the left and right sides of a line. In line with a large body of research (see Jewell and McCourt (2000) for a review) it was expected that the length on the left side would be overestimated due to the effects of pseudoneglect. In the near and far conditions, the lines were superimposed over photographs of near and far objects, respectively. The near conditions showed photographs of objects within reach (a light switch and a filing cabinet drawer). Because these objects are both located in near space, they are also both potentially actionable. Given that the dorsal stream is specialised for processing objects in near space and for programming action in space, it
was expected that this condition would activate the dorsal visual stream. As a result, there should be a leftward attentional bias, which causes the length of left side of the line should be overestimated. The far condition showed photographs of far objects (a classical building facade and a verandah) that were not immediately actionable. It was anticipated that these images would activate the ventral stream, which is specialised for distance perception and perception independent of action. Consistent with research in this area (Longo & Lourenco, 2006), a rightward attentional bias was predicted in this condition. 2. Method 2.1. Participants The sample was drawn from a large undergraduate class (m = 106, f = 358). To exclude individuals who were perhaps not attending to the task, participants with an accuracy score below chance (50%) were excluded. Participants who were not righthanded were also excluded. This left a sample of 372 participants (m = 80, f = 292) with a median age of 19 years. Participants gave informed consent and the study was approved by the Human Research Ethics Committee at the University of Melbourne. 2.2. Apparatus and stimuli Horizontal lines were defined by two short bars (10 mm long and 2 mm thick), which were separated by a horizontal distance of 140 mm. A third bar was placed between the flankers either 2 mm to the left or right of the true horizontal centre. For the baseline condition, three black bars were superimposed over a black horizontal line and placed in the centre of a sheet of white A4 paper. There were four baseline trials, with half of the trials bisected to the left or right (see Fig. 1). For the near and far conditions, three white bars were superimposed over black and white photographic images, which were 250 mm wide and 160 mm high. The three bars were aligned with
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Distance Context Fig. 1. Mean lateral bias (with ±SE bars) for the baseline, near and far conditions. Negative and positive lateral bias scores indicate leftward and rightward biases, respectively. Results for one sample t-tests, comparing each of the conditions with zero are shown along the x axis (# = p < .05; ## = p < .001). Results of post hoc comparison t-tests are shown with the bars above ( = p < .05; = p < .001). Examples of the stimuli used in the baseline, near and far conditions are shown below.
M.E.R. Nicholls et al. / Brain and Cognition 76 (2011) 349–352
a natural horizontal line that ran through the image. There were four images: Two were taken of distant objects (a classical building facade and a verandah) and two were of close objects (a filing cabinet and a light switch). There were four repeats of each image. Two images contained a line bisected to the left and two were bisected to the right. The actual horizontal position of the ‘line’ within the photograph was jittered by 5 mm between trials to discourage the use of landmark cues. Each trial was printed on a separate piece of A4 paper (20 pages in all). The order in which the trials occurred was randomised and the pages were stapled together. Half of the participants received a booklet in which the images were in their normal orientation. The other participants received a booklet where the entire image had been left/right reversed. Mirror imaging was introduced to control for asymmetries that occur in natural images, such as shadows and trees. 2.3. Procedure The experiment was carried out in a class-room setting. Participants sat at a desk with the response booklet placed on the table in front of them, aligned with their midline. Participants were asked to look at each image for 10 s. This relatively long exposure duration was chosen to increase the chance that the near/far pictorial context would influence the participant’s cognitive state. The possibility that such long presentations could induce adaptation after-effects was avoided by introducing a relatively long inter stimulus interval of approximately 20 s. Following the inspection period of 10 s, participants judged whether the left or right side of the line was longer. The response was recorded by the participant below the image. Participants did not move onto the next image until asked to do so by the experimenter after a delay of approximately 20 s. 3. Results Lateral bias was calculated by subtracting the number of ‘left longer’ responses from the number of ‘right longer’ responses and then converting to a percentage of the number of trials in that condition. Scores therefore ranged from 100 (the left side always seems longer) to +100 (the right side always seems longer). Scores approaching zero indicate no bias to either the left or right. The lateral biases for the baseline, near and far conditions are shown in Fig. 1. One sample t-tests comparing the samples with zero, revealed significant leftward biases for the baseline (t(371) = 4.56, p < .001) and near (t(371) = 3.05, p < .001) conditions and a rightward bias for the far condition (t(371) = 2.11, p < .05). A mixedmodel ANOVA with distance (baseline, near & far) and sex (male, female) as within- and between-participants factors was used to analyse the data. There was a significant effect of distance (F (2, 740) = 15.13, p < .001, g2p = .04). Post-hoc comparisons revealed significant differences for all points of comparison between the three conditions (all ps < .05). There was no effect of sex and no interaction. 