Cognitive Science > Action and Cognition II > Q&A - Reference Frames

[edit] Hemineglect

What are the characteristic symptoms of hemineglect?

• Unawareness of one visual hemifield
  • Ignore stimuli in the hemifield contralateral to the lesion's side
  • Impaired eye movements in this direction
  • May not be able to recall information from this side
• Neither purely visual nor purely sensory impairment
• No insight into the disturbance
• More severe and longer lasting with right hemisphere lesion

  May play a stronger role in representation of spatial relationships

Describe an experiment, which demonstrates extinction!

• Extinction: Occurrence of stimuli is not perceived while something else is going on
• Experiment with neglect patient
  • Task: Detect occurrence of stimuli on screen
  • Performance measure: Detection time
• Control condition
  • Occurrence detection of single stimuli everywhere on screen
  • → detection possible (in ignored hemifield slightly impaired performance)
• Main condition
  • Two items shown simultaneously – one left, one right
  • Patient does not perceive item in the neglected hemifield – extinguished by presence of the perceived one.

Describe an experimental setup, which shows that neglect is not purely sensory, but contains a strong motor component. Hint: Choose an experimental setup, where starting points are different.

• Experiment with patient who has a right parietal lesion
• Speeded-reaching-paradigm
  • Reaching target
  • Measure reaction time
  • Possible targets can appear left, middle, right
  • Fixation cross in the middle
  • Three conditions: Index finger fixated either left, middle, right
• Observations
  • Good performance in rightward movements with finger
  • Poor performance in leftward movements

    Reaction time doubles

  • Differences between indexes (left-left better than middle-left & right-left)

    Interpretation: Patient can detect target in left visual hemifield, but has problems in initiating movements relatively to contralateral side.

Which experiment can show you cross-modal effects in hemineglect and how are the effects expressed?

• Ladavas et al. (2000)

  Right parietal hemisphere lesion subjects sitting on a table, palms down on the table

  • Classic tactile extinction
    • Hands covered by cardboard shield
    • Single tactile to the left → detection possible
    • Tactile to left and right simultaneously → left tactile most often neglected
  • Cross-modal extinction
    • Cardboard covers neglected side
    • Simultaneously tactile stimuli to left & visual stimuli to right
    • → still 60% extinction
    • Extinction effect is reduced when the distance between visual stimulus and hand is increased

[edit] Reference Frames

Describe the differences between egocentric and allocentric reference frames.

• Reference frame
  • Set of axes which locates objects in space
  • For each dimension one axis serves as reference metric
• Egocentric

  Locates external objects with respect to agent's position

• Allocentric

  Locates objects independent of observer, selects a different reference point

How could you find out, whether the reference frame of primary visual cortex corresponds to retina positions or head coordinates?

• Experimental condition: Three different fixation points
• Visual targets at different head-centered positions
• Measure firing rates of cells
• Tuning curves show which reference frame is used
  • With head-centered coordinates, a change of fixation point will not change the tuning curves.

    Reason: Only head positions matter

  • With retinal coordinates, different fixation points lead to different tuning curves.

[edit] Neurons in LIP, VIP & PRR

Describe the response properties of neurons in LIP (lateral inferio-parietal cortex) and PRR (parietal reach region) in a combined fixation and reaching task.

• LIP: Specialized for saccadic movements
• PRR: Specialized for reaching movements
• Fixation-and-reaching-task
  • Monkey fixates center light with eyes and one hand
  • Briefly, a spatial location is cued by light
  • Memorizes position
  • After delay period: Center light turned off
  • Then, two conditions:
    1. Reach to the location without changing fixation (reach task)
    2. Change fixation to location without moving the hand (saccade task)
  • Neurons in
    • LIP show higher activity during delay period in saccade task
    • PRR vice versa

Explain the response properties of neurons in LIP or VIP (ventral inferior parietal) with respect to multimodal stimuli.

• Both respond to multimodal stimuli
• Direction selectivity
• Congruence in receptive field location for multimodal cues
• VIP: Selectivity for rightwards moving visual stimuli → selectivity for rightwards moving tactile stimuli
• LIP: Selectivity for auditory memory saccade to the right → selectivity for visual memory saccade to the right
• Example: Visual field moves together with arm, crosses midline (to other hemisphere!) when arm crosses midline.

[edit] Gain fields

Explain the concept of a gain field using the example of a neuron in LIP.

• Gain field: Characterization
  • A special form of receptive field
  • Space-dependent modulation of neuron's response behavior with respect to a non-standard reference frame
  • Found in many neurons in parietal areas
  • Position information in two different coordinate systems is taken into account: Tuning curve local in one stimulus dimension, size modulated by another independent variable (gain).

• Example: LIP neuron
  • Most neurons in LIP encode stimuli in eye-centered coordinates
  • Response behavior is modulated by object's location in head-centered coordinates

    Tonic activity during delay period of saccadic task is linearly modulated by eye position

  • Regulatory effect on eye positioning: Stimulus in preferred eye position → frequency of response ↑