Timing

Psy391D - Class 5

(This page last updated 27 September, 2004.)

Libet's Experiment

• Direct stimulation of primary sensory cortex
• Cortical stimulation can block perception of stimulus on arm.
  • Interfering cortical stimulation can be 500 msec after arm stimulus.
  • Is experience of arm stimulus delayed by 500 msec?
• Cortical stimulation can produce its own sensation.
  • Weak cortical stimulation must continue for 500 msec to trigger sensation.
• Arm stimulation and cortical stimulation together
  • Sensation from cortical stimuluation is delayed by 500 msec
  • No perceived delay for arm stimulation
  • Arm stimulation perceived to occur first, even when it starts after cortical stimulation.
• Libet's conclusions
  • Experience does not arise until some time after neural signals first reach cortex.
    • Sensory signals from arm could be blocked by cortical stimulation up to 500 msec after they occurred.
  • Once the experience is established, it can be "backdated" so that subject believes it occurred when it did.
  • The time assigned to the experience is that of the primary evoked potential that arose just after the stimulation began.

Metacontrast Masking

See http://www.psych.purdue.edu/~coglab/VisLab/Metacontrast/Metacontrast.html

Perception of first stimulus blocked by second stimulus.

How can second stimulus reach back in time to change perception of first stimulus.

 

Two Ways in which Experiences might be Revised

• Stalinesque Revision
  • After a string of sensory events, a coherent set of experiences is created and "played back" for you to experience.
  • If the process is interfered with before playback, then you have no experience of these events.
• Orwellian Revision
  • Events are experienced as they happen.
  • In some cases, memory for past events will be changed to make them more coherent.

Next class: The Cartesian Theater

Psych 391D: Consciousness Kyle Cave Psychology Dept. U. Mass.