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PERCEPTUAL ASYNCHRONY OF COLOUR AND MOTION RESULTS FROM REPETITIVE ALTERNATION

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. Background. Modularity of Vision

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PERCEPTUAL ASYNCHRONY OF COLOUR AND MOTION RESULTS FROM REPETITIVE ALTERNATION

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  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 …. => Can we accurately judge the temporal relationship between events defined by differentattributes? • 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?” => 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 => LARGER DELAY FOR MOTION

  9. Perceptual Task - Slow Repetitive Changes • Task: “Just in-phase?” • Slow alternation • ICI: 2 sec / 0.25 Hz => VERIDICAL JUDGEMENT

  10. 200 150 100 50 0 -50 -100 Perceptual Tasks: Results summary fast slow Repetitive Single changes changes relative to colour (msec) Perceptual delay of motion SN YK RM 100 500 1000 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 => 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 => LARGER DELAY FOR MOTION AT SHORT INTER-CHANGE INTERVALS Error bar: 1SD

  15. 200 150 100 50 0 -50 -100 Motor Tasks: Results summary fast slow Repetitive Single changes changes relative to colour (msec) Response delay of motion SN YK RM Simple RT Choice RT 100 500 1000 2000 Inter-change interval (msec) Inter-change interval (msec)

  16. Colour vs. Luminance/Orientation/Motion Colour Luminance Orientation Motion

  17. Colour vs. Luminance/Orientation/Motion Single changes Repetitive changes (ICI: 250 msec) 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. => 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. => 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

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