Shin'ya Nishida

1
PERCEPTUAL ASYNCHRONY OF COLOUR AND MOTION
RESULTS FROM REPETITIVE ALTERNATION

• Shin'ya Nishida
• NTT Communication Science Laboratories, Japan
• Alan Johnston
• Department of Psychology, University College
  London, UK

2
Background

• Modularity of Vision
• Specialised processing modules for colour,
  motion, pattern .
• gt Can we accurately judge the temporal
  relationship between events defined by different
  attributes?
• Moutoussis Zeki (1997, Proc. R. Soc. Lond, B,
  264, 393-399 1407-1414)
• Oscillations of colour and motion direction are
  not perceived to be in synchrony when they are
  physically in phase.
• Perceptual synchrony requires motion change to
  lead by 100 msec.
• This reflects differences in the time course of
  cortical activation i.e., longer processing
  delay for motion than for colour.

3
Main points

• Question
• Why had we overlooked such a large temporal
  dissociation of colour and motion processing?
• Is it obtained even for non-repetitive changes?
• Experiments
• Perceptual tasks (Expt 1) Motor tasks (Expt 2)
• Answer
• We can veridically judge the temporal
  relationship between colour and direction changes
  when they are non-repetitive.
• Rapid repetitive alternation is a critical factor
  for obtaining the large temporal dissociation of
  colour and motion.

4
Stimuli Apparatus

• A pair of plaids
• 1.4 c/deg
• 100 contrast
• Colour change
• Green - Red
• Equiluminance
• Motion direction change
• Upward - Downward
• Speed 6 deg/sec
• Apparatus
• VSG2/3
• Refresh rate 120Hz

4 deg
4 deg
4 deg
Fixation
5
Expt 1a Perceptual Task - Single Changes

• Single changes
• Colour Green to Red
• Motion Upward to Downward
• Variable
• Relative timing of changes
• Task
• Temporal order judgement Which change occurs
  first?
• Response choice Left/Right

1 - 3 sec
1 sec
Left
Right
Time
6
Perceptual Task - Single Changes

• Task Which is first?
• gt VERIDICAL JUDGEMENT

7
Expt 1b Perceptual Task - Repetitive Changes

• Repetitive changes
• Colour Green - Red
• Motion Upward - Downward
• Variable
• Relative timing of changes
• Task
• Relative phase judgement Are they just
  in-phase?
• (In-phase Red - Downward)
• Response choice Yes/No

Inter-change interval (ICI) 250 - 2000 msec
Left
Right
Time
8
Perceptual Task - Rapid Repetitive Changes

• Task Just in-phase?
• Fast alternation
• ICI 250 msec / 2 Hz
• gt LARGER DELAY FOR MOTION

9
Perceptual Task - Slow Repetitive Changes

• Task Just in-phase?
• Slow alternation
• ICI 2 sec / 0.25 Hz
• gt VERIDICAL JUDGEMENT

10
Perceptual Tasks Results summary
slow
fast
Single
Repetitive
changes
changes
Perceptual delay of motion
relative to colour (msec)
SN
YK
RM
100
1000
500
2000
Inter-change interval (msec)
11
Expt 2a Motor Task - Single Changes
1 - 3 sec

• Simple reaction time
• Press a button immediately after stimulus change
• Colour Green to Red
• Motion Upward to Downward
• Choice reaction time
• Press one of two buttons according to the
  colour/direction of the second stimulus
• Colour Green to Red/Blue
• Motion Upward to Downward/Leftward

Simple (Motion)
Button press
Choice (Colour)
Button(1) press
Left and right plaids were identical
12
Motor Task - Single Changes

• Simple Choice Reaction Times
• gt NO OR SMALL DIFFERENCES

Error bar 1SD
13
Expt 2b Motor Task - Repetitive Changes

• Repetitive changes
• Colour Green - Red
• Motion Upward - Downward
• Synchronous button press
• Press a button while the stimulus is Red/Downward
  
• 1 trial 1 min
• The timing of stimulus changes was predictable.

Inter-change interval (ICI) 250 - 2000 msec
Button press
Left and right plaids were identical
14
Motor Task - Repetitive Changes

• Synchronous button press
• gt LARGER DELAY FOR MOTION AT SHORT INTER-CHANGE
  INTERVALS

Error bar 1SD
15
Motor Tasks Results summary
slow
fast
Single
Repetitive
changes
changes
relative to colour (msec)
Response delay of motion
SN
YK
RM
Simple RT
Choice RT
100
1000
500
2000
Inter-change interval (msec)
Inter-change interval (msec)
16
Colour vs. Luminance/Orientation/Motion
Colour
Orientation
Luminance
Motion
17
Colour vs. Luminance/Orientation/Motion
Motor Tasks
Perceptual Tasks
Error bar 1SD
Colour
Colour
Orientation
Orientation
Luminance
Motion
Luminance
Motion
18
Discussion (1)

• Perceptual asynchrony between colour and motion
  is only seen for rapid repetitive oscillation..
  Why?
• Hypothesis 1 Change in processing time?
• Repetitive changes may selectively elongate the
  processing time for motion direction changes.
• gt Processing time may be increased with the
  processing load when the load exceeds a certain
  level.

Colour
Motion
Repetitive
Single
Repetitive
Single
Processing delay
Processing delay
Processing load
Processing load
19
Discussion (2)

• Hypothesis 2 No change in processing time?
• Repetitive changes may disturb a process that
  enables veridical judgements of the temporal
  order of isolated changes.
• gt For example, the visual system may use
  temporal boundary signals for accurate temporal
  localisation of events.
• Weaker for direction changes than for colour
  changes
• Not available for repetitive direction changes

Temporal boundary signal
Perceptual judgement