Perception and Attention - Computational Explorations in Cognitive Neuroscience - page 246

2 3

0 1

Figure 8.17: Probe stimuli used to test the response proper-
ties of V2 and V4 units in the model.

Probe Stimulus-Based Receptive Field Analysis

For another perspective on how the V2 and V4 units are
encoding the visual inputs, we can use the probe stimu-
lus technique and observe the unit's responses, just like
we did in the previous simulation. In the present case,
we present the 4 different probe stimuli shown in fig-
ure 8.17, and record the V2 and V4 unit's responses,
which can be displayed in a grid log much like the
activation-based receptive fields we just examined.

Select View , PROBE_RF in the control panel. Press
Open in the file selection window that appears as before,
and a grid log window will appear (figure 8.18). Another
file selection window will also appear, and you can again
move that aside while you examine the current display.
This log displays the responses of a hypercolumn of
V2 units to the probe stimuli. To view the responses
to all 4 probes in one display, we presented the probes
in only one quarter of the possible locations (the lower
left set of 8x8 LGN locations), and then plotted the
units' responses for each probe as one quarter of the

Figure 8.18: V2 probe response fields. The large 8 x 8 grid
represents one hypercolumn's worth of units, with each cell
containing the probe receptive field for that unit as described
in the previous figure.

Question 8.7 (a) Based on the number of units that
responded to this small subset of features compared to
those that did not significantly respond (i.e, did not have
at least one yellowish dot of activity), what can you con-
clude about the selectivity of the V2 units for respond-
ing to particular features? (b) For those units that were
significantly active, based on the number of different lo-
cations for which the unit was active (i.e., the area of
colored pixels in the display), would you say that these
units exhibited at least some degree of spatial invari-
ance? Explain. (c) How do these results correspond
with the conclusions based on the activation based re-
ceptive fields examined previously?

grid shown in the display, with the position of
the probe corresponding to those shown in figure 8.17.
Figure 8.19 illustrates this arrangement. Thus, the re-
sponses over the 8 x 8 locations for the lower left hand
probe in the figure (probe 0) are shown in the lower left

cells of the overall grid, and those for the upper
right probe in the figure (probe 3) are in the upper right

8 x 8

cells of the overall grid. Thus, a unit that responded
selectively to only one probe, but did so in all of its lo-
cations (i.e., had a highly spatially invariant represen-
tation), would show up in the display as having a solid
yellow (active) color in the corresponding quarter of its
grid.

8 x 8

Press Open to the next file selection window, which
will bring up the probe receptive fields for the V4 units.