4. Discussion The pictorial context in which the lines were presented had a significant effect on line bisection. Although there is no direct evidence that the near/far images actually affected the cognitive processing of distance, the results are in accord with such a shift. Consistent with the effect of pseudoneglect, participants overestimated the length of the left side of the line in the baseline condition. Unexpectedly, the leftward bias was stronger in the baseline condition compared to the near condition. This effect may be
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related to the nature of the background. Because the baseline condition presented the stimuli against a plain background, it is possible that it provided a measure of pseudoneglect that was free of bilateral distracters, which can reduce the leftward bias commonly found in line bisection tasks (Berti, Maravita, Frassinetti, & Umilta’, 1995). It is also possible the strong leftward bias in the baseline condition reflects the operation of a near/far gradient discussed by Longo and Lourenco (2006, 2007). In this case, the line in the near condition is perceived to be set at the same viewing distance as the baseline condition plus the ‘distance’ from the camera’s lens to the object. As a result, the lines in the near condition are perceived to be further away than the baseline condition, which results in a shift away from a strong leftward bias. The left side of the line was judged to be longer in the near condition whereas the right side was judged to be longer in the far condition and this resulted in a significant difference between the near and far conditions. The difference between the near and far conditions is unlikely to be related to an illusory lengthening effect in the far condition (e.g. the Ames illusion, Gehringer & Engel, 1986). If perceived length increased in the far condition, the leftward bias should have increased in this condition – due to a well known effect of stimulus length on pseudoneglect (Jewell & McCourt, 2000; McCourt & Jewell, 1999). Instead, the results of changing the pictorial context from near to far seem remarkably similar to studies that have physically moved the stimuli from near to far space (Longo & Lourenco, 2006). Previous research has brought distant objects into near space through tool use. It is possible that tool use induces an effect by switching activation from the ventral stream to the dorsal stream, which is specialised for the visual control of action (Milner & Goodale, 1995). The present study is quite different. The task was deliberately designed so that the motor/perceptual demands were identical in the near and far conditions. In addition, no overt response was directed towards the stimuli. The present results therefore cannot be the result of dorsal stream activation through overt motor activity. The results suggest that simply changing the representational context in which the stimuli occurred was sufficient to bring about a shift between ventral and dorsal stream processing. In this regard, the results are similar to perceptual studies of number/space (Nicholls et al., 2008) and spatial embodiment (Estes et al., 2008), which also found that changing the context of a stimulus brings about an automatic shift of attention. The fact that the distribution of attention is affected by pictorial context is also relevant to the discussion of the level at which attention in near/far space is controlled. Although low level cues such as ocular convergence are undoubtedly important (Atchley, Kramer, Theeuwes, & Andersen, 1997), high level cues can also induce a shift of attention along the proximal/distal axis. Robertson and Kim (1999) demonstrated that space, as it is perceived, affected shifts of attention within a depth plane. Our study demonstrates that the perceptual context in which a stimulus occurs shifts attention between depth planes. Besides being close, the near images also showed objects that were actionable. Chao and Martin (2000) demonstrated that images of tools, but not other objects selectively activated dorsal stream mechanisms. The fact that the images of the light switch and filing cabinet were potentially actionable may have also played an important role in the effect observed in the current study. Using classic visual illusions of depth (such as the Ponzo illusion) would be an interesting way of investigating the effect of perceived depth independent of the semantic content of the images. In summary, our results show that pictorial context can influence whether a stimulus is processed as near or far. The shift in attentional asymmetries suggests that near objects were pushed into far space. Unlike previous studies that have brought far space into reach using tools, the current effect is unlikely to reflect overt
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motor activity. Instead, it is possible that context has an effect through covert activation of the dorsal and ventral visual streams.
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