Unequal representation of cardinal and oblique contours in ferret visual cortex

Abstract

We have measured the amount of cortical space activated by differently oriented gratings in 25 adult ferrets by optical imaging of intrinsic signal. On average, 7% more area of the exposed visual cortex was preferentially activated by vertical and horizontal contours than by contours at oblique angles. This anisotropy may reflect the real-world prevalence of contours in the cardinal axes and could explain the greater sensitivity of many animals to vertical and horizontal stimuli.

Figure 1

Optical imaging and analysis of orientation preference maps in the ferret visual cortex. (A) Image of the exposed region of the visual cortex in ferret 22 (see Fig. 2), showing typical appearance of the cortical surface (dotted line indicates the region analyzed in B and C). (B) Orientation preference map of the cortical regions responding best to different stimulus orientations (bar shows the orientation color code; bar length = 3 mm). (C) Histogram of the region of interest (see dotted line in A), showing the number of pixels (i.e., the area of cortex) that responded best to each orientation. Note the peaks near the vertical and horizontal meridians, which indicate that more cortex responded best to contours in the cardinal axes compared with those at oblique angles.

Figure 2

Amount of cortical space devoted to the analysis of different stimulus orientations. The overall profile of orientation versus cortical area: the black line is a polynomial fit to the raw pixel data combined from 25 animals; the gray band represents a similar fit to the mean ± SEM. The smooth profile indicates that we were not undersampling the cortex because of the small number of stimulus angles used.

Figure 3

Percentage difference in the amount of cortical area responding best to cardinal (0° and 90°) and oblique (45° and 135°) angles for each of the 25 animals examined. Data from the angle plot histograms (Fig. 1C) were collapsed around each orientation (±22.5°). Bars show the deviation from the zero line, which represents an equal distribution of cortical space to all orientations. Seventeen of the 25 ferrets examined showed a greater amount of cortex responding best to the cardinal axes.