Figure 1.
Model, receptive field organization and figure-ground segregation.
A: The model consists of two separate feature channels (Feat-1 and Feat-2) with each two layers, which are unidirectional connected (arrows). The white regions in the two lower squares indicate the stimulus input (figure-ground input). Black regions provide no input to the model. In the two layers of the model, the light grey central squares depict the figureregion and dark grey regions the background. B: Layer-1 neurons have a centre receptive field, i.e. they are driven by one input pixel. Layer-2 neurons have an excitatory centre and inhibitory surround receptive field. The central small black circles represent a neuron in the first and second layer of the model. The small grey square represents one input pixel. Blue arrows indicate point-to-point (retinotopic), excitatory connections and orange region represent the inhibitory connections from layer 1 to layer 2.C: Each layer-1 neurons receives inhibitory feedback connections from all layer-2 neurons, indicated by the orange region. Connections are identical for both feature maps. Note that in B,C only one feature map is shown.
Figure 2.
Spike responses of the neurons in the first and second layer to figure-ground stimulus.
Arrows point to the responses of neurons (small circles) lying on the figure (red traces) and background (green traces) regions.
Figure 3.
A, B: Responses without (A) and with (B) feedback. Small circle represents a single neuron. Time is from stimulus onset.
Figure 4.
Average responses to figure, ground and homogeneous input (A,B) and strength of figure-ground segmentation (C) with (red/orange) and without (black) feedback connections.
Table 1.
Number of spikes per second of layer-1 neurons in the two feature maps in the presence or absence of feedback.
Figure 5.
Amount of figure-ground modulation after testing the model with different strengths of feedback connections (A) and feedforward connections (B).
Grey lines indicate the strength of figure-ground modulation observed in the previous experiments and is used as reference.
Figure 6.
Amount of figure-ground modulation for different time delays of feedback inhibition.
Sign indicates whether feedback input arrives before (negative value) or after (positive values) the feedforward input. For each time delay one iteration step should be added.