Jeffery et al. provide evidence that horizontal and vertical coordinates are processed by different systems, to the effect that navigation in a three-dimensional environment requires two codes. The authors summarise evidence from allocentric navigation and single cell recording to support their position. Although we are more familiar with egocentric coordinate systems, we agree with Jeffery et al. that bicoding is likely, and note evidence from perceptual illusions, neurological conditions, studies of stimulus–response compatibility, and biomechanical constraints further supporting this.
Perceptual illusions
Distinctly different codes responsible for horizontal and vertical distances are indicated by perceptual illusions. In the horizontal/vertical illusion, two lines of equal length form an upside-down T. Although of equal length, the vertical line nevertheless appears longer perceptually (Avery & Day Reference Avery and Day1969). Such observations imply that horizontal and vertical distances are processed differently, and this is reinforced by the differential vulnerabilities of horizontal and vertical coordinate systems to neurological disorders.
Neurological disorders
Cerebro-vascular accidents (strokes) affecting the right hemisphere, particularly the parietal lobe, lead to hemineglect (Mattingley et al. Reference Mattingley, Bradshaw and Phillips1992). Hemineglect is a tendency to ignore the left side of space that occurs independently of any deficits in the visual field. These strokes affect the perception of space and the willingness to search and explore the left side of space. Although hemineglect is a relatively common neurological syndrome, a comparable tendency to ignore the lower or upper zones of space is quite rare, and the underlying pathophysiology is not well understood (Shelton et al. Reference Shelton, Bowers and Heilman1990), suggesting that different systems are responsible for horizontal and vertical coordinates, and this is reinforced by a consideration of effector systems.
Stimulus–response compatibility
The transition from a horizontal plane to a three-dimensional coordinate system as occurs in three-dimensional interfaces (Phillips et al. Reference Phillips, Triggs and Meehan2005), poses potential sources of incompatibility between stimulus and response (Worringham & Beringer Reference Worringham and Beringer1989). For instance, in human–computer interfaces, leftwards joystick or leftwards mouse motions both indicate left cursor motion, and remain so in three dimensions. Nevertheless, the use of motions to code for up can be arbitrary, with forward mouse movements coding for upwards cursor motion on computer screens and forward joystick motion coding for downwards cursor movements. Indeed, this arbitrariness is also seen within animals, with locomotor responses operating in opposition to trunk/body responses, such that downward propulsive movements code for up (flying, swimming) or head-down movements code for down (flying, swimming).
Biomechanical constraints
Indeed, navigation is necessarily a corollary of activity and motion, and there are biomechanical constraints that argue for motion to be primarily coded in the horizontal plane. Motion occurs within the context of a constant gravitational field that has implications for movement (Phillips & Ogeil Reference Phillips and Ogeil2010). For most mammals, locomotion is roughly analogous to the oscillations of pendulums suspended vertically from hip or shoulder joints. Such motions are mathematically predictable. However, there are potential sources of problems if motions incorporate additional oscillators to create motion in the third dimension. If motion in the third dimension requires horizontally oriented oscillators, these physical systems have the potential to be chaotic and unpredictable, because the combination of horizontal and vertical oscillators approximates a pendulum (x oscillator) suspended on a rotating driving arm (y oscillator). Such systems are potentially chaotic (Baker & Gollub Reference Baker and Gollub1996) in their behaviour, with system characteristics dependent upon speed and the size of the y oscillator. Given a potential unpredictability inherent in motion in three dimensions, it should not be surprising that motion systems are primarily oriented in accordance with gravity and that coordinate systems are oriented perpendicularly to gravity, with other sources of information being more important for navigation than is altitude.
Coordinate systems
Navigation requires motion, with coordinates updated as time and location change. For example, studies of Monarch butterfly navigation during seasonal migration have demonstrated at the cellular level that the circadian system integrates information about navigating space based on time in addition to an insect's position relative to the sun (Reppert Reference Reppert2006). Central to this is a time-compensated sun compass, which likely receives direct input from the circadian system (Zhu et al. Reference Zhu, Sauman, Yuan, Casselman, Emery-Le, Emery and Reppert2008), to allow correction of flight direction and coordinates relative to changing light levels across the course of the day. This time-compensated sun compass appears to be genetically determined rather than learned (Reppert Reference Reppert2006). Hence, we feel the second code is more likely to involve a time component than an altitude component.
