Direction Coding

Bob Sekuler and coworkers measured the human ability to discriminate the direction of motion of large patches of moving dots. They showed that observers could discriminate directions that were 1 or 2° apart. They estimate that the bandwidth of directionally selective mechanisms is about 60°, and that 12 such mechanisms evenly distributed about 360° can account for human direction discrimination. Humans likely achieve their sensitivity to small direction differences by comparing the patterns of activation across these broadly tuned mechanisms. This psychophysical bandwidth estimate is consistent with physiological

Time (ms)

C Space-time Receptive Field

X-Axis Position

Figure 2 (A) A spatial receptive field. This is an x-y plot of a neuron's response to a spot of light at different positions. In general, the neuron's response to a spatially extended stimulus is computed by multiplying the stimulus contrast at each position by the receptive field value and summing over all receptive field locations. Bright regions indicate excitatory responses, whereas dark areas indicate inhibitory responses. The receptive field shown is a vertical spatial Gabor in cosine phase: R(x,y) = exp(—(x + y2)/s2) ■ cos(2 ■ p ■ f ■ x), where s is related to the spatial width of the Gaussian and f is the spatial frequency of the sine wave. This receptive field responds strongly to bright vertical bars with widths matched to the cell's excitatory center (~0.5f). (B) Two temporal weighting functions described by Adelson and Bergen. The response amplitude as a function of time to a brief pulse at time 0 is plotted. For temporally extended stimuli, the neuron's response at a given time is the sum over previous times of the weighting function multiplied by the stimulus contrast. The two temporal response functions shown have different delays: The dashed line function's response is maximal at 23 msec, whereas the solid line function's response is maximal at 39 msec. (C) An x-t plot of a space-time separable receptive field. This receptive field is unselective for direction of motion and responds best to a static bar. It was constructed by multiplying the spatial receptive field shown in A by the temporal impulse response function shown by the solid line in B and taking an x-t slice through the resulting x-y-t space.

X-Axis Position

Figure 2 (A) A spatial receptive field. This is an x-y plot of a neuron's response to a spot of light at different positions. In general, the neuron's response to a spatially extended stimulus is computed by multiplying the stimulus contrast at each position by the receptive field value and summing over all receptive field locations. Bright regions indicate excitatory responses, whereas dark areas indicate inhibitory responses. The receptive field shown is a vertical spatial Gabor in cosine phase: R(x,y) = exp(—(x + y2)/s2) ■ cos(2 ■ p ■ f ■ x), where s is related to the spatial width of the Gaussian and f is the spatial frequency of the sine wave. This receptive field responds strongly to bright vertical bars with widths matched to the cell's excitatory center (~0.5f). (B) Two temporal weighting functions described by Adelson and Bergen. The response amplitude as a function of time to a brief pulse at time 0 is plotted. For temporally extended stimuli, the neuron's response at a given time is the sum over previous times of the weighting function multiplied by the stimulus contrast. The two temporal response functions shown have different delays: The dashed line function's response is maximal at 23 msec, whereas the solid line function's response is maximal at 39 msec. (C) An x-t plot of a space-time separable receptive field. This receptive field is unselective for direction of motion and responds best to a static bar. It was constructed by multiplying the spatial receptive field shown in A by the temporal impulse response function shown by the solid line in B and taking an x-t slice through the resulting x-y-t space.

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