De dynamica van het visueel systeem - PowerPoint PPT Presentation

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De dynamica van het visueel systeem

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  1. De dynamica van het visueel systeem Victor A.F.Lamme Cognitive Neuroscience group Dept Psychology, University of Amsterdam Eye and Brain Institute, Royal Academy of Sciences VA.F.Lamme@uva.nl

  2. V2: Neurons tuned for the orientation of illusory contours

  3. Cells in V4 and areas of the temporal lobe are selectively responding to complex shapes (a), or higher order shape descriptors, like radial frequency (b)

  4. Tanaka decomposed complex objects into the essential features that are necessary to activate IT cells, and found selectivity for higher order abstract shapes

  5. Cells in Inferotemporal Cortex (IT) respond selectively to faces or hands

  6. Fast Feedforward Sweep • Speeds through the visual system within 80 – 100 ms by means of feedforward connections • Provides the neurons with their receptive field tuning properties • Provides fast detection of ‘hardwired’ features and feature constellations • Provides basic categorizations, i.e. face / non-face etc.

  7. Recurrent processing • Is the processing mediated by the horizontal and feedback connections • Starts as soon as the feedforward sweep reaches an area, and may then take a variable amount of time, depending on distances, complexity etc. • Is expressed by effects from beyond the classical (= feedforward) receptive field

  8. Feedforward connections: carrying orientation information IT Horizontal connections: Lateral inhibition between neurons tuned to the same orientation. This creates the ‘bumps’ that merge when the RF is as large as the figure V4 V2 Feedback connections: Do a retinotopic feedback, so that the regions between the ‘bumps’ get progressively filled in, creating contextual modulation Roelfsema et al., J. Cogn Neurosci, 2002 V1

  9. The recurrent network architecture captures all features of the neurophysiological recordings

  10. Hierarchical detection of texture defined squares V1: Small receptive fields, detection of oriented line segments (Hub&Wies) IT: Large receptive fields, detection of cue invariant shapes (Vogels et al)

  11. Feedforward connections: carrying orientation information IT Horizontal connections: Lateral inhibition between neurons tuned to the same orientation. This creates the ‘bumps’ that merge when the RF is as large as the figure V4 V2 Feedback connections: Do a retinotopic feedback, so that the regions between the ‘bumps’ get progressively filled in, creating contextual modulation Why the recurrent processing? V1

  12. Fast Feedforward Sweep • Unimpaired by backward masking

  13. Monkeys detecting figures on backgrounds • 3 possible locations • Catch trials with homogenous textures • Average reaction 240ms • Used to demonstrate blindsight in monkeys

  14. 80% 8%

  15. Recurrent processing selectively absent for not seen trials

  16. Contextual ModulationRecurrent processing • Interfering with its occurrence (anesthesia, masking, TMS) results in a loss of the figure-ground percept • Spontaneous lapses of its occurrence result in a loss of the figure-ground percept • In all these cases, feedforward processing is unimpaired

  17. TMS of the occipital cortex yields face stimuli invisible, even at latencies beyond the feedforward sweep (120ms) Nevertheless, subjects can guess the emotional expression of that face: Affective blindsight (Jolij & Lamme 2005)

  18. Feedforward activation Unconscious Recurrent processing Conscious