Jeffery et al. provide evidence that horizontal and vertical coordinates are processed by different systems, to the effect that navigation in a three-dimensional environment requires two codes. The authors summarise evidence from allocentric navigation and single cell recording to support their position. Although we are more familiar with egocentric coordinate systems, we agree with Jeffery et al. that bicoding is likely, and note evidence from perceptual illusions, neurological conditions, studies of stimulus–response compatibility, and biomechanical constraints further supporting this.
Perceptual illusions
Distinctly different codes responsible for horizontal and vertical distances are indicated by perceptual illusions. In the horizontal/vertical illusion, two lines of equal length form an upside-down T. Although of equal length, the vertical line nevertheless appears longer perceptually (Avery & Day Reference Avery and Day1969). Such observations imply that horizontal and vertical distances are processed differently, and this is reinforced by the differential vulnerabilities of horizontal and vertical coordinate systems to neurological disorders.
Neurological disorders
Cerebro-vascular accidents (strokes) affecting the right hemisphere, particularly the parietal lobe, lead to hemineglect (Mattingley et al. Reference Mattingley, Bradshaw and Phillips1992). Hemineglect is a tendency to ignore the left side of space that occurs independently of any deficits in the visual field. These strokes affect the perception of space and the willingness to search and explore the left side of space. Although hemineglect is a relatively common neurological syndrome, a comparable tendency to ignore the lower or upper zones of space is quite rare, and the underlying pathophysiology is not well understood (Shelton et al. Reference Shelton, Bowers and Heilman1990), suggesting that different systems are responsible for horizontal and vertical coordinates, and this is reinforced by a consideration of effector systems.
Stimulus–response compatibility
The transition from a horizontal plane to a three-dimensional coordinate system as occurs in three-dimensional interfaces (Phillips et al. Reference Phillips, Triggs and Meehan2005), poses potential sources of incompatibility between stimulus and response (Worringham & Beringer Reference Worringham and Beringer1989). For instance, in human–computer interfaces, leftwards joystick or leftwards mouse motions both indicate left cursor motion, and remain so in three dimensions. Nevertheless, the use of motions to code for up can be arbitrary, with forward mouse movements coding for upwards cursor motion on computer screens and forward joystick motion coding for downwards cursor movements. Indeed, this arbitrariness is also seen within animals, with locomotor responses operating in opposition to trunk/body responses, such that downward propulsive movements code for up (flying, swimming) or head-down movements code for down (flying, swimming).
Biomechanical constraints
Indeed, navigation is necessarily a corollary of activity and motion, and there are biomechanical constraints that argue for motion to be primarily coded in the horizontal plane. Motion occurs within the context of a constant gravitational field that has implications for movement (Phillips & Ogeil Reference Phillips and Ogeil2010). For most mammals, locomotion is roughly analogous to the oscillations of pendulums suspended vertically from hip or shoulder joints. Such motions are mathematically predictable. However, there are potential sources of problems if motions incorporate additional oscillators to create motion in the third dimension. If motion in the third dimension requires horizontally oriented oscillators, these physical systems have the potential to be chaotic and unpredictable, because the combination of horizontal and vertical oscillators approximates a pendulum (x oscillator) suspended on a rotating driving arm (y oscillator). Such systems are potentially chaotic (Baker & Gollub Reference Baker and Gollub1996) in their behaviour, with system characteristics dependent upon speed and the size of the y oscillator. Given a potential unpredictability inherent in motion in three dimensions, it should not be surprising that motion systems are primarily oriented in accordance with gravity and that coordinate systems are oriented perpendicularly to gravity, with other sources of information being more important for navigation than is altitude.
Coordinate systems
Navigation requires motion, with coordinates updated as time and location change. For example, studies of Monarch butterfly navigation during seasonal migration have demonstrated at the cellular level that the circadian system integrates information about navigating space based on time in addition to an insect's position relative to the sun (Reppert Reference Reppert2006). Central to this is a time-compensated sun compass, which likely receives direct input from the circadian system (Zhu et al. Reference Zhu, Sauman, Yuan, Casselman, Emery-Le, Emery and Reppert2008), to allow correction of flight direction and coordinates relative to changing light levels across the course of the day. This time-compensated sun compass appears to be genetically determined rather than learned (Reppert Reference Reppert2006). Hence, we feel the second code is more likely to involve a time component than an altitude component.