In most West European countries pigs are housed indoors under low-light conditions. While former
Dutch National legislation for pigs required 12 lx for 8 h per day, EU legislation requires 40 lx, so pigs
should be able to distinguish small objects and subtle visual signals. However, limited scientific evidence
exists about the effects of light intensity (illumination) and object size on the pigs’ ability to distinguish
visual cues. We studied these two effects using operant conditioning with a Landolt-C symbol as shape
discriminator. Four Landolt-C symbols with different sizes (with gaps of, respectively, 10 mm(L10), 20 mm
(L20), 30 mm (L30) and 40 mm (L40)) were tested under 8 different illuminant levels (0.5, 3, 6, 12, 20, 30,
40 and 80 lx). Following a 4-week training period, 20 female 4-month-old pigs were tested in two batches
and the numbers of correct, hesitant and incorrect choices were recorded. Reduced illumination significantly
increased the number of incorrect choices (P < 0.05). Symbol size also had a significant effect (P < 0.001);
pigs made more mistakes with the two smaller symbols L10 (41.5%) and L20 (28.6%) compared to the
bigger symbols L30 (17.3%) and L40 (16.3%). Furthermore, pigs hesitated less with the smallest symbol
L10 (4.5%) compared to L20, L30 and L40 (17.3, 22.3 and 23.0%, respectively; P < 0.001). Visual acuity
(ability to distinguish details and shapes) varied from 0.001 to 0.03 and pigs failed to discriminate visual
cues below 20 mm. Results indicate that the detail of the visual cue (symbol size) has a more pronounced
effect on the pigs’ ability to distinguish visual cues, compared to the illuminant level.

For more information the full article can be found at http://journals.elsevierhealth.com/periodicals/applan/issues